Luminescent properties of some imidazole and oxazole based heterocycles: Synthesis, structure and substituent effects

A series of 2-R-6-(aryloxazol-/imidazol-2-yl)pyridine and 2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-/oxazol-2-yl)phenol derivatives were synthesized and characterized using elemental and spectroscopic analyses as well as single-crystal X-ray diffraction analysis. The oxazole derivatives displayed higher photoluminescence efficiencies than their imidazole analogues. The 2-(phenanthro[9,10-d]oxazole/imidazol-2-yl)pyridine derivatives displayed quantum yields approaching unity in non-polar solvents but were quenched in polar solvents. The oxazole analogues produced reversible fluorescence photo-switching between 400 nm and 550 nm regions whilst the imidazole analogues underwent an irreversible photo-induced excimer formation in polar solvents. 1H NMR was used to rationalize the proposed mode of intermolecular interactions between excimers of the 2-(phenanthro[9,10-d]imidazol-2-yl)pyridine derivatives.