Photoinduced Intermolecular Electron Transfer Research Articles

Photoinduced intermolecular electron transfer between oligo( p-phenylene vinylene)s and N-methylfulleropyrrolidine in a polar solvent

Photoinduced intermolecular electron transfer in mixtures of oligo( p-phenylene vinylene)s (OPV ns, with n=2–7, the number of phenyl rings) and N-methylfulleropyrrolidine (MP-C 60) is studied in o-dichlorobenzene. Photoinduced absorption (PIA) spectroscopy is used to show that photoexcitation of MP-C 60 generates the corresponding triplet state, which is quenched in a one-electron reduction reaction by OPV n for n>2. The electronic transitions of the photogenerated OPV n + radical cations evolve with conjugation length in a near linear relation with 1/ n. Charge transfer in these donor–acceptor systems for n>2 is in full agreement with the calculated change in free energy for charge separation.

Synthesis of medium and large ring heterocycles by photoinduced intermolecular and intramolecular electron transfer reactions of tetrachlorophthalimides with alkenes

Photoinduced intermolecular and intramolecular electron transfer reactions of N-(ω-hydroxyalkyl)-tetrachlorophthalimide with alkenes led to the formation of medium to large sized heterocyclic rings.

Measurements of the electric-field-induced change in fluorescence decay profile of a mixture of ethylcarbazole and dimethylterephthalate in a PMMA polymer film

Electric field effects on photoinduced intermolecular electron transfer between ethylcarbazole (ECz) and dimethylterephthalate (DMTP) doped in a PMMA film have been investigated by using a picosecond time-correlated single-photon counting system combined with a bipolar sample bias, which enables the direct measurements of the field-induced change in fluorescence decay profile. The difference of the lifetime in the presence and absence of applied electric fields was successfully measured. The obtained results show that the electron transfer rate is enhanced by an electric field in agreement with those by the steady-state fluorescence measurements.

Three-layer organic solar cell with high-power conversion efficiency of 3.5%

To prepare organic solar cells with practical level of energy-conversion efficiency, the following strategies were adopted. By using HD as the photosensitizer, which is heterodimer consisting of a weak electron-donating 5, 10, 15, 20-tetra(2, 5-dimethoxyphenyl)porphyrinatozinc and a weak electron-accepting 5, 10, 15-triphenyl-20-(3-pyridyl)porphyrin, intramolecular photoinduced electron transfer is promoted resulting in effective charge separation. To create an energetically well-arranged system, the HD was placed between an electron-acceptor layer of PV (perylene-3, 4, 9, 10-tetracarboxyl-bis-benzimidazole) and strong electron-donor layer of MC (3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene]-2-thioxo-4-thiazolidinone), where photoinduced intermolecular electron transfer from HD to PV and rapid injection of electrons from MC to HD suppress back electron transfer in the charge-separated HD. As a result of this, the three-layer solar cell Al/PV/HD/MC/Au showed fairly good photovoltaic properties, short-circuit photocurrent quantum yield of 49.2%, open-circuit photovoltage of 0.39 V, fill factor of 0.51, and energy conversion yield of 3.51% when irradiated with 445 nm monochromatic light of 12 μW cm −2 intensity transmitted through the Al/PV interface. Since the photocurrent hardly decreased with age, the photocurrent observed here is really from energy conversion and not from photocorrosion of Al electrode being occasionally responsible for the photocurrent.

Influence of redoxinert counterions on photoinduced intermolecular electron transfer rates in dichloromethane

Rate constants kq of the fluorescence quenching of 9,10-dicyanoanthracene by bromide and chloride ions and of 9-cyanoanthracene by bromide ions were determined in the low polar solvent dichloromethane with a series of tetra-alkylammonium counterions. Cation size dependent electron transfer rate constants ket were derived from kq, and were fitted to a nonadiabatic electron transfer model equation. Because of ion pair formation in dichloromethane electrostatic interactions of the redoxinert countercations during electron transfer were considered. Accordingly, the position of the countercation within the precursor complex had to be described.The electronic coupling matrix element derived by fitting of the experimental data is in satisfactory agreement with results of simple quantum chemical calculations.

Time-resolved Raman study on the photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols

Time-resolved Raman spectroscopy has uncovered a new type of photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols. The Raman bands of the first excited singlet (S1) state, the cation radical and the anion radical of biphenyl were simultaneously observed in the time range 0–200 ns after photoexcitation. The two ion radicals exhibit distinct rise and decay behaviors from each other. A kinetic analysis shows that the electron–transfer reaction consists of the following three elementary processes; (i) monophotonic formation of a biphenyl cation radical and a solvated electron via the S1 state, (ii) migration of the electron in the solvent medium, and (iii) anion radical formation resulting from the capture of the electron by a biphenyl molecule in the ground state. The formation of the cation radical from the S1 state is a slow process with a rate not greater than 10−7 s−1.

