Effect of high-frequency modes and hot transitions on free energy gap dependence of charge recombination rate

The charge recombination (CR) dynamics of geminate ion pairs formed by excitation of the ground-state donor-acceptor complexes in polar solvent have been investigated within the framework of stochastic approach. It is shown that for low exergonic reactions these dynamics critically depend on the reorganization energy of intramolecular high-frequency mode. Even moderate reorganization energies (0.1-0.2 eV) significantly accelerate the excited-state population decay making it nearly exponential. In the solvent-controlled regime, the majority of the excited donor-acceptor complexes recombine at nonthermal (hot) stage when the nonequilibrium initial wave packet passes through a number of term crossings corresponding to the transitions with creation of several vibrational quanta. Analysis of this mechanism allows to conclude (i) the CR in viscous solvents proceeds much faster than the diffusive relaxation of solvent, (ii) under certain conditions, the CR rate becomes practically independent of the diffusive component of solvent relaxation which is determined by solvent viscosity, (iii) in contrast to predictions of Marcus theory, the CR rate decreases monotonically with the rise of reaction exergonicity even at small free energy gaps, in accordance with experimental results. Two semiquantitative approaches providing rather simple analytical expressions for the hot charge recombination dynamics are suggested. These approximations give a good reproduction of the excited-state decay in the wide area of model parameters.     

Nonequilibrium charge recombination from the excited adiabatic state of donor-acceptor complexes

A model of nonequilibrium charge recombination from an excited adiabatic state of a donor-acceptor complex induced by the nonadiabatic interaction operator is considered. The decay of the excited state population prepared by a short laser pulse is shown to be highly nonexponential. The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes is explored. The charge recombination rate constant is found to decrease with increasing excitation frequency. The variation of the excitation pulse carrier frequency within the charge transfer absorption band of the complex can alter the effective charge recombination rate by up to a factor 2. The magnitude of this spectral effect decreases strongly with increasing electronic coupling.     

Simultaneous generation of different types of ion pairs upon charge-transfer excitation of a donor-acceptor complex revealed by ultrafast transient absorption spectroscopy

The excited-state dynamics of the methylperylene/tetracyanoethylene (MPe/TCNE) donor-acceptor complex has been investigated in various solvents using femtosecond transient absorption spectroscopy. The transient spectra reveal the formation of two types of ion pairs: The first (IP1), constituting the major fraction of the total ion-pair population, is characterized by a broad and red-shifted absorption spectrum compared to that of the free MPe cation and by a subpicosecond lifetime, whereas the second (IP2) has a spectrum closer to that of MPe cation and a lifetime of a few picoseconds. A substantial polarization anisotropy was observed with IP1 but not with IP2, indicating a relatively well-defined structure for the former. The reaction scheme that best accounts for the observed dynamics and its solvent dependence involves the simultaneous excitation of complexes that differ by their electronic coupling. The more coupled complexes have a high absorption coefficient and thus yield IP1, which undergoes ultrafast charge recombination, whereas the less coupled complexes have a lower probability to be excited and lead to the longer-lived IP2.     

Effect of the excitation pulse frequency on the ultrafast photoinduced electron transfer dynamics

The dependence of the ultrafast photoinduced electron transfer dynamics in donor-acceptor complexes on the excitation pulse carrier frequency (spectral effect) has been investigated in the framework of a model involving three electronic state. The spectral effect has been shown to strongly depend on the angle theta between the reaction coordinate directions corresponding to optical and charge transfer transitions. Describing the solvent as a linear homogenous polar medium and accounting for Coulombic interaction of the transferred charge with the medium polarization fluctuations, the angle theta has been found out to be typically in the area 40 degrees -85 degrees. Exactly in this area of theta the spectral effect is predicted to be most pronounced.     

Resonance Raman study of a two-chromophore system. The 2:1 complex of hexamethylbenzene with tetracyanoethylene

Raman excitation profiles are presented for the 2:1 electron donor-acceptor (EDA) complex of hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) in cyclohexane. Though the absorption and Raman spectra of the 1:1 and 2:1 complexes are similar, distinct differences are found in the Raman excitation profiles (REPs) of vibrational modes common to both systems. REPs of the 2:1 complex show intensity cancellation that is taken as evidence for interference of two charge-transfer excited states. The implications of the observed spectra concerning excited state electron delocalization are considered.     

Charge recombination in excited donor-acceptor complexes with two absorption bands

The dynamics of charge recombination in a photoexcited donor-acceptor complex comprising 1,2,4-trimethoxybenzene (electron donor) and tetracyanoethylene (electron acceptor) in several polar solvents (acetonitrile, valeronitrile, and octanonitrile) was studied in terms of the stochastic approach. The Gibbs energy of charge recombination and the reorganization energies of the medium and quantum and vibrational degrees of freedom were found by fitting the stationary absorption spectrum. The electronic couplings were determined by analyzing the time dependences of the population of the ionic state in acetonitrile. A comparison of the numerical simulation results with the experimental data showed that the nonstationary model under consideration quantitatively described the dynamics of charge recombination and its dependence on the carrier frequency of excitation pulses and the relaxation properties of solvents. Original Russian Text ? V.N. Ionkin, A.I. Ivanov, E. Vauthey, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 4, pp. 791–797.     

