Designed Intramolecular Competition in a Chromophore−Biquencher Complex

Competitive energy and electron transfer occur following excitation of the chromophore−biquencher [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 (MebpyCH2OCH2An is 4-[(9-anthrylmethoxy)methyl]-4‘-methyl-2,2‘-bipyridine; MQ+ is N-methyl-4,4‘,-bipyridinium ion). Following excitation with a 392-nm, 4-ns laser pulse in 1,2-dichloroethane (DCE), the initial excited state energy is distributed in a ∼2:1 ratio between 3An* and the lower lying ReII(MQ·) metal-to-ligand charge transfer (MLCT) excited state. The appearance of the two states was monitored by transient absorption spectroscopy, which detected the triplet−triplet band for 3An* at λmax = 420 nm (τ = 35 ± 2 μs, k = (2.9 ± 0.2) × 104 s-1) and a π → π* band of MQ• at 610 nm (τ = 37 ± 2 ns, k = (2.7 ± 0.2) × 107 s-1). These states result from a competitive partitioning in the initial excited state or states. They do not interconvert to any significant degree on the time scale of the shorter lived, higher energy ReII(MQ*) state.