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Triplet and singlet energy transfer in carotene-porphyrin dyads: Role of the linkage bonds
Abstract
A series of carotenoporphyrin dyad molecules in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta, or para position of a meso aromatic ring has been prepared, and the molecules have been studied using steady-state and transient fluorescence emission, transient absorption, and H-1 NMR methods. Triplet-triplet energy transfer from the porphyrin moiety to the carotenoid has been observed, as has singlet-singlet energy transfer from the carotenoid polyene to the porphyrin. In addition, the carotenoid quenches the fluorescence of the attached porphyrin by a mechanism which increases internal conversion. The rates of all three of these processes are slower for the meta isomer than for the corresponding ortho and para molecules. Analysis of the data suggests that the triplet-triplet energy transfer is mediated by a through-bond (superexchange) mechanism involving the pi-electrons of the linkage bonds, rather than a direct, through-space coupling of the chromophores. The same appears to be true for the process leading to enhanced internal conversion. The results are consistent with a role for the through-bond mechanism in the singlet-singlet energy transfer as well. Simple Huckel molecular orbital calculations are in accord with the proposed through-bond process.
... Therefore, poor excited-state electronic coupling scenarios, where high kinetic barriers mediate IC/VR (Figure 1, bottom), must be explored to realize anti-Kasha photochemistry and anti-dissipative energy conversion. Observations involving a variety of intramolecular donor/bridge/acceptor processes, ranging from singlet fission to ground-and excited-state intervalence charge transfers, [6][7][8][9][10][11][12][13][14][15] suggest that the topology of the bridge can be exploited to modulate excited-state electronic coupling. 16,17 Prototypical polynuclear ruthenium polypyridines 18,19 provide an ideal platform for these studies because their excited states have clear spectroscopic handles, with well-defined long-lived metal-to-ligand charge transfer (MLCT) transitions and they are widely used in energy conversion schemes and photoredox catalysis. ...
In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the catalytic center formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion and vibrational relaxation, also dissipate as much as 20-30 % of the absorbed photon energy. Minimization of these energy losses, a holy-grail in solar energy conversion and solar fuels production, is a challenging task, because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for internal conversion of around 2000 cm–1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, i.e. a bridging-ligand centered metal-to-ligand charge transfer (MLCTLm), and a 2,2’-bipyridine-centered MLCT (MLCTbpy), which lies 800-1400 cm–1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by tri-tolylamine and produced the corresponding bpy•– and Lm•– centered reduced complexes, as confirmed by transient absorption spectroscopy. This efficiently generated two distinct reduced photosensitizers with different reducing abilities, i.e. –0.93 V and –0.79 V vs NHE for bpy•– and Lm•–, respectively. Hence, this novel strategy not only allows to trap higher energy excited states, before internal conversion and vibrational relaxation set in, saving between 110 and 170 meV and but also leads in fine to 140 meV more potent reductant for energy conversion schemes and solar fuels production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harnesses the excess energy saved by controlling dissipative conversion pathways.
... 29,31,33,36 The linker between the carotenoid and tetrapyrrole can have an effect by partitioning in the energy transfer mechanism, in modulating the distance and electronic coupling, and by influencing the relative orientation of the two chromophores. 33,37,38 Here, we report that the same carotenophthalocyanine dyad with a phenylene linker can undergo quenching by energy transfer, quenching by excitonic coupling, and nonquenching. We propose the coexistence of these three regimes is caused by conformational variation. ...
Under excess illumination, photosystem II of plants dissipates excess energy through the quenching of chlorophyll fluorescence in the light harvesting antenna. Various models involving chlorophyll quenching by carotenoids have been proposed, including (i) direct energy transfer from chlorophyll to the low-lying optically forbidden carotenoid S1 state, (ii) formation of a collective quenched chlorophyll–carotenoid S1 excitonic state, (iii) chlorophyll–carotenoid charge separation and recombination, and (iv) chlorophyll–chlorophyll charge separation and recombination. In previous work, the first three processes were mimicked in model systems: in a Zn-phthalocyanine–carotenoid dyad with an amide linker, direct energy transfer was observed by femtosecond transient absorption spectroscopy, whereas in a Zn-phthalocyanine–carotenoid dyad with an amine linker excitonic quenching was demonstrated. Here, we present a transient absorption spectroscopic study on a Zn-phthalocyanine–carotenoid dyad with a phenylene linker. We observe that two quenching phases of the phthalocyanine excited state exist at 77 and 213 ps in addition to an unquenched phase at 2.7 ns. Within our instrument response of ∼100 fs, carotenoid S1 features rise which point at an excitonic quenching mechanism. Strikingly, we observe an additional rise of carotenoid S1 features at 3.6 ps, which shows that a direct energy transfer mechanism in an inverted kinetics regime is also in effect. We assign the 77 ps decay component to excitonic quenching and the 3.6 ps/213 ps rise and decay components to direct energy transfer. Our results indicate that dual quenching mechanisms may be active in the same molecular system, in addition to an unquenched fraction. Computational chemistry results indicate the presence of multiple conformers where one of the dihedral angles of the phenylene linker assumes distinct values. We propose that the parallel quenching pathways and the unquenched fraction result from such conformational subpopulations. Our results suggest that it is possible to switch between different regimes of quenching and nonquenching through a conformational change on the same molecule, offering insights into potential mechanisms used in biological photosynthesis to adapt to light intensity changes on fast time scales.
... Corrole has pulled out from the backwater after the synthetic breakthroughs by Gross and Paolesse [36,37], which have opened the door to more extensive research. It is important to mention that, the role of either an energy/electron donor or acceptor depends on the nature of counter molecular entity of the porphyrin/corrole (vide infra), by utilizing this, many donor-acceptor (D-A) systems have been constructed with different energy/electron donors such as ferrocene [38], anthracene [39], carbazole [40], coumarin [41], triphenylamine [42], fluorene [15], pyrene [15], BODIPY [43], carotenyls [5], tetrathiafulvalenes [44], oligothiophenes [45], xanthenes [46] and azulene [47] connected either with porpyrin and/or corrole have been studied. On the other hand the efficient electron acceptors such as C 60 , anthraquinone [8], naphthalenediimide (NDI) were also reported [27,28]. ...
Two novel donor-acceptor dyads, in which phenothiazine (PTZ) connected at β-pyrrolic position of either freebase corrole (TPC) or freebase porphyrin (TPP) via vinylic spacer have been synthesized. Both the dyads were characterized by ESI-MS, IR, ¹H NMR (1D and 2D ¹H-¹H COSY and J-Resolved), UV–Vis, Study state Fluorescence, Time-resolved fluorescence (Time-correlated single photon counting (TCSPC), Streak Camera) as well as electrochemical methods. In the absorption spectra of the dyads, both Soret and Q-bands were red shifted by 8–20 nm indicating weak electronic communication between the two chromophores. However, the fluorescence emission from the PTZ of the dyad was efficiently quenched (96–99%) as compared to pristine PTZ where the dyad was excited at 310 nm, which is attributed to singlet-singlet excited energy transfer. Fluorescence emission from porphyrin part of the TPP-PTZ dyad also quenched when excite the dyad at 420 nm, which is ascribed to photoinduced electron transfer from ground state of PTZ to excited state of porphyrin. In contrast, when excite the corrole at 420 nm in TPC-PTZ dyad, we found there are no significant changes in photophysical properties. In both the cases the solvent dependence of the rate of energy and electron transfer was observed.
... The dyad and the two isolated reference compounds, Pc and Car, were synthesized as previously described. 24 The dyad consists of a 10-double-bond polyene terminated with a para-phenylene group that is attached to an amino group at a peripheral position of Pc. The reference Pc is identical to the dyad, but the polyene chain is replaced by a methyl group and in the reference carotenoid the Pc moiety of the Car-Pc is replaced by a tolyl group. ...
In any photosynthetic/photocatalytic device, multiple steps are required between the arrival of a solar photon and the formation of a stable product. Here we explain and demonstrate the target analysis methodology to develop minimal models, identify the steps and estimate the parameters that characterize energy converting devices. With this modelling tool the molecular mechanisms of the loss processes can be identified and quantified. This can then inspire photosynthetic device optimization by precisely targeting those sites involved in the most significant losses. Two case studies of recently published measurements (Pillai et al., 2013) on a carotenoporphyrin dyad and a carotenofullerene dyad are modelled in depth. After carotenoid excitation, no excited state energy transfer (EET) to porphyrin was found, but EET from carotenoid hot S1 to the fullerene moiety occurred with a rate of 1.6/ps. The total radical pair yields of these dyads were found to be, respectively, 46% and 79%. Out of these 79%, 31% were due to electron transfer from the fullerene excited state. The triplet yields were 3.8% and 4.6%. The remainder of the excitations decay to the ground state from the carotenoid hot S1 and S1 states.
... To emulate the light-absorbing property of chlorophylls, many artificial reaction centers feature porphyrins, chlorophyll derivatives, and related cyclic tetrapyrrolic molecules as the primary chromophore and excited-state electron donor. 3,12,[16][17][18]20,31,34,36,37,40,42,[44][45][46][47][48][49] Modeling natural photosynthesis led to the inclusion of quinones as the electron acceptor among some of the earliest synthesized photosynthetic mimics. 19,26,27,29,[50][51][52] Subsequently, fullerenes were found to possess ideal electron acceptor qualities in artificial photosynthetic systems due to their large electron affinity, large charge accumulation capacity, and a small reorganization energy upon electron transfer. ...