Free radical polymerization initiated via photoinduced intermolecular electron transfer process: kinetic study 3

Various electron donors and electron acceptors have been tested in order to examine the possibility of the application of the Marcus equation to the description of the kinetics of free radical polymerization photoinitiated via photoinduced electron transfer (PET). Variations of the driving force of the electron transfer, for selected dyes (xanthene dyes, camphorquinone), were introduced by using: a series of tertiary aromatic amines (TAAs) and N-phenylglycine derivatives; a series of electron acceptors and only one type of electron donor (cyanine cations with tetraorganylborate anion); a mixed system with a series of benzophenones as electron acceptors and TAAs as electron donors. Several important conclusions follow from the experimental data. (1) For the case with the rate of PET much lower than the rate of diffusion controlled processes, the Marcus theory can be used for analysing or predicting the ability of organic redox systems for light-induced free radical polymerization. (2) For a process controlled by diffusion, the reactivity of free radicals formed as a result of PET limits the rate of initiation of polymerization. It is shown that this relationship can be also presented as a function of thermodynamic driving forces of the photoredox reaction ( − G°). For this type of process, the relationship between the rate of polymerization and − ΔG° is linear, indicating the ‘inverted-region-like’ kinetic behaviour. Additionally, it is shown that the bleaching process of the dyes competes with the polymerization photoinitiation in a way that suggests that, after free radical formation, two parallel reactions occur and that the rate of bleaching can be expressed as a fraction of the total rate of electron transfer.

Effect of laser intensity on the determination of intermolecular electron transfer rate constants—Observation of Marcus inverted region in photoinduced back electron transfer reactions

The light intensity and concentration dependence of the photoproduct yield are investigated in a monophotonic process. The relationship of the photoproduct yield with the laser intensity and the complex concentration for a monophotonic process is derived under laser flash photolysis. The relationship is confirmed experimentally in a monophotonic process, i.e., triplet–triplet transition for a Cu(I) complex Cu6(DMNSN′)6 (DMNSN′=4,6-dimethylpyrimidine-2-thiolate). At low light intensity, the relationship can be approximated by a linear inverse square root dependence on the light intensity. Based on this equation, a method is proposed to determine the intrinsic back electron transfer rate constant kETb in photoinduced intermolecular electron transfer reactions, precluding the effect from the diffusional encounter pairs. The Marcus “inverted region” is observed by using the method in photoinduced back electron transfer reactions of [Au2(dppm)2](ClO4)2 (dppm=bis(diphenylphosphino)methane) with a series of substituted pyridinium acceptors.

Investigation of the Excited-State Dynamics of Radical Ions in the Condensed Phase Using the Picosecond Transient Grating Technique

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.

Solvent effects on intermolecular photoinduced electron transfer reactions of aromatic molecules

The electron transfer quenching of excited singlet of pyrene by indole and 3-cyanopyridine, and a,h-dibenzanthracene by o-dicyanobenze was investigated in solvents of different polarity. The electron transfer nature of the quenching reaction in all the solvents was confirmed by the transient absorption spectra of the radical ions formed. Quenching rate constants were determined and their variation with the solvent was compared with the prediction of the current electron transfer theories. It was found that for aprotic solvents the trend may be explained by the semiclassical theory for non-adiabatic electron transfer if a dielectric continuum model is used for the solvent. In hydroxylic solvents the rate constants do not fall on the same correlation, probably due to specific effects not taken into account by the model.

The influence of molecular oxygen on transient absorption and transient photocurrent kinetics

The transient absorption and transient photocurrent kinetics of photoinduced intermolecular electron transfer reactions in solution can show considerable discrepancies at different molecular oxygen concentrations. The lifetimes of the radical anions observed by transient absorption are decreased in the presence of oxygen, and this is explained by a secondary charge shift reaction of the primary radical anion to oxygen, generating the oxygen anion. Transient photocurrent kinetics are only slightly affected, because no net charge loss results from this mechanism and only the mobility of the corresponding charge carriers changes.

