Excited-state dynamics of heteroleptic copper(i) photosensitizers and their electrochemically reduced forms containing a dipyridophenazine moiety - a spectroelectrochemical transient absorption study.

The electrochemically singly-reduced Cu(i) photosensitizers of the type [Cu(xant)(N^N)]+ (with xant = xantphos ligand and N^N = bidentate diimine ligand: dipyrido[3,2-a:2',3'-c]phenazine = dppz or 3,6,11,12-tetramethyl-dipyrido[3,2-a:2',3'-c]phenazine = tmdppz) exhibit a metal-to-ligand charge transfer (MLCT) transition from the Cu(I) center to the reduced dppz˙- ligand. This special behavior makes them promising candidates for two-electron accumulation. Consequently, the photoinduced excited-state processes of [Cu(xant)(dppz)]+ (1) and [Cu(xant)(tmdppz)]+ (2) were investigated in solution by femtosecond transient absorption spectroelectrochemistry. Furthermore, the influence of the methyl substitution at the dppz ligand on the transient dynamics was revealed. Moreover, both singly-reduced species 1- and 2- possess short-lived excited states (10-20 ps) when excited into the MLCTphen or the low-lying states, representing an obstacle for the possible two-electron photoaccumulation.