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Charge separation in a conformationally-flexible porphyrin-fullerene dyad synthesised using cross-metathesis
Abstract
The synthesis of a Zn(II) porphyrin-fullerene dyad in which the two chromophore units are tethered by a conformationally-flexible linker, is described. The synthesis is highlighted by the use of a cross metathesis strategy to prepare the linker between the chromophores. Photoexcitation of the Zn(II) porphyrin unit of the dyad in tetrahydrofuran leads to substantial (77%) quenching of porphyrin fluorescence. The multiple exponentials fluorescence decay kinetics observed are attributed to different rates of electron transfer from photoexcited porphyrin to fullerene in the various conformers present. A charge-separated state with a 330 ns lifetime is observed by transient spectroscopy.
... Fluorescence decay profiles were recorded by the timecorrelated single-photon counting (TCSPC) technique using either a Jobin Yvon Fluorolog-3 spectrofluorometer (with TCSPC accessory) or an in-house-built instrument based on a mode-locked and cavity-dumped Ti:Sapphire laser excitation source (Coherent MIRA/PulseSwitch) that was frequencydoubled to 400 nm. [8] Measurements were performed in degassed solutions. Fluorescence lifetimes were determined by analysis of the fluorescence decay profiles using a sum of exponentials model by iterative reconvolution procedures. ...
... Full details of the instrumentation and data analysis procedures can be found elsewhere. [8] Fluorescence quantum yield measurements (f f ) were carried out using the comparative method against a known fluorescence standard (cresyl violet in methanol f f ¼ 0.53). [3a] The uncertainty in fluorescence quantum yield is AE5 %. ...
A series of seven thiophen-substituted diketopyrrolopyrrole derivatives were synthesised and their solution absorption spectra characterised in a range of solvents of varying polarity. The absorption spectra of these diketopyrrolopyrrole derivatives exhibited significant negative solvatochromism. The behaviour is consistent with results of time-dependent density-functional theory calculations of excitation energies. Calculated electronic structures suggest that there is significant charge transfer between the electron-donating thiophen substituents and the diketopyrrolopyrrole core but that the magnitude of this charge shift is reduced in the excited state compared with the ground state. The resulting reduction in polarity of the excited state accounts for the negative solvatochromism observed. The implications of the results for the potential application of diketopyrrolopyrrole compounds as photovoltaic materials are considered.
... Combination of cross metathesis with the Prato reaction (1,3-dipolar cycloaddition) 405 successfully afforded C 60 -por-phyrin dyad 166.1 (Scheme 166). 406 Enyne metathesis is effective for the construction of a diene skeleton. Sequential enyne metathesis and [4 + 2] cycloaddition with C 60 efficiently provided C 60 -porphyrin dyad 167.1 (Scheme 167). ...
This review focuses on the postfunctionalization of porphyrins and related compounds through catalytic and stoichiometric organometallic methodologies. The employment of organometallic reactions has become common in porphyrin synthesis. Palladium-catalyzed cross-coupling reactions are now standard techniques for constructing carbon–carbon bonds in porphyrin synthesis. In addition, iridium- or palladium-catalyzed direct C–H functionalization of porphyrins is emerging as an efficient way to install various substituents onto porphyrins. Furthermore, the copper-mediated Huisgen cycloaddition reaction has become a frequent strategy to incorporate porphyrin units into functional molecules. The use of these organometallic techniques, along with the traditional porphyrin synthesis, now allows chemists to construct a wide range of highly elaborated and complex porphyrin architectures.
Photoinduced electron and energy transfer pathways are elucidated in a panchromatic light-absorbing, covalently linked triad (BODIPY-C60-distyryl BODIPY, noted as BDP-C60-DSBDP) along with the two reference dyads: BODIPY-C60 (BDP-C60) and distyryl-BODIPY-C60 (DSBDP-C60). The flexible linker between the BODIPY and C60 units leads to different possible conformations with varying donor–acceptor distances. Ultrafast transient absorption along with the time-resolved emission spectroscopies revealed the occurrence of different photoinduced electron/energy transfer processes in these conformers. Photoexcitation of the BODIPY units in the two reference dyads leads to one electron and two energy transfer steps from BODIPY to C60. However, in the BDP-C60-DSBDP triad, additional energy transfer processes from BDP to DSBDP were evidenced upon photoexcitation of the BDP unit. The singlet excited state of DSBDP in the triad then follows the same relaxation route as that of the DSBDP-C60 dyad. The intricate photophysics, particularly the formation of radical ion pairs in these flexible covalently linked donor–acceptor systems contribute to our fundamental understanding of electron and energy transfer mechanisms, which is important to build donor–acceptor assemblies for energy applications.
