Articles Radiative and Nonradiative Excited State Processes for Studying the Sol to Gel Evolution M. Claudia Marchi, Sara A. Bilmes, and R. Martı ´n Negri* Instituto de Quı ´mica Fı ´sica de los Materiales, Medio Ambiente y Energı ´a (INQUIMAE), Departamento de Quı ´mica Inorga ´ nica, Analı ´tica y Quı ´mica Fı ´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabello ´ n II, (C1428EHA) Buenos Aires, Argentina Received December 5, 2001. In Final Form: May 3, 2002 The sol to gel evolution of systems based on the hydrolysis of titanium n-butoxide, Ti(OBu n )4, in 1-butanol was investigated by monitoring the changes of the radiative and nonradiative electronic excited state processes of embedded dyes (cresyl violet and 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)- 4H-pyran). Fluorescence anisotropy experiments (FA) allow determination of changes in the microviscosity of the medium surrounding the fluorophore through the sol-gel evolution. The increase of the anisotropy parameter, 〈r〉, is explained in terms of solvent confinement in cavities enclosed within cross-linked polymeric chains. The acoustic signal recorded in laser-induced optoacoustics experiments (LIOAS) is attenuated as the system loses fluidity, with a minimum at t g, thus providing an alternative method for determining the gelation point. In addition, within the theoretical approach of percolation theory, the exponent of the viscosity power law is obtained from the attenuation of the sound wave. Although both FA and LIOAS provide information on the degree of cross-linking between polymeric chains, there is a clear difference between the behavior of the macroscopic shear viscosity determined by LIOAS and the local friction or microviscosity obtained from FA. Introduction The sol-gel process is a well-known low-temperature synthesis method that uses molecular precursors, typically metal alkoxides or halides, 1 to obtain ceramics, glasses, and nanocomposites with wide applications in optics, 2,3 optoelectronics, 2,4,5 chemical sensors, 1,6 and matrixes for molecules, cell and bacteria encapsulation, 7,8 and photo- catalysts. 9,10 The properties of the final material are related to the formation of a cross-linked network, which in turn is determined by the kinetics of hydrolysis and condensa- tion reactions involved in the sol to gel evolution. Diverse methods have been employed for the study of sol-gel materials, such as NMR with different isotopes, 11 small-angle X-ray or neutron scattering (SAXS or SANS), 12,13 infrared and Raman spectroscopies, 14 and dynamic rheological measurements. 15 However, these methods usually report on average properties and are insensitive to the local microenvironments within a nanocomposite. We propose here a strategy based on the radiative and nonradiative electronic excited-state re- laxation of dyes included in the system, such as steady- state fluorescence anisotropy (FA) and laser-induced optoacoustic spectroscopy (LIOAS). FA of chromophores is a well-known technique for the study of microenvironments in micelles 16,17 that has also been employed for monitoring the structure and aging of sol-gel materials. 18-22 Briefly, the steady-state fluores- cence anisotropy, 〈r〉, is a measure of the fluorescence depolarization that is due to the rotation of the dye (as a rigid body) during its excited-state lifetime. Thus, 〈r〉 gives direct information on the friction exerted by the environ- ment on the fluorophore; large values of 〈r〉 are related to slow rotation of the dye or, in other words, to a high local friction or microviscosity of the molecular environment * To whom correspondence should be addressed. Fax: xx54-11- 4576-3341. E-mail: rmn@qi.fcen.uba.ar. (1) Brinker, C. 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