Resonance Raman evidence for photogeneration of a class II mixed-valence excited state of a binuclear copper(I) complex in solution

Abstract

Pulsed laser resonance Raman (RR) studies of mono- and bin-nuclear complexes of Cu(I) with the bridging ligand 2,3-di-(2-pyridyl)quinoxaline (dpq) and triphenyl phosphine (Ph 3P) are reported. Laser flash photolysis at 416 nm or 532 nm in CHCl 3 solution of the mononuclear complex, [(Ph 3P) 2Cu(dpq)] + ( 1) produced a transient with a lifetime of 37 ± 2 ns (20°C), attributable to the lowest energy metal ligand (dpq) charge-transfer excited state of ( 1). Under similar conditions in CH 2Cl 2 solution, the binuclear complex, [(Ph 3P) 2Cu(dpq)Cu(PPh 3) 2] 2+ ( 2), resulted in a weak transient absorbance signal, convoluted with the laser pulse, indicative of an excited state with a lifetime less than the pulsed duration (circa 8 ns). Pulsed RR spectra generated at 416 nm of the excited state of ( 1) exhibited features attributable to the dpq − radical anion. The much shorter-lived excited state of the binuclear complex ( 2) exhibited a virtually identical RR spectrum suggesting that it is a class II mixed valence species, [(Ph 3P) 2Cu I(dpq − )Cu II(PPh 3) 2] 2+, in which the dpq ligand does not strongly couple the two metal sites.