Fluorescence spectra of methyl salicylate during the sol–gel–xerogel transitions of mixed silicon–aluminium alkoxides

Abstract

The fluorescence and fluorescence-excitation spectra of methyl salicylate (MS) during the sol–gel and gel–xerogel transitions of six systems containing silicon and aluminium alkoxides in different ratios, (Si:Al = 100:0, 99:1, 95:5, 90:10, 80:20 and 50:50) using a small amount of HCI as a catalyst have been observed at constant temperature (300 K) as a function of the sol–gel reaction time. FTIR spectra of the prepared xerogels have also been observed. In the pure Si system, MS exhibits two different fluorescence bands at ca. 360 and 450 nm originating from two different types of closed conformers. In the sol–gel reaction systems containing Al atoms, there are other fluorescence bands at ca. 400 and ca. 430 nm. It is concluded that some of the mixed MS molecules in the ground state form complexes with the Al atoms, which act as Lewis bases. The fluorescence bands at ca. 400 and 430 nm originate from a complex between the MS and Brønsted acid site by the Al atom in an —O—Si—O—Al—O— network and an anionic form of MS originating from the H transfer to the Lewis base site.