Oligo(Cyclohexylidene)S And Oligo(Cyclohexyl)S As Bridges For Photoinduced Intramolecular Charge Separation And Recombination

A series of semirigid donor-bridge-acceptor (D-B-A) molecules was synthesized to study the effect of the position and number of nonconjugated olefinic bonds in the bridge on the photoinduced charge-separation and charge-recombination kinetics. The molecules consist of a phenylpiperidine electron donor, an oligo(cyclohexylidene) or oligo(cyclohexyl) bridge, and a dicyanovinyl acceptor. Partly saturated ter(cyclohexylidene) bridges were used as well. The edge-to-edge donor-acceptor separation of the compounds under study varies between 2.89 and 15.4 Angstrom. The replacement of a C-C single bond by an olefinic bond increases the rate of charge separation with a factor of 3.0 +/- 0.8 per replaced bond. For all D-B-A compounds the extended fully charge-separated state folds to a compact charge-transfer (CCT) conformer. The rate of charge recombination (CR) of the CCT state increases with solvent polarity for those compounds having an olefinic bond located three sigma bonds from the acceptor. Thus, while in cyclohexane the CR rate is equal for all compounds, in benzene the CR rate in compounds with an olefinic bond near the acceptor is 10 times larger than in compounds with a single bond instead. It is believed that a (virtual) charge-transfer state involving the radical cation of the olefinic bond and the radical anion of the acceptor (D-B.+-A(.-)) is responsible for the enhanced CR process.