Enhanced aromaticity of adsorbed dibenzotropone as reflected by electronic spectra and photodimerization studies on silica surfaces

Dibenzotropone adsorbed on dry silica gel exhibits a very large red shift in the electronic spectrum. This surface effect has been studied on other silica surfaces (fluorinated silica, methylated silica, silica heated to 500°C, wet silica, fumed silica). Photodimerization of adsorbed dibenzotropone was used to probe the relative diffusivities on some of the adsorbents. The results are interpreted as reflecting enhancement of aromaticity of the tropone-tropyllium moiety. This enhancement is induced by a synergetic effect of several silanol hydrogen bonds to a single adsorbate molecule, and is dependent on the degree of surface acidity.