Reversible porosity changes in photoresponsive azobenzene-containing periodic mesoporous silicas

Two differently substituted azobenzenes having terminal trimethoxysilyl groups attached to the para position of the phenyl rings by means of different linkers have been used in combination with tetraethyl orthosilicate to prepare two periodic mesoporous organosilicas. As the corresponding precursors in solution, these solids exhibit a photochemical trans-to-cis isomerization of the azo group upon irradiation at 358 nm. The cis-configured isomer reverts to the trans isomer either photochemically upon irradiation at 450 nm or thermally at room temperature within days. As a consequence of the dramatic changes in the molecular dimension of azobenzene upon trans/cis isomerization, the adsorption of gold nanoparticles (1-10 nm) into the mesoporous silica depends reversibly on the trans or cis configuration of the azo groups grafted on the walls, the adsorption capacity of the trans-configured azobenzene being larger.