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

in: Physical Chemistry Chemical Physics (2019)
Zhang, Ying; Zedler, Linda; Karnahl, Michael; Dietzek, Benjamin
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:20,30-c]phenazine = dppz or 3,6,11,12-tetramethyl-dipyrido[3,2-a:20,30-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.

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