We present a detailed study of charge transfer (CT) excited states for a
large number of structural conformations in a light-harvesting
Carotenoid-diaryl-Porphyrin-C60 (CPC60) molecular triad. The molecular triad
undergoes a photoinduced charge transfer process exhibiting a large excited
state dipole moment, making it suitable for application to molecular-scale
opto-electronic devices. An important consideration is that the conformational
flexibility of the CPC 60 triad impacts its dynamics in solvents. Since
experimentally measured dipole moments for the triad of ~110 Debye (D) and ~160
Debye strongly indicate a range in conformational variability for the triad in
the excited state, studying the effect of conformational changes on the CT
excited state energetics furthers the understanding of its charge transfer
states. We have calculated the lowest CT excited state energies for a series of
14 triad conformers, where the structural conformations were generated by
incrementally scanning a 360 degree torsional (dihedral) twist at the
C60-porhyrin linkage and the porphyrin-carotenoid linkage. Additionally, five
different CPC60 conformations were studied to determine the effect of
pi-conjugation and particle-hole Coulombic attraction on the CT excitation
energies. Our calculations show that structural conformational changes in the
triad show a variation of ~0.4 eV in CT excited state energies in the
gas-phase. The corresponding calculated excited state dipoles show a range of
88 D - 188 D.
In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20-30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm-1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(Lm)], and a 2,2'-bipyridine-centered MLCT [MLCT(bpy)], which lies 800-1400 cm-1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding Lm•--centered and bpy•--centered reduced photosensitizers, which involve different reducing abilities, that is, -0.79 and -0.93 V versus NHE for Lm•- and bpy•-, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.
Understanding the dynamics of the back electron transfer (BET) rate of ion pairs from the electronically excited state of donor-acceptor systems is crucial for developing materials for organic electronics. The structure-property relationships in the organic molecular architectures play a key role in controlling the BET rate and have been utilized as a criterion to design systems with a reduced BET rate. Here, we examine the influence of isomerism on the BET rate in anthracene based systems, viz., (E)-2-(2-(anthracen-9-yl)vinyl)benzonitrile (ortho-CN) and (E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile (meta-CN) with N,N-diethylaniline (DEA) in methylcyclohexane using time-resolved spectroscopy. The radical cation (DEA˙+) and the radical anion (ortho-CN˙- or meta-CN˙-) generated after photoexcitation show synchronous decay kinetics, and the rate constant of back electron transfer (kBET) for the DEA/ortho-CN pair was 6.6 × 104 s-1, which is ca. 2 orders of magnitude lower compared with the DEA/meta-CN pair. The role of isomerism in providing resonance stabilization for the organic radicals is expected to have implications for strategies that retard charge recombination in photovoltaics. The role of the molecular structural features that dictate the kinetics for charge recombination has been further identified using quantum calculations.
Carotenoids act as light harvesting accessory pigments in the photosynthetic process. These absorb light to drive photosynthesis as well as protect the photosynthetic machinery from damage due to high intensity of light. This light energy is transferred to the reaction center through either by fluorescence resonance energy transfer or electron exchange between carotenoids and chlorophyll molecules. Both of these energy transfer processes occur due to triplet excited state and singlet excited state of carotenoids respectively. Here, it has been demonstrated through spectroscopic studies that how carotenoids affect the reaction centre while transferring the energy. High intensity of light as well as low intensity of light also affects the process and rate of photosynthesis as well as the physical appearance of the plant. We have attempted to briefly describe the experiments and interpretations by other scientists that dealt with the question of how energy is transferred from the original location of photon absorption in the photosynthetic antenna system into the reaction centers and where it is converted into useful chemical energy.
Natural light-harvesting complexes (LHCs) absorb a broad spectrum of sunlight using a collection of photosynthetic pigments whose spatial arrangement is controlled by a protein matrix and exhibit efficient energy transfer. We constructed a novel light-harvesting protein mimic, which absorbs light in the UV to visible region (280−700 nm) by displaying flavone and porphyrin on a peptoid helix. First, an efficient synthesis of 4'-derivatized 7-methoxyflavone (7-MF, 3 and 4) was developed. The flavone-porphyrin-peptoid conjugate (FPPC) was then prepared via Miyaura borylation on a resin-bound peptoid followed by Suzuki coupling between the peptoid and pigment. Circular dichroism spectroscopy indicated that the FPPC underwent helix-to-loop conversion of the peptoid scaffold upon changing the solvent conditions. A distinct intramolecular energy transfer was observed from 7-MF to porphyrin with greater efficiency in the helix than that in the loop conformation of the peptoid, while no clear evidence of energy transfer was obtained for unstructured FPPC. We thus demonstrate the value of the helical peptoid, which provided a controlled orientation for 7-MF and porphyrin and modulated the energy transfer efficiency via conformational switching. Our work provides a way to construct a sophisticated LHC mimic with enhanced coverage of the solar spectrum and controllable energy transfer efficiency.
The 2,5-diaminoterephthalate structural motif is a powerful chromophore with remarkable fluorescence properties. Containing two carboxylate and two amino functions it defines a colored molecular scaffold which allows for orthogonal functionalization with different functional units. Therefore, different applications in Life Sciences and Materials Science could be addressed. In this study, the two amino functions were alkylated by reductive amination with side chains carrying amino (orthogonally protected as Boc or Alloc) and carboxylate functions (orthogonally protected as tBu or allyl ester). After sequential deprotections, functional units were introduced by amidation reactions. As three examples, the chromophore was coupled to retinoic acid and fullerene C60 in order to obtain a triad for studying photoinduced electron transfer processes. Furthermore, cyclooctyne and azide moieties were introduced as functional units allowing for ligation by Click reactions. These two clickable groups were applied in combination with a maleimide units which are reactive towards thiol residues. The latter dyes define so called "turn on" probes, since the fluorescence quantum yields increased by an order of magnitude upon reaction with the molecular target.
Presently, the world is experiencing an important energetic crisis due to the increasing demands of humanity and the limited resources provided by fossil fuels. In this context, harvesting and converting solar energy via artificial photosynthesis, in a similar way as plants and photosynthetic organisms do, offers an ideal way to limit the dependence of our society on fossil fuel and its myriad consequences. Hence, it is of extreme importance the development and study of molecular artificial photosynthetic reactions centers and antenna complexes to build the understanding required for development of future scalable technologies. This review focuses on the study of molecular complexes, design of which is inspired by the components of natural photosynthesis, and covers research from early triad reaction centers developed in the 80s by the group of Gust, Moore, and Moore to recent hybrid systems capable of reproduce specific functions of the photosynthetic apparatus.
This chapter presents a brief history of molecule-based artificial photosynthesis research. The emphasis is on early work on the subject beginning in the late 1970s, but the evolution of the field up through the present day is discussed. The main subjects covered are artificial reaction centres and photoinduced electron transfer, artificial photosynthetic antennas, mimicry of photosynthetic photoprotection and photoregulation and the construction of systems that couple these processes to the production of fuels.
This chapter discusses multistep electron and energy transfer in artificial photosynthesis. Three general classes of photophysical and photochemical processes are of great importance in bacterial reaction centers. Photosynthetic organisms employ a variety of antenna pigments to absorb light throughout the useful region of the solar spectrum. These antennas convey the resulting excitation to the special pair via singlet-singlet energy-transfer processes. Carotenoid pigments are responsible for a large fraction of the light used in photosynthesis. In the case of carotenoid-to-tetrapyrrole singlet energy transfer, there is a complication because of the lack of complete information concerning the energies and photophysical parameters of the carotenoid-excited states. In principle, singlet-singlet energy transfer may occur by a trivial emission-absorption mechanism, by a long-range columbic mechanism, or by an electron exchange mechanism similar to that involved in triplet-triplet energy transfer.
Connecting electron donors and acceptors to a benzene ring in a meta or para relationship results in quantum interference effects that can strongly influence charge separation (CS) and charge recombination (CR) processes in these systems. We report on the energy and electron transfer behavior of chlorophyll-based para- and meta-linked donor-bridge-acceptor (D-B-A) dyads, where the semi-synthetic chlorophyll a derivative, zinc methyl 3-ethyl-pyrochlorophyllide a (D), is covalently attached at its 20-position to the para position of one phenyl of diphenylacetylene (B). The meta or para position of the phenyl in B distal to the donor is in turn attached to perylene-3,4:9,10-bis(dicarboximide) (PDI) (A). Photoexcitation of the D-B-A dyads produces long-lived radical ion pairs D(+•)-B-A(-•), which recombine to ground state and to both (3*)D-B-A and D-B-(3*)A. Time-resolved optical and electron paramagnetic resonance spectroscopies were used to monitor the charge transfer and triplet energy transfer (TEnT) processes. At longer times, TEnT occurs from (3*)D-B-A to D-B-(3*)A. Surprisingly, the D-B-A molecules linked via the meta linkage exhibit faster CS, CR, and TEnT rates than do those with the para linkage in contrast to most other meta/para-linked D-B-A molecules previously examined.
The kinetics of formation reaction of Zn(II), Cd(II), Hg(II) with metamethyl tetraphenylporphyrin (H2T(m-CH3)PP) in acetone and Zn(II) with tetraphenylporphyrin (H2TPP) in acetone or in DMF have been studied. A general mechanism was proposed and the kinetic parameters were obtained by non-linear least-squares program. The different behavior of the reaction system of Zn(II), Cd(II), Hg(II) have been interpreted reasonably. The effects of temperature and solvent on the reaction rate were also investigated in this paper.
Through-bond triplet exciplex formation in donor-acceptor systems linked through a rigid bile acid scaffold has been demonstrated on the basis of kinetic evidence upon population of the triplet acceptors (naphthalene, or biphenyl) by through-bond triplet-triplet energy transfer from benzophenone.
The initial energy transfer steps in photosynthesis occur on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that Förster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which lead to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited states as part of the system’s Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play the central role in the excited state population transfer to bacteriochlorophyll; resonance between the donor-acceptor energy gap and the vibrational ground state
energies is the physical basis of the ultrafast energy transfer rates in these systems.
To develop novel polypeptide-based thin films, a series of star polypeptides modified with nonlinear optical chromophores has been synthesized. Using amino-substituted tetraphenyl porphyrin as an initiator, the N-carboxy anhydride of γ-benzyl L-glutamic acid was polymerized onto the porphyrin at a monomer to initiator ratio 20:1. The resulting four-branch star was modified with a selection of dyes. Dyes that modified both the N-terminus and benzyl side chain were used. We demonstrated feasibility of insertion of a metal ion into the polypeptide porphyrin core. The polypeptide series was characterized by UV/VIS, FTIR, and CD. The UV/VIS data suggested ease of modification of both the N-terminus and side chains. The FTLR and CD data show the resulting polypeptides were ∝-helical. The results demonstrate the feasibility of modifying the optical properties of a porphyrin by three approaches: insertion of metal, attachment of dye to N-terminus or modification of γ-benzyl L-glutamate side chain.
