OPTICAL SPECTROSCOPY OF SINGLE-CRYSTAL [RE(BPY)(CO)(4)](PF6) - MIXINGBETWEEN CHARGE-TRANSFER AND LIGAND CENTERED EXCITED-STATES

The photochemical, photocatalytic, and luminescent properties of d(6) transition metal complexes are dictated by the character of the lowest electronic transition. Observation of the lowest excited electronic s tate in single crystal [Re(bpy)(CO)(4)](PF6) by high resolution absorp tion spectroscopy at cryogenic temperatures allows assignment as a nom inally (3)LC transition with its electronic origin centered at 22 510 cm(-1). Mixing between the ligand centered and charge transfer excitat ions in transition metal complexes can significantly effect the optica l spectroscopy and decay processes of the complex. The (3)LC state is weakly mixed (3%) with the lowest lying (CT)-C-1 state (31 570 cm(-1)) , resulting in the observation of metal-ligand vibrational side bands, a shortened luminescence decay (tau = 33.0 mu s), and a large zero-fi eld 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 ([/H-50/] = 261 cm(-1)). The presence of a low lying ligand field state can be defini tely ruled out from the observed absorption and luminescence behavior. The observed photochemical properties are likely due to the charge tr ansfer character of the first excited state.