Synthesis and fluorescence properties of novel benzoxazole- and chromone-functionalised bis(arylvinylene)imidazoles



Novel benzoxazole- and chromone-functionalised bis(arylvinylene)imidazoles were synthesised and characterised by proton nuclear magnetic resonance, Fourier Transform–infrared, mass spectrometry and elemental analyses. These dyes were found to exhibit two quite different types of fluorescence behaviour in the alkali examined; i.e. fluorescence quenching for the benzoxazole–imidazole hybrids and fluorescence enhancement for the chromone–imidazole hybrids. The benzoxazole-based imidazoles emit intense green fluorescence, but the fluorescence is remarkably quenched upon addition of alkali. Such a process can be reversibly controlled by simple deprotonation/protonation of the imidazole group; thus, they exhibit efficient fluorescence on/off switchable behaviour. In contrast, significant fluorescence enhancement is observed for the chromone–imidazole hybrids in the presence of alkali. Without alkali, these imidazole dyes are very weakly fluorescent, whereas the addition of alkali leads to an appearance of strong blue fluorescence and a dramatic increase of emission intensity.