The energy demand of humanity is increasing rapidly, and shows no signs of slowing. Alongside this issue, abuse using fossil fuels is one of the main reasons which leads to an increase in atmospheric CO₂ concentration. These problems have to be solved in terms of both limiting CO₂ emission and finding renewable energy sources to replace fossil fuels. Nowadays, solar energy appears as one of the most effective renewable energy sources. Conversion of solar light energy to electricity in photovoltaics or to chemical energy through photocatalytic processes invariably involves photoinduced energy transfer and electron transfer. In this context, the aim of the thesis focuses on studying photoinduced processes in molecular photosystems using laser flash photolysis. The first theme of this thesis focus on studying single electron transfer in Donor-Acceptor Dyad systems towards optimization efficiency of charge separation and application in the photovoltaic organic solar cell. In the second theme of this thesis, two model systems of artificial photosynthesis were investigated to assess the possibility of stepwise charge accumulation on model molecules. A fairly good global yield of approximately 9% for the two charge accumulation on MV²⁺ molecule was achieved. Then, different photocatalytic systems, which have developed for CO₂ reduction, were studied. Understanding of the photoinduced processes is an important step toward improving the efficiency of reduction of CO₂ in practical photocatalytic systems.
A new donor-acceptor dyad comprised of a BODIPY (BDP) donor and a fullerene (C60) acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BDP thiol lead to a BDP-C60 dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, steady-state and time-resolved spectroscopies, the photophysical properties of this new BDP-C60 Dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BDP-C60 Dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer (PET) from the singlet excited state of the BDP to the C60 moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BDP-C60 Dyad, in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BDP moiety, occurrence of both energy and electron transfers were demonstrated for the Dyad by femtosecond and nanosecond transient absorption (TA) spectroscopies. Photoinduced electron transfer occurs in the folded conformer while energy transfer is observed in extended conformers.
The review focuses current research in the rapidly developing field of the chemistry of porphyrin–fullerene complexes. Recent advances in the synthesis, properties, and potential applications of these compounds are considered. An overview of the most popular methods to prepare porphyrin complexes with C60 fullerene is given. The discussion of porphyrin‒fullerene complexes includes the structures of noncovalently linked porphyrin‒fullerenes along with covalently linked complexes. Much attention is paid to potential applications of porphyrin‒fullerene conjugates.
A series of covalently linked axially symmetric porphyrin–fullerene dyads with a rigid pyrrolo[3,4-c]pyrrolic linker enabling a fixed and orthogonal arrangement of the chromophores has been synthesized and studied by means of transient absorption spectroscopy and cyclic voltammetry. The lifetime of the charge-separated state has been found to depend on the substituents on the porphyrin core, reaching up to 4 μs for a species with meso-(p-MeOC6H4) substituents. The ground and excited electronic states of model compounds have been calculated at the DFT and TD-DFT B3LYP(6-31G(d)) levels of theory and analyzed with regard to the effect of the substituent on the stabilization of the charge-separated state in the porphyrin–fullerene ensemble with a view to explaining the observed dependence.
Photo-induced electron transfer in a covalently linked zinc(II) tetraphenylporphyrin-amino naphthalene diimide dyad (ZnTPP-ANDI) is reported. The fluorescence of ZnTPP-ANDI is strongly quenched in both toluene and benzonitrile solvents compared to emission from ZnTPP. Ultrafast pump-probe spectroscopy has identified transient absorptions attributable to the ZnTPP•+ radical cation and the ANDI•− radical anion. It is shown that electron transfer (ET) can occur directly upon excitation to the S2 state of the porphyrin followed by rapid charge recombination to form S1 that subsequently undergoes a further slower ET to ANDI. The kinetics of charge separation (CS) and charge recombination (CR) for the latter ET process are strongly solvent dependent with a dramatically accelerated charge recombination rate (kCR) in the more polar solvent benzonitrile (kCR = 1.59 × 1011 s−1) compared to toluene (kCR = 8 × 109 s−1), in which inter-system crossing (ISC) from the CS state to form the lowest porphyrin triplet state is the dominating decay pathway (kCR/ISC = 3.46 × 1010 s−1). The implications of the results for designing molecular systems to potentially utilise ET following S2 excitation are discussed.
