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

Abstract

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) (). Excitation at 480 nm by a ∼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 ³MLCT state of appear with rise times from 100 to 130 fs. An upper limit for the initial ¹MLCT state lifetime of <30 fs is determined by analysis of the spontaneous emission spectra and quantum yield. The ultrashort lifetime of the ¹MLCT 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.

Keywords:

• Tris(2,2′-bipyridine)ruthenium(II)
• Femtosecond stimulated Raman spectroscopy
• Metal-to-ligand charge transfer
• Intersystem crossing
• Photosensitizer dynamics

Acknowledgements

We gratefully acknowledge the support of the National Institutes of Health (EY02051) in setting up the FSRS apparatus and the Mathies Royalty Fund for supporting this research. We thank Dr. In Su Lee for providing Ru(bpy)₃(PF₆)₂ for the experiments in CH₃CN and C₃H₇CN.