INTRAMOLECULAR ENERGY-TRANSFER IN (DIIMINE)REI(CO)3-[CPMII(ARENE)] DIMERS

This report presents the photochemistry and photophysics of the series of dimeric complexes fac-(b)ReI(CO)3-[CpMII(arene)]2+, where b = a se ries of substituted 2,2'-bipyridine ligands, Cp = eta5-cyclopentadieny l, and M = Fe and Ru. The two metals are linked by 4-benzylpyridine, w hich is coordinated eta1 to Re(I) via the pyridyl nitrogen and eta6 to M(II) via the benzyl group. Near-UV photoexcitation of these complexe s produces the luminescent dpi (Re) --> pi (diimine) metal-to-ligand charge-transfer (MLCT) state. The MLCT state is quenched by intramolec ular exchange energy transfer (E(N)T) to the reactive 3d-d state of Cp M(arene). Evidence that E(N)T occurs is provided by the observation of the following reaction during Re --> diimine MLCT photoexcitation: (b )ReI(CO)3-[CPMII(arene)]2+ --> hv, CH3CN (b)ReI(CO)3(4-benzylpyridine) + + CpM(CH3CN)3+. The driving force for Re --> M E(N)T (DELTAE(E(N)T)) is varied by changing the energy of the Re --> diimine MLCT state and by varying M. For M = Ru, E(N)T is weakly endothermic, and for M = Fe , E(N)T is moderately exothermic. The rate of intramolecular E(N)T is estimated from the quantum efficiency for CpM(arene) loss from the dim ers and From MLCT emission lifetimes. The activation energy and freque ncy factor for E(N)T in the (b)Re-Ru series is determined from the tem perature dependence of the MLCT emission decay rate.