Studies on quenching reactions in the excited electronic states of tetrahydronaphthols both at the ambient temperature as well as at 77 K

Abstract

The present investigation was carried out at the ambient temperature as well as at 77 K both by steady-state and time-resolved luminescence techniques to reveal the mechanisms of non-radiative transitions of the lowest excited singlet (S 1) and triplet (T 1) states of tetrahydronaphthol donors 5,6,7,8-tetrahydro-1-naphthol (THN1OH) and 5,6,7,8-tetrahydro-2-naphthol (THN2OH) in presence of the electron acceptor 2-nitrofluorene (2NF) in solvents of different polarity. At room temperature large negative values of ΔG ° (~ −1.9 eV), Gibbs free energy for photo-induced electron transfer (ET) reaction, indicate highly exothermic ET reaction occurs within the present donor-acceptor systems possibly in the Marcus inverted region. At this temperature evidence of concurrent occurrence of Förster's-type singlet-singlet (S D 1→S A 1) energy transfer process is also found. In presence of the acceptor 2NF, the fluorescence of the donor molecule is strongly quenched 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 −1s −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 highly polar acetonitrile (ACN) this band was absent but instead another broad band at a far longer wavelength region due to formation of donor anion (confirmed from metallic sodium experiment and excitation spectra) was found. It was suggested that this anion was produced from its initially formed cationic species, resulted from ET reaction with 2NF. The reaction scheme showing this mechanism is given. At 77 K it seems the Förster mechanism of singlet-singlet energy transfer is still operative resulting the observed fluorescence quenching of the donor in presence of the acceptor 2NF. A reaction mechanism has been proposed for the observed lowering of donor phosphorescence intensity in presence of the acceptor. The possibility of the occurrence of photo-induced ET reaction between excited (T 1) donor and ground-state acceptor is hinted from time-resolved phosphorescence measurements and thermodynamic considerations.