Diaminoterephthalate was used for the first time as a chromophore in a dyad with [60]fullerene. The synthesis was accomplished from the benzyl methyl diester by hydrogenolytic cleavage of the benzyl group, re-esterification with a benzyl alcohol with protected formyl function, deprotection of the latter and final 1,3-dipolar cyclo-addition with C60 and sarcosine via the azomethine ylide.
It is shown that at 295 K in meso-orthonitrophenyl-substituted octaethylporphyrins and their chemical dimers the steric interactions of the nitro-group and the volume substituents at β-positions of the pyrrole rings favor direct overlapping of molecular orbitals in a donor-acceptor pair. The efficient quenching of fluorescence of the nitroporphyrins in toluene is attributed to direct nonadiabatic electron transfer from the S1-level of a porphyrin to the lower-lying state with charge transfer by the “through-space” mechanism. Quenching of the T1-states is related with heat-stimulated transmission to the higher-lying states with charge transfer of the ion-radical pair as well as with enhancement of the probability of the nonradiative T1→S0-transition.
The excitation energy transfer (EET) between a fluorescent resonance energy transfer (FRET) pair, cyan fluorescent protein chromophore (CFPc)/ green fluorescent protein chromophore (GFPc), is used for measuring molecular proximity in experimental studies. In this article, the role of the bridging protein media has been investigated using the quantum chemical calculations for the EET electronic coupling. We adopted a model peptide connecting GFPc and CFPc, a GFPc-X-CFPc model, and examined the X (amino acids) dependence. The major part of the EET coupling element arises from the direct interaction between GFPc and CFPc via the Förster mechanism. The magnitude of the through-X interactions vary for the type of residues. The contribution of the X was, however, at most 10% of the total value. Together with the contributions from through-peptide and charge-transfer types, X gives minor modifications to the total coupling via the super-exchange mechanism. © 2012 Wiley Periodicals, Inc.
The donor–acceptor electronic coupling for a bridge-mediated triplet excitation energy transfer (EET) was analyzed to determine the tunneling pathway using intergroup tunneling configuration fluxes. In the singlet EET, the electronic coupling is primarily composed of direct coupling between the donor and acceptor with a secondary contribution from bridge-mediated single-step through-exciton pathways. In contrast, the triplet EET is dominated by superexchange interactions due to the Förster-type interaction becoming forbidden. The triplet EET tunneling pathways are characterized as a sequentially coupled hole and electron transfer (CHET) for both direct and bridge-mediated EETs. This is the first report concerning the difference between the triplet and singlet EETs of the electronic mechanism of bridge-mediated superexchange mechanisms.
Singlet excitation energy transfer (EET) generally occurs via a Förster mechanism and originates from direct electronic coupling between the donor (D) and acceptor (A). However, indirect EET (the so-called superexchange mechanism) can be crucial in some situations. In this study, we have explored the contributions of various linker types, linker lengths, and tunneling energies in D–linker–A molecular systems using quantum chemical methods [Kawatsu et al. J. Phys. Chem. A2011, 115, 10814], obtaining conditions in which the superexchange term emerges. To analyze the results in terms of physical parameters, a model Hamiltonian was developed and found to yield results qualitatively consistent with those from the quantum chemical calculations. In a model compound, the superexchange term can be up to 4 times larger than the direct (Förster) term when the energy of the excited state at a linker unit is close to the tunneling energy and when the interactions between these excited states are strong. The ratio of the indirect term to the direct term is greatest at a certain D–A distance because the number of multistep terms increases and the sizes of multistep indirect terms decay exponentially. Single-step indirect terms, which exhibit a polynomial decay, dominate the indirect term for larger D–A distances. The decay behavior with the distance is very different from that of the McConnell type donor–bridge–acceptor superexchange mechanism proposed for the charge transfer (CT). The result of pathway analysis showed that the exciton states mediate EET. The coupling between the local excited states is driven by the pseudo-Coulombic interaction, even in the superexchange terms.
In this work we studied an artificial supramolecule composed of a carotenoids (Car) with 10 conjugated double bonds linked to a phthalocyanine (Pc), mimicking the light harvesting unity. We follow the selective excitation of Car, monitoring the ultrafast rise of the Pc population. The Car→Pc ET process occurs from the bright Car S2 excited state and competes with an internal conversion (IC) process towards the lower-lying dark Car S1 excited state. To establish the relative weight of these two deactivation pathways, we performed pump-probe experiments with sub-20-fs temporal resolution on the isolated Car and on the dyad and compared the excited state dynamics. In addition, following selective excitation of the Pc, we have identified the spectral signatures of the S1 excited state of the Car which appear within the ≈50-fs time resolution of our measurement. This strongly indicates excited state coupling between the S1 state of Car and the S1 state of Pc, with important implications for the regulation of photosynthetic activity.
In this paper, we review the generalized Forster‐Dexter theory to treat photoinduced electronic energy transfer for a system in dense media and for an isolated system (i.e., a system in the collision‐free condition). Instead of expressing the rate of energy transfer in terms of spectral overlap, the expression of the energy‐transfer rate constant is obtained by evaluating a Fourier integral involved in the energy transfer rate constant using the saddle‐point method. In this way, the energy‐gap dependence, and the effect of temperature and the isotope effect on the energy transfer can be easily studied. The effect of bridge groups connecting between donor and acceptor chromophores on the intramolecular energy transfer is also studied.
The development of new hybrid materials that can be integrated into current technologies is one of the most important challenges facing material scientists today. The purpose of this work is to review recent studies in one largely unexplored area of nanobiotechnology: the development of nano-bio hybrid materials that exploit Förster Resonance Energy Transfer (FRET) to enhance the functionalities of technologically-promising photosynthetic biomaterials. One of very promising approaches is to employ semiconductor quantum dots having a broad absorption spectrum as nanoantennae coupled with the natural light-harvesting complexes of photosynthetic reaction centres. This system reveals great potential for the utilization of quantum dots in artificial photosynthetic devices. The second very useful functionality, which is discussed in this review, is the possibility to enhance the efficiency of the main biological function (proton pumping) of the protein bacteriorhodopsin using nonradiative energy transfer from quantum dots. Also recent studies revealed that FRET-based improvement of the biological function of bacteriorhodopsin in the presence of quantum dots allows for strong wavelength-dependent enhancement of the nonlinear refractive index of bacteriorhodopsin. These new hybrid bio-nanomaterials with exceptional light-harvesting and nonlinear properties will have numerous photonic applications employing their photochromic, energy transfer, and energy conversion properties.
Studies on the synthesis, structural characterization and biological evaluation of novel chalcone-porphyrin derivatives are described. The photodynamic effect, intracellular localization and cellular uptake of the new conjugates were evaluated in vitro in COS-7 cells. All derivatives exhibited high stability and yielded good fluorescence under light irradiation. None of the chalcone-porphyrins proved to be cytotoxic and/or phototoxic for COS-7 cells.
Numerous interactions come into play when a carotenoid (Car) and a chlorophyll (Chl) become juxtaposed during the assembly of various pigment-bearing proteins which carry out photosynthesis, and it is widely held that these interactions are crucial to the functioning of the photosynthetic apparatus (1). The manifold roles of photosynthetic Car’s include light-harvesting (Car-to-Chl transfer of singlet excitation energy), photoprotection (which entails quenching of triplet Chl and of singlet oxygen), and involvement in the assembly of LHCII (the Chla/b light-harvesting complex associated with photosystem II of green plants). Recently, the three xanthophylls in LHCII—lutein (Lut), neoxanthin (Neo), and violaxanthin (Vio)—were credited with yet another function (2,3), the quenching of Chla*, where the asterisk signifies occupation of S
1, the lowest electronically excited state of singlet spin multiplicity. The Car-induced quenching, since it lowers the fluorescence yield as well as the triplet formation yield, has been called catalysed internal conversion (CIC). Without touching upon the mechanism which brings CIC into operation, it was proposed that (i) as monomers form trimers, and these in turn aggregate, quenching of the first excited singlet state of Chla by the Xan’s bound to LHCII becomes increasingly important, and (ii) quenching of this type, effected through the modulation of the state of aggregation of the complexes in thylakoid membranes, serves to regulate the dissipation of the excitation energy in chloroplasts, thereby protecting plants against excessive irradiation.
We temporally resolved energy transfer kinetics in an artificial
light-harvesting dyad composed of a phthalocyanine covalently linked to
a carotenoid. Upon carotenoid photo-excitation, energy transfers within
≈100fs (≈52% efficiency) to the phthalocyanine.
We have studied the energy transfer dynamics in an artificial light-harvesting dyad composed of a phthalocyanine (Pc) covalently linked to a carotenoid (Car). The combination of high temporal resolution transient absorption spectroscopy with global and target analysis allowed us to quantify the efficiency of the energy transfer from the S2 excited state of the Car to the Pc at 37%, close to values observed in some natural light-harvesting complexes. In addition, following selective excitation of the Pc, we have identified the spectral signatures of the S1 excited state of the Car which appear within the ≈30-fs time resolution of our measurement. This strongly indicates excited state coupling between the S1 state of Car and the Qx state of Pc, with important implications for the regulation of photosynthetic activity.
