A study of benzazole derivatives showing large stokes shifts

Abstract

A series of benzazoles has been synthesised, generally by reacting a salicylic acid derivative with a 2-substituted aniline in the presence of phosphorus trichloride. Solid state fluorescence properties of benzazoles bearing a phenyl , naphthyl or coumarinyl substituent at the 2-position have been investigated. It has been confirmed that the presence of a 2'-hydroxy group is necessary for fluorescence to occur and a scheme has been proposed of the sequence of events which occurs on absorption of a photon by a solid benzazole. In most cases, the benzazoles are colourless or cream-coloured solids which fluoresce green or greenyellow, indicating a large Stokes shift. This property has been shown to be due to intramolecular hydrogen transfer.
Substituent effects on the various steps in the proposed scheme have been studied. In an attempt to quantify these effects on the emission process, values for the radiative and non-radiative rate constants have been calculated and are presented. The variation of these rates has been discussed in terms of both the electronic properties of the substituent and the position of substitution.
The absorption and fluorescence properties of some 2-arylbenzazoles in solution have been examined. The effects of substitution in the 2-phenyl ring and of changes in solvent on the absorption spectra have been studied. It has been found that the fluorescence properties of 2-(2-hydroxyphenyl)benzothiazoles in solution depend critically on the solvent and on the nature of other substituents present in the 2-phenyl ring. Each of the main emission peaks in the fluorescence spectra has been attributed to particular species.
Fluorescence decay time data have been obtained for three pyridyl substituted benzimidazoles. The anomalous behaviour of 2-(2-pyridyl)- benzimidazole in alcohols relative to the 3- and 4-pyridyl analogues has been explained by facilitation of hydrogen transfer from the imidazole ring to the pyridine heteroatom by the alcohol . Rate constants for this process have been evaluated by studying the quenching of the fluorescence of 2-(2-pyridyl)benzimidazole in cyclohexane by ethanol