Photoinduced Electron Transfer and Geminate Recombination in Liquids

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between donors (rubrene) and acceptors (duroquinone) are studied in two molecular liquids: dibutyl phthalate and diethyl sebacate. Time-correlated single-photon counting and fluorescence yield measurements give information about the depletion of the donor excited state due to forward transfer, while pump−probe experiments give direct information about the radical survival kinetics. A straightforward procedure is presented for removing contributions from excited-state−excited-state absorption to the pump−probe data. The data are analyzed with a previously presented model that includes solvent structure and hydrodynamic effects in a detailed theory of through-solvent electron transfer. Models that neglect these effects are incapable of describing the data. When a detailed description of solvent effects is included in the theory, agreement with the experimental results is obtained. Forward electron transfer is well-described with a classical Marcus form of the rate equation, though the precise values of the rate parameters depend on the details of the solvents' radial distribution function. The additional experimental results presented here permit a more accurate determination of the forward transfer parameters than those presented previously.1 The geminate recombination (back transfer) data are highly inverted and cannot be analyzed with a classical Marcus expression. Good fits are instead obtained with an exponential distance dependence model of the rate constant and also with a more detailed semiclassical treatment suggested by Jortner.2 Analysis of the pump−probe data, however, suggests that the geminate recombination cannot be described with a single solvent dielectric constant. Rather, a time-dependent dielectric constant is required to properly account for diffusion occurring in a time-varying Coulomb potential. A model using a longitudinal dielectric relaxation time is presented. Additionally, previously reported theoretical results3 are rederived in a general form that permits important physical effects to be included more rigorously.

Generalization of the Kinetic Scheme for Photoinduced Polymerization via an Intermolecular Electron Transfer Process. 2. Application of the Marcus Theory

This paper presents a theoretical description of the kinetics of free radical dye initiated photopolymerization via an intermolecular electron transfer process. Analysis considers the properties of organic redox pair forming initiating radicals. An application of the Marcus theory gives the kinetic scheme, which considers both the theromodynamic and kinetic aspects of the electron transfer process. The analysis shows that both the reactivity of free radicals resulting from the photoinduced intermolecular electron transfer (PET) process and the rate of the PET process can limit the rate of the polymerization initiation process. The theory is supported by experimental data. Several organic redox pairs forming free radicals have been tested. As the electron-accepting molecules, xanthene dyes and camphorquinone have been tested. As the electron donors, tertiary aromatic amines (TAAs) and N-phenylglycines (NPGs) were used. Several important conclusions are drawn from the theoretical and experimental data: (i) For the process with the rate of PET much lower than the rate of the diffusion-controlled process, the Marcus theory can be used for analyzing or predicting the ability of organic redox systems for light-induced free radical polymerization. (ii) For the process controlled by the diffusion, the reactivity of radicals formed as a result of the PET process limits the rate of the polymerization initiation. It is shown that this relationship can also be presented as a function of thermodynamic driving forces of the photoredox reaction (−ΔG°). (iii) Experimental results show that one can describe the rate of photoinitiated polymerization as a function of thermodynamic driving forces of the photoredox reaction (−ΔG°; described by the Rehm−Weller equation) of the organic donor−acceptor pair. For the photoinduced electron transfer occurring much slower than diffusion-controlled processes, the relationship between the rate of polymerization and −ΔG° presents a classical Marcus parabolic relationship. For PET controlled by diffusion, the relationship between the rate of polymerization and −ΔG° is dependent on the reactivity of free radicals resulting from the PET process and gives a linear relationship indicating the “inverted region-like” kinetic behavior.

Intramolecular and intermolecular photoinduced electron transfer in isomeric mesoporphyrin nitrobenzyl esters: structure and solvent effects

Photoinduced intramolecular electron transfer was studied in a series of flexible dyads consisting of the isomeric ortho-, meta- and para- nitrobenzyl monoesters and diesters of mesoporphyrin existing preferentially in folded conformations. This process was compared with the intermolecular electron transfer between mesoporphyrin dimethyl ester and the 2-, 3- and 4-nitrobenzyl acetates. In both cases, electron transfer occurs from the porphyrin first excited singlet state, and the rate constant is lower in the ortho isomer, although intermolecular electron transfer is less isomer selective. The selectivity in the intermolecular case can be related to the isomer redox potentials, but in the dyads an ortho effect must also be taken into account; this effect produces a deviation from coplanarity between the nitro group and the phenyl ring. Electron paramagnetic resonance (EPR) in a low temperature matrix permits the charge transfer product to be detected. The process does not lead to the production of long-lived species, as shown by optoacoustic calorimetry. The efficiency of photoinduced electron transfer strongly depends on the ability of the solvent to stabilize the radical ion pair; this process occurs in CH 3OH, CH 3CN and CH 2Cl 2, but is inhibited in toluene and benzene. In the last two solvents, singlet oxygen is formed instead via energy transfer from the triplet state, with the same quantum yield as for the nitrobenzyl-free porphyrin.

