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Quantum Chemical Evidence for an Intramolecular Charge-Transfer State in the Carotenoid Peridinin of Peridinin−Chlorophyll−Protein
Abstract
We present theoretical confirmation of an intramolecular charge-transfer (CT) state in peridinin in agreement with experimental results of Frank and co-workers (J. Phys. Chem. B 1999, 103, 8751 and J. Phys. Chem. B 2000, 104, 4569). Quantum chemical calculations using time-dependent density functional theory under the Tamm−Dancoff approximation were made on two structures: fully optimized peridinin and a molecule from the crystal structure of peridinin−chlorophyll−protein. The CT state appears as the third and second excited singlet state, respectively, for the two structures. A dipole-in-a-sphere model is used to estimate the solvation stabilization energies of each state in a variety of solvents. The energy of the CT state is shown to decrease dramatically in solvents of increasing polarity while the energy of the dark S1 (Ag--like) state remains comparatively constant.
... Quantum chemical calculations using the time-dependent density functional theory with the Tamm−Dancoff approximation [14][15][16] demonstrate presence of CT state, which appears as the third and second excited singlet state, respectively. Energy of the CT state has been shown to decrease dramatically in solvents of increasing polarity, while the energy of the dark S 1 state remains comparatively constant 17 . Several other types of electronic excited states have been suggested, however, their existence and involvement in relaxation process is still debatable 18 . ...
... The resulting operators in Eqs. (16)(17) read ...
... here f ep is the electron-phonon coupling strength of the pth phonon mode to the electronic state e. The electronic ground state is taken as the reference point so it is not affected by bath fluctuations f gp = 0. Notice, that the system-bath coupling has the same form as the last term in Eq. (17). The electronic state energy modulation by the intramolecular and intermolecular vibrations is treated equivalently. ...
Electronic absorption spectrum of beta-carotene (b-Car) is studied using quantum chemistry and quantum dynamics simulations. Vibrational normal modes were computed in optimized geometries of the electronic ground state S0 and the optically bright excited S2 state using the time-dependent density functional theory. By expressing the S2 state normal modes in terms of the ground state modes, we find that no one-to-one correspondence between the ground and excited state vibrational modes exists. Using the ab initio results, we simulated b-Car absorption spectrum with all 282 vibrational modes in a model solvent at 300K using the time-dependent Dirac-Frenkel variational principle (TDVP) and are able to qualitatively reproduce the full absorption lineshape. By comparing the 282-mode model with the prominent 2-mode model, widely used to interpret carotenoid experiments, we find that the full 282-mode model better describe the high frequency progression of carotenoid absorption spectra, hence, vibrational modes become highly mixed during the S0 -> S2 optical excitation. The obtained results suggest that electronic energy dissipation is mediated by numerous vibrational modes.
... Nonetheless, this framework ignores the unique chemical nature of the peridinin Car-containing an allene tail and a lactone ring-that may support intramolecular-charge-transfer (ICT) character and even a separate ICT state [20]. Yet much like the claims of spectral evidence for dark intermediate states, the nature of ICT in peridinin remains controversially assigned, as evidenced by twenty-first-century experimental [21][22][23] and computational [24][25][26][27] investigations. On one hand, red-shifted emission with diminished lifetime in the face of polar solvent [20] and a distinct low-lying electronic excited state with ICT character [24] have been reported; on the other hand, conformational bright-state local minima [21,22] and bond-vibration-driven interplay between the lowest-lying bright-and dark-state properties [25] have been proposed as explanations for peridinin's anomalous photophysics. ...
... Yet much like the claims of spectral evidence for dark intermediate states, the nature of ICT in peridinin remains controversially assigned, as evidenced by twenty-first-century experimental [21][22][23] and computational [24][25][26][27] investigations. On one hand, red-shifted emission with diminished lifetime in the face of polar solvent [20] and a distinct low-lying electronic excited state with ICT character [24] have been reported; on the other hand, conformational bright-state local minima [21,22] and bond-vibration-driven interplay between the lowest-lying bright-and dark-state properties [25] have been proposed as explanations for peridinin's anomalous photophysics. Whether ICT character exists as its own independent electronic excited state, and whether it impacts S 2 /S 1 state properties, may have important implications in energy transfer. ...
... Interface 17: 20190736 multichromophoric states. As summarized in table 4 and figure 3, these states, which we call S* and CT (not the intramolecular CT state intrinsic to peridinin [24], but rather a dimeric CT) that appear in the respective Car-Car and Car-Chl systems (vide infra), open up alternative pathways to intrinsic S 2 -S 1 Car internal conversion by spreading electronic excitation energy across multiple chromophores. ...
It has long been recognized that visible light harvesting in Peridinin-Chlorophyll-Protein is driven by the interplay between the bright (S2) and dark (S1) states of peridinin (carotenoid), along with the lowest-lying bright (Qy) and dark (Qx) states of chlorophyll-a. Here, we analyse a chromophore cluster in the crystal structure of Peridinin-Chlorophyll-Protein, in particular, a peridinin-peridinin and a peridinin-chlorophyll-a dimer, and present quantum chemical evidence for excited states that exist beyond the confines of single peridinin and chlorophyll chromophores. These dark multichromophoric states, emanating from the intermolecular packing native to Peridinin-Chlorophyll-Protein, include a correlated triplet pair comprising neighbouring peridinin excitations and a charge-transfer interaction between peridinin and the adjacent chlorophyll-a. We surmise that such dark multichromophoric states may explain two spectral mysteries in light-harvesting pigments: the sub-200-fs singlet fission observed in carotenoid aggregates, and the sub-200-fs chlorophyll-a hole generation in Peridinin-Chlorophyll-Protein.
... On the same optimized geometries, we have performed TD-DFT calculations [23,24] to obtain S 0 → S 2 transition energies, with the B3LYP functional and 6-311G(d,p) basis set (TD-B3LYP). Also the Tamm-Dancoff approximation [25] was used in TD-DFT calculations (TDA-B3LYP), since Vaswani et al. [26] have reported that TDA-DFT was better able to reproduce results on another carotenoid (peridinin) than normal TD-DFT. The transition to the 1 1 B u + bright excited state, corresponding to what observed experimentally, has mainly a HOMO → LUMO character and thus TD-DFT is able to characterize it correctly. ...
... TDA-B3LYP results are in better agreement with experiments than TD-B3LYP, but the trend observed is the same. Note that this is not surprising since Vaswani et al. [26] have already reported that the Tamm-Dancoff approximation (TDA) improves the reliability of TD-DFT methods in the case of carotenoids. Here and hereafter we use TDA-B3LYP results. ...
Orange carotenoid protein (OCP) is a cyanobacterial photoactive protein which binds echinenone as a chromophore; it is involved in photoprotection of these photosynthetic organisms against intense illumination. In its resting state, OCP appears orange (OCPo), and turns into a red form (OCPr) when exposed to blue-green light. Here we have combined resonance Raman spectroscopy and molecular modeling to investigate the mechanisms underlying the electronic absorption properties of the different forms of OCP. Our results show that there are at least two carotenoid configurations in the OCPo, suggesting that it is quite flexible, and that the OCPo to OCPr transition must involve an increase of the apparent conjugation length of the bound echinenone. Resonance Raman indicates that this chromophore must be in an all-trans configuration in OCPo. Density functional theory (DFT) calculations, in agreement with the Raman spectra of both OCP forms, show that the OCPo to OCPr transition must involve either an echinenone s-cis to s-trans isomerization which would affect the position of its conjugated end-chain rings, or a bending of the echinenone rings which would bring them from out of the plane of the C=C conjugated plane in the OCPo form into the C=C plane in the OCPr form.
Copyright © 2015. Published by Elsevier B.V.
... The higher energy-absorption peaks between l = 366 and 388 nm are assigned to the pp* transitions, whereas the lower energy-absorption peaks at l = 456 and 480 nm are attributed to the ICT transitions. [18] The 78, see below) between the DTQ moiety and the phenyl rings may be beneficial for inhibiting intermolecular H aggregation as a result of the large dimensions of the DTQ core and may facilitate photoelectronic performance, similarly to that reported for 2,3-diphenylquinoxaline-based dyes. [11b] Nevertheless, incorporation of the n-hexyloxy chains on the phenyl rings was found to be necessary for decent solubility of the dyes in common organic solvents. ...
... There exist a number of previous works that employed TD-DFT to characterize excited states with charge-transfer character. [18,30] The excitation energies are underestimated in some cases. [31] Thus, TD-DFT was used to extract the extent of the transition moments, as well as their chargetransfer characters, without drawing conclusions from the excitation energy. ...
New donor-acceptor'-acceptor-type sensitizers (QBT dyes), comprising arylamine as the electron donor, dithieno[3,2-f:2',3'-h]quinoxaline as the internal acceptor, and 2-cyanoacrylic acid as both the acceptor and anchor, have been synthesized. The QBT dyes have broad absorption spectra covering the range of λ=368-487 nm with a highest molar extinction coefficient of up to approximately 40 000 M(-1) cm(-1) . The light-to-electricity conversion efficiencies of the dye-sensitized solar cells (DSSCs) fabricated from the dyes range from 6.11 to 7.59 % under simulated AM 1.5 G illumination. Upon addition of a threefold concentration of chenodeoxycholic acid as the co-adsorbent, the best performance cell has a power-conversion efficiency of 8.41 %, which is higher than that of the N719-based standard DSSC (8.27 %).
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
... There exist a number of previous works that employed TD−DFT to characterize excited states with charge-transfer character. 41,42 In some cases underestimation of the excitation energies was seen. 43 Therefore, TD−DFT was used to visualize the extent of transition moments as well as their chargetransfer characters without drawing conclusions from the excitation energy. ...
... The high energy absorption peaks between 366 and 380 nm are assigned to the π−π* transitions, whereas the low energy peak at 462 and 464 nm are attributed to the intramolecular charge transfer (ICT) transitions. 41 Due to the large dimension of DTBF, incorporating the long hydrocarbon chains at the thienyl rings is necessary for good solubility of the dyes in common organic solvents and suppression of dye aggregation. However, the presence of these chains leads to large dihedral angles (33.1−44.2°, ...
New D-π-A'-π-A type sensitizers (JH dyes), comprised arylamine as the electron donor, dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c]furazan (DTBF) in the conjugated spacer, and 2-cyanoacrylic acid as both the acceptor and anchor, have been synthesized. The JH dyes have broad absorption spectra covering the range of 350 to 600 nm with the highest molar extinction coefficient up to >40000 M-1cm-1. The dye-sensitized solar cells (DSSCs) fabricated from the dyes exhibited light-to-electricity conversions ranging from 1.42 to 6.18% under simulated AM 1.5 G illumination. Upon adding 10 mM CDCA as the co-adsorbent, the best performance cell has the power conversion efficiency of 7.33%, which is close to that of N719-based standard DSSC (7.56%).
... [22] There exist a number of previous works that employed TDDFT to characterize excited states with charge-transfer character. [23,24] In some cases, underestimation of the excitation energies was seen. [23][24][25] Therefore, we avoid drawing conclusions from the excitation energy, though TDDFT was used to characterize the extent of the charge-shift. ...
... [23,24] In some cases, underestimation of the excitation energies was seen. [23][24][25] Therefore, we avoid drawing conclusions from the excitation energy, though TDDFT was used to characterize the extent of the charge-shift. ...
Metal-free dyes (MD1 to MD5) containing an anthracene/phenothiazine unit in the spacer have been synthesized. The conversion efficiency (7.13 %) of the dye-sensitized solar cell using MD3 as the sensitizer reached approximately 85 % of the N719-based standard cell (8.47 %). The cell efficiency (8.42 %) of MD3-based dye-sensitized solar cells (DSSCs) with addition of chenodeoxycholic acid is comparable with that of N719-based standard cell. The MD3 water-based DSSCs using a dual-TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)/iodide electrolyte exhibited very promising cell performance of 4.96 % with an excellent Voc of 0.77 V.
... In Fx, an additional excited state with an intramolecular charge transfer (ICT) character exists. This ICT state can be coupled to the S 1 state and plays a major role in carotenoid-Chl energy transfer (Bautista et al. 1999;Vaswani et al. 2003;Zigmantas et al. 2004;Papagiannakis et al. 2005;Premvardhan et al. 2005;Gelzinis et al. 2015;West et al. 2018). In addition, Fx displays an extreme bathochromic shift upon protein binding, extending the absorption from 390 nm up to 580 nm. ...
