Intramolecular Energy Transfer in (diimine)ReI(CO)3-[CpMII(arene)] Dimers

Abstract

This report presents the photochemistry and photophysics of the series of dimeric complexes fac-(b)Re[sup I](CO)[sub 3[minus]] [CpM[sub II](arene)][sup 2+], where b = a series of substituted 2,2[prime]-bipyridine ligands, Cp = [eta][sup 5]-cyclopentadienyl, and M = Fe and Ru. The two metals are linked by 4-benzylpyridine, which is coordinated [eta][sup 1] to Re(I) via the pyridyl nitrogen and [eta][sup 6] to M(II) via the benzyl group. Near-UV photoexcitation of these complexes produces the luminescent d[pi] (Re) [yields] [pi][sup *] (diimine) metal-to-ligand charge-transfer (MLCT) state. The MLCT state is quenched by intramolecular exchange energy transfer (E[sub N]T) to the reactive [sup 3]d-d state of CpM(arene). Evidence that E[sub N]T occurs is provided by the observation of the following reaction during Re [yields] diimine MLCT photoexcitation: (b)Re[sub I](CO)[sub 3[minus]][CpM[sup II]-(arene)][sup 2+] [yields][sup hv,CH[sub 3]CN] (b)Re[sub I](CO)[sub 3](4-benzylpyridine)[sup +] + CpM(CH[sub 3]CN)[sub 3[sup +]]. The driving force for Re [yields] M E[sub N]T ([Delta]E[sub E[sub N]]T) is varied by changing the energy of the Re [yields] diimine MLCT state and by varying M. For M = Ru, E[sub N]T is weakly endothermic, and for M = Fe, E[sub N]T is moderately exothermic. The rate of intramolecular E[sub N]T is estimated from the quantum efficiencymore » for CpM(arene) loss from the dimers and from MLCT emission lifetimes. The activation energy and frequency factor for E[sub N]T in the (b)Re-Ru series is determined from the temperature dependence of the MLCT emission decay rate.« less