Solvent effects nonexponential kinetics

An unusual feature is that the luminescence intensity and lifetime of 12b increases substantially in nonpolar solvents relative to the lifetime in polar solvents. Moreover, as the emission lifetime increases, the decay kinetics become distinctly nonexponential. The explanation for the unusual solvent-dependent luminescence properties of 12b is that in a low dielectric solvent the MLCT and charge separated states (14 and 15, respectively, in Scheme 8) are similar in energy and equilibrium is established between the two states. In polar solvents 15 is stabilized with respect to 14, and photoinduced ET is irreversible. Similar, but attenuated, solvent dependent luminescence is observed for 12c and 12d. The effect is attenuated in these complexes because the amine donors are easier to oxidize and thus the charge separated state is stabilized with respect to the MLCT state. 

The rebinding dynamics to microperoxidase are nonexponential due to a solvent cage effect. To deduce the time constant for rebinding CO from the solvent cage, the recombination kinetics were modeled according to... 

Chemical kinetics is all about bottlenecks. They determine the reaction mechanism on different timescales. We have seen that ET can be hmited by nonadiabatic transitions, solvent dynamics, intramolecular vibrational relaxation, translational diffusion, and conformational fluctuations either of the reactants themselves or of the embedding medium. The interplay between different mechanisms as well as nonequilibrium initial condition may result in rich kinetic behaviors, with strong nonexponentiality and coherence effects observed in recent experiments. 

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