Photocoloring Reaction\nBehavior in Spironaphthooxazine\nNanoparticles under Nanosecond Pulse Laser Irradiation

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