... In Fx, an additional excited state with an intramolecular charge transfer (ICT) character exists. This ICT state can be coupled to the S 1 state and plays a major role in carotenoid-Chl energy transfer (Bautista et al. 1999;Vaswani et al. 2003;Zigmantas et al. 2004;Papagiannakis et al. 2005;Premvardhan et al. 2005;Gelzinis et al. 2015;West et al. 2018). In addition, Fx displays an extreme bathochromic shift upon protein binding, extending the absorption from 390 nm up to 580 nm. ...
Light harvesting and photochemistry is performed by photosystems coupled to specific antennae embedded in the thylakoid membrane, a common principle across diatoms, plants, and green algae. Still, unique features of diatoms within this common principle have been unraveled in recent decades, likely resulting from the complex evolutionary history of diatoms. These unique features are found in (1) the lipid composition of the thylakoid membrane, (2) the spatial organization of the light-harvesting complexes, and (3) their protein and pigment composition. This chapter summarizes current knowledge of these three specific features, with a focus on structural and functional properties.KeywordsDiatomsFCPLHCLight harvestingLipidsThylakoidsXanthophyll cycle
... The energy of the CT state has been shown to decrease dramatically in solvents of increasing polarity, while the energy of the dark S 1 state remains comparatively constant. 17 Several other types of electronic excited states have been suggested; however, their existence and involvement in the relaxation process are still debatable. 18 Specific spectral features have been assigned to S 1 and CT states, and these may play an important role in de-excitation processes. ...
The electronic absorption spectrum of β-carotene (β-Car) is studied using quantum chemistry and quantum dynamics simulations. Vibrational normal modes were computed in optimized geometries of the electronic ground state S0 and the optically bright excited S2 state using the time-dependent density functional theory. By expressing the S2-state normal modes in terms of the ground-state modes, we find that no one-to-one correspondence between the ground- and excited-state vibrational modes exists. Using the ab initio results, we simulated the β-Car absorption spectrum with all 282 vibrational modes in a model solvent at 300 K using the time-dependent Dirac–Frenkel variational principle and are able to qualitatively reproduce the full absorption line shape. By comparing the 282-mode model with the prominent 2-mode model, widely used to interpret carotenoid experiments, we find that the full 282-mode model better describes the high-frequency progression of carotenoid absorption spectra; hence, vibrational modes become highly mixed during the S0 → S2 optical excitation. The obtained results suggest that electronic energy dissipation is mediated by numerous vibrational modes.
... Photoexcitation into a bright S 2 excited state of carotenoids converts on ultrafast time scales (less than a few hundred of femtoseconds) to an intermediate dark S 1 excited state, which subsequently converts to the ground state, S 0 . Although excited state dynamics of carotenoids have been extensively studied experimentally by time-resolved electronic and vibrational spectroscopy [9][10][11][12][13][14][15][16] and theoretically by time-dependent density functional theory methods [17][18][19][20] , details of the excited state dynamics of carotenoids, for example, the existence of intermediate dark states between the S 2 and S 1 state and the electronic structure and dynamics in the hydrogen bonding environment of the photosynthetic proteins, are still in question 6 . ...
Metal-enhanced fluorescence of carotenoids, all-trans-beta-carotene and 8'-apo-beta-carotene-8'-al dispersed in thin layers of polystyrene and polyethylene glycol were investigated by time-resolved fluorescence spectroscopy. The weak emission signals of carotenoids in polymer films were increased by 4-40 times in the presence of a silver island film and the emission lifetimes of both carotenoids were measured as significantly shortened. The energy transfer from the intermediate states of carotenoids to the silver islands and the subsequent surface plasmon coupled emission were proposed for the mechanisms of metal-enhanced fluorescence. The fluorescence enhancements of carotenoids in the polymer films were also investigated statistically over a wide area of the silver island films.
... There are two major peaks in the visible region: the high energy absorption due to the π−π* transition and the low energy peak ascribed to the intramolecular charge transfer (ICT) transitions with π−π* transition character. 20 In order to alleviate dye aggregation and improve the solubility of the CTY dyes in a common solvent, a 2-ethylhexyl substituent is incorporated at QBT-3, CTY-1 has slightly red-shifted absorption and has a higher molar extinction coefficient (ε). This is in accordance with the report that a 2-thienyl group is inductively more electron-withdrawing than a phenyl group, leading to a lower LUMO energy level. ...
Metal-fee D–π–RS–π–A type sensitizers, consisting of triphenylamine as the electron donor, 2,3-bis(3-(2-ethylhexyl)-5-methylthiophen-2-yl)dithieno[3,2-f:2',3'-h]quinoxaline (DTQT) as the rigidified conjugation spacer (RS), thiophene as the π-spacer, and 2-cyanoacrylic acid as the acceptor/anchor, have broad absorption spectra ranging from 350 to 550 nm and high molar extinction coefficient up to >46,200 M-1cm-1. Under simulated AM 1.5 G illumination, the dye-sensitized solar cells (DSSCs) fabricated from the dyes exhibited light-to-electricity conversions in the range of 6.78 to 8.27%. The best efficiency is slightly higher than that of N719-based standard DSSC (7.92%). The efficiency can be further boosted to 8.51% by optimizing the concentration of LiI electrolyte.
... Electronic properties were investigated by performing theoretical calculations using the B3LYP/6e31G* level of density functional theory [32] (DFT; Fig. 3a) and the optimized structures of the compounds at the DFT level are shown in Fig 3b. Long alkyl chains were replaced by ethyl groups for computational simplicity. ...
... In general, we have found that TDA provides a slightly 190,191] We therefore continue below with the TDA and note that the CT excited state energies are found to have even smaller difference between the TDA and the full TD-DFT energies. [52,[91][92][93][190][191][192][193][194] The current study is focused on octahedral silsesquioxane (OHSQ) functionalized with trans-stilbene that has been shown to result in the large shift of the emission spectrum of up to 0.9 eV. [132] The systems studied via experimental methods are assumed to be fully functionalized by eight chromophores, as shown in Figure 3. red-shifted emission. ...
A fundamental understanding of charge separation in organic materials is necessary for the rational design of optoelectronic devices suited for renewable energy applications and requires a combination of theoretical, computational, and experimental methods.
Density functional theory (DFT) and time-dependent (TD)DFT are cost effective ab-initio approaches for calculating fundamental properties of large molecular systems, however conventional DFT methods have been known to fail in accurately characterizing frontier orbital gaps and charge transfer states in molecular systems. In this dissertation, these shortcomings are addressed by implementing an optimally-tuned range-separated hybrid (OT-RSH) functional approach within DFT and TDDFT.
The first part of this thesis presents the way in which RSH-DFT addresses the shortcomings in conventional DFT. Environmentally-corrected RSH-DFT frontier orbital energies are shown to correspond to thin film measurements for a set of organic semiconducting molecules. Likewise, the improved RSH-TDDFT description of charge transfer excitations is benchmarked using a model ethene dimer and silsesquioxane molecules.
In the second part of this thesis, RSH-DFT is applied to chromophore-functionalized silsesquioxanes, which are currently investigated as candidates for building blocks in optoelectronic applications. RSH-DFT provides insight into the nature of absorptive and emissive states in silsesquioxanes. While absorption primarily involves transitions localized on one chromophore, charge transfer between chromophores and between chromophore and silsesquioxane cage have been identified. The RSH-DFT approach, including a protocol accounting for complex environmental effects on charge transfer energies, was tested and validated against experimental measurements.
The third part of this thesis addresses quantum transport through nano-scale junctions. The ability to quantify a molecular junction via spectroscopic methods is crucial to their technological design and development. Time dependent perturbation theory, employed by non-equilibrium Green???s function formalism, is utilized to study the effect of quantum coherences on electron transport and the effect of symmetry breaking on the electronic spectra of model molecular junctions.
The fourth part of this thesis presents the design of a physical chemistry course based on a pedagogical approach called Writing-to-Teach. The nature of inaccuracies expressed in student-generated explanations of quantum chemistry topics, and the ability of a peer review process to engage these inaccuracies, is explored within this context.
... The same functional was also applied for the calculation of excited states using time-dependent density functional theory (TD−DFT). TD−DFT was employed to characterize excited states with charge-transfer character in a number of previous works, 46 and underestimation of the excitation energies was seen in some cases. 47 Therefore, we use TD−DFT to visualize the extent of transition moments as well as their charge-transfer characters, and avoid drawing conclusions from the excitation energy. ...
Dipolar dyes comprising an arylamine as the electron donor, a cyanoacrylic acid as electron acceptor, and an electron deficient naphtho[2,3-c][1,2,5]thiadiazole (NTD) or naphtho[2,3-d][1,2,3]triazole (NTz) entity in the conjugated spacer, were developed and used as the sensitizers in dye-sensitized solar cells (DSSCs). The introduction of the NTD unit into the molecular frame distinctly narrows the HOMO/LUMO gap with electronic absorption extending to >650 nm. However, significant charge trapping and dye aggregation were found in these dyes. Under standard global AM 1.5 G illumination, the best cell photovoltaic performance achieved 6.37% and 7.53% (~94% relative to N719-based standard cell) without and with CDCA co-adsorbent, respectively. Without CDCA, the NTz dyes have higher power conversion efficiency (7.23%) than NTD dyes due to less charge trapping, dye aggregation and better dark current suppression.
... A number of previous works employed TD-DFT to characterize the excited states with charge-transfer character. 37,38 The excitation energies were underestimated in some cases. 37−39 In the present work, we therefore use TD-DFT to visualize the extent of transition moments, as well as their charge-transfer character, and avoid drawing conclusions from the excitation energy. ...
New D‒A‒π‒A based isomeric sensitizers, PTNn (n = 1-2) and NPTn (n = 1-5), were synthesized using 2H-[1,2,3]triazolo[4,5-c]pyridine (PT) as an auxiliary acceptor, triphenylamine or N,N-bis(4-(hexyloxy)phenyl)aniline as the donor, furan, thiophene, phenyl or 3-hexylthiophene as the conjugated spacer, and the 2-cyanoacrylic acid as the acceptor and anchor as well. They were used as the sensitizers of dye-sensitized solar cells. The NPTn dyes show better performance than the PTNn dyes. Among them the best efficiency of 7.92% (~96%, N719) was obtained with the NPT5 dye, indicating that PT core could be used as a new building block for the design of high-performance sensitizers in the future. The negative Mulliken charge from the auxiliary acceptor was found useful as a semi-empirical index for correlation of molecular structure with the cell efficiency among structurally similar D-A-π-A type congeners.
... The same functional was also applied for the calculation of excited states using time-dependent density functional theory (TD-DFT). There exist a number of previous works that employed TD-DFT to characterize excited states with charge-transfer character [37,38]. In some cases underestimation of the excitation energies was seen [36,39]. ...
A series of new organic dyes comprising different amines as electron donors, 2-(6-substituted-anthracen-2-yl)-thiophene as the π-conjugated bridge, and cyanoacrylic acid group as an electron acceptor and anchoring group, have been synthesized. There exists charge transfer transition from arylamine and anthracene to the acceptor in these compounds, as evidenced from the photophysical measurements and the computational results. Under one sun (AM 1.5) illumination, dye-sensitized solar cells (DSSCs) using these dyes as the sensitizers exhibited efficiencies ranging from 1.62% to 2.88%, surpassing that using 9,10-difunctionalized anthracene-based sensitizer.
... 7,8,10,16,21 Although it is commonly accepted that the shortening of peridinin excited state lifetime is ascribable to the occurrence of the ICT state, the exact electronic nature and formation mechanism of ICT are still debated. 9,10,15,22,23 The presence of the carbonyl group, primarily responsible for setting up and stabilizing the charge transferred electronic structure, has a great influence on both the stationary and transient absorption spectra of peridinin. In the ground state absorption spectrum, the clear vibronic progression observed in non-polar solvents is smeared out in polar media, where the absorption band broadens and shifts to the red. ...
By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relax- ation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C==C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitationwavelength.
... Carotenoids with carbonyl substitution such as fucoxanthin [59] and peridinin [58,[60][61][62] would be expected to be especially prone to the formation of an enhanced ICT character because the addition of an electron-withdrawing group in conjugation increases the tendency to fully transfer charge across a twisting C@C bond. In peridinin, for example, Stark absorption experiments [63,64] reveal that a very large change in permanent dipole moment accompanies the S 0 ? ...
