Femtosecond Raman structural studies of interfacial electron transfer in Grätzel cell sensitizers

Femtosecond stimulated Raman spectroscopy(FSRS) is used to investigate the excited state structural changes in Gratzel cell sensitizers in solution and in titania colloids to reveal the vibrational motions and structural properties that promote efficient electron transfer. FSRS studies of Fe(tren(py)₃)²⁺ (Smeigh, A. L., et al., JACS, 130, 14105-14107, 2008) reveal a 190 fs formation time for the quintet state, proving that these relaxation process occur on a timescale which competes with electron injection. Second, we directly examined the electron transfer process in dye-sensitized TiO₂ nanoparticles (Frontiera, R. R. et al., JACS, in press). Resonance Raman spectra of the ground state of Coumarin-343 sensitized nanoparticles show high Franck-Condon activity in C-C stretching and breathing modes. Excited state spectra monitor the formation of the radical cation structure; the dominant single C=C stretching mode indicates that the hole resides throughout the carbon backbone. This suggests that extending Coumarin's conjugated ring system could result in higher efficiency solar cells with slower back electron transfer processes. Future work will include studies on charge-transfer complexes with varying electron injection times.