Long-Lived Mixed <sup>2</sup>MLCT/MC States in Antiferromagnetically Coupled d<sup>3</sup> Vanadium(II) Bipyridine and Phenanthroline Complexes
Ryan D. Dill, Romeo I. Portillo, Samuel G. Shepard, Matthew P. Shores, Anthony K. Rappé, Niels H. Damrauer
Abstract
Exploration of [V(bpy)3]2+ and [V(phen)3]2+ (bpy = 2,2′-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4MLCT ← 4GS (ground-state) absorption band (ε400–700 nm = 900–8000 M–1 cm–1), the 4MLCT state rapidly (τisc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5–3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)3]2+ and [V(phen)3]2+, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)3]3+, which rapidly forms a long-lived doublet metal-centered (2MC) state following photoexcitation and lacks strong visible GS absorption character. 2MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)3]3+, where the GS and 2MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2MLCT state at lower energy than the 4MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal–ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.