Temperature and specific anion effects in photo-induced intermolecular electron-transfer reactions

The rate constants for the bimolecular oxidative quenching of *Ru(bpy)32+ by methylviologen (kq) and the electron-transfer recombination (krec) reaction of the redox products were determined as a function of the ionic strength and temperature in aqueous solution. At 25°C for Na+ salts at constantμ, kq andkrec are in the order H2PO4−<CH3CO2−∼HPO42−∼SO42−≪ClO4−; activation energies are independent of μ and the anion (∼16kJ/mol), except for ClO4−, whereEa is ∼30% lower. The trends inkq andkrec are attributed to the existence of ion-paired aggregates of the cationic reactants and anions; correlations with the sizes and hydration enthalpies of the anions can be made. It is suggested that ClO4−, with the smallest hydration radius and the least negative hydration enthalpy, is more closely ion-paired within the Ru(bpy)32+ (or Ru(bpy)33+) interligand pockets, “tethering” the MV2+ (or MV.+) into closer proximity, thereby lowering the activation barrier and increasing the rate of electron transfer.

Nitrobenzene "Caged" Compounds as Irreversible Photoreductants: A Rational Approach to Studying Photoinduced Intermolecular Electron-Transfer Reactions in Proteins

Nitrobenzene "Caged" Compounds as Irreversible Photoreductants: A Rational Approach to Studying Photoinduced Intermolecular Electron-Transfer Reactions in Proteins

Photoinduced intermolecular electron transfer reaction between N,N-dimethylaniline and anthracene in acetonitrile solution: A theoretical study

The photoinduced intermolecular electron transfer (ET) reaction between N,N-dimethylaniline and excited state anthracene in acetonitrile solution is studied theoretically. A solvation coordinate s which represents stochastic one-dimensional dynamics of the solution phase reaction is defined and a Hamiltonian in terms of s and perpendicular bath modes is derived from the spin-boson Hamiltonian. This has an advantage that the dynamics of the transferring electron is influenced by the bath only through coupling with the coordinate s. Intra- and intermolecular potentials are constructed by using ab initio molecular orbital methods, and a series of molecular dynamics simulation analysis is performed. Mean force potentials as a function of the donor–acceptor distance R are computed and the bimolecular encounter dynamics is investigated. Diabatic free energy curves for the coordinate s are computed and shown to be well approximated by parabolas, indicating that the dielectric saturation effect is negligible. The dependence of the free energy relationships on R is examined. It is shown that the present system corresponds to the increasing region of the rate constant, in contrast with the conventional picture. The electronic coupling of the ET is evaluated by the method of corresponding orbitals. The R dependence of the ET rate is evaluated and the reaction adiabaticity and mechanism are discussed. Dynamical solvent effects are taken into consideration in terms of the generalized Langevin equation formalism.

Nonadiabaticity in the photoinduced electron transfer reactions of metal complexes

The rate of electron transfer (ET) in a variety of chemical and biological processes is influenced by factors like the free energy change (†G), the donor-acceptor electronic coupling and the medium. The effect of donor-acceptor electronic coupling on the rate of photoinduced intermolecular electron transfer is considered by taking Ru(II) and Cr(III) metal complexes in the excited state as electron acceptors and organic compounds as electron donors. The electronic coupling between the donor and acceptors depends strongly on donor-acceptor distance. The electron transfer distance is varied by introducing alkyl groups of different sizes either on the bipyridine ligand of the metal complex or on the quencher. The semiclassical theory of electron transfer expresses kET as the product of a nuclear and an electronic transmission coefficient (K n andK el respectively) and an effective nuclear-vibration frequency (v n),k ET =v nKel, Kn. The electron transfer reaction becomes nonadiabatic if the donor-acceptor distance is long. The change of electron transfer mechanism from adiabatic to nonadiabatic due to the introduction of bulky groups is explained in terms of semiclassical theory and from the temperature-dependence study of photoinduced electron transfer reactions of metal complexes.

PHOTO-INDUCED INTERMOLECULAR AND INTRAMOLECULAR ELECTRON-TRANSFER REACTIONS BETWEEN XANTHENE DYES AND DONORS/ACCEPTORS

A number of electron donors, acceptors and diads containing xanthene dyes were sythesized. When the dyes were excited, the rate constants and the efficiencies of the intermolecular and intramolecular photo-induced electron transfer reactions were determined and calculated. It is found that the photo-induced electron transfer reactions occurred between xanthene dyes and many, including very weak donors or acceptors. The rate constants of intermolecular reactions were controlled by diffusion, and influenced by the reactant concentrations. The laser flash experiments showed that for low reactant concentrations, this kind of reactions took place mainly via the triplet excited state of the dyes. If different electric charges exist with dyes and donors/acceptors, there will be static quenching of the dyes' fluorescence. The intramolecular electron transfer reactions are independent of the solution concentrations, and they may directly proceed via the singlet excited state of the dyes effectively.

Absorption band shape analysis for photoinduced intermolecular electron transfer in solution: application to a new near-infrared transition between free hexacyanoferrate(II) and hexacyanoferrate(III)

Absorption band shape analysis for photoinduced intermolecular electron transfer in solution: application to a new near-infrared transition between free hexacyanoferrate(II) and hexacyanoferrate(III)