Treatment of 1,4-bis(tetraethylene glycol)-substituted zinc(II) phthalocyanine with p-toluenesulfonyl chloride and triethylamine followed by 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin and K2CO3 resulted in the formation of both a covalently linked phthalocyanine–porphyrin dyad and triad. The photophysical properties of these hetero-arrays were studied in detail using steady-state and time-resolved spectroscopic methods. Upon excitation at the porphyrin moiety, both compounds exhibited predominantly highly efficient excitation energy transfer from the excited porphyrin to the phthalocyanine core. The rate of energy transfer (2.4 × 1010 s−1) was more than 20-fold faster than the rate of electron transfer (1.1 × 109 s−1). When the phthalocyanine core of the dyad and triad was excited either directly or via excitation energy transfer, its singlet excited state underwent charge transfer leading to reduction in the fluorescence quantum yield and fluorescence lifetime. The rate of charge transfer for the triad (2.8 × 108 s−1) was about two-fold higher than that for the dyad (1.5 × 108 s−1) due to the presence of an additional porphyrin moiety in the triad.
The synthesis of a porphyrin star-pentamer bearing a free-base porphyrin core and four zinc(II) metalloporphyrins, which are tethered by a conformationally flexible linker about the central porphyrin's antipody, is described. The synthetic strategy is highlighted by the use of olefin cross metathesis to link the five chromophores together in a directed fashion in high yield. Photoexcitation into the Soret absorption band of the zinc porphyrin chromophores at 425 nm leads to a substantial enhancement of central free-base porphyrin fluorescence, indicating energy transfer from the photoexcited zinc porphyrin (outer periphery) to central free-base porphyrin. Time-resolved fluorescence decay profiles required three exponential decay components for satisfactory fitting. These are attributed to emission from the central free-base porphyrin and to two different rates of energy transfer from the zinc porphyrins to the free-base porphyrin. The faster of these decay components equates to an energy-transfer rate constant of 3.7 x 10(9) s(-1) and an efficiency of 83%, whereas the other is essentially unquenched with respect to reported values for zinc porphyrin fluorescence decay times. The relative contribution of these two components to the initial fluorescence decay is similar to 3:2, similar to the 5: 4 ratio of cis and trans geometric isomers present in the pentamer.
Trimer, tetramer, and pentamer oligomers based on the polymer backbone structure of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) have been synthesized by Horner-Wadsworth-Emmons reactions. The fluorescence spectra, emission quantum yields, and lifetimes of the oligomers have been characterized in dilute chloroform solutions. The oligomers exhibit a sequential increase in absorption and emission wavelength maxima and a decrease in fluorescence lifetime as the π conjugation length is increased. The shortening in excited state lifetime is shown to be due to an increase in the rates of both radiative and nonradiative processes. The absence of a mirror-image relationship for the absorption and fluorescence spectra of the oligomers is attributed to the photoexcitation of a range of torsional configurations followed by relaxation to a more planar arrangement that then emits.
The synthesis of a series of polymers with appended pendant MEH-PPV oligomers of different conjugation length (trimer, tetramer and pentamer) is described. The synthesis involves RAFT polymerization of 4-chloromethylstyrene followed by coupling with the appropriate functionalized oligomer. The polymers have been characterized by GPC, UV-Vis and fluorescence measurements. The polymers exhibit a sequential red-shift in absorption and fluorescence spectra with increasing length of the MEH-PPV oligomer side chain. Temperature dependent measurements demonstrate that the non-mirror image relationship between absorption and fluorescence spectra in room temperature solutions can be attributed to the presence of torsional isomers in the side-chain chromophores. Efficient intramolecular energy transfer between the pendant chromophores is confirmed by fluorescence anisotropy measurements. The polymerization procedure outlined provides a useful template for providing polymers with tunable absorption and emission properties.
The review focuses current research in the rapidly developing chemistry of porphyrin-fullerene complexes, and highlights recent advances in the synthesis, properties, and potential applications of this family. An overview of the most popular methods to prepare porphyrin complexes with C 60 fullerene exhibiting various types of linkage is given with the main focus on the Prato reaction and its modifications. The discussion of porphyrin-fullerene complexes will include the structures of noncovalently linked porphyrin-fullerenes along with the covalently linked complexes; the chemistry and design of supramolecular ensembles will be covered rather comprehensively. Finally, the potential applications for porphyrin-fullerene conjugates will be discussed.
An olefination approach to the construction of covalently linked cyclic metalloporphyrin trimers is presented using fullerenes such as C(60) or C(70) as a template. Yields of the trimer approach 60%. In the absence of a template, the major product is the cyclic dimer (50% yield) with only a small amount of trimer (<10%) formed, indicating this is a template-directed approach.
