2-Phenylbenzoxazole derivatives as solid-state fluorescence emitters: Influence of steric hindrance and hydrogen bonding on the optical properties.

Six new 2-phenylbenzoxazole (PBO) derivatives, bearing either a methoxy or hydroxy group at position 5, were prepared via an efficient one-step synthesis that allowed both analogues to be obtained in equal proportions. These PBO derivatives also differed by the presence or absence of an alkyl substituent at the para-position of the phenyl group. In organic solutions, all six compounds were strongly fluorescent in the near-UV range. In the solid state, the 5-methoxy derivatives emitted bright light, ranging from violet to deep blue according to the substitution of the phenyl group. The presence of a bulky tert-butyl group indeed resulted in the separation of molecules, but also led to a deviation from molecular planarity. Remarkably, the introduction of a methyl group had a far more beneficial effect on the optical properties. With regard to the hydroxy derivatives, none of them was photoluminescent, probably due to strong intermolecular hydrogen bonding in the crystals. The 50:50 mixtures of methoxy and hydroxy analogues showed acceptable emission properties in the solid state. The substitution pattern also influenced the crystal habit of the pure compounds and the crystallinity of the mixtures. These cheap molecules could be adapted to suit a variety of applications in the field of photoluminescent materials.