Theoretical and Experimental Chemistry, Vol. 32, No. 5, 1996 PHOTOCHEMICAL TECHNIQUE FOR STUDYING SOL-GEL-XEROGEL TRANSITIONS IN SILICA N. P. Smirnova, T. A. Kikteva, A. M. Eremenko, and V. M. Ogenko UDC 535.372:541.183.5 The sol-gel-xerogel transition in the hydrolysis of tetraethoxysilane (TEOS) was studied following the phototransfer of an electron between Py* and cupric ions. The quenching efficiency of Py in the xerogels depends on the presence of a dopant (Triton X-IO0 or polyethylene oxide) and of intermicellar liquid in the pores. In the past decade, special attention has been given to sol-gel processes in the hydrolysis of alkoxysilanes and in accompanying the formation of glassy or film silica, citanosilica, and alumosilica materials [1-3]. The sol-gel method permits the insertion of light-sensitive organic molecules into an inorganic matrix during hydrolysis and polycondensation of the alkoxysilanes. In order to study the structural changes during silica formation, a series of rheological investigations was carried out. In particular, we studied the quantitative changes in viscosity during the sol-gel transition [2, 3], and the gelation time was determined. The optimal viscosity and the concentration and ratio of the components required for the formation of spherical particles, films, and fibers were found. However, according to Sacks [3], most viscosity measurements are insufficiently precise since the state of the system during the sol-gel transition changes from viscous to elastoviscous. Thus, spectroscopic methods are used in all the recent studies of silicate formation: small-angle x-ray scattering, 29Si NMR and SH NMR spectroscopy [2], and the fluorescent probe method [4, 5]. The introduction of a molecular probe into a reacting sol-gel system permits us to obtain information on both the silicate matrix and state of the organic molecule, its microenvironment [5], and nature of its interaction with the environment. The latter becomes especially important in light of the use of matrices obtained by the sol-gel method in the manufacture of detectors and redox systems [6]. Pyrene (Py) is a convenient and often used photochemical probe for characterization of the medium [2, 3]. In the present work, the diffusion-controlled quenching of Py* fluorescence by cupric ions is used to study the sol-gel transition. We might expect that change in the viscosity of a solution of tetraethoxysilane (TEOS) in the sol-gel transition step will be accompanied by a change in the reaction course due to diminution in the diffusion rate of the fluorophor and quencher, which, in turn, would affect the quenching characteristics. The gel- xerogel transition (aging, shrinkage, and dehydration) alters the conditions for diffusion of Py and Cu 2 + ions in the intrapore space, possibly by encapsulating Py in the matrix. The addition of film-forming agents, Triton X-100 (TX-100) and polyethylene oxide (PEO) used for the preparation of strong, transparent porous films from TEOS, affect this process [1]. The starting solutions for the sol-gel transition contained 5 ml TEOS, 5 ml ethanol, 2 ml 1.10 -4 M Py in ethanol, 0.05 ml 1 M hydrochloric acid, 0.1 M Cu(NO3)2-3H20, and 4.5 ml aqueous ammonium hydroxide. The pyrene fluorescence lifetime was recorded during maturation of a silica gel. Transparent silicate films were obtained from the starting solution by adding 1% aqueous TX-100 according to Avnir et al. [1] (film I) or PEO-US-P-301 (film II). Py was added to the composition as a solution in ethanol, TX-100, or PEO. After two hours maturation of the mother solution, the film was deposited on a degreased glass surface by immersion. The samples were dried at room temperature and then at 90°C. The copper salt was introduced into the starting composition as an ethanolic solution or by adsorption onto the surface of the formed film containing Py. The fluorescence of quenching kinetics was measured on a laser fluorimeter. A nitrogen laser was used for the excitation pulse. Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 31 Prospect Nauki, 252039 Kiev, Ukraine. Translated from Teoreticheskaya i l~ksperimental'naya Khimiya, Vol. 32, No. 5, pp. 311-314, September-October, 1996. Original article submitted January 15, 1995. 272 0040-5760/96/3205-0272515.00 ©1997 Plenum Publishing Corporation