A significant number of quadrupolar dyes with a D-�€-A-�€-D or  A-�€-D-�€-A  structure, where D and A are electron donor and acceptor groups, were shown to undergo symmetry breaking (SB) upon optical excitation. During this process, electronic excitation, originally distributed evenly over the molecule, concentrates on one D−Ã�€â€“A branch, and the molecule becomes dipolar. This process can be monitored by time-resolved infrared spectroscopy and causes significant spectral dynamics. A theoretical model of excited-state SB developed earlier (Ivanov, A. I. J. Phys. Chem. C2018,122, 29165–29172) is extended to account for the temporal changes taking place in the IR spectrum upon SB. This model can reproduce the IR spectral dynamics observed in the -C≡C- stretching region with a D-�€-A-�€-D dye in two polar solvents using a single set of molecular parameters. This approach allows estimating the degree of asymmetry of the excited state in different solvents and its change during SB. Additionally, the relative contribution of different mechanisms responsible for the splitting of the symmetric and antisymmetric -C≡C-  stretching bands, which are both IR active upon SB, can be determined.

A simple model has been developed to describe the symmetry-breaking of the electronic distribution of A_L–D–A_R type molecules in the excited state, where D is an electron donor and A_L and A_R are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed. At the same time, the electronic coupling between the two degenerate states, which correspond to the transferred charge being localised either on A_L or A_R, favours a quadrupolar excited state with equal amount of charge-transfer on both sides. Because of these counteracting effects, symmetry breaking is only feasible when the electronic coupling remains below a threshold value, which depends on the solvation energy and the Coulomb repulsion energy between the charges located on A_L and A_R. This model allows reproducing the solvent polarity dependence of the symmetry-breaking reported recently using time-resolved infrared spectroscopy.

A model for simulating the transient electronic absorption spectra of donor–acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed.

Terms of donor-acceptor complex states participating in transitions with the excitation of the CT1 and CT2 bands (vertical arrows). The dotted arrow is the radiationless transition. Dotted lines are vibrational sublevels of the ground electronic state. Dashed arrows show the directions of system relaxation before and after nonthermal transitions.

The charge recombination dynamics of excited donor−acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent. It appears that nearly 100% of the initially excited donor−acceptor complexes recombine in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state. Once all parameters of the model have been obtained, the influence of the dynamic solvent properties (solvent effect) and of the carrier frequency of the excitation pulse (spectral effect) on the charge recombination dynamics have been explored. The main conclusions are (i) the model provides a globally satisfactory description for the IDU/TCNE complex although it noticeably overestimates the spectral effect, (ii) the solvent effect is quantitatively well described for the PMB/TCNE and HMB/TCNE complexes but the model fails to reproduce their spectral effects, and (iii) the positive spectral effect observed with the HMB/TCNE complex cannot be described within the framework of two-level models and the charge redistribution in the excited complexes should most probably be taken into account.

The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.

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.

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.

The excitation of a charge transfer band by a laser pulse of finite duration and the ensuing charge recombination are calculated in the framework of the perturbation theory. The influence of the spectral characteristics of the laser pulse on the charge recombination dynamics is investigated for models including several nuclear modes that differ greatly in their timescales. It is shown that, in the area of applicability of the perturbation theory, the variation of the pulse carrier frequency inside the absorption band can significantly change the effective charge recombination rate constant.