Probing Interfacial Electron Transfer in Coumarin 343 Sensitized TiO2 Nanoparticles with Femtosecond Stimulated Raman

Femtosecond Stimulated Raman Spectroscopy (FSRS) is used as a structural probe of the interfacial electron transfer between Coumarin 343, a solar cell sensitizer, and TiO2 nanoparticles. Resonance Raman spectra of the charge-transfer complex reveals multiple vibrational motions, consisting primarily of carbon backbone and Ti-O binding modes, that direct the molecule out of the Franck-Condon region on the excited state potential energy surface. The vibrational spectrum and decay dynamics of the radical cation are also reported. These spectra indicate that the hole is localized on the conjugated carbon rings and that the donation mechanism involves multiple vibrational motions. We conclude with a discussion of prospects for improving the solar cell sensitization properties of Coumarin 343 through structural modification.