Photoinduced Charge Separation Promoted by Ring Opening of a Piperazine Radical Cation

摘要

Metal complex dyads, M-D, comprise a transition metal chromophore (M) covalently linked to an organic electron donor (D). Photochemical excitation of these assemblies produces a charge-separated state, M~(._)—D~(+.) which has a lifetime that is controlled by the dynamics of the highly exothermic (inverted region) charge recombination reaction. In previous studies it has been shown that the lifetime of the charge-separated state in M—D assemblies increases with the driving force for charge recombination or the separation distance between M and D. Herein we report a new approach to increasing the lifetime of a charge-separated state in a metal complex dyad which relies on the reversible ring opening of the radical cation of a piperazine electron donor. Thus, metal complex dyad c-1 contains the (bpy)Re~I(CO)_3- (Re) chromophore covalently linked to a 2,3-diaryl-l,4-dimethylpiperazine electron donor. Intramolecular electron transfer from piperazine to photoexcited Re produces a charge-separated statein which Re is reduced and the piperazine is a cation radical. Carbon—carbon bond fragmentation in the piperazine cation radical produces a new charge-separated state in which the donor exists as an open-chain distonic cation radical. The net result is the production of a charge-separated state having a lifetime which is 5—10-fold longer than that observed in structurally related metal complex dyads.