Optical Spectroscopy of Single Crystal [Re(bpy)(CO)4](PF6): Mixing between Charge Transfer and Ligand Centered Excited States

The photochemical, photocatalytic, and luminescent properties of d6 transition metal complexes are dictated by the character of the lowest electronic transition. Observation of the lowest excited electronic state in single crystal [Re(bpy)(CO)4](PF6) by high resolution absorption spectroscopy at cryogenic temperatures allows assignment as a nominally 3LC transition with its electronic origin centered at 22 510 cm−1. Mixing between the ligand centered and charge transfer excitations in transition metal complexes can significantly effect the optical spectroscopy and decay processes of the complex. The 3LC state is weakly mixed (3%) with the lowest lying 1CT state (31 570 cm−1), resulting in the observation of metal-ligand vibrational side bands, a shortened luminescence decay (t = 33.0 μs), and a large zero-field splitting (ZFS) of the electronic origin (ZFS = 7.2 cm−1). These observations are interpreted using a mechanism in which the coupling arises from a large spin-orbit coupling matrix element (<|Hso|>) = 261 cm−1). The presence of a low lying ligand field state can be definitely ruled out from the observed absorption and luminescence behavior. The observed photochemical properties are likely due to the charge transfer character of the first excited state. © 1995, American Chemical Society. All rights reserved.