A consideration of the excited state potential energy surfaces of carotenoids develops a new hypothesis for the nature of the conformational motions that follow optical preparation of the S2 (1(1)Bu(+)) state. After an initial displacement from the Franck-Condon geometry along bond length alternation coordinates, it is suggested that carotenoids pass over a transition-state barrier leading to twisted conformations. This hypothesis leads to assignments for several dark intermediate states encountered in femtosecond spectroscopic studies. The Sx state is assigned to the structure reached upon the onset of torsional motions near the transition state barrier that divides planar and twisted structures on the S2 state potential energy surface. The X state, detected recently in two-dimensional electronic spectra, corresponds to a twisted structure well past the barrier and approaching the S2 state torsional minimum. Lastly, the S(∗) state is assigned to a low lying S1 state structure with intramolecular charge transfer character (ICT) and a pyramidal conformation. It follows that the bent and twisted structures of carotenoids that are found in photosynthetic light-harvesting proteins yield excited-state structures that favor the development of an ICT character and optimized energy transfer yields to (bacterio)chlorophyll acceptors.
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The water–gas shift reaction (WGSR) is important in industries because it can reduce the CO content of syngas to produce purified H2, which can be used as fuel or to make ammonia (NH3). Supported noble metal catalysts have been widely studied for the WGSR because they exhibit high reactivity. However, the role of a metal–support interface in the WGSR has not yet been revealed and remains elusive. Density functional theory (DFT) calculations were performed for a model system of Co3O4-supported Pd (Pd/Co3O4) catalysts. The presence of the interface was found to promote the H2O dissociation step, which is crucial for improving WGSR activity. Thus, the WGSR activity was predicted to be enhanced by an increased number of interfaces, which could be achieved by controlling the size of the supported Pd nanoparticles (NPs). Furthermore, electronic metal–support interactions (MSIs) were found to be a source of the promoted H2O dissociation at the interface. The DFT-predicted promotion of H2O dissociation was further experimentally validated using Pd/Co3O4 catalysts that were size-controlled with calcination temperatures, and the total length of the interface was shown to have a direct correlation with the WGSR rate. Theoretical insights into the role of the interface and the enhancement of WGSR activity due to increased interface sites, which can be achieved by size control, are believed to be useful in the design of efficient supported metal catalysts for the WGSR.
The coding regions for the N-domain, and full length peridinin–chlorophyll a apoprotein (full length PCP), were expressed in Escherichia coli. The apoproteins formed inclusion bodies from which the peptides could be released by hot buffer. Both the above constructs were reconstituted by addition of a total pigment extract from native PCP. After purification by ion exchange chromatography, the absorbance, fluorescence excitation and CD spectra resembled those of the native PCP. Energy transfer from peridinin to Chl a was restored and a specific fluorescence activity calculated which was ~86% of that of native PCP. Size exclusion analysis and CD spectra showed that the N-domain PCP dimerized on reconstitution. Chl a could be replaced by Chl b, 3-acetyl Chl a, Chl d and Bchl using the N-domain apo protein. The specific fluorescence activity was the same for constructs with Chl a, 3-acetyl Chl a, and Chl d but significantly reduced for those made with Chl b. Reconstitutions with mixtures of chlorophylls were also made with eg Chl b and Chl d and energy transfer from the higher energy Qy band to the lower was demonstrated.
Carotenoids are naturally occurring pigments and essential for light-harvesting and photoprotection in photosynthesis of plants, algae, and bacteria. Carbonyl-containing carotenoids exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In the present study, we performed femtosecond pump-probe spectroscopy on the short-chain polyene compound 2-(all-trans-β-ionylideneetinylidene)-indan-1,3-dione with three conjugated double bonds and two carbonyl groups, denoted as C15Ind in acetone and in n-hexane. The spectroscopic properties of C15Ind both in polar and nonpolar solvents suggests an ICT character based on the observations of the stimulated emission due to the coupled ¹Ag⁻/ICT state. In acetone, an additional excited state was found to be located below the ¹Ag⁻/ICT state. The ultrafast triplet generation via the ¹nπ* state was also observed both in acetone and in n-hexane, while the amplitude of the triplet transient absorption of C15Ind was rather small compared with those in retinal and Retinyl-1 reported previously. Thus, an ICT character of carbonyl-containing carotenoids may prevent a triplet generation upon excitation into the optically allowed ¹Bu⁺ (S2) state.
Dye aggregation causes poor performance of dye-sensitized solar cell (DSSC) applications through faster charge recombination of the photosensitizer with electrolyte. Triphenylamine (TBA)-based dyes feature a higher molar absorption coefficient and broadened wavelength but cannot absorb sunlight in the near-infrared (NIR) region. In contrast, the squaraine (SQ) photosensitizer, which is also called an NIR photosensitizer, has a maximum wavelength in the NIR region with high intensity. However, SQ dye suffers from dye aggregation due to its planar structure. The use of a co-sensitizer is one well-tested way to increase the power conversion efficiency (η) of solar cells by reducing dye aggregation and charge recombination. Using density functional theory (DFT) and time-dependent DFT (TDDFT), this work explains from a theoretical perspective the higher η values of the TZC1 and TZC2 dyes compared to that of asymmetric the SQ sensitizer (YR6) as free dyes. The electronic properties, reorganization energies, absorption and emission spectra, ICT parameters, and photovoltage parameters of the TZC1, TZC2, and YR6 dyes were computed using the M06/6-31G(d,p) level of theory in the gas phase and CH2Cl2 solvent (CPCM method). Additionally, the mono- and co-adsorption processes of TZC-based sensitizers with YR6 on the anatase (001) surface were investigated using periodic DFT calculations with the PBE+U/PAW method and the dispersion correction of the Grimme method D3. The results reveal that the use of the co−sensitized led to significant stabilization of the formed complexes by at least 1.21 eV, the panchromatic effect on the absorption spectra, and an increase in the light-harvesting ability in the NIR region, which improves the performance of DSSCs.
Cyanobacteria are photosynthetic organisms capable of CO₂ conversion into organic compounds and production of O2 by using light energy. Nevertheless, high light intensities saturate the photosynthetic apparatus leading to production of reactive oxygen species, which are dangerous for the cell. To cope with this, the photoactive Orange Carotenoid Protein (OCP) induces thermal dissipation of the excess energy harvested by the antenna complex, the phycobilisome (PBS) to decrease the energy arriving at the photochemical centers. The OCP is composed of two domains connected by a flexible linker, the C-terminal domain (CTD) and the N-terminal domain (NTD). During photoactivation, the carotenoid is translocated to the NTD, the domains separate and the NTD is able to interact with the PBS. Three OCP families co-exist (OCPX, OCP1 and OCP2) in modern cyanobacteria. In addition to the OCP, many cyanobacteria also contain homologs of OCP domains, the CTDH and HCP. The HCPs are a family of carotenoid proteins with different photoprotective traits. Most of them are very good singlet oxygen quenchers, and one sub-clade is able to interact with the PBS and to induce thermal energy dissipation like OCP. The role of CTDH was unknown. The presence of these homologs in parallel to the OCP supported the general idea that the OCP has a modular evolutionary origin and that the CTDH and HCP can interact forming an OCP-like complex with similar characteristics and function than the OCP.In this thesis, I present the first characterization of the CTDH proteins. CTDHs are dimers binding a carotenoid molecule. The main role of the CTDH is to transfer its carotenoid to the HCP. In addition, CTDHs are able to uptake carotenoids from membranes but not HCPs. These results strongly suggested that the CTDHs are carotenoid carriers that ensure the proper carotenoid loading into HCPs. This novel carotenoid translocation mechanism could be multidirectional. The resolution of two tridimensional structures of the ApoCTDH from Anabaena showed that the C-terminal tail of the CTDH (CTT) can populate different conformations. Moreover, mutational analysis demonstrated that the CTT has an essential role in carotenoid translocation. Finally, I report a molecular characterization of the flexible linker connecting the domains of different modern OCPs and its role during the evolution of the OCP. First, I characterized OCPs from the three subclades, including the uncharacterized OCPX. OCPX and OCP2 present a fast deactivation compared with OCP1. While OCP1 and OCPX can dimerize, OCP2 is stable as monomer. Finally, I found that the linker is essential for the OCP deactivation and it regulates the photoactivation. In OCP1 and OCPX the linker slows down the photoactivation, while in OCP2 it increases the photoactivation rate. Bioinformatic analysis complements this characterization and provides a clear picture of the evolution of the OCP to respond efficiently to stress conditions.
The driving force of electron transfer is one of important factors for initializing inter‐ and intramolecular charge separation. In this work, the main goal is to understand how driving force determines electron transfer pathway in subphthalocyanine‐AzaBODIPY‐C60 supramolecular triad. Experimental observations have suggested that there are only two intramolecular charge transfer states (subPC+‐AzaBODIPY−‐C60 and subPC+‐AzaBODIPY‐C60−) after photon absorption, where subPC is the donor. Through the calculations by using tuned long range corrected density functionals with polarizable continuum model, we find two more new intramolecular charge transfer states: subPC−‐AzaBODIPY+‐C60 and subPC‐AzaBODIPY+‐C60−, where AzaBODIPY is the donor. We compare the HOMO/LUMO energy of subPC, AzaBODIPY, and C60 monomers to their corresponding orbital energy in the triad. The results indicate that the driving force (HOMO/LUMO energy offsets) is not enough for electron transfer from AzaBODIPY to subPC or C60, which can explain why subPC−‐AzaBODIPY+‐C60 and subPC‐AzaBODIPY+‐C60− intramolecular charge transfer states cannot be observed in the experiment. In addition, this work may provide a simple and practical method to find the intramolecular charge transport pathway of a supramolecule. Experimental observations have suggested that there are only two intramolecular charge transfer states (subPC+‐AzaBODIPY−‐C60 and subPC+‐AzaBODIPY‐C60−) after photo absorption in this triad. Calculated results indicates that driving force (HOMO/LUMO energy offsets) is not enough for electron transfer from AzaBODIPY to subPC or C60, which explains why subPC−‐AzaBODIPY+‐C60 and subPC‐AzaBODIPY+‐C60− states cannot be observed in experiment. This work may provide a simple and practical method to find intramolecular charge transport pathway in supramolecules.
To prevent irreversible damage caused by an excess of incident light, the photosynthetic machinery of many cyanobacteria uniquely utilizes the water-soluble orange carotenoid protein (OCP) containing a single keto-carotenoid molecule. This molecule is non-covalently embedded into the two OCP domains which are interconnected by a flexible linker. The phenomenon of OCP photoactivation, causing significant changes in carotenoid absorption in the orange and red form of OCP, is currently being thoroughly studied. Numerous additional spectral forms of natural and synthetic OCP-like proteins have been unearthed. The optical properties of carotenoids are strongly determined by the interaction of their electronic states with vibrational modes, the surrounding protein matrix, and the solvent. In this work, the effects of the pigment-protein interaction and vibrational relaxation in OCP were studied by computational simulation of linear absorption. Taking into account Raman spectroscopy data and applying the multimode Brownian oscillator model as well as the cumulant expansion technique, we have calculated a set of characteristic microparameters sufficient to demarcate different carotenoid states in OCP forms, using the model carotenoids spheroidene and spheroidenone in methanol/acetone solution as benchmarks. The most crucial microparameters, which determine the effect of solvent and protein environment, are the Huang-Rhys factors and the frequencies of C=C and C-C stretching modes, the low-frequency mode and the FWHM due to inhomogeneous line broadening. Considering the difference of linear absorption between spheroidene and spheroidenone, which remarkably resembles the photoinduced changes of OCP absorption, and applying quantum chemical calculations, we discuss structural and functional determinants of carotenoid binding proteins.
