1- and 2-Photon Fluorescence Anisotropy Decay in Silicon Alkoxide Sol−Gels:  Interpretation in Terms of Self-assembled Nanoparticles

We have studied the one- and two-photon induced fluorescence anisotropy decay of rhodamine 6G (R6G) during polymerization of tetramethyl orthosilicate (TMOS) approaching the sol-to-gel transition, a time denoted t g, using time-correlated single-photon counting and femtosecond Ti:sapphire laser excitation. A biexponential decay of fluorescence anisotropy is observed at all times. We propose a different interpretation to the widely accepted view, that fluorescence anisotropy reports solely on molecular viscosity in sol−gels. We think our results are consistent with the presence of both free dye and dye bound to nm-size silica particles rather than just the coexistence of different discrete viscosity domains as reported previously. A corollary of our interpretation is that the microviscosity changes very little from that of the initial bulk sol throughout the sol−gel polymerization. Nanometer-size particles are known from small angle scattering studies to be precursors to gelation in sol−gels over a wide range of conditions and our interpretation might prove to be an important step toward understanding the self-assembly mechanisms of silicon alkoxide based materials at the molecular level. According to our measurements and interpretation, for TMOS at pH 2.3 for example, primary silica particles of ≈0.8-nm mean radius grow by monomer−monomer or monomer−cluster addition to produce larger structures ≈1.1-nm mean radius after one month.