Photophysical properties of some phenetidines in non-polar and hydrogen bonding solvents at room temperature and 77 K

Abstract

The present study was undertaken to investigate the photophysical processes in o-, m- and p-phenetidines, when dissolved in nonpolar and hydrogen bonding solvents, in their ground state and excited electronic state S 1, both at 300 and 77 K. In the ground as well as in the S 1 state it is proposed that the o-phenetidine molecule possesses a structure in which NH 2 and OC 2H 5 groups are away from each other, both in nonpolar cyclohexane (CH) and H-bond acceptor solvent triethylamine (TEA). The formation of a transient or nonemissive charge transfer (CT) complex resulting from strong excited state hydrogen bonding interaction with TEA is found to be responsible for the observed fluorescence quenching of the proton donor phenetidines at 300 K. From the room as well as low (77 K) temperature electronic absorption and steady state fluorescence studies, it was deduced that nonplanarity in the structure of the molecules increases as one moves from aniline to the phenetidines. It is suggested that in the solvent stiffening temperature 77 K, triplet states of all the phenetidines ( o-, m- and p-) acquire some nπ* character due to conformational changes, whereas ππ* character is retained in their S 1 state. This facilitates a larger intersystem crossing (ISC) rate in phenetidines relative to the situation in aniline where both S 1 and T 1 possess the same nπ* nature at 77 K due to its more planar structure. However, ISC efficiency in phenetidines at 77 K is found to be impeded, especially in the case of o- and m-isomers, in the presence of TEA as inferred from the lowering of φ p /φ ⨍ values and the increment of τ p . In p-phenetidine, rapid equilibrium between a triplet state hydrogen bonded species and free molecules during the triplet excited state lifetime is suggested.