The steady-state absorption, fluorescence, and excitation spectra and upper excited-state temporal fluorescence decay profiles of 11 tetrapyrroles in several fluid solvents are presented and analyzed to ascertain the factors that control their S2 population decay times. The S2 lifetimes, which vary by more than 2 orders of magnitude, are controlled exclusively by their rates of radiationless decay. The only important electronic relaxation path is S2-S1 internal conversion, the efficiency of which is near 1.0 in all compounds studied (except CdTPP where it is 0.69). The rate of S1 population rise equals the rate of S2 population decay in all cases. Among the compounds studied, only MgTPP exhibits S2-S1 decay behavior that corresponds to the weak coupling limit of radiationless transition theory; all zinc metalloporphyrins exhibit intermediate to strong coupling. Perdeuteration of ZnTPP produces no significant change in the rate of S2 decay or in the quantum yield of S2-S0 fluorescence, indicating that in-plane C-C and C-N vibrations are the accepting modes in S1 with the largest Franck-Condon factors. The initial vibrational energy content of the S2 states (0 < E(vib) < 3500 cm(-1) over the range of compounds) plays no significant role in determining their overall population decay rates in solution. The S2 population decay rates of these tetrapyrroles are controlled by two factors: the Franck-Condon factor, which is inversely proportional to the exponent of the S2-S1 electronic energy spacing and the S2-S1 coupling energy. The S2-S1 electronic energy spacing is determined in solution by the difference in the polarizabilities of the S2 and S1 states and can be controlled by varying the polarizability of the solvent. The S2-S1 coupling energy is influenced by the nature, location, and effect of the substituents, with beta-alkyl substitution and reduction of symmetry in the tetrapyrrole--for example by loss of planarity--increasing the interstate coupling energy.
A covalently linked porphyrin–fullerene C 60 dyad 6 was conveniently synthesized by 1,3‐dipolar cycloaddition using 5‐(4‐carbonylphenyl)‐10,15,20‐tris(4‐methoxylphenyl)porphyrin 5, N ‐methylglycine and fullerene C 60 . Spectroscopic studies show that dyad 6 is a promising architecture with potential application as photoactive organic material.
A dyad with terminal porphyrin (P) and viologen (MV2+) chromophores covalently linked by a flexible seven-atom hexanoate chain has been synthesized. Ab initio and semi-empirical solvent continuum calculations suggest that the preference for ‘extended’ or ‘folded’ conformations of the dyad depend strongly on solvent. Fluorescence lifetime and transient absorption studies in acetonitrile indicate that approximately 80% of the dyad population are in a suitable conformation to undergo photoinduced electron transfer (PET) to produce P+–MV+ with a rate of 5×108s−1. A long-lived charge-separated state lifetime of 170ns is observed. It is proposed that electrostatic repulsion between the positively charged end groups resulting from PET increases the average donor–acceptor separation thus retarding the charge recombination process.
Long-range photoinduced energy and electron transfer have been investigated in a novel dyad containing a zinc tetraarylporphyrin (PZn) donor and a C60 acceptor separated by a saturated norbornylogous bridge nine sigma bonds in length (PZn-9σ-C60). In non-polar solvents singlet-singlet energy transfer from PZn to C60 is observed but in benzonitrile efficient (> 90%) charge separation occurs with a rate constant of 1 × 1010 sec−1 to yield PZn.+-9σ-C60.−. The charge separated state exhibits a remarkably long lifetime for a dyad of 420 ns. This behaviour is discussed with reference to the energetics, dynamics and orbital symmetry properties of the states involved.
A simple photovoltaic device in which two chromophoric components are assembled by Zn-N coordination yields a charge-separated state with microsecond lifetime upon photoexcitation in non-polar solvents. Characterization of the electron transfer dynamics using time-resolved fluorescence and transient absorption spectroscopy suggests that the unusual longevity is due to charge recombination occurring between states with different electron spin character. Control of electron spin may provide a novel paradigm for optimizing light-induced charge-separation processes.
Photoinduced electronic energy transfer (EET) and electron transfer (PET) have been investigated in a multichromophoric system 2, that comprises zinc porphyrin (ZnP), dimethoxynaphthalene, and [60]fullerene (C60) chromophores separated by norbornylogous bridge sections of six and five bond lengths, respectively. In toluene as solvent, EET from ZnP to C60 occurs in folded and extended conformations of the molecule with rate constants of 1.3 × 1010 and 8.1 × 108 s-1, respectively. Evidence is presented for the existence of a third collapsed conformation of 2 which can undergo PET with a rate constant of 1.1 × 1012 s-1. In benzonitrile solvent, PET can also occur in the folded and extended geometries leading to a charge separated lifetime of 460 ns. The mechanisms for the unusually complex solvent and conformationally dependent excited-state processes in this molecule are discussed.
