STUDIES ON QUENCHING REACTIONS IN THE EXCITED ELECTRONIC STATES OF TETRAHYDRONAPHTHOLS BOTH AT THE AMBIENT-TEMPERATURE AS WELL AS AT 77 K

The present investigation was carried out at the ambient temperature a s well as at 77 K both by steady-state and time-resolved luminescence techniques to reveal the mechanisms of non-radiative transitions of th e lowest excited singlet (S-1) and triplet (T-1) states of tetrahydron aphthol donors 5,6,7,8-tetrahydro-1-naphthol (THN1OH) and 5,6,7,8-tetr ahydro-2-naphthol (THN2OH) in presence of the electron acceptor 2-nitr ofluorene (2NF) in solvents of different polarity. At room temperature large negative values of Delta G(0) (similar to-1.9 eV), Gibbs free e nergy for photo-induced electron transfer (ET) reaction, indicate high ly exothermic ET reaction occurs within the present donor-acceptor sys tems possibly in the Marcus inverted region. At this temperature evide nce of concurrent occurrence of Forster's-type singlet-singlet (S-1(D) -->S-1(A)) energy transfer process is also found. In presence of the a cceptor 2NF, the fluorescence of the donor molecule is strongly quench ed and the large dynamic (as there is absence of ground-state complex) fluorescence quenching rate constant k(q), of the order of 10(12) M-1 s(-1) is observed. This quenching has been ascribed mainly due to two concurrent processes: non-radiative energy transfer and ET. Radiative energy transfer is found to have negligible role. In 2NF, apart from the quenching in fluorescence spectra of the present donor molecules a longer wavelength band of broad nature was observed due to formation of contact charge transfer (CT) complex in non-polar solvent. In highl y polar acetonitrile (ACN) this band was absent but instead another br oad band at a far longer wavelength region due to formation of donor a nion (confirmed from metallic sodium experiment and excitation spectra l was found. It was suggested that this anion was produced from its in itially formed cationic species, resulted from ET reaction with 2NF. T he reaction scheme showing this mechanism is given. At 77 K it seems t he Forster mechanism of singlet-singlet energy transfer is still opera tive resulting the observed fluorescence quenching of the donor in pre sence of the acceptor 2NF. A reaction mechanism has been proposed for the observed lowering of donor phosphorescence intensity in presence o f the acceptor. The possibility of the occurrence of photo-induced ET reaction between excited (T-1) donor and ground-state acceptor is hint ed from time-resolved phosphorescence measurements and thermodynamic c onsiderations.