DESIGNED INTRAMOLECULAR COMPETITION IN A CHROMOPHORE-BIQUENCHER COMPLEX

Competitive energy and electron transfer occur following excitation of the chromophore-biquencher CH(2)An)(CO)(3)(MQ(+))](PF6)(2)(MebpyCH(2) OCH(2)An is -anthrylmethoxy)methyl]-4'-methyl-2,2'-bipyridine; MQ(+) i s N-methyl-4,4',-bipyridinium ion). Following excitation with a 392-nm , 4-ns laser pulse in 1,2-dichloroethane (DCE), the initial excited st ate energy is distributed in a similar to 2:1 ratio between (3)An and the lower lying Re-II(MQ(.)) metal-to-ligand charge transfer (MLCT) e xcited state. The appearance of the two states was monitored by transi ent absorption spectroscopy, which detected the triplet-triplet band f or (3)An at lambda(max) = 420 nm (tau = 35 +/- 2 mu s, k = (2.9 +/- 0 .2) x 10(4) s(-1)) and a pi --> pi band of MQ(.) at 610 nm (tau = 37 +/- 2 ns, k = (2.7 +/- 0.2) x 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 th e shorter lived, higher energy Re-II(MQ) state.