Four different kinds of Cââ-linked zincporphyrins have been prepared by changing systematically the linking position at meso-phenyl ring from ortho to para and their photophysical properties have been investigated. Regardless of the linkage between the two chromophores, photoinduced charges separation (CS) and subsequent charge recombination (CR) were observed in a series of zincporphyrin-Cââ dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. In THF the CS occurs from both the excited singlet state of the porphyrin and the Cââ moieties, implying that the absorption cross section by both the chromophores results in the efficient formation of the ion pair (IP) state. On the other hand, in benzene the IP state generated by the photoinduced CS from the excited singlet state of the porphyrin to the Cââ produces or energetically equilibrates with the locally excited singlet state of the Cââ. Both the CS and CR rates for the meta isomer are much slower than those for the other porphyrin-linked Cââ. Linkage dependence of the electron transfer (ET) rates can be explained by superexchange mechanism via spacer. These results demonstrate that Cââ is a new promising building block as an acceptor in artificial photosynthetic models. 32 refs., 14 figs., 3 tabs.
Two new pyrrolidinofullerenes, 1 and 2, have been synthesized, and their photophysical properties have been investigated. The pyrrolidinofullerene 1 has three phenyl groups attached to the 1, 2, and 5 positions of the pyrrolidine ring. The pyrrolidinofullerene 2, on the other hand, has a flexible attachment with an N-methylaniline end group and is the prototype of a fullerene−aniline dyad. Singlet and triplet excited-state properties of these two functionalized fullerene derivatives have been examined with picosecond and nanosecond laser flash photolysis. The singlet and triplet excited states of these fullerenes exhibit characteristic absorption bands in the vis-IR region of the spectrum. The functionalization of C60 with pyrrolidine groups shift the excited-state absorption maxima to the blue. Three different quenchers, O2, TEMPO, and ferrocene, are employed to investigate the reactivity of triplet excited states. The bimolecular quenching rate constants determined for these quenchers were in the range of 5.3 × 108 to 7.7 × 109 M-1 s-1. The excited-state interactions of these functionalized fullerenes are compared to that of C60.
Photoactive fulleropyrrolidine−perylenetetracarboxylic diimide−porphyrin triad (FPP) and its zinc analogue (ZnFPP) with expanded absorptions in the visible spectral region were synthesized. Steady absorption spectra and cyclic voltammetry (CV) measurement showed rather weak electronic interactions among the component chromophores in the ground state within the triads. Preliminary fluorescence measurements indicated the occurrence of a photoinduced electron-transfer process within the triads. These processes can be elicited either in the excited state of the perylenetetracarboxylic diimide chromophore or in the excited state of the porphyrin chromophore. The existence of photoinduced charge-separated states within triads in solution was confirmed by steady-state photolysis in the presence of methyl viologen (MV2+) as a sacrificed electron acceptor and 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificed electron donor. The photovoltaic devices have been fabricated on the basis of the triads and have shown clear photovoltaic behavior under illumination of white light. Though the power conversion efficiencies under simulated solar illumination of 68 mW·cm-2 were found to be moderate (0.028% for FPP and 0.035% for ZnFPP), it is an encouraging result for application of such molecular donor−acceptor ensembles to photovoltaics.
Zinc meso-tetraphenylporphyrin (ZnTPP) is known to have a relatively slow S2 → S1 internal conversion rate, (1−3 ps)-1, so that electron transfer from S2 to a nearby acceptor is possible. We have synthesized two zinc porphyrins in which a pyromellitimide (PI) acceptor is attached to the para position of a meso-phenyl group of the porphyrin (ZnTPP−PI) or to that of a β-phenyl group of the porphyrin (ZnTPP−β-PhPI). We report here on competitive electron transfer from the S2 (excitation at 420 nm) and S1 states (excitation at 550 nm) of these zinc porphyrins to PI in 2-methyltetrahydrofuran (MTHF) and toluene. Our transient absorption studies show that efficient electron transfer occurs from S2 of ZnTPP to PI in these porphyrins despite the presence of the intervening phenyl between the zinc porphyrin macrocycle and the PI acceptor. Moreover, the results show that while charge separation from S1 is about 6 times faster for PI attached to the meso-phenyl than for PI attached to the β-phenyl, the opposite is observed for charge separation from S2. Subsequent charge recombination is 2.6−2.8 times faster for PI attached to the meso-phenyl relative to that of the β-phenyl. Comparisons of rates between MTHF and toluene show that the ordering of rates is the same for both solvents, although the rate ratios are larger in MTHF than in toluene.
A new conjugated porphyrin−fullerene dyad featuring a styrene linkage has been prepared in 11% overall yield from previously synthesized starting materials. In contrast to the singlet and triplet excited states commonly observed in analogous systems with zinc and free base porphyrins, the present investigation involves doublet and quartet ground and excited states. The photophysical pathways in the present system include intramolecular electron and energy transfer events, whose outcome depends predominantly on the solvent polarity.