Solar light harvesting begins with electronic energy transfer in structurally complex light-harvesting antennae such as the peridinin chlorophyll-a protein from dinoflagellate algae. Peridinin chlorophyll-a protein is composed of a unique combination of chlorophylls sensitized by carotenoids in a 4:1 ratio, and ultrafast spectroscopic methods have previously been utilized in elucidating their energy-transfer pathways and timescales. However, due to overlapping signals from various chromophores and competing pathways and timescales, a consistent model of intraprotein electronic energy transfer has been elusive. Here, we used a broad-band two-dimensional electronic spectroscopy, which alleviates the spectral congestion by dispersing excitation and detection wavelengths. Interchromophoric couplings appeared as cross peaks in two-dimensional electronic spectra, and these spectral features were observed between the peridinin S2 states and chlorophyll-a Q
x
and Q
y
states. In addition, the inherently high time and frequency resolutions of two-dimensional electronic spectroscopy enabled accurate determination of the ultrafast energy-transfer dynamics. Kinetic analysis near the peridinin S1 excited-state absorption, which forms in 24 fs after optical excitation, reveals an ultrafast energy-transfer pathway from the peridinin S2 state to the chlorophyll-a Q
x
state, a hitherto unconfirmed pathway critical for fast interchromophoric transfer. We propose a model of ultrafast peridinin chlorophyll-a protein photophysics that includes (1) a conical intersection between peridinin S2 and S1 states to explain both the ultrafast peridinin S1 formation and the residual peridinin S2 population for energy transfer to chlorophyll-a, and (2) computationally and experimentally derived peridinin S2 site energies that support the observed ultrafast peridinin S2 to chlorophyll-a Q
x
energy transfer.
Besides the so-called ‘green lineage’ of eukaryotic photosynthetic organisms that include vascular plants, a huge variety of different algal groups exist that also harvest light by means of membrane intrinsic light harvesting proteins (Lhc). The main taxa of these algae are the Cryptophytes, Haptophytes, Dinophytes, Chromeridae and the Heterokonts, the latter including diatoms, brown algae, Xanthophyceae and Eustigmatophyceae amongst others. Despite the similarity in Lhc proteins between vascular plants and these algae, pigmentation is significantly different since no Chl b is bound, but often replaced by Chl c, and a large diversity in carotenoids functioning in light harvesting and/or photoprotection is present. Due to the presence of Chl c in most of the taxa the name ‘Chl c-containing organisms’ has become common, however, Chl b-less is more precise since some harbour Lhc proteins that only bind one type of Chl, Chl a. In recent years huge progress has been made about the occurrence and function of Lhc in diatoms, so-called fucoxanthin chlorophyll proteins (FCP), where also the first molecular structure became available recently. In addition, especially energy transfer amongst the unusual pigments bound was intensively studied in many of these groups. This review summarises the present knowledge about the molecular structure, the arrangement of the different Lhc in complexes, the excitation energy transfer abilities and the involvement in photoprotection of the different Lhc systems in the so-called Chl c-containing organisms.
This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.
The attribution of quantum beats observed in the time-resolved spectroscopy of photosynthetic light-harvesting antennae to nontrivial quantum coherences has sparked a flurry of research activity beginning a decade ago. Even though investigations into the functional aspects of photosynthetic light-harvesting were supported by X-ray crystal structures, the non-covalent interactions between pigments and their local protein environment that drive such function has yet to be comprehensively explored. Using symmetry-adapted perturbation theory (SAPT), we have comprehensively determined the magnitude and compositions of these non-covalent interactions involving light-harvesting chromophores in two quintessential photosynthetic pigment-protein complexes — peridinin chlorophyll-a protein (PCP) from dinoflagellate Amphidinium carterae and phycocyanin 645 (PC645) from cryptophyte Chroomonas mesostigmatica. In PCP, the chlorophylls are dispersion-bound to the peridinins, which in turn are electrostatically anchored to the protein scaffold via their polar terminal rings. This might be an evolutionary design principle in which the relative orientation of the carotenoids towards the aqueous environment determines the arrangement of the other chromophores in carotenoid-based antennas. On the other hand, electrostatics dominate the non-covalent interactions in PC645. Our ab initio simulations also suggest full protonation of the PC645 chromophores in physiological conditions, and that changes to their protonation states result in their participation as switches between folded and unfolded conformations.
To demonstrate the value of the multi-pulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of S1 and ICT state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump-probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states which reveals that lower polarity, proticity, and temperature delays S1/ICT equilibration. Based upon a two-state model representing the S1 and ICT states on the same S1/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based upon this model allowed us to quantify the population transfer rates throughout the life of the S1/ICT state as well as separate the S1 and ICT spectral signatures. The results are consistent with the concept that the S1 and ICT states are part of one potential energy surface.
The nature of intramolecular charge transfer (ICT) and the mechanism of intramolecular singlet fission (SF) in peridinin remain open research questions. Obtaining an understanding of the population evolution from the bright state to dark states following a photoinduced electronic transition is critical. Unambiguously describing this evolution in peridinin, and light-harvesting carotenoids in general, has proven elusive experimentally and computationally. To offer a balanced description of bright- and dark-state electronic structure, we here apply ab initio multireference perturbation theory quantum chemistry—the density matrix renormalization group self-consistent-field (DMRG-SCF) and complete-active space self-consistent field (CASSCF) with second-order N-electron valence perturbation theory (NEVPT2). At traditional bright- (S2) and dark-state (S1) optimized geometries, we find that an additional correlated triplet pair (CTP) state and ICT state are derived from the canonical polyene Bu (S3) and 3Ag (S4) dark singlet excited states, respectively. While the S3 state’s physical properties are insensitive to peridinin’s allene-tail donor and lactone-ring acceptor functionalization, the S4 state exhibits a markedly enhanced oscillator strength and HOMO-LUMO transition density. These changes suggest that ICT character stems from mixing between the bright S2 and putatively dark S4.
The central-cis isomer of the carotenoid peridinin, presumably 13-cis, was separated and studied with spectroscopic methods including static absorption, fluorescence and femtosecond time-resolved absorption. The investigations exposed differences in the photophysical properties of this isomer in respect to all-trans peridinin. Steady-state absorption spectroscopy revealed the presence of an additional weak absorption band at the long wavelength tail of the main S0 → S2 transition. Modeling of the hypothetical vibronic progression of the S0 → S1 electronic transition demonstrated that this weak band can be associated with a higher (0-2) vibronic band of the transition and that lower vibronic bands have negligible intensities due to a large displacement between the S0 and S1 states energy curves as also suggested by the spectral shape of steady-state fluorescence emission. Transient absorption studies demonstrated that the lifetime of the S1 state of the central-cis isomer is shorter compared to the all-trans counterpart by 6-16%, depending on the polarity of the solvent. On the other hand, molecular isomerization negligibly affects the lifetime of intramolecular charge transfer (ICT), which for both isomers is ∼10 ps in the polar solvent methanol.
A series of phenothiazine based dyes (OMS1–3), comprising different conjugation lengths and numbers of electron deficient (cyanovinyl) moieties with cyanoacrylic acid as an anchor, have been synthesized. The dyes display broad UV-visible absorption, from 389 nm to 484 nm. The higher molar extinction coefficient and longer absorption peak are achieved as the conjugation length and numbers of electron deficient units increase. The cell performance based on these dyes exhibits efficiencies ranging from 0.68–4.00%, compared to a standard N719-based device (PCE = 7.49%) fabricated under similar conditions. Although the OMS3 dye has two electron deficient units between phenothiazine units, an insignificant electron trapping effect is observed. From the results, the OMS3 based cell exhibits the highest short circuit current (JSC) at 8.72 mA cm⁻² and the highest open-circuit voltage (VOC) at 0.66 V, together with the best cell performance at 4.00%.
New fluorene dyes tethering a perfluorohexyl or n-hexyl side chain in the conjugated spacer (thiophene) were successfully synthesized for DSSCs. The perfluoro- dyes more effectively suppress the charge recombination than their congeners with a hexyl chain and lead to a higher open-circuit voltage (VOC). The more efficient dark current suppression of perfluorohexyl-containing dyes was also confirmed by electrochemical impedance spectroscopy (EIS). The perfluoro- containing dye (PFT4) exhibited better power conversion efficiency, which further improved upto 6.73% by the addition of CDCA (10 mM). The hydrophobic perfluorohexyl-containing dyes are more potent in resisting dye leaching due to moisture, leading to higher temporal stability of the DSSCs.
The stereocontrolled convergent synthesis of 19′-deoxyperidinin, 2, which might be a useful peridinin analog to understand the ICT characteristics, was efficiently achieved by sequential Pd-catalyzed cross-coupling reactions using bidirectionally extensible conjugated C5 olefin segments. The crucial 5(2H)-ylidenedihydrofuran function of 2 was successfully constructed by the Au-catalyzed regio- and stereoselective 5-exo-dig etherification.
The Raman and infrared spectra of all-trans-astaxanthin (AXT) in dimethyl sulfoxide (DMSO) solvent were investigated experimentally and theoretically. Density functional calculations of the Raman spectra predict the splitting of the ν1 band into ν1-1 and ν1-2 components. The absence of splitting in Raman experimental spectra is ascribed to the competition between the two symmetric C=C stretching vibrations of the backbone chain. The ν1 band is very sensitive to the excitation wavelength: resonance excitation stimulates the higher-frequency ν1-2 mode, and off-resonance excitation corresponds to the lower-frequency ν1-1 mode. Analyses of the intramolecular hydrogen bonding between C=O and O−H in the AXT/DMSO system reveal that the C4=O1⋯ H1−O3 and C4′=O2⋯ H2−O4 bonds are strengthened and weakened, respectively, in the electronically excited state compared with those in the ground state. This result reveals significant variations of the AXT molecular structure in different electronic states.
Carotenoids can play multiple roles in biological photoreceptors thanks to their rich photophysics. In the present work, we have investigated six of the most common carbonyl containing carotenoids: Echinenone, Canthaxanthin, Astaxanthin, Fucoxanthin, Capsanthin and Capsorubin. Their excitation properties are investigated by means of a hybrid density functional theory (DFT) and multireference configuration interaction (MRCI) approach to elucidate the role of the carbonyl group: the bright transition is of {\pi}{\pi}* character, as expected, but the presence of a C=O moiety reduces the energy of n{\pi}* transitions which may become closer to the {\pi}{\pi}* transition, in particular as the conjugation chain decreases. This can be related to the presence of a low-lying charge transfer state typical of short carbonyl- containing carotenoids. The DFT/MRCI results are finally used to benchmark single- reference time-dependent DFT-based methods: among the investigated functionals, the meta- GGA (and in particular M11L and MN12L) functionals show to perform the best for all six investigated systems.
A new series of benzimidazole (BIm) based dyes (SC32 and SC33) and pyridoimidazole (PIm) based dyes (SC35 and SC36) were synthesized as the sensitizers for dye-sensitized solar cells. The N-substituent and C-substituent at the BIm and PIm cores was found to be the dominating factor in deciding the electronic properties of the dyes and their DSSCs performance. The efficiency of BIm-based dyes (SC35 and SC36) was found higher than the BIm-based dyes (SC32 and SC33) due to better light harvesting. The C-substituent in SC36, 4-hexylloxybenzene, is beneficial to dark current suppression, and hence SC36 achieves the best efficiency of 7.38% (~85% of N719). The two BIm dyes have better cell efficiencies than their congeners with a bithiophene entity between the BIm and the anchor due to better light harvesting of the former.
Photoinitiated phenomena play a crucial role in many living organisms. Plants, algae, and bacteria absorb sunlight to perform photosynthesis, and convert water and carbon dioxide into molecular oxygen and carbohydrates, thus forming the basis for life on Earth. The vision of vertebrates is accomplished in the eye by a protein called rhodopsin, which upon photon absorption performs an ultrafast isomerisation of the retinal chromophore, triggering the signal cascade. Many other biological functions start with the photoexcitation of a protein-embedded pigment, followed by complex processes comprising, for example, electron or excitation energy transfer in photosynthetic complexes. The optical properties of chromophores in living systems are strongly dependent on the interaction with the surrounding environment (nearby protein residues, membrane, water), and the complexity of such interplay is, in most cases, at the origin of the functional diversity of the photoactive proteins. The specific interactions with the environment often lead to a significant shift of the chromophore excitation energies, compared with their absorption in solution or gas phase. The investigation of the optical response of chromophores is generally not straightforward, from both experimental and theoretical standpoints; this is due to the difficulty in understanding diverse behaviours and effects, occurring at different scales, with a single technique. In particular, the role played by ab initio calculations in assisting and guiding experiments, as well as in understanding the physics of photoactive proteins, is fundamental. At the same time, owing to the large size of the systems, more approximate strategies which take into account the environmental effects on the absorption spectra are also of paramount importance. Here we review the recent advances in the first-principle description of electronic and optical properties of biological chromophores embedded in a protein environment. We show their applications on paradigmatic systems, such as the light-harvesting complexes, rhodopsin and green fluorescent protein, emphasising the theoretical frameworks which are of common use in solid state physics, and emerging as promising tools for biomolecular systems.
