Energy gap law of electron transfer in nonpolar solvents

We investigate the energy gap law of electron transfer in nonpolar solvents for charge separation and charge recombination reactions. In polar solvents, the reaction coordinate is given in terms of the electrostatic potentials from solvent permanent dipoles at solutes. In nonpolar solvents, the energy fluctuation due to solvent polarization is absent, but the energy of the ion pair state changes significantly with the distance between the ions as a result of the unscreened strong Coulomb potential. The electron transfer occurs when the final state energy coincides with the initial state energy. For charge separation reactions, the initial state is a neutral pair state, and its energy changes little with the distance between the reactants, whereas the final state is an ion pair state and its energy changes significantly with the mutual distance; for charge recombination reactions, vice versa. We show that the energy gap law of electron-transfer rates in nonpolar solvents significantly depends on the type of electron transfer.     

Dynamics of photoinduced electron transfer in a molecular donor-acceptor quartet

The electronic structures and dynamics of photoinduced charge separation and recombination in a new donor/acceptor quartet molecule with bis-oligothiophene (BOTH) and bis-perylenediimide (BPDI) blocks attached to a benzene ring were described. Detailed transient spectroscopic studies were carried out on this compound and reference compounds at isolated molecular levels in solution. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. These results were discussed based on Marcus theory and ascribed to both through-bond and through-space electron-transfer processes associated with two different orientations of the acceptors relative to the donor group. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and nonpolar organic solvents, indicating that the material is an interesting candidate for photovoltaic studies in solid state.     

Corrole-fullerene dyads: formation of long-lived charge-separated states in nonpolar solvents

The first example of covalently linked free-base corrole-fullerene dyads is reported. In the newly synthesized dyads, the free-energy calculations performed by employing the redox and singlet excited-state energy in both polar and nonpolar solvents suggested the possibility of electron transfer from the excited singlet state of corrole to the fullerene entity. Accordingly, steady-state and time-resolved emission studies revealed efficient fluorescence quenching of the corrole entity in the dyads. Further studies involving femtosecond laser flash photolysis and nanosecond transient absorption studies confirmed electron transfer to be the quenching mechanism, in which the electron-transfer product, the fullerene anion radical, was able to be spectrally characterized. The rate of charge separation, kCS, was found to be on the order of 10(10)-10(11) s(-1), suggesting an efficient photoinduced electron-transfer process. Interestingly, the rate of charge recombination, kCR, was slower by 5 orders of magnitude in nonpolar solvents, cyclohexane and toluene, resulting in a radical ion-pair lasting for several microseconds. Careful analysis of the kinetic and thermodynamic data using the Marcus approach revealed that this novel feature is due to appropriately positioning the energy level of the charge-separated state below the triplet states of either of the donor and acceptor entities in both polar and nonpolar solvents, a feature that was not evident in donor-acceptor dyads constructed using symmetric tetrapyrroles as electron donors.     

Photoinduced Electron Transfer in a Distyryl BODIPY–Fullerene Dyad

A novel distyryl BODIPY–fullerene dyad is prepared. Upon excitation at the distyryl BODIPY moiety, the dyad undergoes photoinduced electron transfer to give a charge‐separated state with lifetimes of 476 ps and 730 ps in polar (benzonitrile) and nonpolar (toluene) solvents, respectively. Transient absorption measurements show the formation of the triplet excited state of distyryl BODIPY in the dyad, which is populated from charge‐recombination processes in both solvents.     

A High‐Energy Charge‐Separated State of 1.70 eV from a High‐Potential Donor–Acceptor Dyad: A Catalyst for Energy‐Demanding Photochemical Reactions

A high potential donor–acceptor dyad composed of zinc porphyrin bearing three meso‐pentafluorophenyl substituents covalently linked to C₆₀, as a novel dyad capable of generating charge‐separated states of high energy (potential) has been developed. The calculated energy of the charge‐separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin–fullerene dyad. Intramolecular photoinduced electron transfer leading to charge‐separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto‐ to nanosecond transient absorption techniques. The high energy stored in the form of charge‐separated states along with its persistence of about 50–60 ns makes this dyad a potential electron‐transporting catalyst to carry out energy‐demanding photochemical reactions. This type of high‐energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light‐to‐fuel products.     

An ab initio description of the excited states of the reaction O(3P, 1D) + H2 → OH(2Π, 2Σ+) + H. An attempt to describe several potential energy surfaces with constant accuracy

A new aspects of the role of the solvent mode in the photoinduced electron-transfer process of electron donor and acceptor system in polar solvents has been exploited. Taking into account the important fact that the vibrational frequency of the solvent mode in the initial neutral state of the reactants is considerably smaller than that in the final ionic state, we have derived a new formula for the energy-gap dependence of the electron-transfer rate. In this formulation, the activation energy is greatly reduced and the electron-transfer rate is almost independent of the energy gap over a wide down-hill energy region. This qualitative feature explains the experimental results for the relation between the bimolecular quenching rate constant k_w and the standard free-energy change ΔG° associated with electron transfer in the “anomalous region”.     

