Acceleration of photochromism and negative photochromism by the interactions with mesoporous silicas

The adsorption of merocyanine onto mesoporous silicas with varied pore sizes (5.5, 9.4 and 2.2 nm) from toluene solution of 1,3,3-trimethylindolino-6'-nitrobenzopyrylospiran under UV irradiation was investigated quantitatively. The photoinduced adsorption of the merocyanine onto the SBA-15 with the pore diameter of 9.4 nm followed the pseudo-second order kinetics and the rate constant was larger than that observed for MCM-41 (pore size of 2.2 nm) thanks to the efficient diffusion of the merocyanine. The maximum adsorbed amounts of the merocyanine was 152 mg/g of SBA-15, which corresponded to the quite high concentration of the merocyanine in the pore (0.376 mol/L of pore). The resulting red-colored hybrids (SBA-15 containing merocyanine) showed decoloration in the solid-state by the visible light irradiation (negative photochromism). The conversion was high (about 80 % at the photostationary state) under the visible light irradiation at room temperature using a solar simulator (100 W). The red color was re-generated by storing the photochemically formed colorless samples in the dark at room temperature. The half-lives of the thermal coloration were 2.6, 1.9 and 1.3 h for the MCM-41, SBA-15s with the BJH pore sizes of 5.5 and 9.4 nm, respectively. Since the coloration included the diffusion of the molecules in the pore, larger pore accelerated the reaction to lead faster reactions.