Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S 1) and triplet (T 1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D +H⋯A −), causes the formation of the neutral radical, 3(D+HA)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.