We present results from fluorescence excitation and transient absorption spectroscopy on a series of artificial light harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked through a phenylamine to carotenoids (Car) with 8, 9, 10 or 11 conjugated double bonds, referred to as dyads 8–11. In dyad-10 and dyad-11, the energy transfer efficiency from the carotenoid S2 state to Pc was shown to depend on the amount of excess vibrational energy in the carotenoid S2 state. The carotenoid S2 state lifetimes in dyad-9 and dyad-10 were several times shorter than those of model carotenoids (car-n) in solution, indicating that energy transfer to Pc occurs from the S2 state. The S2 lifetimes lengthen with excess vibrational energy, and this correlates with a higher efficiency of energy transfer. We hypothesize that the higher energy transfer efficiency on excess vibrational excitation results either from a decreased internal conversion rate to lower-lying optically forbidden states, or from an enhanced coupling between vibrationally hot S2 states with Pc. Ultrafast transient absorption studies revealed S* state features with unprecedented characteristics: In dyad-9, the S* state had a lifetime of only 100 fs and was observed to operate as the major mediator of energy transfer between Car and Pc. In contrast, the contribution to the energy transfer process by the optically forbidden S1 state was negligible (5%). In dyad-10, neither S* nor S1 appear to play a role in the energy transfer process to Pc, and all Car to Pc energy transfer proceeds through S2. Many of the observed phenomena may be a consequence of the unusually strong electronic coupling between Car and Pc observed in the past on these particular systems.
A fluorimetric/colorimetric mercury(II) sensor based on porphyrin-functionalized Fe3O4@SiO2 core/shell magnetic microspheres has been developed and demonstrated by sol–gel grafting reaction. These multifunctional microspheres show excellent fluorescence sensitivity and selectivity towards Hg2+ over other metal ions (K+, Na+, Ba2+, Mn2+, Ca2+, Co2+, Cu2+, Ag+, Mn2+, Ni2+ and Pb2+). Upon addition of Hg2+, the color of porphyrin-functionalized Fe3O4@SiO2 microspheres changes from red to green within 1 min and the fluorescence of microspheres becomes obviously weak. Conversely, no significant changes in fluorescence emission or color are observed in the parallel experiment with other metal ions. Regarding the reversibility of the microspheres, the color and fluorescence of the porphyrin-functionalized Fe3O4@SiO2 microspheres in the presence of Hg2+ ion are found to be almost reversible when the microspheres are treated with EDTA solution. Furthermore, the used microspheres can efficiently remove Hg2+ ions in aqueous solution and easily separated from the mixture by adding an external magnetic field. Theses results suggest that functionalized Fe3O4@SiO2 core/shell magnetic microspheres are potentially useful materials for simultaneously detecting and removing environmental pollutants.
The kinetics of fluorescence polarization in intense pulse excitation of solid disordered solutions of bichromophores that consist of complex molecules of two types between which there can be inductiveresonance transfer of electron-excitation energy is theoretically investigated. Variants of fluorescence excitation by single pulses and pulse trains are considered. The lifetime of the fluorescence of a given solution increases with the intensity of the exciting pulses. The possibility of controlling the duration of fluorescence attenuation for donor molecules incorporated into the bichromophores by the action of luminescence radiation at the frequency of acceptor-molecule absorption on the solution is demonstrated.
Optical limiting properties of organic reverse saturable absorbers, fullerenes, and nanoscopic materials are reviewed. The strongly nonlinear absorptive organic dyes are discussed in terms of the potential and limitation in their further improvement. The principle and practice for the development of supramolecular nonlinear absorbers based on a new molecular engineering approach are described along with future prospects. The fullerene based materials are discussed by emphasizing the mechanistic issues of their optical limiting properties and the potential for improving their optical limiting performance through fullerene cage derivatizations and through the incorporation of fullerene cages into polymeric structures. Finally, the discussion on the development of nano-materials as a new class of strong optical limiters is centred on the optical limiting performance and mechanism of metal and metal sulphide nanoparticles, and on a comparison with strongly nonlinear scattering materials such as suspensions of carbon black particles. The review also includes a comprehensive list of references on optical limiting materials.
It is shown that at 295 K in meso-orthonitrophenyl-substituted octaethylporphyrins and their chemical dimers the steric interactions of the nitro-group and the volume substituents at β-positions of the pyrrole rings favor direct overlapping of molecular orbitals in a donor-acceptor pair. The efficient quenching of fluorescence of the nitroporphyrins in toluene is attributed to direct nonadiabatic electron transfer from the S1-level of a porphyrin to the lower-lying state with charge transfer by the “through-space” mechanism. Quenching of the T1-states is related with heat-stimulated transmission to the higher-lying states with charge transfer of the ion-radical pair as well as with enhancement of the probability of the nonradiative T1→S0-transition.
Two artificial photosynthetic reaction centers consisting of a porphyrin (P) covalently linked to both a carotenoid polyene (C) and a fullerene derivative (C60) have been prepared and found to transfer triplet excitation energy from the fullerene moiety of C-P-3 C60 to the carotenoid polyene, yielding 3C-P-C6. The transfer has been studied both in toluene at ambient temperatures and in 2-methyltetrahydrofuran at lower temperatures. The energy transfer is an activated process, with Ea=0.17 eV. This is consistent with transfer by a triplet energy transfer relay, whereby energy first migrates from C-P-3C60 to the porphyrin, yielding C-3P-6060 in a slow, theramally activated step. Rapid Rapid energy transfer from the porphyrin triplet to the carotenoid gives the final state. Triplet relays of this sort have been observed in photosynthetic reaction centers, and are part of the system that protects the organism from damage by singlet oxygen, whose production is sensitized by chlorophyll triplet states. The fullerene-containing triads can also demonstrate stepwise photoinduced electron transfer to yield long-lived C.+-P-C60-− charge-separted states. Electron transfer occurs even at 8 K. Charge recombination of C.+-P-C60.− yeilds 3C-P-C60, rather than the molecular ground state. These protochemical events are reminiscent of photoinduced electron transfer in photosynthetic reaction centers.
Data on the synthesis, steric structures, and photochemical properties of molecular diad systems based on porphyrins as synthetic models of the reaction centre in photosynthesis are considered and treated systematically. The bibliography includes 102 references.
Sensitized photocurrent generation is observed with a porphyrin dyad (PZn-P) and its structural moieties: 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin (PZn). The dyes were adsorbed to saturation on a nanocrystalline SnO2 thin film, employed as working electrode in a photoelectrochemical cell. The metallized and unmetallized moieties possess different singlet state energies and redox properties. In both, solution and adsorbed state, nearly complete singlet−singlet energy transfer from the PZn to P has been determined in the dyad. PZn is less efficient than P in the photocurrent generation, but is a suitable energy donor in the dyad molecule. The generation of photoelectrical effects by the dyad is less effective in comparison with P. Considering the oxidation potentials of the two moieties in PZn−P, a mechanism is proposed where the oxidized metallized porphyrin enhances the back electron-transfer process.
Absorption and fluorescence spectra of several dyad compounds consisting of 1,8-naphthalimide and 1,3,4-oxadiazole moieties have been investigated in different solvents. The intramolecular energy transfer from oxadiazole to naphthalimide was observed. The fluorescence decay lifetimes of these dyad systems were measured by nanosecond time-resolved laser photolysis.
A tetraphenylporhyrin bearing four anthracene donor moieties (5,10,15,20-tetra(3-(9-methyloxyanthracenyl)phenyl)porphyrin, I) was synthesized and fully characterized. Spectroscopic and electrochemical data and results of metallation and intermolecular fluorescence quenching experiments suggest that there is no appreciable interaction between the porphyrin and anthracene moieties in I. Although intermolecular quenching of anthracene fluorescence by 5,10,15,20-(tetraphenyl)porphyrin is not obvious, the fluorescence of the anthracene donors in I is quenched by more than 90%. On the basis of the fluorescence excitation pectral data, the quenching observed is attributed to intramolecular singlet-singlet energy transfer from anthracene to the porphyrin. However, neither Förster's dipole-dipole formalism nor Dexter's electron exchange mechanism can explain adequately this intramolecular energy transfer. Thermodynamic considerations and solvent-dependent fluorescence data indicate that photoinduced electron transfer from singlet anthracene to the porphyrin can compete with energy transfer in this supramolecular system.
Isomeric donor-acceptor (DA) dyads in which an anthracene donor moiety is covalently linked, via a short ether bridge, to either orhto, metha or para position of one of the aryl groups of 5, 10, 15, 20-tetraphenylporphyrin have been synthesized and characterized by spectral and electrochemical methods. UV-visible and 1H nuclear magnetic resonance data of these DA systems suggest the presence of weak intramolecular π-π interaction between the porphyrin and the anthracene. Fluorescence from the anthracene subunit in each dyad in found to be quenched in comparison with the flourescence of free anthracene. Excitation spectral data provide evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin and the energy transfer efficiency is found to be dependent on the site of attachment (i.e. ortho > meta > para) of the donor to the acceptor. Detailed analysis of the data suggests that Förster's dipole-dipole mechanism does not adequately explain this energy transfer and that an electron-exchange-mediated mechanism can, in principle, contribute to the intramolecular EET in these short ether bridged dyads. Furthermore, arguments based on the thermodynamic considerations and also the solvent-dependent fluorescencedata reveal that, while quenching of the fluorescence in the ortho isomer could be explained solely on the basis of EET, invoking an intramolecular electron transfer can rationalize the observed quenching in the meta and para isomers. Finally, a comparison is made of the EET reactions in these isomeric dyads with those observed for the previously reported porphyrin-based energy transfer systems which include a supramolecular, D4-A pentad porphyrin bearing four anthracene donor subunits.