磺酰脒基桥连的锌卟啉-富勒烯化合物构型对光诱导电子转移机理的影响

通过"一锅法"多组分偶联反应合成了一种新型磺酰脒基桥连的卟啉-富勒烯化合物ZnP-H-C₆₀. 该化合物具有Z式和E式2种异构体, 其中Z式异构体中含有分子内氢键. 光物理研究结果表明, 2种异构体中的卟啉与富勒烯之间都可以发生光诱导电子转移, 但其相应的电子转移机理却完全不同. 在Z式异构体中, 卟啉或富勒烯被激发后直接发生电荷分离而形成电荷分离态, 其电荷分离机理是通过氢键进行电子传递; 在E式异构体中, 由于卟啉和富勒烯之间存在空间电子相互作用, 被激发后先形成卟啉-富勒烯激基复合物, 再进一步发生电荷分离形成电荷分离态, 电荷分离通过空间电子转移实现.     

Study of photoinduced electron transfer between [60]fullerene and proton-sponge by laser flash photolysis: addition effects of organic acid

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.     

Competing electron-transfer pathways in hydrocarbon frameworks: short-circuiting through-bond coupling by nonbonded contacts in rigid U-shaped norbornylogous systems containing a cavity-bound aromatic pendant group

This work explores electron transfer through nonbonded contacts in two U-shaped DBA molecules 1DBA and 2DBA by measuring electron-transfer rates in organic solvents of different polarities. These molecules have identical U-shaped norbornylogous frameworks, 12 bonds in length and with diphenyldimethoxynaphthalene (DPMN) donor and dicyanovinyl (DCV) acceptor groups fused at the ends. The U-shaped cavity of each molecule contains an aromatic pendant group of different electronic character, namely p-ethylphenyl, in 1DBA, and p-methoxyphenyl, in 2DBA. Electronic coupling matrix elements, Gibbs free energy, and reorganization energy were calculated from experimental photophysical data for these compounds, and the experimental results were compared with computational values. The magnitude of the electronic coupling for photoinduced charge separation, /V(CS)/, in 1DBA and 2DBA were found to be 147 and 274 cm(-1), respectively, and suggests that the origin of this difference lies in the electronic nature of the pendant aromatic group and charge separation occurs by tunneling through the pendant group, rather than through the bridge. 2DBA, but not 1DBA, displayed charge transfer (CT) fluorescence in nonpolar and weakly polar solvents, and this observation enabled the electronic coupling for charge recombination, /V(CR)/, in 2DBA to be made, the magnitude of which is approximately 500 cm(-1), significantly larger than that for charge separation. This difference is explained by changes in the geometry of the molecule in the relevant states; because of electrostatic effects, the donor and acceptor chromophores are about 1 A closer to the pendant group in the charge-separated state than in the locally excited state. Consequently the through-pendant-group electronic coupling is stronger in the charge-separated state--which controls the CT fluorescence process--than in the locally excited state--which controls the charge separation process. The magnitude of /V(CR)/ for 2DBA is almost 2 orders of magnitude greater than that in DMN-12-DCV, having the same length bridge as for the former molecule, but lacking a pendant group. This result unequivocally demonstrates the operation of the through-pendant-group mechanism of electron transfer in the pendant-containing U-shaped systems of the type 1DBA and 2DBA.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Fullerenes linked to photosynthetic pigments

The synthesis and photochemical characterization of two porphyrin-fullerene dyads, two zinc porphyrin-fullerene dyads, and a carotenobuckminsterfullerene are reviewed. In these molecules, the fullerene first excited singlet state may be formed by direct excitation or by singlet-singlet energy transfer from the attached pigment. In polar solvents, the dominant singlet-state decay pathway is photoinduced electron transfer to yield the pigment radical cation and fullerene radical anion. This charge-separated state has a long lifetime relative to the time constant for charge separation. In toluene, in cases where photoinduced electron transfer is slow for thermodynamic reasons, the fullerene singlet state decays by intersystem crossing, and the resulting triplet energy is partitioned between the components of the dyad according to their triplet energies. The results suggest that fullerenes can be valuable components of photochemically active multicomponent molecular systems.     

Efficient charge separation in C60-based dyads: triazolino

Triazoline[4,5][60]fullerenes are strong electron acceptors that form with tetrathiafulvalene (TTF), a novel type of donor-acceptor dyad exhibiting efficient improved electron-transfer dynamics. In particular, a rapid photoinduced intramolecular electron transfer, forming a charge-separated state, is followed by a slow charge recombination to generate the fullerene triplet excited state in moderate quantum yields.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Photoinduced energy and electron-transfer processes in porphyrin-perylene bisimide symmetric triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.     