New heterocyclic quinoid-based hole transporting materials (HTMs) with a rigid quinoid core [3,6-di(2H-imidazol-2-ylidene)cyclohexa-1,4-diene] have been synthesized. The new HTMs have good hole mobility (>10(-4) cm(2) V(-1) s(-1) ) and very intense absorption in the near-infrared region extending to >800 nm. High performance perovskite solar cells can be fabricated using these HTMs without dopant. The best cell efficiency under simulated AM 1.5 G illumination reaches 12.22 %, which is comparable with that (12.58 %) using doped 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) as the HTM.
A series of indeno[2,1-b]thiophene-containing organic dyes (IDT1–IDT5) have been synthesized, and their UV/Vis spectra extend beyond 700 nm. These dyes were employed as sensitizers for dye-sensitized solar cells (DSSCs). Among them, dyes IDT3–IDT5 with a thiophene unit conjugated with an anchor have better light harvesting ability and higher J SC values. Dye aggregation contributes substantially to the photocurrent in the near infrared region. The dye IDT5, with chenodeoxycholic acid (CDCA) as the co-adsorbent, has the highest power conversion efficiency (5.06 %) under AM 1.5G simulated sunlight irradiation, which is 68 % of the standard device based on N719 dye.
Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S2 (11Bu+) state to the S1 (21Ag−) state of peridinin in methanol solution. Just as previously observed by this laboratory for β-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S2 state decays in 12 fs to produce an intermediate state, Sx. The excited state absorption spectrum from the Sx state of peridinin, however, is found to be markedly blue shifted from that of β-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S2 state rather than to a vibrationally excited (or hot) S1 state. The Sx state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation-energy barrier between planar and twisted structures on the S2 potential surface. The lengthened lifetime of the Sx state of peridinin in methanol, 900 ± 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, ~150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system–bath coupling is significantly enhanced over that in β-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the Sx state and subsequent distorted S2 structures may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin–chlorophyll a protein from dinoflagellates.
The design of optimal light-harvesting (supra)molecular systems and materials is one of the most challenging frontiers of science. Theoretical methods and computational models play a fundamental role in this difficult task, as they allow the establishment of structural blueprints inspired by natural photosynthetic organisms that can be applied to the design of novel artificial light-harvesting devices. Among theoretical strategies, the application of quantum chemical tools represents an important reality that has already reached an evident degree of maturity, although it still has to show its real potentials. This Review presents an overview of the state of the art of this strategy, showing the actual fields of applicability but also indicating its current limitations, which need to be solved in future developments.
Photosynthesis begins when photons are absorbed by the light-harvesting system. The light-harvesting system then transfers the absorbed energy to the reaction center (RC), where it is trapped and initiates the primary redox reactions of photosynthesis. The light-harvesting system therefore acts to increase the effective cross-sectional area of each RC for light absorption. This means that RCs can be kept supplied with sufficient numbers of photons even when the incident light intensity is rather low. The size of the antenna system and the types of pigments that are used depend on the ecological niche in which the particular photosynthetic species lives.
New imidazole-based organic dyes with two 2-cyanoacetic acid acceptors/anchors connected at the C-4 and C-5 positions, and a carbazole or arylamine donor connected at the C-2 position of the imidazole entity were synthesized and used as sensitizers for dye-sensitized solar cells. The dyes exhibit optical absorption from 300–600 nm with high molar extinction coefficients. The inclusion of a hexyl chain at the nitrogen atom of the imidazole was found to result in a large twist of the C-4 segment from the imidazole and, hence, to less efficient electron injection. Although dye aggregation results in light harvesting at longer wavelength regions, it also leads to excited-state quenching and hinders electron injection. Consequently, only moderate conversion efficiencies of DSSCs were achieved. Upon addition of CDCA coadsorbents, up to ca. 30 % improvement of the cell efficiency was achieved. The best cell efficiency reached 4.97 % with addition of 50 mM CDCA.
Femtosecond transient grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast timescale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the 1(1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition state structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C=C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared, Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient towards a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy transfer mechanisms to (bacterio)chlorophyll acceptors.
Three new organic dyes containing a 5,5'-bithiazole or 2,2'-bithiazole entity have been synthesized for dye-sensitized solar cell (DSSC) applications. The conversion efficiencies of DSSCs fabricated are moderate due to serious dye aggregation. Upon addition of CDCA co-adsorbents, the optimized cell efficiencies were improved by 43-86%. The best efficiency was 6.31%, which reached 86% of N719-based DSSC fabricated and measured under similar condition. The efficiency can be further improved to 6.45% by adding a near-IR absorbing dye as the co-sensitizer due to broadened IPCE spectrum.
Carotenoids are known to play a fundamental role in photosynthetic light-harvesting (LH) complexes; however, an accurate quantum-mechanical description of that is still missing. This is due to the multideterminant nature of the involved electronic states combined with an extended conjugation which limits the applicability of many of the most advanced approaches. In this study, we apply a multireference configuration interaction extension of density functional theory (DFT/MRCI) to describe transition energies and densities as well as the corresponding excitonic couplings, for the three lowest singlet excited states of nine carotenoids present in three different LH complexes of algae and plants. These benchmark results are used to find an approximated computational approach, which could be used to quantitatively reproduce the key quantities at a reduced computational cost. To this end, we tested the Tamm-Dancoff approximation (TDA) to time-dependent density functional theory in combination with different functionals. By analyzing the errors with respect to DFT/MRCI-TDA results for the full set of electronic properties, we conclude that TDA-TPSS with small basis sets indeed represents an effective approach to investigate LH processes that involve carotenoids.
In this work we analyzed the infrared and visible transient absorption spectra of all trans-β-apo-8'-carotenal in several solvents, differing both in polarity and polarizability at different excitation wavelengths. We correlate the solvent dependence of the kinetics and the bandshape changes in the infrared with that of the excited state absorption bands in the visible, and we show that the information obtained in the two spectral regions is complementary. All the collected time-resolved data can be interpreted in the frame of a recently proposed relaxation scheme, according to which the major contributor to ICT state is the bright 1Bu+ state, which, in polar solvents, is dynamically stabilized through molecular distortions and solvent relaxation. A careful investigation of the solvent effects on the visible and infrared excited state bands demonstrate that both solvent polarity and polarizability have to be considered in order to rationalize the excited state relaxation of trans-8'-apo-ß-carotenal and clarify the role and the nature of the ICT state in this molecule. The experimental observations reported in this work can be interpreted by considering that at the Frank Condon geometry the wavefunctions of the S1 and S2 excited states have a mixed ionic/covalent character. The degree of mixing depends on solvent polarity, but can be dynamically modified by the effect of polarizability. Finally the effect of different excitation wavelengths on the kinetics and spectral dynamics can be interpreted in terms of photoselection of a sub-population of partially distorted molecules.
Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another chlorophyll, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically chlorophyll b and lutein, and the group is present in the ‘green lineage’. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind chlorophyll c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution.
We present and review the results of fluorescence upconversion and photon echo experiments, and ab initio calculations performed in our group within the last few years with respect to the light harvesting process in purple bacteria. Carotenoids transfer energy to bacteriochlorophyll (BChl) mainly via the carotenoid S-2 --> BChl Q(x) pathway on a similar to 100 fs timescale. This transfer is reasonably reproduced by considering the Coulombic coupling calculated using the transition density cube method which is valid at all molecular separations. Carotenoids may also serve a role in mediating B800 --> B850 energy transfer in LH2 by perturbing the transition density of the B850 as shown by ab initio calculations on a supermolecule of two B850 BChls, one carotenoid and one B800 BChl. Further calculations on dimers of B850 BChl estimate the intra- and interpolypeptide coupling to be 315 and 245 cm(-1), respectively. These interactions are dominated by Coulombic coupling, while the orbital overlap dependent coupling is similar to 20% of the total. Photon echo peak shift experiments (3PEPS) on LH1 and the B820 subunit are quantitatively simulated with identical parameters aside from an energy transfer time of 90 fs in LR1 and infinity in B820, suggesting that excitation is delocalized over roughly two pigments in LH1. 3PEPS data taken at room and low temperature (34 K) on the B800-B820 suggest that static disorder is the dominant mechanism localizing excitation in LR1 and LH2. We suggest that the competition between the delocalizing effects of strong electronic coupling and the localizing effects of disorder and nuclear motion results in excitation in the B850 and B875 rings being localized on 2-4 pigments within approximately 60 fs.
In photosynthetic light-harvesting systems carotenoids and chlorophylls jointly absorb light and transform its energy within about a picosecond into electronic singlet excitations of the chlorophylls only. This paper investigates this process for the light-harvesting complex II of the purple bacterium Rhodospirillum molischianum, for which a structure and, hence, the exact arrangement of the participating bacteriochlorophylls and carotenoids have recently become known. Based on this structure and on CI expansions of the electronic states of individual chromophores (bacteriochlorophylls and carotenoids) as well as on an exciton description of a circular aggregate of bacteriochlorophylls, the excitation transfer between carotenoids and bacteriochlorophylls is described by means of Fermi’s golden rule. The electronic coupling between the various electronic excitations is determined for all orders of multipoles (Coulomb mechanism) and includes the electron exchange (Dexter mechanism) term. The rates and efficiencies for different pathways of excitation transfer, e.g., 11Bu+(carotenoid)→bacteriochlorophyll aggregate and 21Ag-(carotenoid)→ bacteriochlorophyll aggregate, are compared. The results show that in LH-II the Coulomb mechanism is dominant for the transfer of singlet excitations. The 11Bu+→Qx pathway appears to be partially efficient, while the 21Ag-→Qy pathway, in our description, which does not include vibrational levels, is inefficient. An improved treatment of the excitation transfer from the 21Ag- state is required to account for observed transfer rates. Exciton splitting of bacteriochlorophyll Qy excitations slightly accelerates the excitation transfer from the 21Ag- state, while it plays a crucial role in accelerating the transfer from the B800BChlQy state. Photoprotection of bacteriochlorophylls through triplet quenching is investigated, too. The results suggest that eight of the 16B850 bacteriochlorophylls in LH-II of Rhodospirillum molischianum are protected well by eight carotenoids observed in the x-ray structure of the protein. The remaining eight B850 bacteriochlorophylls can transfer their triplet excitation energy efficiently to their neighboring protected bacteriochlorophylls. Eight B800 bacteriochlorophylls appear not to be protected well by the observed carotenoids.
Peridinin-chlorophyll-protein, a water-soluble light-harvesting complex that has a blue-green absorbing carotenoid as its
main pigment, is present in most photosynthetic dinoflagellates. Its high-resolution (2.0 angstrom) x-ray structure reveals
a noncrystallographic trimer in which each polypeptide contains an unusual jellyroll fold of the α-helical amino- and carboxyl-terminal
domains. These domains constitute a scaffold with pseudo-twofold symmetry surrounding a hydrophobic cavity filled by two lipid,
eight peridinin, and two chlorophyll a molecules. The structural basis for efficient excitonic energy transfer from peridinin to chlorophyll is found in the clustering
of peridinins around the chlorophylls at van der Waals distances.
A new density-functional approach to calculate the excitation spectrum of many-electron systems is proposed. It is shown that the full linear density response of the interacting system, which has poles at the exact excitation energies, can rigorously be expressed in terms of the response function of the noninteracting (Kohn-Sham) system and a frequency-dependent exchange-correlation kernel. Using this expression, the poles of the full response function are obtained by systematic improvement upon the poles of the Kohn-Sham response function. Numerical results are presented for atoms. {copyright} {ital 1996 The American Physical Society.}
Blue and green sunlight become available for photosynthetic energy conversion through the light-harvesting (LH) function of carotenoids, which involves transfer of carotenoid singlet excited states to nearby (bacterio)chlorophylls (BChls). The excited-state manifold of carotenoids usually is described in terms of two singlet states, S(1) and S(2), of which only the latter can be populated from the ground state by the absorption of one photon. Both states are capable of energy transfer to (B)Chl. We recently showed that in the LH1 complex of the purple bacterium Rhodospirillum rubrum, which is rather inefficient in carotenoid-to-BChl energy transfer, a third additional carotenoid excited singlet state is formed. This state, which we termed S*, was found to be a precursor on an ultrafast fission reaction pathway to carotenoid triplet state formation. Here we present evidence that S* is formed with significant yield in the LH2 complex of Rhodobacter sphaeroides, which has a highly efficient carotenoid LH function. We demonstrate that S* is actively involved in the energy transfer process to BChl and thus have uncovered an alternative pathway of carotenoid-to-BChl energy transfer. In competition with energy transfer to BChl, fission occurs from S*, leading to ultrafast formation of carotenoid triplets. Analysis in terms of a kinetic model indicates that energy transfer through S* accounts for 10-15% of the total energy transfer to BChl, and that inclusion of this pathway is necessary to obtain a highly efficient LH function of carotenoids.
