Photoinduced organic donor to metal electron transfer across a rigid spacer

Two compounds (b)ReCH-DTF, which contain the metal complex chromophore (b)Re{sup I}(CO){sub 3} (where b = 2,2{prime}-bipyrazine or 5,5{prime}-bis(N,N-diethylcarbamido)-2,2{prime}-bipyridine) covalently linked to a 1,3-benzodithiafulvene (DTF) electron donor via a trans-1,4-cyclohexane (CH) spacer, have been prepared and studied by emission and transient absorption spectroscopy. Steady-state and time-resolved emission data indicate that the metal-to-ligand charge-transfer (MLCT) excited state of the (b)ReCH-DTF complexes is quenched compared to the model compounds (b)ReCH which contain the (b)Re{sup I}(CO){sub 3} chromophore but not the DTF donor. The MLCT quenching is ascribed to a rapid intramolecular DTF {yields} Re electron transfer (ET) which competes with normal radiative and nonradiative MLCT decay. The occurrence of intramolecular ET is confirmed by nanosecond transient absorption studies which reveal that the charge-transfer state, (b{sup {sm bullet}{minus}})Re{sup I}(CO){sub 3}-CH-DTF{sup {sm bullet}+}, is formed rapidly following photoexcitation.