Nanoscale Properties and Matrix−Dopant Interactions in Dye-Doped Organically Modified Silicate Thin Films

Abstract

The nanoscale properties of organically modified sol−gel-derived silicate thin films are studied in detail by single-molecule spectroscopic methods. For these studies, the solvent-sensitive probe Nile Red is doped into the films at nanomolar concentrations. Spectroscopic data are obtained for films prepared from sols containing different mole fractions of isobutyltrimethoxysilane and tetraethoxysilane. The data are analyzed using a model based on Marcus theory, providing important new information on static local film properties such as polarity and the extent of specific dopant−matrix interactions. Data on dynamic phenomena related to local matrix rigidity is also obtained. In general, throughout the range of samples studied, the most polar environments are also found to be the most rigid. With regard to their static properties, broad heterogeneous distributions are found in films of predominantly inorganic composition. In several instances, bimodal distributions are also observed, which result from specific chemical interactions and likely involve hydrogen bonding of the dye to the silicate matrix and/or to entrapped solvent. As the organic content of the film is increased, the film environments become less polar, less rigid, and more homogeneous. In addition, the effects of specific chemical interactions become dramatically less apparent. With respect to dynamic film properties, two distinct distributions are observed in films of intermediate organic/inorganic composition, reflecting the presence of environments differing in their rigidity. Studies of time-dependent single-molecule fluorescence fluctuations provide support for the conclusions derived from the spectroscopic data.