Hole transfer in a C-shaped molecule: conformational freedom versus solvent-mediated coupling

The electronic coupling matrix elements attending the charge separation reactions of a C-shaped molecule containing an excited pyrene as the electron acceptor and a dimethylaniline as the donor are determined in aromatic, ether, and ester solvents. Band shape analyses of the charge-transfer emission spectra (CT --> S(0)) provide values of the reaction free energy, the solvent reorganization energy, and the vibrational reorganization energy in each solvent. The free energy for charge separation in benzene and toluene solvents is independently determined from the excited state equilibrium established between the locally excited pyrene S(1) state and the charge-transfer state. Analyses of the charge separation kinetics using the spectroscopically determined reorganization energies and reaction free energies indicate that the electronic coupling is solvent independent, despite the presence of a cleft between the donor and acceptor. Hence, solvent molecules are not involved in the coupling pathway. The orientations of the donor and acceptor units, relative to the spacer, are not rigidly constrained, and their torsional motions decrease solvent access to the cleft. Generalized Mulliken-Hush calculations show that rotation of the pyrene group about the bond connecting it to the spacer greatly modulates the magnitude of through-space coupling between the S(1) and CT states. The relationship between the torsional dynamics and the electron-transfer dynamics is discussed.     

A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

What factors control product yield in charge separation reaction from second excited state in zinc-porphyrin derivatives?

Intramolecular charge separation from the second singlet excited state of directly linked Zn-porphyrin-imide dyads and following charge recombination into the first singlet excited state has been investigated in the framework of a model involving three electronic states (the first and the second singlet excited and charge separated states) as well as their vibrational sublevels. Kinetics of the transitions between these states are described in terms of the stochastic point-transition approach. The influence of the model parameters (free energy change of charge separation, magnitude of the reorganization energies of the medium and the high frequency intramolecular vibrations, the rate of relaxation of the medium and the intramolecular high frequency vibrational mode) on the kinetics of population of both the charge separated and the first singlet excited states has been explored. Simulations of the kinetics of the charge separated state population have allowed reproducing the distinctive features of the kinetics observed in the experiment [Wallin, S.; Monnereau, C.; Blart, E.; Gankou, J.-R.; Odobel, F.; Hammarstr?m, L. J. Phys. Chem. A 2010, 114, 1709]: (i) two maxima on short time scale (hundreds of femtoseconds) and long time scale (tens of picoseconds), (ii) the magnitudes of both maxima, and (iii) the depth of the notch between the maxima.     

Nonadiabatic proton-coupled electron transfer reactions: impact of donor-acceptor vibrations, reorganization energies, and couplings on dynamics and rates

Fundamental aspects of proton-coupled electron transfer (PCET) reactions in solution are analyzed with molecular dynamics simulations for a series of model systems. The analysis addresses the impact of the solvent reorganization energy, the proton donor-acceptor mode vibrational frequency, and the distance dependence of the nonadiabatic coupling on the dynamics of the reaction and the magnitude of the rate. The rate for nonadiabatic PCET is expressed in terms of a time-dependent probability flux correlation function. The time dependence of the probability flux correlation function is determined mainly by the solvent reorganization energy and is not significantly influenced by the proton donor-acceptor frequency or the distance dependence of the nonadiabatic coupling. The magnitude of the PCET rate becomes greater as the solvent reorganization energy decreases, the proton donor-acceptor frequency decreases, and the distance dependence of the nonadiabatic coupling increases. The approximations underlying a previously derived analytical PCET rate expression are also investigated. The short-time approximation for the solvent is valid for these types of systems. In addition, solvent damping effects on the proton donor-acceptor motion are not significant on the time scale of the probability flux. The rates calculated from the molecular dynamics simulations agree well with those calculated from the analytical rate expression.     

Pathways for photoinduced charge separation and recombination at donor-acceptor heterojunctions: the case of oligophenylenevinylene-perylene bisimide complexes

Semiempirical Hartree-Fock techniques have been applied to assess the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The corresponding rates have been estimated in the framework of the Marcus-Levich-Jortner formalism for different geometries of the complexes. The results indicate that dissociation pathways involving the lowest two charge transfer excited states contribute significantly to the dynamics of the whole process. The rates are found to be strongly sensitive to the relative position of the donor and acceptor units and can be rationalized in terms of symmetry arguments applied to relevant electronic levels.     

Solvent Effects on Excited-State Relaxation Dynamics of Paddle-Wheel BODIPY-Hexaoxatriphenylene Conjugates: Insights from Non-adiabatic Dynamics Simulations

Understanding the excited state dynamics of donor-acceptor (D-A) complexes is of fundamental importance both experimentally and theoretically. Herein, we have first explored the photoinduced dynamics of a recently synthesized paddle-wheel BODIPY-hexaoxatriphenylene (BODIPY is the abbreviation for BF\begin{document}$ _2 $\end{document}-chelated dipyrromethenes) conjugates D-A complexes with the combination of both electronic structure calculations and non-adiabatic dynamics simulations. On the basis of computational results, we concluded that the BODIPY-hexaoxatriphenylene (BH) conjugates will be promoted to the local excited (LE) states of the BODIPY fragments upon excitation, which is followed by the ultrafast exciton transfer from LE state to charge transfer (CT). Instead of the photoinduced electron transfer process proposed in previous experimental work, such a exciton transfer process is accompanied with the photoinduced hole transfer from BODIPY to hexaoxatriphenylene. Additionally, solvent effects are found to play an important role in the photoinduced dynamics. Specifically, the hole transfer dynamics is accelerated by the acetonitrile solvent, which can be ascribed to significant influences of the solvents on the charge transfer states, i.e. the energy gaps between LE and CT excitons are reduced greatly and the non-adiabatic couplings are increased in the meantime. Our present work not only provides valuable insights into the underlying photoinduced mechanism of BH, but also can be helpful for the future design of novel donor-acceptor conjugates with better optoelectronic performance.     

Temperature dependence of charge separation and recombination in porphyrin oligomer-fullerene donor-acceptor systems

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor-acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1-4, 6) appended to C(60) were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5-25 ps) electron transfer to C(60) followed by slower (200-650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor-acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C(60)-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.