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

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexy l (2DA) substituted with an anilino electron donor and a dicyano-ethylene e lectron acceptor have been studied. Quenching of local donor emission is ob served for these compounds as well as quenching of the "pseudo-local" accep tor emission. Transient absorption spectra show dialkylanilino-type radical -cation and dicyanoethylene-type radical-anion absorptions. These results s how that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by lime-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. T ime-resolved fluorescence spectroscopy revealed that in the charge separate d state in cyclohexane for 2DA, molecular folding takes place on a nano-sec ond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) f olding takes place subsequent to charge separation. In addition to this dif ference in conformational behavior, the presence of the exocyclic double bo nd between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination ra te by a factor of 20.