Designed Intramolecular Competition in a Chromophore−Biquencher Complex

Competitive energy and electron transfer occur following excitation of the chromophore−biquencher [Re I (MebpyCH 2 OCH 2 An)(CO) 3 (MQ + )](PF 6 ) 2 (MebpyCH 2 OCH 2 An 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 3 An* and the lower lying Re II (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 3 An* at λ max = 420 nm (τ = 35 ± 2 μs, k = (2.9 ± 0.2) × 10 4 s -1 ) and a π → π* band of MQ • at 610 nm (τ = 37 ± 2 ns, k = (2.7 ± 0.2) × 10 7 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 Re II (MQ*) state.