Carotenoids are, along with chlorophylls, crucial pigments involved in light-harvesting processes in photosynthetic organisms. Details of carotenoid to chlorophyll energy transfer mechanisms and their dependence on structural variability of carotenoids are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to reveal energy transfer pathways in the peridinin-chlorophyll-a-protein (PCP) complex containing the highly substituted carotenoid peridinin, which includes an intramolecular charge transfer (ICT) state in its excited state manifold. Extending the transient absorption spectra toward near-infrared region (600-1800 nm) allowed us to separate contributions from different low-lying excited states of peridinin. The results demonstrate a special light-harvesting strategy in the PCP complex that uses the ICT state of peridinin to enhance energy transfer efficiency.
We present the first direct evidence of the presence of an intermediate singlet excited state (Sx) mediating the internal conversion from S2 to S1 in carotenoids. The S2 to Sx transition is extremely fast and is completed within approximately 50 femtoseconds. These results require a reassessment of the energy transfer pathways from carotenoids to chlorophylls in the primary step of photosynthesis.
The origin and distance dependence of the electronic interactions which promote energy transfer within photosynthetic Light-harvesting complexes is investigated. A model based on localized molecular orbitals is related to canonical molecular orbital calculations, therefore demonstrating its practical utility and allowing us to interpret the results of CAS-SCF calculations of the coupling between donor-acceptor pairs. We then focus on the mechanism of energy transfer involving the carotenoid 2(1)A(g) (S-1) electronic state: [carotenoid (2(1)A(g)) (Car) to carotenoid (2(1)A(g))] and [carotenoid (2(1)A(g)) to bacteriochlorophyll (Q(y)) (Bchl)] interactions. The Car-Car coupling is found to involve reasonably long-range interaction terms, with a primary contribution from dispersion-type interactions, which have an R-6 distance dependence. The primary contributor to the Car-Bchl S-1 --> S-1 energy transfer mechanism is suggested to be proportional to the product of dipole-dipole and polarization interactions. In neither case does the electronic interaction resemble the Dexter exchange integral in origin or distance dependence. Some model CAS-SCF calculations of electronic interactions in 2,4,6-octatriene dimers are presented which support the predictions of the theory: the calculated interaction is found to be (i) small in comparison to the overlap-dependent triplet-triplet interaction at close separations; (ii) small in comparison to a dipole-dipole (S-2) interaction at all separations; and (iii) quite weakly distance dependent at larger separations. The implications for the role of carotenoids in photosynthetic light-harvesting complexes are discussed.
The Protein Data Bank (PDB; http://www.rcsb.org/pdb/ ) is the single worldwide archive of structural data of biological macromolecules. This paper describes the goals of the PDB, the systems in place for data deposition and access, how to obtain further information, and near-term plans for the future development of the resource.
A correct description of the electronic excitations in polyenes demands that electron correlation is accounted for correctly. Very large expansions are necessary including many-electron configurations with at least one, two, three, and four electrons promoted from the Hartree–Fock ground state. The enormous size of such expansions had prohibited accurate computations of the spectra for polyenes with more than ten π electrons. We present a multireference double excitation configuration interaction method (MRD-CI) which allows such computations for polyenes with up to 16 π electrons. We employ a Pariser–Parr–Pople (PPP) model Hamiltonian. For short polyenes with up to ten π electrons our calculations reproduce the excitation energies resulting from full-CI calculations. We extend our calculations to study the low-lying electronic excitations of the longer polyenes, in particular, the gap between the first optically forbidden and the first optically allowed excited singlet state. The size of this gap is shown to depend strongly on the degree of bond alternation and on the dielectric shielding of the Coulomb repulsion between the π electrons.
A 4 ps, 510 nm laser pulse was used to electronically excite toluene solutions of all-trans-β-carotene, canthaxanthin, and β-8'-apocarotenal. The 430–510 nm electronic absorption band of each carotene bleaches immediately upon excitation and recovers with single-exponential kinetics: τ = 8.4 ± 0.6 ps for all-trans-β-carotene, 5.2 ± 0.6 ps for canthaxanthin, and 25.4 ± 0.2 ps for β-8'-apocarotenal. The results are discussed in terms of the excited singlet state properties of carotenoids.
Formalism of the excitation transfer matrix element applicable for any multiconfigurational wave functions is made. On the basis of the resultant formulas, the excitation transfer matrix elements between the S2 or S1 state of a carotenoid, neurosporene, and the S2 or S1 state of bacteriochlorophyll a are calculated at various stacked configurations of the two molecules. The results show that the excitation transfer from the carotenoid S1 state to the bacteriochlorophyll S1 state via the Coulomb mechanism including multipole–multipole interactions takes place very efficiently in a speed more rapid than that via the electron-exchange mechanism. The results also show that the excitation transfer from carotenoid to bacteriochlorophyll occurs directly from the carotenoid S2 state, as well as from the carotenoid S1 state. Furthermore, it is shown that the excitation transfer matrix element due to the electron-exchange interaction has an oscillatory dependence on the displacement of one molecule from the other when the distance between the planes of the π systems is kept constant. Based on these results, a possible mechanism of the excitation transfer from carotenoid to bacteriochlorophyll invivo is discussed.
The energies of the optically forbidden 2Ag- and 1Bu- states of crystalline carotenoids were determined by measurements of resonance-Raman excitation profiles together with those of the optically allowed 1Bu+ state. The 1Bu+, 1Bu-, and 2Ag- state energies (for the 0 ← 0 vibronic transition) were 18 600, 15 770, and 13 200 cm-1 in lycopene; 19 150, 16 550, and 14 670 cm-1 in β-carotene; and 20 900, 19 700 and 15 750 cm-1 in mini-9-β-carotene, respectively. Comparison between lycopene (the number of the conjugated double bonds, n = 11) and spheroidene (n = 10) (Sashima et al. Chem. Phys. Lett. 1999, 299, 187) as well as between β-carotene (n = 11) and mini-9-β-carotene (n = 9) lead us to the following conclusions: (i) the ordering of the singlet states is 1Bu+ (S3) > 1Bu- (S2) > 2Ag- (S1), (ii) all the state energies decrease when n increases, and (iii) the dependence of the state energy on n is the strongest for the 1Bu- state. All of these observations agree with extrapolation of theoretical prediction by Tavan and Schulten for shorter polyenes (Tavan, P.; Schulten, K. J. Chem. Phys. 1986, 85, 6602).
This work reports the first density-functional theory (DFT) treatment of excited-state potential energy surfaces exhibiting avoided crossings. Time-dependent DFT (TD-DFT) results, using a recently proposed asymptotically corrected local density approximation functional, are compared with multireference doubles configuration interaction (MRD-CI) results for the 1A1 manifold of the CO stretching curves of planar formaldehyde. TD-DFT is found to reproduce the qualitative features essential for understanding the spectroscopy of this manifold, specifically the strong mixing of the 1(π, π*) with Rydberg transitions and the resultant avoided crossings. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 933–941, 1998
Both the ethenyl and phenyl peroxyl radical absorb in the visible region in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were initially surprising. Ab initio determination of the electronic structure of the ground and excited states of the ethynyl, ethenyl, and phenyl peroxyl radicals provides a fundamental understanding of these electronic transitions. The electronic excited states of these radicals are low in energy because the pi-type open-shell orbital localized on the oxygen atoms in the ground state couples to a relatively low-energy pi or conjugated orbital system in the excited state. In the case of both the ethynyl and phenyl peroxyl radicals, the excitation of the C-C bond pi orbital is relatively high in energy, and the in vacuo prediction for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because all the radicals are polar in the ground state, and the dipole moment, in the relevant excited state is substantially larger. In addition, for the ethenyl peroxyl radical there are two isomeric forms whose ground states are close in energy, and the observed spectrum can be assigned to the convoluted spectra of both isomers.
TDDFT singlet excitation energies calculations have been performed for a series of donor-acceptor para-substituted N,N-dimethyl-anilines (4DMAB-CHO, 4DMAB-COMe, 3DMAB-CHO, 3DMAB-COMe, 4DMAB-COOH, 4DMAB-COOMe, 4DMAB-CONH2, 4DMAB-CON(CH3)2, and DMAPY) using both B3LYP and MPW1PW91 functionals with a 6-311*)(2p,d basis set. The aim of this study is to investigate the influence of the variation of the acceptor group on the fluorescence behavior of these molecules. This work is a complementary investigation to part I [J. Chem. Phys. 117, 4146 (2002), preceding paper], where variation of the donor group has been studied. Ground-state geometries were optimized using density-functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For all molecules, the potential energy surface has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Computed vertical absorption energies using the MWP1PW91 and B3LYP functionals compare within 0.02 and -0.09 eV in average with gas-phase experimental data (4DMAB-COMe and 4DMAB-COOMe). Almost all para compounds are predicted to be dual fluorescent within the TICT model, in agreement with experiments. The present calculations predict 4DMAB-CONH2 to be dual fluorescent in nonpolar solvent in disagreement with experimental results. For meta compounds, the literature is sparse and no fluorescence spectra have been reported for these systems. Our results indicate that these molecules should exhibit only one band in the fluorescence spectra. Our calculations reinforce the validity of the TICT model as a possible mechanism to explain the fluorescence activity of these donor-acceptor systems.
The asymptotic correction approach was used to determine the local density approximation (LDA) potential. The approach is crucial in improving the approximate exchange-correlation potentials in the time-dependent density functional theory (TD-DFT) for calculating the excitation spectra. The asymptotic correction approach involves a constant shift to incorporate the effect of the derivative discontinuity (DD) in the bulk region of finite systems, and a spliced asymptotic correction in the large region.
The assessment of time dependent density functional theory for the calculation of the critical features in the absorption spectra of a series of aromatic donor-acceptor systems was discussed. Geometry optimization and excitation energy calculations were performed. The locally excited state and the charge transfer state of NC-C6H4-N(CH3)2 were found to be 0.1 and 0.04 eV. The charge transfer excitation energy and its relative position to the locally excited state agreed with the acceptor strength concept.
Singlet excitation energies for a series of acceptor para-substituted
N,N-dimethyl-anilines that are dual (4DMAB-CN, 3M4MAB-CN, MHD) and
nondual (4AB-CN, 3M4AB-CN, 4MAB-CN, 3M4DMAB-CN, HHD, and MMD)
fluorescent have been performed using the TDDFT method. The aim of this
study is to investigate the influence of changing donor groups as well
as the addition of methyl groups to the benzene moiety, on the
fluorescence behavior of these molecules. Calculations of excitation
energies have been performed with both B3LYP and MPW1PW91 functionals
using a 6-311*)(2p,d (Bg) basis set. For all systems,
ground-state geometries were optimized using density-functional theory
with the Becke three parameter Lee-Yang-Parr functional combined with a
6-31G(d) (Sm) basis set. In addition, 4AB-CN, 4DMAB-CN, and MMD
ground-state geometry has also been optimized using the MPW1PW91
functional with the Sm basis set. For all molecules, the potential
energy surface (PES) has been investigated following the twisting
intramolecular charge transfer (TICT) model proposed in the literature
as a possible mechanism to explain the fluorescence behavior. Both
4AB-CN and HHD molecules have been computed to be nondual fluorescent in
full agreement with experimental spectra. The single band observed in
the gas-phase fluorescence spectra of 3M4DMAB-CN, and MMD is clearly
understood by the form of the PES of the charge transfer excited state
that presents a minimum for the perpendicular structure. The qualitative
picture of the PES along the twisting angle is in full agreement with
experimental observations. The dual fluorescence of 4DMAB-CN and
3M4MAB-CN is explained, within the TICT model, by a double mechanism
proposed by Serrano [et al.] [J. Am. Chem. Soc. 117, 3189 (1995)], that
involves the presence of two low-lying states close enough in energy.
