Long-Lived Mixed 2 MLCT/MC States in Antiferromagnetically Coupled d 3 Vanadium(II) Bipyridine and Phenanthroline Complexes.

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 ⁴ MLCT ← ⁴ GS (ground-state) absorption band (ε _400-700 nm = 900-8000 M ^-1 cm ^-1 ), the ⁴ MLCT state rapidly (τ _isc < 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 ( ² MC) state following photoexcitation and lacks strong visible GS absorption character. ² MLCT 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 ² MC 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 ² MLCT state at lower energy than the ⁴ MLCT 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.