Synthesis, characterisation, and study of the photophysical properties of highly stable imidazole‐based novel solid‐state fluorescent azo colourants

Abstract

Novel solid‐state fluorescent azo colourants derived from 5‐amino‐2‐(4‐dimethylaminophenyl) benzimidazole as electron donor were synthesised. The colourants were characterised by means of elemental analysis, 1H NMR, and mass spectrometry. These colourants showed molar extinction coefficients in the range 12 000–48 000 dm3 mol−1 cm−1. These compounds exhibited solid‐state fluorescence under short UV (254 nm). Electron coupling originating from broad π‐electron delocalisation and keto–enol form is responsible for the large Stokes shift as indicated by absorption and fluorescence spectra. These colourants were soluble in polar aprotic high‐boiling solvents such as N,N‐dimethylformamide, N,N‐dimethylacetamide, and dimethyl sulphoxide, whereas they were insoluble in other common organic solvents. Thermogravimetric analysis of solid‐state fluorescent colourants showed that two of the colourants showed thermal stability in the range 230–240 °C, whereas for the rest of the colourants it was found to be lower, and hence the two colourants could find application in the coloration of polymers.