Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha-Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis-trans isomerizations, respectively.

Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equilibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics.

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.

Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 10(8) and 3.2 ± 0.2 × 10(8) M(-1) s(-1), in toluene and acetonitrile, respectively. This is ~1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge-O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.

The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate