Photoinduced intramolecular charge separation in donor/acceptor-substituted bicyclohexylidene and bicyclohexyl

The photophysical properties of a bicyclohexylidene (1 DA) and a bicyclohexyl (2 DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the “pseudo-local” acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1 DA and 2 DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1 DA and 2 DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2 DA, molecular folding takes place on a nanosecond timescale. For 1 DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.