Non-equilibrium effects in ultrafast photoinduced charge transfer kinetics

[Display omitted] •Influence of intramolecular vibrations and non-equilibrium solvent on the free energy gap law.•Charge transfer from the second excited electronic state.•The factors controlling ultrafast non-equilibrium charge transfer. Modern laser-based spectroscopy has provided methods for detection ultrafast photochemical transformations occurring on the timescale of intramolecular and solvent reorganization. Such processes usually proceed in non-equilibrium regime, in parallel with nuclear relaxation, and often manifest strong deviations from the Kasha–Vavilov rule. In particular, they offer a possibility to control the yield of photoinduced electron transfer (ET) by using different excitation wavelengths. In the last decade the non-equilibrium charge transfer (CT) processes have attracted considerable interest from the scientific community due to their determining role in photosynthesis, dye-sensitized solar cells and various molecular electronic devices. Non-equilibrium of nuclear (intramolecular and solvent) degrees of freedom can be created by a pump pulse or by photoreaction itself at some of its stages. In this review both situations are considered and illustrated by examples in which non-equilibrium effects are pronounced. It is shown that ultrafast charge recombination in photoexcited donor–acceptor complexes and photochemical processes in donor–acceptor1–acceptor2 molecular compounds proceed predominantly in non-equilibrium (hot) regime. It is important that kinetics and product yields of these reactions demonstrate regularities that considerably differ from that observed in thermal reactions. Among them, the lack of the Marcus normal region in the free energy gap law for charge recombination of the excited donor–acceptor complexes, extremely low quantum yields of the thermalized charge separated states in ultrafast CT from the second excited state of the donor are most known. Although there have been many efforts to clarify microscopic mechanisms of non-equilibrium photoreactions by using ultrafast time-resolved spectroscopy techniques, control of the rate and efficiency of photoinduced charge transfer reactions is still an open challenge. One of the most important applications here is a suppression of ultrafast charge recombination in CT systems, formed either by direct optical excitation or by the preceding ET step. In these systems charge recombination is often regarded as undesirable process, leading to the loss of energy and selectivi