The generation of photoelectrical effects through the spectral sensitization of wide band gap (SnO2) nanostructured semiconductor electrode by the excitation of a novel porphyrin−fullerene (P−C60) dyad is reported. P−C60 was synthesized from 5-(4-amidophenyl)-10,15,20-tris(4-methoxylphenyl) porphyrin (P) linked to 1,2-dihydro-1,2-methoxyphenyl [60]-61-carboxilic acid (acid-C60) by an amide bond. Anodic photocurrents and photovoltages are observed under visible irradiation of ITO/SnO2/ P−C60 electrodes, although the porphyrin fluorescence is strongly quenched by the C60 moiety in the dyad. The photocurrent generation quantum yield of the dyad P−C60 is around twice higher than the yield of the porphyrin moiety at the same wavelength (Soret band). A mechanism involving the formation of an intramolecular photoinduced charge-transfer state is proposed to explain the efficiency in the generation of photoelectrical effect.
Diels−Alder reaction of C60 with the 1,3-dienes syn-5 and syn-20 affords the giant trichromophoric systems syn-C60-5-DMN-6-PZn and syn-C60-9-DMN-6-DMA, respectively, in which the three chromphores in each system are linked via rigid, hybrid saturated polynorbornane-bicyclo[2.2.0]hexane (“norbornylogous”) hydrocarbon bridges. The norbornylogous bridge is a strong mediator of electron and energy transfer via a through-bond coupling mechanism. Hence, the giant trichromophores described herein illustrate the necessary methodology for the synthesis of efficient molecular systems that are capable of generating long-lived charge-separated states. The 1,3-dienes syn-5 and syn-20 were prepared in a straightforward manner from two fragments containing a diene and a dienophile, and thus a simple building-block approach to such sophisticated systems is described. Semiempirical AM1 MO SCF calculations were carried out on the trichromophoric systems to quantify their ability to adopt two nondegenerate boat conformations, i.e., extended and folded conformers.
The efficient synthesis as well as computational and photophysical studies of two novel classes of prophyrin-C6-hybrids is reported. A new synthetic methodology which can be efficaciously exploited in the construction of structurally unique conformationally flexible prophyrin-fullerene hybrids in which intramolecular interactions are enhanced as a result of complexation with metal cations is developed.
Zn(II) porphyrin–C60 dyad (7) was synthesized by the coupling reaction between Zn(II) 5-(4-aminophenyl)-10,15,20-tris(4-methoxylphenyl)porphyrin (4) and 1,2-dihydro-1,2-methanofullerene[60]-61-carboxylic acid (6). The electron-donor capacity of the porphyrin moiety in dyad 7 is enhanced by methoxy group substitution on the macrocycle peripheries and complexation with Zn(II). Spectroscopic studies showed only a very weak interaction between the chromophores in the grown state. The emission of the porphyrin moiety in dyad 7 is strongly quenched by the attached fullerene C60 moiety. Thermodynamically, dyad 7 shows a higher capacity to form a photoinduced charge-separated state than the same dyad with free-base porphyrin. This was confirmed by steady-state photolysis in the presence of methylviologen as electron acceptor and N,N,N′,N′-tetramethyl-1,1′-naphthidine as electron donor. Dyad 7 sensitizes a tin dioxide (SnO2) electrode and the generation of photoelectrical effect shows that a large photocurrent is generated for the dyad 7 in the region where C60 is mainly responsible for light absorption. Also, dyad 7 produces a higher photoelectric effect than the corresponding amidoporphyrin model. An alternative photoelectric mechanism, other than direct electron injection from the excited porphyrin to SnO2, could be occurring, probably involving a charge-separated state of dyad 7. These results indicate that dyad 7 shows interesting properties for possible application in artificial solar energy conversion devices. Copyright © 2002 John Wiley & Sons, Ltd.
Abstract Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn-C60) and free base (P-C60) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+-C60bull;-. In toluene, P-1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1C60 underoes photoinduced electron transfer to give P•+-C60bull;-. The electron transfer rate constant is −2 × 1011 s−1.
The electron spin dynamics associated with the photoinduced intramolecular electron transfer (ET) in a covalently linked malonate-strapped zinc porphyrin-fullerene (ZnP-C 60) dyad with "parachute" geometry was studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Studies were carried out in widely different media, including solvents of different polarities, such as isotropic toluene and tetrahydrofuran, and anisotropic nematic liquid crystals, LCs (E-7 and ZLI-4389). Photoexcitation of the donor, ZnP, results in ET to the acceptor, C 60 , in all solvents used over a wide range of temperatures. The generated radical pairs (RPs) either decay to the ground state or produce the triplet state of the acceptor moiety, ZnP-3 *C 60 . Temperature-dependent studies permit the determination of the RP energy levels relative to the donor and acceptor singlet and triplet states. Thus, the results enable determination of the genesis of the ET routes as a function of the solvent polarity. Due to their unique dielectric properties, the LCs behave as solvents of low polarity at low temperatures and as solvents of high polarity at elevated temperatures. Finally, the triplet EPR line shape of ZnP-3 *C 60 oriented in the LCs verifies the asymmetric three-dimensional structure calculated recently, while the ZnP moiety in the dyad dictates the orientation of the bulky C 60 part in the LC matrixes.
