Fluorescence properties of benzoxazole type dyes entrapped in a silica matrix by the sol-gel method

For the first time, the incorporation of 2,5-bis(benzoxazol-2'-yl)-4-methoxyphenol (BM) in a silica matrix by the sol-gel method has been accomplished, producing transparent and translucent monoliths with fluorescence properties. The intensity and wavelength range of the fluorescence bands F-1 (primary structures) and F-2 (proton-transferred forms) were extremely sensitive to the presence of solvents and to the dye concentration in the silica pores. The F-2 band was observed when the silica pores contained solvent; however, after the solvent evaporation, the interactions of the dye with the silica host hindered the proton-transfer reaction, eliminating the F-2 band and indicating the polar and acidic character of this host. Nevertheless, it was observed that the proton-transfer could be recovered after insertion of suitable solvents inside the matrix, like water, ethanol or DMF. Changes in the fluorescence spectra due to the use of different dye concentrations were observed when the monoliths were prepared with ethanol as solvent. During the aging process, the F-2 band persists at high dye concentrations, but it disappears for low dye concentrations. The high sensitivity of the proton-transfer reaction of this dye in a particular environment, where the solvent and the host play important roles, suggests the possible application of this dye as a photoprobe for this kind of system.