Photolysis Reaction of 2-Phenylbenzo[d]oxazole

The temperature-resilient, high tensile-strength fiber poly[p-phenylene benzobisoxazole] (PBO) is light-unstable and it degrades under ultraviolet radiation. In this paper we study the photolytic mechanism of the PBO monomer, 2-phenylbenzo[d]oxazole (PO). Following absorption of a photon and excitation into the first excited state (S₁), the molecule overcomes an energy barrier of 25.59 kJ·mol^-1 to enter the transition state; the oxazole ring is then opened and both benzene rings form a dihedral angle of about 90° to obtain the product, which undergoes further addition reaction with water. Calculations reveal that ring-opening is easily achieved in the potential surface of S₁. However, the pathway through which the oxazole ring opens in the ground state remains obscure. The topological properties of these compounds are in good agreement with the expected bond orders and the photolytic mechanism.