This review describes light induced energy or electron transfer reactions in self-assembled supramolecular zinc porphyrin/zinc phthalocyanine and fullerene bearing donor–acceptor systems. The self-assembled supramolecular dyads and triads are formed by using one or two types of the binding mechanisms including metal–ligand axial coordination. The photochemical properties of the metal porphyrin/phthalocyanine and fullerene moieties are shown to be tuned in a controlled manner upon coordination of metal center. The nature of the linker between the donor and acceptor entities influences the overall self-assembly process followed by the photochemical reactivity. In these self-assembled supramolecular systems, the photoinduced charge separation occurs mainly from the excited singlet state of the donor; and the back electron transfer rates generally occurs giving reversible systems. In some of the reported donor–acceptor conjugates, the predicted acceleration of the charge separation process and deceleration of the charge recombination process have been clearly observed, mainly due to the small reorganization energies of fullerenes in electron transfer reactions. Elegantly designed supramolecular triads to achieve sequential electron transfer to obtain the charge-separated states, and sequential energy transfer followed by electron transfer to mimic the photosynthetic ‘antenna-reaction center’ have also been developed and studied. The relations between structures and photochemical reactivities of these novel supramolecular systems are discussed in relation to the efficiency of forward electron transfer and slowing down the charge recombination process.
This review presents some of the efforts that have been made over the last decades to produce systems in which photo-excitation leads to one or more intramolecular electron transfer events ultimately resulting in a charge-transfer (CT) excited state with a relatively long lifetime. This process is generally considered as a mimic of natural photosynthesis and is not only of relevance in relation to solar energy conversion but also in relation to perspectives such as molecular information storage, molecular electronics, and molecular photonics. A long-lived CT state in general may be considered as a weakly coupled radical (ion)pair and in this review we focus especially on the consequences of the eventual electron spin correlation in that radical (ion)pair. If substantial spin–spin interaction is still present, such as in compact dyads, CT states can be assigned pure singlet or triplet configurations (1CT, 3CT) and as we demonstrate this configuration has significant influence on the CT lifetime because charge recombination from 3CT is spin forbidden. For small spin–spin interaction such as is typical for CT states in which the radical sites are further removed from each other – e.g., in triads, tetrads, etc., – rapid interconversion of 1CT and 3CT becomes possible especially via a hyperfine interaction (HFI) driven mechanism. This HFI driven mechanism is strongly influenced by external magnetic fields, which allows sensitive detection of the actual spin–spin interaction via magnetic field effects on the electron transfer kinetics, as well as via time-resolved EPR and field-dependent CIDNP. Examples of such studies on artificial multichromophoric electron transfer systems are presented and the results are discussed.
The synthesis and photophysics of a series of porphyrin–fullerene (P–C60) dyads in which the two chromophores are linked by conformationally flexible polyether chains is reported. Molecular modeling indicates the two moieties adopt a stacked conformation in which the two chromophores are in close proximity. Photoexcitation of the free base dyads in polar solvents such as tetrahydrofuran and benzonitrile, causes electron transfer (ET) to generate charge-separated radical pair (CSRP) states, which were directly detected using transient absorption (TA) techniques. In nonpolar solvents such as toluene, where CSRP states were not directly detected, fullerene triplet state states were formed, according to TA studies as well as singlet oxygen sensitization measurements. The low value of the quantum efficiency for sensitized formation of singlet molecular oxygen [O2(1Δg)] in toluene and chloroform indicates that singlet energy transduction to give H2P–1C60*, followed by intersystem crossing to H2P–3C60* and energy transfer to 3O2, is not the operative mechanism. Rather, a mechanism is proposed involving ET to give CSRP states followed by exergonic charge recombination to eventually generate fullerene triplets. Such a mechanism has been demonstrated experimentally for structurally related P–C60 dyads. For the corresponding ZnP–C60 dyads with flexible linkers, only photoinduced ET to generate long-lived CSRP states is observed. Photoinduced charge separation in these dyad systems is extremely rapid, consistent with a through space rather than through-bond mechanism. Charge recombination is up to three orders of magnitude slower, indicating this process occurs in the inverted region of the Marcus curve that relates ET rates to the thermodynamic driving force. These observations once again demonstrate the advantages of incorporating fullerenes as electron acceptor components in photosynthetic model systems.