Thermally and photochemically stable catalysts in systems that convert the energy of visible light into the chemical energy of dihydrogen gas have yet to be fully developed and all proposed systems suffer, to varying extents, from loss of activity with use. We have attempted to model nature's photochemical reaction center through the use of TiO2 antenna catalysts and very small amounts of Pt that form active centers on the TiO2 semiconductor surface. This system is relatively inexpensive, reproducible, extremely stable, efficient in conversion of light to dihydrogen in aqueous solutions and displays higher linearity in dihydrogen evolution with greater efficiency in recycling than do other previously reported catalysts. Here we report the self-hydrogenation properties of our TiO2Pt catalysts in comparison with well known Pt, Ru and Os catalysts as well as their catalytic efficiency in systems for the sacrificial photoreduction of water. The sensitizers employed in this study were a recently synthesized new class of bis-heteroleptic complexes [Ru(bpy)2(PP)]2+, [Ru(bpy)2(PPB)]2+, [Ru(bpy)2(PPB-pCl)]2+ containing one pyridyl—pyrimidine ligand and [Ru(bpy)3]2+ and [Ru(bpz)3]2+ as well known standard sensitizers. In order to obtain maximum information about the TiO2Pt catalysts we varied the components and conditions in our sacrificial systems for water reduction: MV2+, PVS MPVS were used as electron relays, EDTA and TEOA as sacrificial electron donors. Furthermore, the dependence of the dihydrogen evolution rates on the pH of the photolysis solution and its ionic strength were investigated. The techniques used in this study are cyclic voltammetry, steady state and time resolved luminescence spectroscopy, Stern—Volmer quenching, measurements of the quantum yields for PVS reduction, and volumetric measurements of dihydrogen photoproduction. A linear dependence was found between the dihydrogen production rate of the TiO2Pt catalysts and the quantum yield for the MV2+ and PVS photoreduction at low ionic strength, showing that the new catalysts are reduced by the electron relays and convert protons into dihydrogen in a highly efficient manner
A carotenobenzoporphyrin has been prepared by covalently joining a benzoporphyrin derivative and an anilinocarotenoid by an amide linkage. A synthetic meso-diphenylporphyrin is the precursor of the benzoporphyrin moiety. The carotenobenzoporphyrin is highly fluorescent and does not sensitize measurable amounts of singlet oxygen.
The electrochemical properties of a new class of cyanine dyes with and without a covalently linked viologen group, 2-[1-ethyl-1,2-dihydro-quinolinyliden-(2)-methyl]-1-ethylquinolinium chloride (1), 1-[3-[2-[(1-ethyl-2(1H)-quinolinylidene) methyl]quinolinium-1-yl]pentyl]-1′-methyl-4,4′-bipyridinium trichloride (2) and 1,1′-(4,4′-bipyridinium-1,1′-diyl-5,1-pentandiyl)bis[2-[(1-ethyl-2(1H)-quinolinylidene) methyl]chinolinium]tetra chloride (3) have been investigated both in dilute solutions (monomers) and in organized assemblies (J-aggregates). Fully reversible redox reactions of the viologen group were observed in acetonitrile while consecutive reactions (protonation) were found in aqueous solutions, resulting in the irreversibility of the second reduction step. Reduction potentials of the viologen unit in 2 and 3 were comparable with those of methyl viologen (MV2+). In acetonitrile and at low concentrations in water, the electrochemistry (redox potentials and reversibility) of the viologen units was not affected by the covalently bonded cyanine dye group. When J-aggregates are formed at sufficiently high concentrations, different behavior was found for MV2+ incorporated in aggregates of 1 and for the viologen units in 2 and 3. While MV2+ and 3 showed a decrease in the rate constant of the heterogeneous electron transfer, aggregates of 2 did not. Furthermore, adsorption of the cyanine dye was observed in some cases, leading to the passivation of the electrode.
The photophysical properties of meso-nitro-phenyl-octaethylporphyrins and their dimers with electron-accepting NO2 groups in the para-, meta- and ortho-positions of the phenyl ring were studied. For the ortho-NO2 case in deaerated toluene at 295 K, strong fluorescence quenching is caused by the intramolecular electron transfer from the porphyrin S1 state in the absence of phenyl ring librations around the single C–C bond (`normal' region, non-adiabatic case). T1 state lifetime shortening for the same compounds is explained by thermally activated transitions to upper-lying charge-transfer states of the radical ion pair as well as by the rise of the intersystem crossing T1⤳S0 rate constants caused by T1 states mixing with charge-transfer states.
A series of anthrylpolyynes (α-anthryl-ω-(formylphenyl)polyynes) and anthrylpolyynylporphyrins (5-[(anthrylpolyynyl)phenyl]porphyrins) have been synthesized, and their photochemical properties and photoinitiated intramolecular excitation energy transfer from anthracene to porphyrin were studied by picosecond time-resolved fluorescence spectroscopy. Although anthrylpolyynes have strong fluorescence emission and fluorescence high quantum yield, anthrylpolyynylporphyrins, in which porphyrin links to the other side of polyyne, show only typical fluorescence emission of porphyrin and almost no fluorescence emission in the spectral region of anthrylpolyynes. The excitation of anthryl substituent brings about an increase in the fluorescence emission of porphyrin on the picosecond time scale. The mechanism of quantitative energy transfer from the anthryl moiety to the porphyrin is discussed on the basis of the photochemical properties of the component molecules.
A series of bichromophoric molecules was studied containing a 1,4-dimethoxynaphthalene donor (D) chromophore and a 1,1-dicyanoethylene acceptor (A) chromophore, interconnected by five different, rigid, saturated hydrocarbon bridges. The length of the bridges varies with increments of two sigma bonds from 4 to 12 to provide donor-acceptor separations up to 15 A. In all cases excitation of D is followed by rapid intramolecular electron transfer from D to A. Through-bond interaction involving sigma / pi interaction between the bridges and the chromophores is proposed to explain the very high rates of electron transfer. The observation of intramolecular charge transfer absorption and emission confirms the operation of such through-bond interaction.
Several photochemical and spectral properties of maize (Zea mays) bundle sheath and mesophyll chloroplasts are reported that provide a better understanding of the photosynthetic apparatus of C(4) plants. The difference absorption spectrum at 298 K and the fluorescence excitation and emission spectra of chlorophyll at 298 K and 77 K provide new information on the different forms of chlorophyll a in bundle sheath and mesophyll chloroplasts: the former contain, relative to short wavelength chlorophyll a forms, more long wavelength chlorophyll a form (e.g. chlorophyll a 693 and chlorophyll a 705) and less chlorophyll b than the latter. The degree of polarization of chlorophyll a fluorescence is 6% in bundle sheath and 4% in mesophyll chloroplasts. This result is consistent with the presence of relatively high amounts of oriented long wavelength forms of chlorophyll a in bundle sheath compared to mesophyll chloroplasts. The relative yield of variable, with respect to constant, chorophyll a fluorescence in mesophyll chloroplasts is more than twice that in bundle sheath chloroplast. Furthermore, the relative yield of total chlorophyll a fluorescence is 40% lower in bundle sheath compared to that in mesophyll chloroplasts. This is in agreement with the presence of the higher ratio of the weakly fluorescent pigment system I to pigment system II in bundle sheath than in mesophyll chloroplast. The efficiency of energy transfer from chlorophyll b and carotenoids to chlorophyll a are calculated to be 100 and 50%, respectively, in both types of chloroplasts. Fluorescence quenching of atebrin, reflecting high energy state of chloroplasts, is 10 times higher in mesophyll chloroplasts than in bundle sheath chloroplasts during noncyclic electron flow but is equal during cyclic flow. The entire electron transport chain is shown to be present in both types of chloroplasts, as inferred from the antagonistic effect of red (650 nm) and far red (710 nm) lights on the absorbance changes at 559 nm and 553 nm, and the photoreduction of methyl viologen from H(2)O. (The rate of methyl viologen photoreduction in bundle sheath chloroplasts was 40% of that of mesophyll chloroplasts.).
Recent efforts in these laboratories have been directed toward understanding the factors governing long distance intramolecular electron transfer (ET). In the models chosen for study the electronic coupling between donor and acceptor is sufficiently weak to assure nonadiabatic reactions. It occurred to them that long distance triplet energy transfer by the Dexter mechanism (TT) of which there are several examples in the literature should exhibit features similar to nonadiabatic electron transfer because both reactions are governed by the same theory of radiationless transitions. They therefore have started a program aimed at finding quantitative similarities and differences in these two processes when studied on directly comparable systems. Also, with one exception, the absolute rates of intramolecular triplet energy transfer have never been measured directly in liquid solution. In this communication they report their first results and conclusions.
Carotenoporphyrins, consisting of carotenoid polyenes linked covalently to porphyrins, are known to mimic both the antenna function and the photoprotection from singlet oxygen damage provided by carotenoids in photosynthetic organisms. A series of Carotenoporphyrins whose conformations, as determined from 1H NMR studies, range from folded (with the carotenoid π-electron system stacked over that of the porphyrin) to extended (with the chromophores widely separated) has been prepared. Time-resolved spectroscopic studies have revealed intramolecular triplet energy transfer from porphyrin to carotenoid. Two distinct pathways for such transfer (presumably occurring via an electron-exchange mechanism) were observed: (a) static transfer which does not require significant intramolecular motions; (b) dynamic transfer mediated by intramolecular motions. The relative importance of these pathways is a function of molecular structure and dynamics. The results for this series of carotenoporphyrins help define the photochemical and photophysical requirements for protection from singlet oxygen damage both in photosynthetic organisms and in other biological systems.
The authors have carried out an extensive photophysical analysis of a tetraarylporphine linked through a single amide bridge to either methyl-p-benzoquinone (PAQ) or the corresponding hydroquinone (PAQHâ) in benzonitrile as the solvent. The photophysical properties of PAQHâ are closely similar to those of nonlinked tetraarylporphine species, while for PAQ significant lifetime quenching of both the lowest excited singlet and triplet states is observed. Picosecond transient absorption spectroscopy and fluorescence lifetime measurements were used top show that quenching of the excited singlet state of PAQ is due to intramolecular electron transfer to the singlet radical ion pair ¹(P/sup .+/AQ/sup .-/) with the rate constant of 4.1 (+-0.3) x 10⸠sâ»Â¹. ¹(P/sup .+/AQ/sup .-/) subsequently decays to the ground state by reverse electron transfer with a rate constant of 1.6 (+- 0.2) x 10⸠sâ»Â¹. This reaction has ..delta..G° approx. = -1.4 eV and is predicted by Marcus theory. Nanosecond flash photolysis studies show that the lowest triplet state of PAQ is also quenched, most likely by electron transfer to the triplet radical ion pair ³(P/sup .+/ AQ/sup .-/, with a rate constant of 4.6 (+-0.2) x 10â´ sâ»Â¹. They suggest that ³(P/sup .+/AQ/sup .-/), which then decays rapidly to the ground state.
