Electronic Structure in Pyridinium-Based Metal-to-Ligand Charge-Transfer Excited States by Step-Scan FTIR Time-Resolved Spectroscopy

Step-scan FTIR time-resolved spectroscopy (S 2 FTIR TRS) has been applied to a series of Re I and Os II polypyridyl complexes that contain N -methyl-4,4‘-bipyridinium (MQ + ) cation as an electron acceptor. Changes in ν(CO) and ring-stretching vibrations of the acceptor ligands in the fingerprint region provide direct insight into electronic structure both in metal-to-ligand charge transfer (MLCT) excited states and 1-electron reduced complexes. The measurements reveal: (1) changes in metal−ligand back-bonding between the ground and excited state, (2) special characteristics of MQ + as an acceptor ligand, including the appearance of an intense Δ A band at ∼1610 cm -1, which appears to be vibronically coupled to the internal charge distribution of the reduced ligand, and (3) elucidation of a competition that exists between MQ + and 4,4‘-(CO 2 Et) 2 bpy as acceptor ligands in fac- [Re(4,4‘-(CO 2 Et) 2 bpy)(CO) 3 (MQ + )] n + ( n = 1, 2).