PHOTOINDUCED GIANT CHARGE-SEPARATED STATES IN A NEW SERIES OF COMPLETELY RIGID COVALENTLY-LINKED TRIADS

Preliminary fluorescence and time-resolved microwave conductivity (TRMC) measurements are reported for the novel triad, DMA[4]DMN[8]DCV, in which the three chromophores, DMA (N,N-dimethylaniline), DMN (dimethoxynaphthalene), and DCV (dicyanovinyl), are connected via bridges comprising linearly fused norbornyl and bicyclo[2.2.0]hexyl units, four and eight bonds in length. The triad, DMA[4]DMN[8]DCV, exists in two noninterconvertible syn and anti diastereomeric forms, the former having a U-shaped geometry and the latter having an S-shaped geometry. Samples of the two stereoisomers, labeled H and L, were obtained, but the actual stereochemistry, syn or anti, of each isomer is presently unknown. Time-resolved fluorescence studies in benzene reveal that photoinduced electron transfer (ET) in the model dyad, [2]DMN[8]DCV, to generate the charge-separated (CS) species, [2]DMN+[8]DCV-, occurs rapidly (3.3 X 10(10) s-1) and with near unit efficiency. Similar rates of quenching were found for triads H- and L-DMA[4]DMN[8]DCV. TRMC studies on the flash photolysis of H-DMA[4]DMN[8]DCV in benzene suggest formation of the giant CS state, DMA+[4]-DMN[B]DCV-, with near unit efficiency. Intriguingly, the L-DMA[4]DMN[8]DCV diastereomer gave little or no TRMC signal following flash photolysis. This observation is tentatively explained in terms of a rapid benzene solvent-mediated charge-recombination process in the L-isomer which is assumed to possess the syn, or U-shaped, geometry. This geometry should encourage congregation of benzene molecules in the molecular cavity spanning the DMA and DCV chromophores.