Direct observation of the ultrafast intersystem crossing in tris(2,2′-bipyridine) ruthenium(II) using femtosecond stimulated Raman spectroscopy

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is used to explore the ultrafast intersystem crossing between the metal-to-ligand charge-transfer (MLCT) states of tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)(3)(2+)). Excitation at 480nm by a similar to 35 fs actinic pump pulse initiates electron transfer from the metal to the bipyridine ligands and the subsequent changes in the vibrational structure of the ligands are probed by FSRS with high spectral (10 cm(-1)) and temporal (70 fs) resolution. The unique Raman spectral features of the (MLCT)-M-3 state of Rudbpy(3)(2+) appear with rise times from 100 to 130 fs. An upper limit for the initial (MLCT)-M-1 state lifetime of 530 fs is determined by analysis of the spontaneous emission spectra and quantum yield. The ultrashort lifetime of the (MLCT)-M-1 state is attributed to fast Franck-Condon vibrational and solvent relaxation of the excited singlet state into near degeneracy with the triplet state, leading to fast and efficient intersystem crossing.