COMPARISON BETWEEN SIME(2) AND CME(2) SPACERS AS SIGMA-BRIDGES FOR PHOTOINDUCED CHARGE-TRANSFER

The potential of dimethylsilylene and isopropylidene sigma-spacers as bridges for photoinduced charge transfer (CT) in 4-cyano-4'-(dimethyla mino)- and 4-cyano-4'-methoxy-substituted diphenyldimethylsilanes and 2,2-diphenylpropanes was studied. Fluorescence solvatochromism and tim e-resolved microwave conductivity measurements show that upon photoexc itation a charge separated state (D(.+)sigma A(.-)) is populated in a ll compounds. Excited state dipole moments for a given donor-acceptor combination are, irrespective of the bridge, equal. The CT states of t he silanes are however lying at lower energies, implying that the pres ence of silicon thermodynamically facilitates the CT process. Cyclic v oltammetry data of model compounds show that this is a consequence of the lowering of the acceptor reduction potential by the silicon bridge . It was however inferred from radiative decay rates that the electron ic coupling between the CT and locally excited states as well as the c oupling between the ground and CT state is larger for the carbon-bridg ed compounds. As shown by both solution and solid state electronic spe ctra and radiative decay rates, the photophysics of the D sigma A comp ounds are dominated by intensity borrowing of the CT transitions from transitions localized in the D sigma and sigma A chromophores.