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The Strathprints institutional repository is a digital archive of University of Strathclyde's Open Access research outputs. Strathprints provides access to thousands of Open Access research papers by University of Strathclyde researchers, including by researchers from the Department of Computer & Information Sciences involved in mathematically structured programming, similarity and metric search, computer security, software systems, combinatronics and digital health.

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1 and 2-photon fluorescence anisotropy decay in silicon alkoxide sol-gels: Interpretation in terms of self- assembled nanoparticles

Geddes, C.D. and Karolin, J. and Birch, D.J.S. (2002) 1 and 2-photon fluorescence anisotropy decay in silicon alkoxide sol-gels: Interpretation in terms of self- assembled nanoparticles. Journal of Physical Chemistry B, 106 (2002). pp. 3835-3841. ISSN 1520-6106

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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 tg, 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.