Franck-Condon state evaluation in metal-to-ligand charge transfer chromophores: Influence of electronic structure, ligand structure, and solvent environment on photo-induced excited state dynamics

The study of photophysical and photochemical transformations in transition metal chromophores has been important for understanding excited-state relaxation processes in electronically complex molecular systems. While a great deal is known about the reactivity of the thermalized excited states of photo-active species, relatively little is known about the primary processes leading to the formation of these states. This dissertation work focuses on the study of primary processes in metal-to-ligand charge transfer (MLCT) complexes of ruthenium (II). Specific attention is paid to the roles that ligand structure and solvent environment play in determining relaxation dynamics.; The starting place for this work is a comparison of the photo-physical properties of [Ru(dmb)₃]2+ and [Ru(dpb)₃] 2+, where dmb = 4,4′-dimethyl-2,2′ -bipyridine and dpb = 4,4′-diphenyl-2,2 ′-bipyridine. A model is proposed that the 3MLCT excited state of [Ru(dpb)₃]2+ is characterized by extensive electron delocalization on aryl substituents leading to differences in excited-state properties in comparison to [Ru(dmb)₃] 2+.; Spectroscopic measurements, structural data, and theoretical efforts to study ligand structure suggest that intraligand electron delocalization is a time-dependent process requiring aryl-ring rotation in the primary relaxation dynamics. Extensive experimental efforts are then made to identify and characterize the excited-state conformational and electronic evolution in these systems.; The initial studies of ultrafast kinetic processes described herein focus on [Ru(dpb)₃]2+ and [Ru(dmb)₃] 2+ in acetonitrile solution at room temperature. A rise signal is observed at λ_probe = 532nm for [Ru(dpb)₃] 2+ following ∼120fs excitation at 400nm. This signal, whose evolution is complete within ∼2ps, is assigned as reflecting the process and time scale of ring-rotation concomitant with electron delocalization. Decay kinetics (∼5ps) observed for [Ru(dmb)₃]2+ at this same probe wavelength are assigned as reflecting the time-scale for vibrational relaxation in these types of chromophores. (Abstract shortened by UMI.)...