Dual role of the unexcited p- N, N-dimethylaminobenzonitrile (DMABN) molecule in photoinduced electron transfer reactions with different excited state (S 1) partners

Abstract

Steady state and time correlated single photon counting techniques have been used to reveal the nature of the dual role of ground state p- N, N-dimethylaminobenzonitrile (DMABN) in photoinduced electron transfer (ET) reactions with different excited state (S 1) partners e.g. pyrene, 9-cyanoanthracene (9CNA) and 9-cyanophenanthrene (9CNP). Redox potential results indicate that DMABN may act as both electron acceptor or donor with pyrene. However, from the measured ΔG values it may be proposed that the possibility of occurrence of photoinduced ET reactions in which DMABN will serve as an acceptor with pyrene (which in turn will behave as an electron donor) would be thermodynamically more favourable. With 9CNA or 9CNP, DMABN was found to behave as a donor. In the presence of DMABN significant quenching of the fluorescence emission of its interacting partners was noticed due to ET reaction. The observed larger quenching rate constants ( k q ) in the cases of pyrene ∗-DMABN and 9CNP ∗-DMABN systems relative to the corresponding value obtained in the 9CNA ∗-DMABN pair have been ascribed to the presence of two processes; one is ET and the other is intersystem crossing (ISC) which is influenced by the formation of stable exciplex in the lowest excited singlet state of the fluorescer molecule. In the case of ET reaction between an excited cyano derivative acceptor (9CNA or 9CNP) and an unexcited DMABN donor it seems that electron delocalization of the donor might also be responsible, with ΔG, for determining the quenching rate.