The nondual fluorescence of 4MAB-CN is explained by the height of the
energy barrier (larger than for 4DMAB-CN and 3M4MAB-CN). Finally, the
dual fluorescence of the MHD molecule can be fully understood by a
double mechanism within the TICT model. 3M4AB-CN is computed nondual
fluorescent like 4AB-CN and HHD, but no experimental data has been
reported in the literature so far. Our calculations give new evidence in
favor of the TICT model as an explanation for the occurrence of dual
fluorescence.
We assess various approximate forms for the correlation energy per particle of the spin-polarized homogeneous electron gas that have frequently been used in applications of the local spin density approximation to the exchange-correlation energy functional. By accurately recalculating the RPA correlation energy as a function of electron density and spin polarization we demonstrate the inadequacies of the usual approximation for interpolating between the para- and ferro-magnetic states and present an accurate new interpolation formula. A Padé approximant technique is used to accurately interpolate the recent Monte Carlo results (para and ferro) of Ceperley and Alder into the important range of densities for atoms, molecules, and metals. These results can be combined with the RPA spin-dependence so as to produce a correlation energy for a spin-polarized homogeneous electron gas with an estimated maximum error of 1 mRy and thus should reliably determine the magnitude of non-local corrections to the local spin density approximation in real systems.
Using time-dependent density functional theory (TDDFT), we obtained the excitation energy transfer coupling (Coulombic coupling) between the S1 state of rhodopin glucoside (RG) and the Qy state of bacteriochlorophylls (BChl) in the light-harvesting complex II (LH2) of purple photosynthetic bacterium Rhodopseudomonas (Rps.) acidophila. Our results suggest that the small mixing of S2 character arising from symmetry-breaking of the carotenoid plays an important role in the Coulombic coupling. As a result the carotenoid (car) S1 couplings to chlorophylls are similar to a set of scaled down Car(S2)−BChl(Qy) couplings. We also report results for 6,10,15,19-tetramethyl-2-cis-4,6,8,10,12,14,16,18,20-all trans-22-cis-tetracosaundecaene, the polyene backbone of RG with six methyl groups attached, in two different structures: an optimized planar structure and the crystal structure of RG with hydrogen atoms replacing the two end groups, which is distorted from its planar structure. The mixing of S2 configuration is strictly forbidden in the planar structure due to symmetry. In this case the polyene still couples moderately strongly to the nearby BChls. In the distorted structure derived from RG crystal structure, coupling strengths and the role of S2 character mixing are similar to those of the full RG. Using an exciton model simulation, the calculated coupling strengths yield Car(S1)-to-BChl(Qy) excitation energy transfer times that are in good agreement with recent experimental results.
The excited-state dynamics of the carotenoids (Car) in light-harvesting complex II (LHC II) of Chlamydomonas reinhardtii were studied by transient absorption measurements. The decay of the Car S1 population ranges from 200 fs to over 7 ps, depending on the excitation and detection wavelengths. In contrast, a 200 fs Car S1→Chlorophyll (Chl) energy transfer component was the dominant time constant for our earlier two-photon fluorescence up-conversion measurements (Walla, P. J.; et al. J. Phys. Chem. B 2000, 104, 4799−4806). We also present the two-photon excitation (TPE) spectra of lutein and β-carotene in solution and compare them with the TPE spectrum of LHC II. The TPE-spectrum of LHC II has an onset much further to the blue and a width that is narrower than expected from comparison to the S1 fluorescence of lutein and β-carotene in solution. Different environments may affect the shape of the S1 spectrum significantly. To explain the blue shift of the TPE spectrum and the difference in the time constants obtained from two-photon vs one-photon methods, we suggest that a major part of the Car S1→Chl electronic energy transfer (EET) is due to efficient EET from hot vibronic states of the Cars. We also suggest that the subpicosecond kinetics has a very broad distribution of EET time scales due to EET from hot states.
Spectroscopic properties as well as excited state dynamics of the carotenoid peridinin in several solvents of different polarities were investigated by time-resolved fluorescence and transient absorption techniques. A strong dependence of the peridinin lowest excited states dynamics on solvent polarity was observed after excitation into the strongly allowed S2 state. Peridinin relaxes to the ground state within 10 ps in the strongly polar solvent methanol, while in the nonpolar solvent n-hexane a 160 ps lifetime was observed, thus confirming the previous observations revealed by transient absorption spectroscopy in the visible region (Bautista, J. A.; et al. J. Phys. Chem. B 1999, 103, 8751). In addition, the solvent dependence in the near-IR region is demonstrated by a strong negative feature in the transient absorption spectrum of peridinin in methanol, which is not present in n-hexane. This band, characterized by a 1 ps rise time, is ascribed to stimulated emission from an intramolecular charge-transfer (ICT) state. Time-resolved fluorescence data support assignment of this band to the emissive singlet state, whose dynamic characteristics depend strongly on the dielectric strength of the medium. On the basis of all our time-resolved measurements combined with simulations of the observed kinetics, we propose the following model: the initially populated S2 state decays to the S1 state within less than 100 fs for both solvents. Then, the population is transferred from the S1 state to the S0 and ICT states. The S1 → ICT transfer is controlled by a solvent polarity dependent barrier. In n-hexane the barrier is high enough to prevent the S1 → ICT transfer and only S1 → S0 relaxation characterized by a time constant of 160 ps is observed. An increase of solvent polarity leads to a significant decrease of the barrier, enabling a direct quenching of the S1 state by means of the S1 → ICT transfer, which is characterized by a time constant of 148 ps for tetrahydrofuran, 81 ps for 2-propanol, and 11 ps for the most polar solvent methanol. The ICT state is then rapidly depopulated to the ground state. This relaxation also exhibits solvent dependence, having a time constant of 1 ps in methanol, 2.5 ps in 2-propanol, and 3.5 ps in tetrahydrofuran.
Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of trans-1,3-butadiene, trans−trans-1,3,5-hexatriene, all-trans-1,3,5,7-octatetraene, and all-trans-1,3,5,7,9-decapentaene. Attachment and detachment densities for transitions in butadiene and decapentaene from the ground state to the 2 1Ag and 1 1Bu excited states are also calculated and analyzed. Based on comparisons with experimental results and high level ab initio calculations in the literature, significant improvement over configuration−interaction singles is observed for the 2 1Ag state of the polyenes, which has been known to have significant double excitation character. For the 1 1Bu state, TDDFT underestimates the excitation energy by 0.4−0.7 eV. In this case we have observed a significant difference between the results for TDDFT and TDDFT within the Tamm−Dancoff approximation, both in excitation energies and, at least for butadiene, in the character of the excited state.
A new method for analyzing calculations of vertical electronic transitions in molecules is proposed. The one-electron difference density matrix between the two states is decomposed into the negative of a `detachment density` describing removal of charge from the initial state plus an `attachment` density describing its new arrangement in the excited state. This approach relates closely to the simple picture of excited states as electron promotions from occupied to unoccupied orbitals, and yet it can be applied to arbitrarily complex wave functions. The trace of the attachment and detachment densities is a measure of the number of electrons promoted in a transition. Attachment and detachment densities are calculated and analyzed for electronic transitions in formaldehyde and the nitromethyl radical. 36 refs., 2 figs., 3 tabs.
The spectroscopic properties and dynamics of the lowest excited singlet states of peridinin, fucoxanthin, neoxanthin, uriolide acetate, spheroidene, and spheroidenone in several different solvents have been studied by steady-state absorption and fast-transient optical spectroscopic techniques. Peridinin, fucoxanthin, uriolide acetate, and spheroidenone, which contain carbonyl functional groups in conjugation with the carbon-carbon pi-electron system, display broader absorption spectral features and are affected more by the solvent environment than neoxanthin and spheroidene, which do not contain carbonyl functional groups. The possible sources of the spectral broadening are explored by examining the absorption spectra at 77 K in glassy solvents. Also, carotenoids which contain carbonyls have complex transient absorption spectra and show a pronounced dependence of the excited singlet state lifetime on the solvent environment. It is postulated that these effects are related to the presence of an intramolecular charge transfer state strongly coupled to the S-1 (2(1)A(g)) excited singlet state. Structural variations in the series of carotenoids studied here make it possible to focus on the general molecular features that control the spectroscopic and dynamic properties of carotenoids.
The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observations. The charge transfer state most probably results from the presence of the lactone ring in the pi-electron conjugation of peridinin analogous to previous findings on aminocoumarins and related compounds. The behavior of peridinin reported here is highly unusual for carotenoids, which generally show little dependence of the spectral properties and lifetimes of the lowest excited singlet state on the solvent environment.
A practical method for accurate evaluation of the Coulombic contribution to the electronic coupling for energy transfer at any donor-acceptor separation is reported. The method involves the exact interaction between transition densities of each chromophore which are calculated ab initio and may include electron correlation. The method is used to calculate coupling strengths between the pigments of the bacterial light-harvesting complex, LH2, and to compare with results using the ideal dipole approximation (IDA). The results suggest that the relatively symmetric transitions of bacteriochlorophyll a (Bchla) pigments are reasonably well described by the IDA for separations >15 Å, although deviations are significant at smaller separations. The less symmetric transition of the twisted carotenoid pigment is rather poorly described by the IDA and shows significant deviation even at separations of well over 20 Å. The calculated coupling strengths are combined with estimates of the spectral overlap integral to estimate energy-transfer rates and time scales. The total depopulation time scale of the carotenoid S 2 state is estimated to be 85 fs, in reasonable agreement with experiment. The B800-B850 transfer time is estimated to be 1.3 ps (a factor of 2 slower than experiment). Rapid (<400 fs) B800-B800 energy transfer is also predicted. Moreover, the calculations suggest that energy flows both from the carotenoid and the B800 Bchla into pigments of several different protomer units, indicating that interaction between protomer units is important in the LH2 function.
Measurements of resonance-Raman excitation profiles of the CC and C–C stretching Raman lines for crystalline all-trans-spheroidene in KBr disc at 77 K identified a new singlet state (17600 cm−1) that is located in-between the 2Ag− state (14200 cm−1) and the 1Bu+ state (19700 cm−1). The particular state is tentatively assigned to the 1Bu− state on the basis of the extrapolation of the PPP-MRD-CI calculations for the low-lying singlet states of shorter polyenes [P. Tavan, K. Schulten, J. Chem. Phys. 85 (1986) 6602], and its possible role in mediating the rapid 1Bu+ to 2Ag− internal conversion is discussed.
Ultrafast fluorescence upconversion has been used to probe electronic excitation transfer within the B800-B820 light-harvesting antenna of Rhodopseudomonas acidophila strain 7050, Emission from the carotenoid St band decays in 54 +/- 8 fs, and the bacteriochlorophyll B820 Q(y) band rises in approximately 110 fs. The B820 Q(y) rise time is wavelength-dependent. Energy-transfer rates between the carotenoid and several neighboring bacteriochlorophyll are calculated. Coupling strengths are estimated through transition dipole-transition dipole, polarization, and higher-order Coulombic coupling along with a new transition density volume coupling calculation. Data are compared to calculated energy-transfer rates through the use of a four-state model representing direct carotenoid to B820 energy transfer. The carotenoid emission data bound the S-2 to Q(x) transfer time between 65 and 130 fs. The S-1 to Q(y) transfer is assumed to be mediated by polarization and Coulombic coupling rather than by exchange; the transfer time is estimated to be in the picosecond regime, consistent with fluorescence quantum yield data.
The energy transfer from the carotenoid okenone to bacteriochlorophyll a (Bchl a) in the light harvesting complex B800–830 and chromatophores of Chromatium purpuratum was studied by steady-state fluorescence and fermtosecond transient absorption spectroscopy. By comparing the fluorescence excitation and the absorption spectra of the B800–830 antenna complex the total energy transfer efficiency of the okenone-to-Bchl a transfer was determined to 95 ± 5%. A fast (<200 fs) transfer from (at least) one carotenoid was observed to the B830 chlorophylls. This transfer was found by fs anisotropy measurements to be a transfer from the okenone S2 state to the Bchl a Qx state, probably caused by the dipole-dipole (Förster) mechanism. Later the low-lying okenone S1 state, which has a lifetime of ∼8ps in solution, was observed transferring energy to the B830 pool in ∼3.8 ps, while a second carotenoid transferred its energy also from the S1 state to B800 in ∼0.5 ps. The results are discussed in relation to the crystal structure of the B800–850 complex of Rhodopseudomonas acidophila [McDermott et al. Nature 374 (1995)].
