Photocoloring Reaction Behavior in Spironaphthooxazine Nanoparticles under Nanosecond Pulse Laser Irradiation

Abstract

The photochromic behavior of aqueous nanocolloids of T-type photochromic spironaphthooxazine (SNO) was studied by continuous wave (CW) light and intense nanosecond pulse laser light irradiation. When a single shot of the 355 nm nanosecond pulse having a pulse duration of 6 ns was irradiated to the SNO nanocolloids and the solution, the nanocolloids remarkably increased in the conversion yield from a colorless spiro (SP) form to blue-colored merocyanine (MC) form in the low excitation intensity region compared to the solution. The amplified mechanism was explained by the time lags between four processes: the temperature elevation (subns) of the nanoparticle and the cooling (ns) by water, the photoisomerization reaction to the MC form (ps) of SNO molecules, and their thermal back reaction to the SP form (>ms). During the 6 ns laser pulse exposure, the nanoparticle temperature could be elevated above room temperature and rapidly drop to room temperature. SNO molecules in the high-temperature nanoparticle advantageously underwent the photoisomerization reaction to the MC form in the picosecond time scale rather than the thermal back reaction to the SP form in the millisecond time scale, resulting in a greater increase in the conversion yield than the solution. The rapid cooling process can work as an inhibitor to decrease the conversion yield by the thermal back reaction. The combination of the transient heating and cooling by water in a nanosecond laser pulse opens a novel amplified photoreaction model in aqueous colloidal solution of photofunctional organic nanomaterials.