Long live the state! Photoexcitation of a zinc chlorin-fullerene dyad with a short linkage results in the formation of the ultra-long-lived charge-separated (CS) state by a one-step photoinduced electron transfer without loss of energy, which is inevitable for charge separation by multistep electron-transfer processes. The lifetime of the charge-separated state was 120 s in frozen PhCN at -150°C (see picture).
A molecular double-throw switch that employs a photochromic moiety to direct photoinduced electron transfer from an excited state donor down either of two pathways has been prepared. The molecular triad consists of a free base porphyrin (P) linked to both a C(60) electron acceptor and a dihydroindolizine (DHI) photochrome. Excitation of the porphyrin moiety of DHI-P-C(60) results in photoinduced electron transfer with a time constant of 2.3 ns to give the DHI-P(*)(+)-C(60)(*)(-) charge-separated state with a quantum yield of 82%. UV (366 nm) light photoisomerizes the DHI moiety to the betaine (BT) form, which has a higher reduction potential than DHI. Excitation of the porphyrin of BT-P-C(60) is followed by photoinduced electron transfer with a time constant of 56 ps to produce BT(*)(-)-P(*)(+)-C(60) in 99% yield. Isomerization of BT-P-C(60) back to DHI-P-C(60) may be achieved with visible light, or thermally. Thus, photoinduced charge separation originating from the porphyrin is reversibly directed down either of two different pathways by photoisomerization of the dihydroindolizine. The switch may be cycled many times.
Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element can be used to construct discrete host-guest complexes, as well as ordered arrays of interleaved porphyrins and fullerenes. The fullerene-porphyrin interaction underlies successful chromatographic separations of fullerenes, and there are promising applications in the areas of porous framework solids and photovoltaic devices.
The electrochemical and photophysical properties of molecular architectures consisting of oligomeric meso,meso-linked oligoporphyrin rods linked at both extremities to methanofullerene moieties are presented in comparison to those of model systems. Cyclic voltammetry data evidence the presence of a strong intramolecular electronic coupling along the porphyrin oligomers that varies slightly with their length. This interaction affects the redox potentials of both fullerene and porphyrin moieties. The electronic coupling between the two chromophores is confirmed by comparing the redox potentials of porphyrin arrays before and after attachment of the carbon sphere. Electronic absorption, fluorescence, and phosphorescence spectra of the porphyrin oligomers in toluene are reported, which provide the energy of the lowest singlet and triplet electronic excited states. In the fullerene-porphyrin conjugates, ground-state charge-transfer (CT) interactions are evidenced by low-energy absorption features above 750 nm. These systems also exhibit near-infrared (NIR) CT luminescence in toluene with lifetimes shorter than 1000 ps. On increasing the solvent polarity (from toluene to Et2O and THF), CT emissions become progressively weaker, red-shifted, and shorter lived, which reflects the energy-gap law and Marcus inverted region effects. Luminescence is not detected in benzonitrile. Picosecond transient absorption spectroscopy of the porphyrin-fullerene conjugates allows detection of the porphyrin cation as a clear fingerprint for electron transfer. The rate of charge recombination is in agreement with CT luminescence lifetimes, which confirms the occurrence of NIR radiative back-electron transfer.
In recent years, olefin cross metathesis (CM) has emerged as a powerful and convenient synthetic technique in organic chemistry; however, as a general synthetic method, CM has been limited by the lack of predictability in product selectivity and stereoselectivity. Investigations into olefin cross metathesis with several classes of olefins, including substituted and functionalized styrenes, secondary allylic alcohols, tertiary allylic alcohols, and olefins with alpha-quaternary centers, have led to a general model useful for the prediction of product selectivity and stereoselectivity in cross metathesis. As a general ranking of olefin reactivity in CM, olefins can be categorized by their relative abilities to undergo homodimerization via cross metathesis and the susceptibility of their homodimers toward secondary metathesis reactions. When an olefin of high reactivity is reacted with an olefin of lower reactivity (sterically bulky, electron-deficient, etc.), selective cross metathesis can be achieved using feedstock stoichiometries as low as 1:1. By employing a metathesis catalyst with the appropriate activity, selective cross metathesis reactions can be achieved with a wide variety of electron-rich, electron-deficient, and sterically bulky olefins. Application of this model has allowed for the prediction and development of selective cross metathesis reactions, culminating in unprecedented three-component intermolecular cross metathesis reactions.
Selective cross-metathesis of type I and type II meso-functionalized porphyrin olefins afforded alkenyl-coupled dimeric and trimeric porphyrin systems in good yield with excellent E/Z selectivity. The synthetic utility of the method is demonstrated through the preparation of mixed metalated (M = 2H, Zn) porphyrin dimer and trimer. [reaction: see text]