The electronic state dynamics and absorption properties of the carotenoid fucoxanthin are examined, both in vitro and in vivo. An S2 lifetime of 0.15 ps and an S1 lifetime of 40 ps are measured for fucoxanthin dissolved in ethanol. Fluorescence emission from both S2 and S1 is observed, allowing corroboration of the direct measure of the S2 lifetime and determination of the S1→S0 oscillator strength. Emission from the S2 state of carotenoids in thylakoid membranes of Phaeodactylum tricornutum is also seen, but S1 emission in vivo is obscured by chlorophyll a (chl a) fluorescence. These new results, together with previous time-resolved and two-photon excitation results, are discussed and the importance of considering Coulomb coupling as a mechanism for carotenoid-to-chl a energy transfer is demonstrated, at least for Phaeodactylum tricornutum.
Fluorescence excitation and emission spectra are reported from the carotenoids β-carotene, rhodopin and spheroidenone in carbon disulfide. From the small Stokes shift and the high emission anisotropy it is concluded that the fluorescence is emitted during the 1Bu→1Ag transition. From the small quantum yields, of about 3×10−5 for spheroidenone and 6×10−5 for β-carotene, and the 15 ps ground-state recovery lifetime for spheroidenone combined with a natural radiative lifetime of 1–10 ns, it is concluded that a fast (30–300 fs) non-radiative relaxation occurs. This probably represents the transition from the 1Bu to a low-lying 2Ag state. Anomalities in the excitation spectra also indicate that this process competes efficiently with vibrational relaxation within the 1Bu state.
The key steps in the photosynthetic conversion of light to chemical potential energy include not only photodriven charge separation, but also prevention of the back-reaction (charge recombination). Although the first of these steps has been achieved in several biomimetic solar energy conversion systems, retarding the back-reaction has proved more difficult. This may be accomplished by rapidly moving the electron, the hole, or both away from the site of excitation to more stabilizing environments. In photosynthetic membranes, the electron is transferred sequentially over several closely coupled molecules, including tetrapyrroles and quinones1-3. In semiconductor/liquid interfacial systems both the electron and the hole migrate following excitation4,5. We now report that substantial slowing of the back-reaction has been achieved with a tripartite molecule in which a long-lived photodriven charge-separated state of relatively high potential is formed from an excited singlet state in accordance with the above principles. This molecular triad (compound I) consists of a tetraarylporphyrin covalently linked to both a carotenoid and a quinone. In solution, excitation of the porphyrin moiety by visible light results in the rapid (
CAROTENOID pigments are closely associated with chlorophyll molecules in the chloroplast structure1,2. Some carotenoids, for example fucoxanthol3, can act as efficient sensitisers for the formation of the excited singlet state of chlorophyll although others, such as β-carotene, are very inefficient2,4,5. Other functions of carotenoids include acting as a deactivator of triplet chlorophyll (and hence protect the chlorophyll from photodegradation and also intercept the production of singlet oxygen) and as a deactivator of singlet oxygen6. Here we report in vitro experiments which unequivocally demonstrate that β-carotene quenches the excited singlet state of chlorophyll.
A synthetic molecular triad consisting of a porphyrin P linked to both a quinone Q and a carotenoid polyene C has been prepared as a mimic of natural photosynthesis for solar energy conversion purposes. Laser flash excitation of the porphyrin moiety yields a charge-separated state C+·—P—Q−· within 100 ps with a quantum yield of more than 0.25. This charge-separated state has a lifetime on the microsecond time scale in suitable solvents. The triad also models photosynthetic antenna function and photo-protection from singlet oxygen damage. The successful biomimicry of photo-synthetic charge separation is in part the result of multistep electron transfers which rapidly separate the charges and leave the system at high potential, but with a considerable barrier to recombination.
Although the concept of an artificial photosynthetic reaction center that mimics natural electron-and energy-transfer processes
is an old one, in recent years major advances have occurred. In this review, some relatively simple molecular dyads that mimic
certain aspects of photosynthetic electron transfer and singlet or triplet energy transfer are described. Dyads of this type
have proven to be extremely useful for elucidating basic photochemical principles. In addition, their limitations, particularly
in the area of temporal stabilization of electronic charge separation, have inspired the development of much more complex
multicomponent molecular devices. The use of the basic principles of photoinitiated electron transfer to engineer desirable
properties into the more complex species is exemplified. The multiple electrontransfer pathways available with these molecules
make it possible to fine-tune the systems in ways that are impossible with simpler molecules. The study of these devices not
only contributes to our understanding of natural photosynthesis, but also aids in the design of artificial solar energy harvesting
systems and provides an entry into the nascent field of molecular electronics.
The term ``sensitized luminescence'' in crystalline phosphors refers to the phenomenon whereby an impurity (activator, or emitter) is enabled to luminesce upon the absorption of light in a different type of center (sensitizer, or absorber) and upon the subsequent radiationless transfer of energy from the sensitizer to the activator. The resonance theory of Förster, which involves only allowed transitions, is extended to include transfer by means of forbidden transitions which, it is concluded, are responsible for the transfer in all inorganic systems yet investigated. The transfer mechanisms of importance are, in order of decreasing strength, the overlapping of the electric dipole fields of the sensitizer and the activator, the overlapping of the dipole field of the sensitizer with the quadrupole field of the activator, and exchange effects. These mechanisms will give rise to ``sensitization'' of about 103−104, 102, and 30 lattice sites surrounding each sensitizer in typical systems. The dependence of transfer efficiency upon sensitizer and activator concentrations and on temperature are discussed. Application is made of the theory to experimental results on inorganic phosphors, and further experiments are suggested.
The decay of donor luminescence in a rigid solution when modified by electronic energy transfer by the exchange mechanism is treated theoretically. The rate constant for the elementary process of energy transfer is taken to be of the Dexter form, const exp(−2R/L), where R is the donor—acceptor distance and L is a positive constant. Calculations are made of the yield and decay time of the donor luminescence as functions of the acceptor concentration. The resulting relationship among the above quantities enables one to analyze experimental data in a quantitative manner, and thereby to obtain information about an intermolecular exchange interaction. As an example of such an analysis, Ermolaev's data on triplet—triplet transfer between some aromatic molecules are compared with our results, and very good agreement is found with a choice of the single parameter L.
Carotenoid pigments, ubiquitous in photosynthetic membranes, are essential for the survival of green plants1. Two facets of carotenoid function are recognized in photosynthetic membranes. First, carotenoids prevent the chlorophyll-photosensitized formation of highly destructive singlet oxygen by intercepting the chlorophyll triplet states2–10 and may also scavenge additional singlet oxygen present11,12. Second, carotenoids perform an antenna function by transferring the energy of absorbed light at the singlet excited state level to the chlorophyll system for the execution of photosynthetic work13–16. Nevertheless, the mechanisms by which carotenoids perform these functions are poorly understood. We now report that a unique synthetic carotenoporphyrin I consisting of a carotenoid part covalently linked to a synthetic tetraarylporphyrin successfully mimics both the photophysical functions of carotenoids in photosynthesis. The explanation for this seems to be the close interaction of the carotenoid and porphyrin π-electron systems.
The primary steps in photosynthesis involve very rapid (sub-nanosecond) electron transfer between molecular entities that are rigidly embedded within a lipid membrane and separated from each other by well-defined distances on the order of 10 Å. In an attempt to simulate such systems we have studied photon-induced electron transfer within specially synthesized molecular assemblies in which a donor moiety is separated from an electron acceptor by a rigid, saturated hydrocarbon framework of variable length, from 5 to 13 Å. We find charge separation to occur on a sub-nanosecond timescale with close to unit quantum efficiency in all cases. The lifetimes of the resulting charge-transfer states, with dipole moments approaching 70 debye units, can extend to several hundred nanoseconds. Non-conjugated hydrocarbon bridges may be important in determining the rate and direction of electron transfer in photo-excited natural or artificial molecular systems.
IN many applications of conjugated polyenes for nonlinear optoelectronics and as probes of biophysical systems, the orientation of the electronic transition dipole moment relative to the long axis of the chains is an important quantity. Simple models predict that the transition moment lies closely along the chain axis1,2 or at an angle of about 30° to this axis3, but the difficulty of preparing perfectly oriented samples has made these predictions hard to test. Here we report the results of polarized single crystal spectroscopy of a linear conjugated tetraene in the highly aligned configuration made possible by incorporating these molecules as guests in the channels of urea crystals. The angular dependence of the absorption spectrum indicates that the transition moment lies at an angle of 15° to the chain axis. Molecular-orbital calculations can reproduce this value when they include the effects of electron correlation.
Absorption, fluorescence, and fluorescence excitation spectra of all-trans-1,3,5,7,9,11,13-tetradecaheptaene have been obtained in room-temperature solutions and 77 K glasses. The heptaene, unlike shorter fluorescent polyenes, does not exhibit the characteristic gap between the origins of absorption (11Ag- → 11Bu+) and emission. Excitation spectra and solvent shift studies lead to the assignment of two distinct emissions, S2 → S0 (11Bu+ → 11Ag-) and S1 → S0 (21Ag- → 11Ag-), with the ratio of S2 to S1 emission increasing with the S2-S1 energy gap. Extrapolation of room-temperature solution data gives a gas-phase S2-S1 difference of 8400 cm-1. Tetradecaheptaene's "anomalous" S2 → S0 emission, the first observed for a linearly conjugated molecule, is compared to similar violations of Kasha's rule by azulene and other polyarenes. The spectroscopic and photochemical implications of these findings for other long polyenes also are discussed.
