Diffuse reflectance UV-Visible spectroscopy for the qualitative and quantitative study of chromophores adsorbed or grafted on silica

Diffuse reflectance UV-Visible spectroscopy (DRUV) is used to study the adsorption and grafting of the pyrene derivative 1 on different types of silicas. For the adsorbed silicas, the formation of dimeric aggregates is evidenced both by DRUV and by fluorescence spectroscopy. The processing of the UV spectrophotometric data allows, as in solution. the determination of apparent equilibrium constants and of the spectra of the dimer. From these results, two sets of silicas are distinguished according to the amount of dimer formed and to its UV spectrum. Lower amounts of dimeric species are found on silica with either large pore size or low particle size. The spectrum in this case is only slightly redshifted relative to that of the monomeric compound. When the pore size decreases, dimer formation is more significant and its spectrum is more characteristic. The results obtained for adsorbed silicas were quantitatively treated and the Kubelka-Munk function, F(R), plotted against the concentration. Attempts to estimate the loading of silicas grafted with 1 were undertaken. It appears that small particle size silica gives rise to the highest loading and to the best quality DRUV spectra as a consequence of a minor amount of fluorescent dimer. On the other hand, it is shown that the loading of grafted silica is difficult to control by the used grafting process and that milder conditions are required. (C) 2001 Elsevier Science B.V. All rights reserved.