Hybrid spiropyran–silica nanoparticles with a core-shell structure: sol–gel synthesis and photochromic properties

Abstract

Photochromic hybrid spiropyran–silica nanoparticles with a core-shell structure were synthesized via a two step sol–gel procedure using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) as silica precursors. The chemical nature and porosity of the materials were modified by the precursor ratio and the silylated spiropyran derivative chromophore was grafted and confined inside the nanoporous shell producing photoresponsive nanomaterials with a tunable dye photochromic response. When the MTEOS content increases in the matrices, the optical response was tuned from reverse to direct photochromism. Thermal bleaching after UV irradiation exhibits a blue shift of the reflectance maximum in the visible region assuming a modification of the dye populations towards open forms stabilized in more polar environments. The kinetic data were finally studied through a Gaussian model to evaluate the decay rates and to give an indication of the degree of heterogeneity of the materials.