INVESTIGATIONS OF EXCITED-STATE QUENCHING REACTIONS BETWEEN P-TOLUIDINE, ITS N,N-DIMETHYL DERIVATIVE AND THE ACCEPTOR DIMETHYLTEREPHTHALATEAT DIFFERENT TEMPERATURES

Electronic absorption and steady state and time-resolved emission spec troscopy were used to investigate the occurrence of several nonradiati ve processes, e.g. excitational energy transfer, photoinduced electron transfer, etc., between the electron donors p-toluidine (p-tol) and i ts N,N-dimethyl derivative (N,N-DM-p-tol) and the well known acceptor dimethylterephthalate (DMTP) in solvents of different polarity at 296 K and in ethanol (EtOH) rigid glassy matrix at 77 K. From the room tem perature experimental results, the strong dynamic fluorescence quenchi ng phenomena observed for the toluidine donors in the presence of the acceptor DMTP may be due mainly to the combined effect of two non-radi ative processes: (1) photoinduced electron transfer from the excited ( S-1) donor to the ground state acceptor; (2) enhanced intersystem cros sing (isc) within the donor as the S-1 state acquires n pi character due to conformational changes in the presence of DMTP. Lack of spectra l overlap between the donor emission and acceptor absorption excludes the possibility of the occurrence of singlet-singlet Forster-type ener gy transfer within the reactants. In addition, no static quenching was observed. At 77 K, the isc process seems to be primarily responsible for the observed decrease in the donor fluorescence emission band in t he presence of DMTP, whereas triplet-triplet (T-1(D) curved right arro w T-1(A)) energy transfer appears to be responsible for donor phosphor escence quenching. (C) 1998 Elsevier Science S.A.