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 ((SFTIR)-F-2 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 v(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 Delta A band at similar to 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'-(CO2Et)(2)bpy as acceptor ligands in fac-[Re(4,4'-(CO2Et)(2)bpy)(CO)(3)(MQ+)](n+) (n = 1, 2).