Photosolvolysis of cis-[Ru(α-diimine)2(4-aminopyridine)2]2+ Complexes: Photophysical, Spectroscopic, and Density Functional Theory Analysis

The photochemical and photophysical properties of the cis-[Ru-II(alpha-diimine)(2)(4-APy)(2)](2+) complexes, where alpha-diimine = 1,10-phenanthroline (phen) and 4-APy = 4-aminopyridine I, 4,7-diphenyl-1,10-phenanthroline (Ph(2)phen) II, 2,2'-bipyridine (bpy) III, and 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy) IV, are reported. The four complexes were characterized using high-performance liquid chromatography, H-1 NMR, UV-visible, emission, and transient absorption spectroscopy. Upon photolysis in acetonitrile solution these complexes undergo 4-APy dissociation to give the monoacetonitrile complex (for II, III, and IV) or the bis(acetonitrile) complex (for I). A fairly wide range of excitation wavelengths (from 420 to 580 nm) were employed to explore the photophysics of these systems. Quantum yields and transient spectra are provided. Density functional theory (DFT) and time-dependent DFT analysis of singlet and triplet excited states facilitated our understanding of the photochemical behavior. A detailed assessment of the geometric and electronic structures of the lowest energy spin triplet charge transfer state ((MLCT)-M-3) and spin triplet metal centered state ((MC)-M-3) (d pi -> sigma* transitions) for species I-IV is presented. A second, previously unobserved, and nondissociative, (MC)-M-3 state is identified and is likely involved in the primary step of photodissociation. This new (MC)-M-3 state may indeed play a major role in many other photodissociation processes.