Photoinduced intra- and intermolecular electron transfer in solutions and in solid organized molecular assemblies

The present paper highlights results of a systematic study of photoinduced electron transfer, where the fundamental aspects of the photochemistry occurring in solutions and in artificially or self-assembled molecular systems are combined and compared. In photochemical electron transfer (ET) reactions in solutions the electron donor, D, and acceptor, A, have to be or to diffuse to a short distance, which requires a high concentration of quencher molecules and/or long lifetimes of the excited donor or acceptor, which cannot always be arranged. The problem can partly be avoided by linking the donor and acceptor moieties covalently by a single bond, molecular chain or chains, or rigid bridge, forming D-A dyads. The covalent combination of porphyrin or phthalocyanine donors with an efficient electron acceptor, e.g. fullerene, has a two-fold effect on the electron transfer properties. Firstly, the electronic systems of the D-A pair result in a formation of an exciplex intermediate upon excitation both in solutions and in solid phases. The formation of the exciplex accelerates the ET rate, which was found to be as fast as >10(12) s(-1). Secondly, the total reorganization energy can be as small as 0.3 eV, even in polar solvents, which allows nanosecond lifetimes for the charge separated (CS) state. Molecular assemblies can form solid heterogeneous, but organized systems, e.g. molecular layers. This results in more complex charge separation and recombination dynamics. A distinct feature of the ET in organized assemblies is intermolecular interactions, which open a possibility for a charge migration both in the acceptor and in the donor layers, after the primary intramolecular exciplex formation and charge separation in the D-A dyad. The intramolecular ET is fast (35 ps) and efficient, but the formed interlayer CS states have lifetimes in microsecond or even second time domain. This is an important result considering possible applications.     

Hydrogen-bond-mediated photoinduced electron-transfer: novel dimethylaniline-anthracene ensembles formed via Watson-Crick base-pairing.

The synthesis of a new, noncovalent anthracene-dimethylaniline dyad (ensemble I) held together via guanosine-cytidine Watson-Crick base-pairing interactions is reported. Upon excitation at 420 nm, photoinduced electron-transfer from the dimethylaniline donor to the singlet excited state of the anthracene acceptor occurs, as inferred from a combination of time-resolved fluorescence quenching and transient absorption measurements. In toluene at room temperature, the rate constants for photoinduced intraensemble electron-transfer and subsequent back-electron-transfer (charge recombination) are k(CS) = (3.5 +/- 0.03) x 10(10) s(-1) and k(CR) = (1.42 +/- 0.03) x 10(9) s(-1), respectively.     

Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor–Acceptor Dyads Capable of Harvesting Near‐Infrared Light

To harvest energy from the near‐infrared (near‐IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near‐IR and IR light‐absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF₂‐chelated azadipyrromethene (ADP; to extend absorption and emission into the near‐IR region) and fullerene as electron‐donor and electron‐acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near‐IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated‐ADP and benzanulated thiophene‐ADP in the respective dyads, and triplet state of C₆₀ in the case of BF₂‐chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near‐IR region of the solar spectrum and for building fast‐responding optoelectronic devices operating under near‐IR radiation input.     

Photochemical hole burning by photoinduced electron transfer. Effects of sacrificially consumable molecules

Photochemical hole burning by photoinduced electron transfer was studied with emphasis on the effects of sacrificially consumable molecules. A backward electron-transfer process reduces the total efficiency of electron transfer. In order to enhance the effective electron transfer and to suppress the backward electron transfer, sacrificially consumable molecules were introduced to the donor—acceptor electron-transfer systems. In the presence of sacrificially consumable molecules, the efficiency of hole formation increased remarkably. The two-color enhancement of hole formation was also observed for a suitable acceptor.     

Excitation energy dependence of photoinduced processes in pentathiophene-perylene bisimide dyads with a flexible linker

In the present paper, photoinduced processes in the dyad molecules of pentathiophene (5T) and perylene-3,4:9,10-bis(dicarboximide) (PDI) with a flexible alkyl linker (propyl or hexyl) were investigated by using femtosecond laser flash spectroscopy in various solvents. Since absorption of 5T covers the wavelength region where absorption of PDI has minima and fluorescence of 5T overlaps with absorption of PDI, combination of 5T and PDI is favorable to achieve light energy harvesting as well as efficient electron transfer. When the sample was excited at the PDI moiety of the dyad, charge separation occurred almost quantitatively even in nonpolar solvent as well as in polar solvents. When the 5T moiety of the dyad was excited, efficient energy transfer to the PDI moiety from which charge separation occurred was confirmed, indicating that 5T acts as an antenna of the charge separation system, like a photosynthesis system of a plant. On the basis of Forster and Marcus theories and the estimated energy and electron-transfer rates, it was indicated that these dyads tend to take a folded structure in all solvents investigated.     

Femtosecond-Picosecond Laser Photolysis Studies on Photoinduced Electron Transfer Phenomena in Solutions

Results of our femtosecond-picosecond laser photolysis studies on photoinduced electron transfer phenomena in solutions including exciplex dynamics and its solvent dependences, energy gap dependences of photoinduced charge separation and charge recombination of various geminate ion pairs, mechanisms of chemical reactions via exciplexes and ion pairs, dynamics of photoinduced election transfer in hydrogen bonding complexes, dynamics and mechanisms of photoinduced electron transfer in fixed distance donor acceptor dyads and photosynthetic reaction center models, and mechanisms of electron ejection from solute fluorescent state in polar solutions are summarized and discussed.