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 ads orption and grafting of the pyrene derivative 1 on different types of silic as. For the adsorbed silicas, the formation of dimeric aggregates is eviden ced both by DRUV and by fluorescence spectroscopy. The processing of the UV spectrophotometric data allows, as in solution. the determination of appar ent equilibrium constants and of the spectra of the dimer. From these resul ts, 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 o n 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 com pound. When the pore size decreases, dimer formation is more significant an d its spectrum is more characteristic. The results obtained for adsorbed si licas were quantitatively treated and the Kubelka-Munk function, F(R), plot ted 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 Els evier Science B.V. All rights reserved.