DUAL ROLE OF THE UNEXCITED P-N,N-DIMETHYLAMINOBENZONITRILE (DMABN) MOLECULE IN PHOTOINDUCED ELECTRON-TRANSFER REACTIONS WITH DIFFERENT EXCITED-STATE (S-1) PARTNERS

Steady state and time correlated single photon counting techniques hav e 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 poten tial results indicate that DMABN may act as both electron acceptor or donor with pyrene. However, from the measured Delta G values it may be proposed that the possibility of occurrence of photoinduced ET reacti ons in which DMABN will serve as an acceptor with pyrene (which in tur n will behave as an electron donor) would be thermodynamically more fa vourable. With 9CNA or 9CNP, DMABN was found to behave as a donor. In the presence of DMABN significant quenching of the fluorescence emissi on of its interacting partners was noticed due to ET reaction. The obs erved larger quenching rate constants (k(q)) in the cases of pyrene-D MABN and 9CNP-DMABN systems relative to the corresponding value obtai ned 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 excit ed singlet state of the fluorescer molecule. In the case of ET reactio n between an excited cyano derivative acceptor (9CNA or 9CNP) and an u nexcited DMABN donor it seems that electron delocalization of the dono r might also be responsible, with Delta G, for determining the quenchi ng rate.