Excited-state intramolecular charge transfer in donor acceptor-substituted aromatic hydrocarbons and in biaryls The significance of the redox potentials of the D/A subsystems

TLDR: In this article, the authors show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups.

Abstract: The energy of the charge transfer (CT) emission maximum of a series of dual fluorescent 4-aminobenzonitriles in diethyl ether and acetonitrile does not show a correlation with the redox potentials of the amino (D) and benzonitrile (A) subgroups. It is therefore concluded that these electron donor and acceptor subgroups cannot be treated in the same way as the A and D molecules in exciplexes 1(A−D+). This means that in the intramolecular charge transfer (ICT) state of the aminobenzonitriles, a considerable electronic coupling between the A− and D+ parts exists, in contradiction with the twisted intramolecular charge transfer (TICT) hypothesis. For 9,9′-bianthryl, 10-cyano-9,9′-bianthryl and 10,10′-dicyano-9,9′-bianthryl, photostationary and time-resolved measurements are presented, which show that the influence of the cyano substituents on the ICT reaction in the excited state can be understood by considering the redox potentials of the anthryl groups. The introduction of an additional cyano group in an aminobenzonitrile, however, leads to the disappearance of ICT and dual fluorescence, indicating that the photophysics of these molecules cannot be understood on the basis of the redox potentials of the D and A constituents.