6730
Langmuir 2002, 18, 6730-6735
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(OBun)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 tg, 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 photocatalysts.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 condensation 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 * To whom correspondence should be addressed. Fax: xx54-114576-3341. E-mail:
[email protected]. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (2) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (3) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007. (4) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. (5) Sakka, S. Struct. Bonding 1996, 85, 1. (6) Bronshtein, A.; Aharonson, N.; Avnir, D.; Turniansky, A.; Altstein, M. Chem. Mater. 1997, 9, 2632. (7) Livage, J.; Corandin, T.; Roux, C. J. Phys.: Condens. Matter 2001, 13, 2673. (8) Avnir, D.; Braun, S.; Lev, O.; Ottolenghi, M. Chem. Mater. 1994, 6, 1605. (9) Bilmes, S. A.; Mandelbaum, P.; Alvarez, F.; Victoria, N. J. Phys. Chem. B 2000, 104, 9851. (10) Calvo, M. E.; Candal, R. J.; Bilmes, S. A.; Environ. Sci. Technol. 2001, 35, 4132. (11) Blanchard, J.; Ribot, F.; Sanchez, C.; Bellot, P.-V.; Trokiner, A. J. Non-Cryst. Solids 2000, 265, 83.
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 relaxation of dyes included in the system, such as steadystate fluorescence anisotropy (FA) and laser-induced optoacoustic spectroscopy (LIOAS). FA of chromophores is a well-known technique for the study of microenvironments in micelles16,17 that has also been employed for monitoring the structure and aging of sol-gel materials.18-22 Briefly, the steady-state fluorescence 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 environment 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 (12) Krakovsky, I.; Urakawa, H.; Kajiwara, K.; Kohjiya, S. J. NonCryst. Solids 1998, 231, 31. (13) Margaca, F. M. A.; Miranda Salvado, I. M.; Teixeira, J. J. NonCryst. Solids 1997, 209, 143. (14) Schraml-Marth, M.; Walther, K. L.; Wokaun, A.; Handy, B. E.; Baiker, A. J. Non-Cryst. Solids 1992, 143, 93. (15) Panton, A.; Barboux-Doeuff, S.; Sanchez, C. Colloids Surf., A 2000, 162, 177. (16) Laia, C. A. T.; Costa, S. M. B. Langmuir 2002, 18, 1494. (17) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Plenum Press: New York, 1999. (18) Dunn, B.; Zink, J. I. J. Mater. Chem. 1991, 1, 903. (19) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17. (20) Marchi, M. C.; Bilmes, S. A.; Negri, R. M. Langmuir 1997, 13, 3665. (21) del Monte, F.; Ferrer, M. L.; Levy, D. J. Mater. Chem. 2001, 11, 1745. (22) Keeling-Tucker, T.; Brennan, J. D. Chem. Mater. 2001, 13, 3331.
10.1021/la0117597 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/08/2002
Excited State Processes in Sol to Gel Evolution
surrounding the dye. On the other side, the principle of LIOAS is the measurement of the thermal energy produced by radiationless decay of electronic excited states of a dye where the resulting volume change, usually an expansion, induces a pressure change measured as an acoustic wave.23 This technique has been widely employed for the study of photophysics of dyes in systems with constant viscosity.23,24 However, the parameters of acoustic waves, that is, amplitude and sound velocity, are very sensitive to the viscoelastic properties of a system,25 and we propose here its use for monitoring the viscoelastic changes, at the macroscopic level, throughout the sol to gel evolution. Although acoustic waves, generated either by an appropriate oscillator26 or by impinging light onto an absorber (usually carbon films) close to the material under study,27 have already been used for the study of sol-gel systems, the concept introduced in this work is the generation of acoustic waves by chromophores included in the system, which has not yet been explored in the sol-gel field. The aim of this work is to exploit the photophysics of dyes for monitoring the evolution of physicochemical properties of a system that evolves from a sol to a gel. The goal is to provide fast and nonexpensive experiments for the determination of the degree of cross-linking during the sol-gel process in systems containing dyes, such as those employed for the design of photochromic devices, optical filters, and lasers.3,28 Moreover, as the dye concentration introduced in the system is very low (
