화학공학소재연구정보센터
Inorganic Chemistry, Vol.59, No.20, 14706-14715, 2020
Long-Lived Mixed (MLCT)-M-2/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 (MLCT)-M-4 <- (4)GS (ground-state) absorption band (epsilon(400-700 nm) = 900-8000 M-1 cm(-1)), the (MLCT)-M-4 state rapidly (tau(isc) < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (tau similar to 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 tau = 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 ((MC)-M-2) state following photoexcitation and lacks strong visible GS absorption character. (MLCT)-M-2 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 (MC)-M-2 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 (MLCT)-M-2 state at lower energy than the (MLCT)-M-4 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.