Excitation of carotenoid-porphyrin-quinone (C-P-Q) triad molecules initiates a two-step electron transfer to yield a final charge separated state of the form C/sup .+/-P-Q/sup .-/. This state has a very long lifetime in solution (10â»â·â»Â¹Â°sup -6/ s), and the nature of the ultimate charge recombination reaction has not previously been investigated. Nanosecond transient absorption spectroscopic studies have been performed on a series of such triad molecules wherein the nature of the linkages joining the porphyrin to the quinone and/or carotenoid moieties is varied systematically. The results reveal that charge recombination in C/sup .+/-P-Q/sup .-/ does not occur in a single step but rather via an unusual two-step electron transfer involving an intermediate C-P/sup .+/-Q/sup .-/ species. The temperature dependence is consistent with the formation of C-P/sup .+/-Q/sup .-/ via a thermally activated process, and measurements over the range 221-296 K yield ..delta..H double dagger = 2.7 kcal/mol and ..delta..S double dagger = -20 cal/(deg x mol). The transient absorption measurements also reveal that the quantum yield of the C/sup .+/-P-Q/sup .-/ state is a function of three electron transfer rate constants and that it can therefore be maximized by controlling the ratios of these rate constants to one another and to the rates of other pathways which depopulate the relevant excited states.
In natural photosynthesis membranes, chlorophyll molecules serve as the site of the initial photodriven charge separation. In addition, they play a role in subsequent electron-transfer steps, accept singlet excitation energy from carotenoid antenna molecules, and transfer triplet energy to carotenoid acceptors (thereby preventing sensitized singlet oxygen production and subsequent photodamage to the organism). The authors report herein the synthesis and study of chlorophyll-based carotenopyropheophorbide-quinone triad molecules which mimic all of these natural processes.
A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) an a 1,1-dicyanoethylene acceptor (a) interconnected by five different, rigid, nonconjugated bridges. The length of the bridges varies with increments of two sigma-bonds from four in 1(4) to 12 sigma-bonds in 1(12), to provide donor-acceptor center-to-center separations (R/sub c/) ranging from 7.0-14.9 A. In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer. The rate constant (k/sub et/) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >10¹¹ sâ»Â¹ for a four-bond separation to approx. =4 x 10⸠sâ»Â¹ for a 12-bond separation. In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10 and 12-bond separations, while it is not significantly decreased for the shorter homologues. Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect. In solvents where electron transfer is thermodynamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridges were used or intermolecular electron transfer was studied. Through-bond interaction involving sigma/..pi.. interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).
Carotenoid-porphyrin-quinone triad molecules undergo a photodriven two-step electron-transfer reaction which results in the generation of a high-energy charge-separated state with a lifetime on the microsecond time scale at ambient temperatures in fluid solution. These systems mimic the initial charge separation steps of photosynthesis. A series of these tripartite molecules which differ systematically in the nature of the linkages joining the porphyrin to the quinone and carotenoid moieties has been synthesized in order to investigate the effect of structure on the yield and lifetime of the charge-separated state. The time-averaged solution conformations of these molecules have been determined from porphyrin ring current induced shifts in the ¹H NMR resonances of the carotenoid and quinone moieties. Studies of the triads and related molecules in dichloromethane solution using time-correlated single photon counting fluorescence lifetime techniques have yielded the rate constant for the first of the photoinitiated electron-transfer steps as a function of the linkage joining the porphyrin and the quinone. The rate constants range from 1.5 x 10⸠to 9.7 x 10â¹ sâ»Â¹. For most members of the series, the results are consistent with an exponential dependence of the electron-transfer rate on the experimentally determined donor-acceptor separation, with the exponential factor ..cap alpha.. = 0.6 Aâ»Â¹.
Over the past few years there has been an active program in their laboratories on long-range intramolecular electron transfer and its dependence on energetics, solvent, distance, and stereochemistry. More recently the authors have extended this study to include positive ions, or hole transfer, and triplet energy transfer. The compounds in the three studies were the same or closely related, and the processes can be summarized and designated as in (1). Dâ»-Sp-A â D-Sp-Aâ» (ET); D{sup +}-Sp-A â D-Sp-A{sup +} (HT); Dâ²Â³-Sp-A â D-Sp-A³ (TT). In the three series A = 2-naphthyl, and D = 4-biphenylyl in the Et and HT series and 4-benzophenoyl in the TT series.
Intramolecular singlet energy transfer can be detected in a series of rigid bichromophoric molecules (1(n)) where a dimethoxynaphthalene chromophore and a carbonyl chromophore are separated by extended all-trans arrays of up to eight C-C Ï bonds (1(8)). In the series of compounds 2(n) kinks are introduced in the array of Ï bonds of the saturated hydrocarbon system, which bridges the chromophores. Singlet energy transfer is then much less efficient (i.e. in 2(6)) or even absent (i.e. in 2(8)), which supports the earlier interpretation of the energy transfer mechanism in 1(n) as being mainly mediated by through-bond exchange interaction and furthermore explains the virtual absence of such interaction in more flexible systems where the chromophores are linked by polymethylene bridges.
A study of intramolecular energy transfer (intra-ET) in a series of bichromophoric molecules consisting of cyclic α-diketones incorporating an ortho-, meta-, or para-substituted benzene ring is reported. Most spectroscopic properties of these molecules are described by a superposition of those of their constituent chromophores. Unique for the bichromophore molecule is the fact that, depending on the molecular geometry, energy absorbed by the aromatic chromophore is transferred in part to the α-diketone and both chromophores emit their characteristic fluorescence spectra. An extensive study was made of the intramolecular electronic energy transfer process in solution as a function of temperature. The results indicate that the transfer efficiency is strongly structure dependent suggesting that a Dexter type exchange interaction is responsible for singlet-singlet intra-ET between close chromophores in a bichromophoric molecule. The thermal dependence observed in some cases is attributed to conformational factors. A general theoretical analysis of intra-ET in bichromophoric molecules provides expressions for donor fluorescence decay and for its fluorescence quantum yield in terms of the average distance between donor and acceptor moieties and the flexibility of the chains connecting donor and acceptor. Comparison with the present experimental data supports the predictions of this analysis. It is concluded that intra-ET in bichromophoric molecules is indeed governed by short-range exchange interactions.
A series of carotenols with from 7 to 11 conjugated double bonds have been synthesized and purified by using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra have been obtained in 77 K glasses. The shorter members of this series exhibit the Stokes-shifted, S{sub 1} {yields} S{sub 0} emissions seen in previous studies of model polyenes. For carotenols with more than eight conjugated double bonds, however, the fluorescence is dominated by anti-Kasha, S{sub 2} {yields} S{sub 0} fluorescence. these findings in part can be attributed to a larger S{sub 2}-S{sub 1} energy difference and the resultant decrease in S{sub 2} {yields} S{sub 1} radiationless decay rates in longer polyenes. The precipitous crossover from S{sub 1} {yields} S{sub 0} to S{sub 2} {yields} S{sub 0} emission, however, cannot be fully accounted for by the energy gap law, which predicts only modest changes in radiative and nonradiative decay rates with increasing polyene length. The implications of large S{sub 2}-S{sub 1} energy gaps for the spectroscopy and photochemistry of {beta}-carotene and other long polyenes also are discussed.
A series of compounds, each containing a triplet energy donor and an acceptor separated by a rigid spacer, has been designed and synthesized. The 1,4-cyclohexanediyl moiety is employed as the spacer for the series. The rates of intramolecular triplet energy transfer (TT) have been measured for the series. The rate of TT shows an inverted parabolic, i.e., Marcus, dependence on the thermodynamic driving force for a selected subset of the compounds wherein the donor is maintained constant throughout and the acceptors are "rigid", having no low-frequency internal degrees of freedom. The internal low-frequency torsional mode of a biphenylyl acceptor can be accounted for quite satisfactorily as an additional contribution to the solvent reorganization energy, lambda-s. The driving force dependence of the rate of TT is not modeled well by the conventional Marcus-Jortner equation for weakly coupled nonadiabatic electron transfer. Generalization of the Marcus-Jortner equation to include coupling to a high-frequency harmonic mode which is both displaced and distorted along the reaction coordinate provides a somewhat better fit to the experimental data with fewer adjustable parameters.
Analysis of photosynthetic light reactions in terms of physical chemistry suggests a set of design criteria which would be desirable for solar energy convertors for commercial power production. These include (1) spectral absorption of nearly all wavelengths of photochemically active light incident at the surface of the earth; (2) efficient conversion of electronic excitation into separation of electrical charge; (3) stabilization against wasteful charge recombination, by separating chemical species across an impermeable membrane with a minimum expenditure of the stored energy and entropy; and (4) conversion of a high fraction of electrical and ion gradient forms of chemical potential into stable chemical products obtained from water and carbon dioxide. 9 figures, 1 table.
A digital software‐controlled back‐off system for probe lamp dc signal nullification has been designed and incorporated into a laser flash photolysis spectrometer. This method takes advantage of modern high‐speed operational amplifiers already employed in the detection circuitry and the dedicated small computer that runs the spectrometer. The determination of I 0 , necessary for absorbance calculations, is inherent in the method.
The review opens by presenting the absorption spectra for three series of porphyrins derived from the basic skeleton: (a) compounds obtained by simple substitution; (b) compounds obtained by reduction of one or more pyrrole rings; and (c) compounds obtained from fusion of aromatic rings onto the basic skeleton. The spectra are discussed in terms of a four orbital model—that is intensity changes and energy shifts are related to the properties of two top filled and two lowest empty pi orbitals. Emission spectra of metal porphyrins are then discussed, three metal series being distinguished: (1) In closed shell metals, the continuous enhancement of phosphorescence at the expense of fluorescence is attributed to spin-orbit coupling. (2) In paramagnetic metals, observed effects are attributed to the existence of a state at the same energy as the usual triplet but with multiplicity the same as the ground state; its intensity is ascribed to exchange interactions. (3) In diamagnetic metals with unfilled d shells, peculiar emission properties are attributed to enhanced spin orbit coupling due to low lying metal triplets. The review closes by discussing n-π transitions and triplet-triplet spectra.
The S2 lifetime of β-carotene has been determined by both femtosecond transient absorption experiments and emission yield studies. For β-carotene dissolved in ethanol and in CS2, the initially excited S2 was found to live for 200 to 250 fs at room temperature. Implications of this result for carotenoid-to-chlorophyll energy transfer in photosynthetic systems are discussed.
