The Fast Sol-Gel Synthetic Route to Supported Glass Films: Synthetic Features, Scope, Applications and Mechanistic Studies

ABSTRACTThe fast sol-gel synthetic route to glass films is facilitated by a solvent-free reaction of trialkoxysilane monomers at 60–80°C and near-stoichiometric water-to-siloxane ratios. The viscous polymer formed within several minutes is spin-cast onto a support (typical thickness 10–25µm), cured within a few hours at 60–70°C or 10–40 min. under an intense electric field at room temperature, and can be characterized on the same day. Multi-layered glass-film assemblies of varying indices of refraction and a total thickness up to 80µm are prepared by the same fast process with slight modifications.Guest molecules such as laser dyes for 2D lasers and chromophores for second-harmonic-generation are incorporated into these glass films at high concentrations by direct addition to the reaction mixture. Their absorbance and fluorescence spectra are typical of monomeric chromophores up to concentrations of ∼10-2M. Crystallites that may appear at higher concentrations exhibit unusual polymorphism. The smoothness of such films enables surface anchoring of iso-environment photoactive molecules and investigation of their photophysics.29Si-NMR studies of the hydrolysis pattern in our acid-catalyzed fast sol-gel reaction prove that uniform hydrolysis of the monomer, necessary for a single-phase polymerization, is attained only when the hydrolysis is fast enough to become kinetically controlled, e.g. at elevated temperature or at high acid-concentration. Our reaction pattern and the data of others can be interpreted in terms of a single mechanism of hydrolysis in which two different rate-determining-steps can prevail, depending predominantly on the reactivity of the alkoxysilane and the ratio of the acid-catalyst to the (apparently-inert) proton-binding solvent.