Disentangling the 1MLCT transition of [Ru(bpy)3]2+ by Stark absorption spectroscopy

Abstract

The metal-to-ligand charge transfer (MLCT) transition of [Ru(bpy)3]2+ was investigated using Stark absorption spectroscopy, where bpy is the abbreviation of 2,2′-bipyridyl ligand. The magnitude and direction of the photoinduced intramolecular charge transfer were precisely determined for the 1MLCT transition of [Ru(bpy)3]2+. The 1MLCT absorption band of [Ru(bpy)3]2+, observed in the 18,000–30,000 cm−1 spectral region, is composed of several sub-bands that can be approximated with Gaussian profiles. In particular, three distinct major 1MLCT bands of [Ru(bpy)3]2+ (g4, 21272 cm−1; g5, 22026 cm−1; g7, 23448 cm−1) could be distinguished by the direction of the charge transfer of each transition. The experimentally determined directions of charge transfer showed good agreement with the theoretical prediction by Kober and Meyer. We also re-examined the phosphorescence and the excitation spectra of [Ru(bpy)3]2+. The 1MLCT excited states of the g5 and g7 bands almost completely transform to 3MLCT excited states, and then 40% of the 3MLCT state relaxes to the ground state by emitting phosphorescence. 46% of 1MLCT excited state of the g4 band non-radiatively relaxes to the ground state. These results provide good support for the assignment of the different origins of the g4 and other two Gaussian sub-bands (g5 and g7).