Electron transfer processes charge recombination lifetimes

Fig. 16.8 Charge recombination lifetimes in the compounds shown in the inset in dioxane solvent. (J. M. Warman, M. P. de Haas, J. W. Verhoeven, and M. N. Paddon-Row, Adv. Chem. Phys. 106, Electron transfer—from isolated molecules to bio-molecules, Part I, edited by J. JortnerandM. Bixon (Wiley, New York, 1999). The technique used is time-resolved microwave conductivity (TRMC), in which the change in dielectric response of a solution is monitored following photoinduced electron transfer—a charge separation process that changes the solute molecular dipole. The lifetimes shown as a function of bridge length (number of a-bonds separating the donor and acceptor sites in the compounds shown in the inset) are for the back electron transfer (charge recombination) process.

The lifetimes of the final charge-separated states in the pentads are far longer than those observed for the simpler molecular devices discussed above, but are substantially shorter than would be expected for charge recombination via direct electron transfer from the benzoquinone to the carotenoid radical cation, based on results for related species. This may well signify that charge recombination occurs via a multistep pathway as was observed for some of the triad species discussed above. Additional studies will be necessary to elucidate the details of the various charge separation and charge recombination processes in the pentads. 

---