Reinvestigation of the photophysics of the 2-naphthol-amine hydrogen-bonded system. I

Abstract

he exciplex emission in the 2-naphthol-triethylamine (2N-P-TEA) system is actually due to excitation of the ground-state hydrogen-bonded complex (GHBC) except in polar solvents, where the exciplex is formed through encounter in the excited state. The dipole moments in the ground and excited states have been calculated for the GHBC. The GHBC emission has interesting characteristics even in nonpolar solvents. In alkanes and cycloalkanes, viz. methylcyclohexane (MCH), the locally excited (LE) and the ion-pair (IP) emissions are observed in equilibrium at 290 K. In benzene (BENZ) and ethyl ether the LE emission is absent. The disappearance of the LE emission and the red shift of the IP emission have been investigated in a binary mixture of MCH-BENZ. The behaviour of the lifetime and the shift shows that the more polar moiety entre the first coordination sphere preferentially. At 80 K, the LE emission is observed with a trace of the IP emission. At 140 K, spontaneous CT reaction takes place and is predominant in benzene while in methylcyclohexane, a fast excited-state equilibrium is attained and both the emissions have the same lifetime even upto 290 K. Moreover, in polar solvents (ethanol, methanol), the situation is similar to that in nonpolar solvents below 140 K; between 150 sad 200 K, the emission is observed due to the IP (GHBC) state and at about 220 K, the exciplex emission (with a risetime) appears which predominates the GHBC emission at higher temperatures.