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 A. 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 σ bonds from the acceptor. Thus, while in cyclohexane the...