Stable ring-opened forms of a spiropyran in the confined space of nanoporous silicas

Abstract

The structural change of a spiropyran (1,3,3-trimethylindolino-8′-methoxybenzopyrylospiran (MBPS)) to ring-opened forms, such as merocyanine and protonated merocyanine, is spontaneously induced in the confined space of a nanoporous silicas. In general, the ring closing of MBPS occurs easily in liquid media under blue-light illumination or under basic conditions. However, the ring-opened forms are tightly fixed and stable in the nanopores of diameters similar of the molecules owing to the hydrogen-bonding with silanol groups and the strong steric-restriction. The color of the ring-opened forms in the confined space varies with the protonation; therefore, the MBPS trapped in the nanoporous silica with open pores is sensitive to the pH of the surrounding solution.