Photoinduced Electron Transfer in Naphthalimide-Pyridine Systems: Effect of Proton Transfer on Charge Recombination Efficiencies

We studied the effect of proton-coupled electron transfer on lifetimes of the charge-separated radicals produced upon light irradiation of the thiomethyl-naphthalimide donor SMe-NI-H in the presence of nitro-cyano-pyridine acceptor (NO2-CN-PYR). The dynamics of electron and proton transfer were studied using femtosecond pump-probe spectroscopy in the UV/vis range. We find that the photoinduced electron transfer between excited SMe-NI-H and NO2-CN-PYR occurs with a rate of 1.1 x 10(9) s(-1) to produce radical ions SMe-NI-H center dot+ and NO2-CN-PYR center dot-. These initially produced radical ions in a solvent cage do not undergo a proton transfer, possibly due to unfavorable geometry between N-H proton of the naphthalimide and aromatic N-atom of the pyridine. Some of the radical ions in the solvent cage recombine with a rate of 2.3 x 10(10) s(-1), while some escape the solvent cage and recombine at a lower rate (k = 4.27 x 10(8) s(-1)). The radical ions that escape the solvent cage undergo proton transfer to produce neutral radicals SMe-NI center dot and NO2-CN-PYR-H-center dot. Because neutral radicals are not attracted to each other by electrostatic interactions, their recombination is slower that the recombination of the radical ions formed in model compounds that can undergo only electron transfer (SMe-NI-Me and NO2-CN-PYR, k = 1.2 x 10(9) s(-1)). The results of our study demonstrate that proton-coupled electron transfer can be used as an efficient method to achieve long-lived charge separation in light-driven processes.