Triplet state electron spin resonance (ESR) spectroscopy has been used to study triplet-triplet energy transfer in B800–850 light-harvesting complexes and carotenoporphyrin molecules. The B800–850 complexes were isolated from the photosynthetic bacteria Rhodobacter sphaeroides GA, aerobically and anaerobically grown Rb. sphaeroides wild type and Rhodopseudomonas acidophila 7750. Free-base and zinc-substituted carotenoporphyrins featuring various linkage structures and different orientations of the pigments were studied. The carotenoporphyrins which contain a short bridging link display ESR spectra which resemble those of the B800–850 complexes. The results indicate a close spatial proximity between the bacteriochlorophyll and carotenoid pigments in the B800–850 complexes which leads to efficient triplet-triplet energy transfer. Computer simulations of the observed ESR spectral line-shapes yielded values for the zero-field splittings which can be understood in terms of the varying extents of π-electron conjugation in the carotenoids. The rate constants for population and decay of the observed triplet states were also obtained from the computer simulations. All of the carotenoid triplet states exhibit similar ESR spectral lineshapes and are characterized by the spin polarization pattern eae aea. The molecular basis for the spectral uniformity may be explained by triplet energy transfer according to the exchange mechanism and conformational changes of the carotenoid which lead to triplet spin relaxation.
We analyse the superexchange model for the primary charge separation from the electronically excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to the bacteriopheophytin (H) across the A branch of the bacterial photosynthetic reaction centers, which is mediated by the accessory bacteriochlorophyll (B). The dominant contribution to the superexchange electronic interaction between the initial 1P* BH and the final P+BH− states originates from the mixing with the mediating electronic state P+P−H, the energy of which is above 1P*. The superexchange electronic interaction is V=VPBVBH/δE, where VPB and VBH are the electronic couplings of 1P*BH with P+B−H and P+B−H with P+BH−, respectively, while δE is the vertical energy difference. The nonadiabatic electron-transfer rate is proportional to V2F, where F is the nuclear Franck-Condon factor, which is determined by the (free) energy gap ΔG=−2000 cm−1, the medium reorganization energy λ (λ<2500 cm−1) and the medium characteristic frequency ω≈100 cm−1. Indirect information on the constituents of the effective electronic coupling V≈25 cm−1 was inferred from the ration |VBH/VPB| calculated from the intermolecular overlap approximation in conjunction with an activated sequential channel and the utilization of kinetic constraints on the dynamics of the primary electron transfer, which result in VPB≥60 cm−1, VBH≥360 cm−1 and δE≥1100 cm−1. We discuss several physical phenomena and observables, i.e., electric field effects on the prompt fluorescence, the unidirectionality of charge separation across the A branch and magnetic interactions in the primary radial pair in the framework of the superexchange mechanism. The electric field (∈) dependence of the fluorescence quantum yield (Yf(∈)) for isotropic samples at 75 K predicts Yf(∈)=5 mV/Å)/Yf(0)=1.39 and Yf(∈=9 mV/Å)/Yf(0)=3.5. The fluorescence polarization data at constant field (Lock-hart, D.J., Goldstein R.F. and Boxer, S.G. (1988) J. Chem. Phys. 89, 1408–1415) can be well accounted for in terms of the energetic parameters λ=1600 cm−1 and ΔG=−2000 cm−1 together with the value ψ=61o for the angle between the dipole P+H− and the transition moment of P. The unidirectionality of the charge separation across the A branch originates predominantly from structural symmetry breaking, which affects the electronic coupling, while the contribution of the nuclear contribution has been shown to be small. The predicted ratio of the electronic transfer rates k(A)/k(B)=82(+190; −70) at T=80 K is consistent with the recent experimental result k(A)/k(B)≥25 at this temperature. Finally we examined magnetic interactions of the primary P+H− radical pair, establishing the interrelationship between the singlet energy shifts and the triplet energy shift with the primary electron transfer rate, k, and the triplet recombination rate kT whereupon the singlet-triplet splitting of P+H− is J=αk−βkT where the coefficients α and β depend on energetic parametes and Franck-Condon factors. The estimate of J within the superexchange mechanism rests on the incorporation of an assumed configurational relaxation and essential cancellation effects.
Intramolecular singlet-singlet energy transfer is reported in a series of compounds containing a 1,4-dimethoxy-naphthalene chromophore as the energy donor and a cyclic ketone as the energy acceptor connected by rigid, elongated, saturated hydrocarbon bridges with an effective length of 4, 6, and 8 CC σ bonds. The rate of energy transfer is found to be proportional to the spectral overlap - as varied by solvent variation - and to show an exponential distance dependence while its magnitude significantly exceeds that predicted for a dipole-dipole coupling mechanism. From this it is concluded that energy transfer occurs predominantly via an exchange mechanism. Exchange integrals of 60, 10, and 2.5 cm−1 across 4, 6, and 8 σ bonds are calculated. The magnitude of these is proposed to signify through-bond exchange interaction between symmetry-matched donor (ππ*) and acceptor (nπ*) states.
Carotenoids serve as light-harvesting pigments and as photoprotective agents in photosynthetic organisms1-9. Their role as antenna pigments involves absorption of photons in the blue-green spectral region followed by highly efficient singlet-singlet energy transfer to a neighbouring chlorophyll. The dependence of both the rate and mechanism of energy transfer on carotenoid-chlorophyll distance and orientation is Unknown. Here, we have directly measured both the rate and efficiency of singlet energy transfer from a carotenoid covalently linked to pyropheophorbide a (PPheo a) in two model compounds, using picosecond transient absorption spectroscopy. In one model the pi systems of the carotenoid and PPsheo a possess a maximum edge-to-edge distance of 5 Å, while in the other model this distance is only 2Å. Energy transfer occurs from the carotenoid to PPheo a at the 2-Å distance with a rate constant of 7 +/- 2 × 1010 s-1 and 53+/-5% efficiency, while energy transfer at the 5-Å distance occurs at a rate constant of
Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers.
The B800-to-B850 energy transfer time in the purified B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1 is determined to be 0.7 ps at room temperature. The electronic state dynamics of the principal carotenoid of this species, spheroidene, are examined, both in vivo and in vitro, by direct femtosecond time-resolved experiments and by fluorescence emission yield studies. Evidence is presented which suggests that carotenoid-to-bacteriochlorophyll energy transfer may occur directly from the initially excited carotenoid S2 state, as well as from the carotenoid S1 state. Further support for this conjecture is obtained from calculations of energy transfer rates from the carotenoid S2 state. Previous measurements of in vivo carotenoid and B800 dynamics are discussed in light of the new results, and currently unresolved issues are described.
Tetraarylporphyrins substituted with nitro groups at beta-pyrrolic positions are potential candidates for electron-accepting pigments in model systems for photosynthesis. The photophysics of 2-nitro-5,10,15,20-tetra-p-tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet-triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature- and solvent-dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+.-NO2-. in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.
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.
Metalloporphyrins, most notably the iron porphyrins, observe clearly defined, internally consistent, structural principles that promise to be fully applicable to the hemes in the several families of the hemoproteins.
From fluorescence action spectra, fluorescence spectra and absorption spectra measured at room temperature and at 77 °K of light petroleum (b.p. 40–60°)-treated and normal chloroplasts, it is concluded that:
1. 1. In blue-green and red algae energy transfer from β-carotene to chlorophyll occurs in Photosystem I exclusively.
2. 2. In green algae and greening bean leaves energy transfer from β-carotene to chlorophyll occurs in both Photosystem I and II.
3. 3. Light absorbed by β-carotene is transferred to chlorophyll with nearly 100% efficiency.
4. 4. Light energy absorbed by xanthophylls is not transferred to chlorophyll.
Energy transfer between carotenoid and bacteriochlorophyll has been studied in isolated B-800-850 antenna pigment-protein complexes from different strains of Rhodopseudomonas sphaeroides which contain different types of carotenoid. Singlet-singlet energy transfer from the carotenoid to the bacteriochlorophyll is efficient (75-100%) and is rather insensitive to carotenoid type, over the range of carotenoids tested. The yield of carotenoid triplets is low (2-15%) but this arises from a low yield of bacteriochlorophyll triplet formation rather than from an inefficient triplet-triplet exchange reaction. The rate of the triplet-triplet exchange reaction between the bacteriochlorophyll and the carotenoid is fast (Ktt greater than or equal to 1.4 . 10(8) S-1) and also relatively independent of the type of carotenoid present.
Two carotenoids, neurosporene and spheroidene, have been successfully added to chromatophores from the carotenoidless mutant of Rhodopseudomonas sphaeroides R26. Carotenoids reconstituted in this way into the B-850 light-harvesting pigment-protein complex both sensitive bacteriochlorophyll fluorescence and protect the complex from the photodynamic reaction.
Intramolecular long-distance electron transfer (EI) has been actively studied in recent years in order to test existing theories in a quantitative way and to provide the necessary constants for predicting ET rates from simple structural parameters. Theoretical predictions of an "inverted region," where increasing the driving force of the reaction will decrease its rate, have begun to be experimentally confirmed. A predicted nonlinear dependence of ET rates on the polarity of the solvent has also been confirmed. This work has implications for the design of efficient photochemical charge-separation devices. Other studies have been directed toward determining the distance dependence of ET reactions. Model studies on different series of compounds give similar distance dependences. When different stereochemical structures are compared, it becomes apparent that geometrical factors must be taken into account. Finally, the mechanism of coupling between donor and acceptor in weakly interacting systems has become of major importance. The theoretical and experimental evidence favors a model in which coupling is provided by the interaction with the orbitals of the intervening molecular fragments, although more experimental evidence is needed.
A new carotenoporphyrin has been prepared in which a synthetic carotenoid is joined to a tetraarylporphyrin through a flexible
trimethylene linkage. This molecule exists primarily in an extended conformation with the carotenoid chromophore far from
the porphyrin π-electron system. In benzene solution, where large-amplitude molecular motions are rapid, the molecule can momentarily assume
less stable conformations which favor triplet energy transfer, and quenching of the porphyrin triplet by the carotenoid is
fast. In a polystyrene matrix or frozen glass such motions are slow, and energy transfer cannot compete with other pathways
for depopulating the triplet state. These observations help establish the requirements for biological photoprotection.