Photophysical properties of some methylindoles and studies on quenching reactions in their excited singlet and triplet states in presence of the electron acceptor 2-nitrofluorene at 296 K as well as at 77 K

Abstract

Electronic absorption, steady state and time resolved, in the domain of nanosecond order, luminescence techniques were employed to study the photophysical properties of some electron donors methylindoles, e.g. 1-methylindole (1MI), 2-methylindole (2MI), 3-methylindole (3MI) and 5-methylindole (5MI) in nonpolar and polar environments at 296 K and in ethanol (EtOH) rigid glassy matrix at 77 K in the presence of electron acceptor 2-nitrofluorene (2NF). It has been proposed that at room temperature and in polar acetonitrile (ACN) solvent, MI (1, 2 or 3) emission originates mainly from the closely lying lowest excited states S 1 ( 1L b), S 2 ( 1L a) and the charge transfer (CT) exciplex state which is formed due to solute-highly polar solvent ACN interaction. This CT state has its origin in 1L a. The room temperature fluorescence quenchings of the MI donors in presence of acceptor 2NF are found to be mainly due to photoinduced electron transfer (ET), excitational energy transfer and transient quenching processes. Out of the three processes Förster's long range energy transfer seems to be the most dominant one. At 77 K singlet–singlet and triplet–triplet energy transfer processes seem to be responsible for the observed quenching in fluorescence and phosphorescence spectra of the donor molecules in presence of the acceptor 2NF.