An energy transfer pathway in a carotenoid-chlorophyll a protein complex of dinoflagellates was studied by the femtosecond up-conversion method. The energy levels of the S2 state of peridinin and their lifetime were essentially identical in methanol and in the complex. The S1 lifetime of peridinin in solvents was more than 30-fold longer than in the complex. These results account for an observed transfer efficiency (higher than 85%) and indicate that an energy transfer occurs between the S1 states of peridinin and chlorophyll a after a rapid internal conversion. This pathway is unique in photosynthetic organisms.
A computationally simple method for molecular excited states, namely, the Tamm–Dancoff approximation to time-dependent density functional theory, is proposed and implemented. This method yields excitation energies for several closed- and open-shell molecules that are essentially of the same quality as those obtained from time-dependent density functional theory itself, when the same exchange-correlation functional is used.
Transient absorption spectroscopy was used to measure the S1 dynamics of four all-trans-carotenoids: 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene and spheroidene. After excitation into the S2 state and subsequent relaxation to the S1 state, the S1 → Sn electronic absorption bands in the region 460–520 nm were probed and observed to decay with single-exponential kinetics: τ = 407 ± 23 ps for 3,4,7,8-tetrahydrospheroidene, 85 ± 5 ps for 3,4,5,6-tetrahydrospheroidene, 25.4 ± 0.9 ps for 3,4-dihydrospheroidene and 8.7 ± 0.1 ps for spheroidene. These data are discussed in three contexts: (1) as a test of the adherence of carotenoids to the energy gap law; (2) as a means of determining the S1 state energy of spheroidene, which appears to be at 14100 cm−1; and (3) in terms of the constraints placed on the efficiencies of carotenoid-to-chlorophyll energy transfer in photosynthetic systems.
The ground and excited-state properties of peridinin in solution and in the peridinin-chlorophyll-protein (PCP) complex are studied by using one-photon and two-photon spectroscopy, solvent effects, and quantum theory. Two-photon excitation spectra, two-photon polarization data, and fluorescence spectra in CS2 reveal three low-lying excited singlet states in peridinin: a lowest-excited(1)A(g)(*-)-like state with a system origin at similar to16 200 cm(-1), a 1B(u)(*+)-like S-2 state with a system origin at similar to19 300 cm(-1), and a B-1(u)*--like S-3 state with a system origin at similar to22 000 cm(-1). The B-1(u)*+-like S-2 state dominates the two-photon excitation spectrum of peridinin in solution and in PCP because of type 11 enhancement associated with a large oscillator strength (f approximate to 1.6) coupled with a change in dipole moment upon excitation (Deltamu approximate to 3 D). Thus, the two-photon spectrum looks very much like the one-photon spectrum although weak vibronic bands of the (1)A(g)(*-)-like state are observed at similar to17, similar to18.1, and similar to19.2 kK in the two-photon spectrum of peridinin in CS2. MNDO-PSDCI theory and solvent effect studies indicate that the S-1 (Ag-1(*-)-like) state has a large dipole moment (mu(aa) approximate to 16 D, Deltamu approximate to 8 D) in both polar and nonpolar environments, much larger than the ground-state dipole moment [mu(00) approximate to 6 (non polar media) - 8 (polar media) D]. Thus, the (1)A(g)(*-)-like state is assigned as the charge-transfer state observed in previous studies. The suggestion that the charge-transfer character is induced in polar solvent is not supported by these studies. We conclude that some of the studies that have suggested an increase in the charge-transfer character of S, with solvent polarity are based on experiments that are more sensitive to (mu(aa) - mu(00))mu(00) than (mu(aa) - mu(00)). Peridinin exists as a mixture of all-trans and 14-s-cis (single bond to allene moiety) conformers in ambient temperature hexane solution, with a predominance of the former. Polar solvents such as methanol and high-dielectric solvents such as CS2 preferentially stabilize the all-trans conformer relative to the 14-s-cis. MNDO-PSDCI calculations on the minimized peridinin molecules within PCP indicate that most of the chromophores have excited state properties similar to those observed for the isolated all-trans chromophore. However, the chromophores occupying sites 612 and 622 are not only blue shifted but have inverted S-1 and S-2 singlet states. Our studies provide both support and additional perspective on the PCP energy transfer model proposed by Damjanovic et al. (Biophys. J. 2000, 79, 1695-1705) in which the peridinin molecules in these sites transfer energy to other peridinin chromophores rather than directly to chlorophyll. We conclude that the 612 and 622 sites optimize energy transfer by increasing the population of the lowest-excited B-1(u)*+-like state, which provides enhanced dipolar coupling to the remaining peridinin set. An analysis of the, PCP complex spectrum in terms of component spectra of the pigments indicates that two peridinin molecules have. unique, blue-shifted spectra.
The Hohenberg-Kohn theorem is extended to fractional electron number N, for an isolated open system described by a statistical mixture. The curve of lowest average energy EN versus N is found to be a series of straight line segments with slope discontinuities at integral N. As N increases through an integer M, the chemical potential and the highest occupied Kohn-Sham orbital energy both jump from EM-EM-1 to EM+1-EM. The exchange-correlation potential Excn(r) jumps by the same constant, and limr Excn(r)>~0.
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
The protection action of carotenoids against irreversible photodestruction was discovered in photosynthetic bacteria by Stanieda and coworkers. In green plant material it was found by Wolff and Witt (1969) Z. Naturforsch, 24b, 1031-1037 and (1972) Proc. 2nd. Int. Congr. Photosynthesis Res. Stresa (Forti, G., Avron, M. and Melandri, A., eds.), Vol. 2, pp. 931-936, Dr. W. Junk, N. V. Publ. The Hague) that the formation of special carotenoid triplet states (via very rapid energy transfer from excited chlorophylls) and their fast radiationless decay in tau1/2 approximately 3 microns is at least one mechanism for the protective action of carotenoids to irreversible photooxidation of the chlorophylls. Hence, it is anticipated that the same mechanism might be realized also in bacteria. The present study gives evidence for such a "triplet valve" to be established also in bacteria. This conclusion was derived from the following observations: 1. The light-induced difference spectrum shows a bleaching of a carotenoid at three characteristic wavelength between 400 and 500 nm. A positive peak around 533 nm indicates the formation of a carotenoid triplet state. 2. The absorption changes can be induced by red light which excites only bacteriochlorophyll. This indicates an energy transfer from bacteriochlorophyll to carotenoids. 3. The light-induced carotenoid triplets decay radiationless in 3 microns in air-saturated aqueous suspensions of the chromatophores. 4. The carotenoid triplet formation occurs only at actinic flash intensities where the photosynthesis becomes saturated. 5. Addition of dithionite, which blocks photosynthesis, markedly increases the extent of carotenoid triplet formation. The different types of exciton migration within the photosynthetic unit are discussed, especially the routes leading to the dissipation of excess excitation energy.
Carotenoids are essential for the survival of photosynthetic organisms. They function as light-harvesting molecules and provide photoprotection. In this review, the molecular features which determine the efficiencies of the various photophysical and photochemical processes of carotenoids are discussed. The behavior of carotenoids in photosynthetic bacterial reaction centers and light-harvesting complexes is correlated with data from experiments carried out on carotenoids and model systems in vitro. The status of the carotenoid structural determinations in vivo is reviewed.
Carotenoids are abundant in many fruits and vegetables and they play diverse roles in photobiology, photochemistry and medicine. This review concerns the reactivity of carotenoids with singlet oxygen and the interaction of carotenoids with a range of free radicals. Mechanisms associated with the anti- and pro-oxidant behaviour of carotenoids are discussed including carotenoid interactions with other anti-oxidants.
Time-resolved excited-state absorption intensities after direct two-photon excitation of the carotenoid S(1) state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S(1) state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S(2) state or the recently discovered dark state situated between S(1) and S(2). The lifetimes of the carotenoid S(1) states in the B800-B850 complex and B800-B820 complex of Rhodopseudomonas acidophila are 7+/-0.5 ps and 6+/-0.5 ps, respectively, and in the light-harvesting complex 2 of Rhodobacter sphaeroides approximately 1.9+/-0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S(2) excitation. In Rps. acidophila the carotenoid S(1) to bacteriochlorophyll energy transfer is found to be quite inefficient (phi(ET1) <28%) whereas in Rb. sphaeroides this energy transfer is very efficient (phi(ET1) approximately 80%). The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S(1) --> B800 electronic energy transfer (EET) pathway ( approximately 85%) dominates over Car S(1) --> B850 EET ( approximately 15%) in Rb. sphaeroides, whereas in Rps. acidophila the Car S(1) --> B850 EET ( approximately 60%) is more efficient than the Car S(1) --> B800 EET ( approximately 40%). The individual electronic couplings for the Car S(1) --> BChl energy transfer are estimated to be approximately 5-26 cm(-1). A major contribution to the difference between the energy transfer efficiencies can be explained by different Car S(1) energy gaps in the two species.
Peridinin-chlorophyll-protein (PCP) is a unique light-harvesting protein that uses carotenoids as its primary light-absorbers. This paper theoretically investigates excitation transfer between carotenoids and chlorophylls in PCP of the dinoflagellate Amphidinium carterae. Calculations based on a description of the electronic states of the participating chromophores and on the atomic level structure of PCP seek to identify the mechanism and pathways of singlet excitation flow. After light absorption the optically allowed states of peridinins share their electronic excitation in excitonic fashion, but are not coupled strongly to chlorophyll residues in PCP. Instead, a gateway to chlorophyll Q(y) excitations is furnished through a low-lying optically forbidden excited state, populated through internal conversion. Carbonyl group and non-hydrogen side groups of peridinin are instrumental in achieving the respective coupling to chlorophyll. Triplet excitation transfer to peridinins, mediated by electron exchange, is found to efficiently protect chlorophylls against photo-oxidation.
The peridinin chlorophyll-a protein (PCP) of dinoflagellates differs from the well-studied light-harvesting complexes of purple bacteria and green plants in its large (4:1) carotenoid to chlorophyll ratio and the unusual properties of its primary pigment, the carotenoid peridinin. We utilized ultrafast polarized transient absorption spectroscopy to examine the flow of energy in PCP after initial excitation into the strongly allowed peridinin S2 state. Global and target analysis of the isotropic and anisotropic decays reveals that significant excitation (25-50%) is transferred to chlorophyll-a directly from the peridinin S2 state. Because of overlapping positive and negative features, this pathway was unseen in earlier single-wavelength experiments. In addition, the anisotropy remains constant and high in the peridinin population, indicating that energy transfer from peridinin to peridinin represents a minor or negligible pathway. The carotenoids are also coupled directly to chlorophyll-a via a low-lying singlet state S1 or the recently identified SCT. We model this energy transfer time scale as 2.3 +/- 0.2 ps, driven by a coupling of approximately 47 cm(-1). This coupling strength allows us to estimate that the peridinin S1/SCT donor state transition moment is approximately 3 D.
We investigate the assignment of electronic transitions in alkyl peroxy radicals. Past experimental work has shown that the phenyl peroxy radical exhibits a transition in the visible region; however, previous high level calculations have not reproduced this observed absorption. We use time dependent density functional theory (TDDFT) to characterize the electronic excitations of the phenyl peroxy radical as well as other hydrocarbon substituted peroxy radicals. TDDFT calculations of the phenyl peroxy radical support an excitation in the visible spectrum. Further, we investigate the nature of this visible absorption using electron attachment/detachment density diagrams of the peroxy radicals and present a qualitative picture of the origin of the visible absorption based on molecular orbital perturbations. The peroxy radical substituent is also compared against isoelectronic radical groups. The visible absorption is determined to be dependent on mixing of the alkyl and radical substituent orbitals.
Nonphotochemical quenching (NPQ) refers to a process that regulates photosynthetic light harvesting in plants as a response to changes in incident light intensity. By dissipating excess excitation energy of chlorophyll molecules as heat, NPQ balances the input and utilization of light energy in photosynthesis and protects the plant against photooxidative damage. To understand the physical mechanism of NPQ, we have performed femtosecond transient absorption experiments on intact thylakoid membranes isolated from spinach and transgenic Arabidopsis thaliana plants. These plants have well defined quenching capabilities and distinct contents of xanthophyll (Xan) cycle carotenoids. The kinetics probed in the spectral region of the S(1) --> S(n) transition of Xans (530-580 nm) were found to be significantly different under the quenched and unquenched conditions, corresponding to maximum and no NPQ, respectively. The lifetime and the spectral characteristics indicate that the kinetic difference originated from the involvement of the S(1) state of a specific Xan, zeaxanthin, in the quenched case.
- Kong J.