Charge-Transfer Interactions in 4-Donor 4-Acceptor Substituted 1,1-Diphenylethenes

Two 1,1-diphenylethenes bearing either a 4-dimethylamino or 4-methoxy group as electron donor and a 4′-cyano group as electron acceptor, as well as compounds containing only a donor or acceptor functionality, were synthesized. The observation of strong fluorescence solvatochromism (originating from a biradicaloid twisted intramolecular charge-transfer state) of the donor-acceptor compounds reveals that photoinduced charge separation through the cross-conjugated 1,1-diphenylethene spacer occurs. The presence of weak charge-transfer absorption bands in the UV spectra reflects modest ground state donor-acceptor coupling. These results show that charge transport through a branched conjugation path is possible. 1-[4-(Dimethylamino)phenyl]-1-phenylethene was found to fluoresce from two different states. In nonpolar solvents the source is a local aniline-like state, whereas in polar solvents the fluorescence originates from a twisted intramolecular charge-transfer state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)