Wetting Properties of Flat and Porous Silicon Surfaces Coated with a Spiropyran

We report the immobilization and characterization of a spiropyran (SP) derivative (1) on smooth Si(100) and porous H-terminated silicon surfaces through a thermal hydrosilylation protocol. Under visible light exposure the SP is in a closed, hydrophobic form, whereas under UV irradiation it converts to a polar, hydrophilic open form named merocyanine (MC). The SP-MC photoinduced isomerization gives a small contact angle (CA) change of 9 degrees for smooth Si(100) samples under sequential irradiation cycles with white and UV light. Irradiation of porous silicon (PS) surfaces, under the same conditions, gave a CA change of 11 degrees. Treatment of PS surfaces, bearing the MC form of chromophore 1, with cobalt(II) ions enhances the wettability switching of the PS surface to a much larger extent, giving rise to a CA variation as high as 32 degrees.