Microviscosity in the cavities of titania gels studied by steady-state fluorescence anisotropy

M. CLAUDIA MARCHI; ALDABE BILMES, S; RM NEGRI

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 1997 vol. 13 p. 3665 - 3673

0743-7463

A series of fluorescent compounds (rhodamine 101, cresyl violet, oxazine 1, and tris(2,2'-bipyridyl)-ruthenium(II) chloride hexahydrate) were used as molecular probes in titania gels. Sols and gels of pH = 2,5 and 3.5 were prepared using standard methods. Absorption and emission spectra and fluorescence quantum yields of the probes were measured at roam temperature. Fluorescence intensity and steady-state fluorescence anisotropy, [r], were measured for cresyl violet and oxazine 1 at controlled temperature between 25 and 95 degrees C as function of aging. The local environment of Rh101, Rbpy, CV, and Ox1 in the gels at 25 degrees C is essentially aqueous. Among these compounds, only CV and Ox1 are appropriate for fluorescence anisotropy studies in titania gels of low pH. Parameters for the calculation of the local friction, xi, inside the cavities were obtained from previous experimental data or by modeling a symmetrical molecule as prolate ellipsoid. For CV, the comparison of the fluorescence anisotropy data in microheterogeneous systems and homogeneous solutions can be directly performed, because its rotational dynamics has been very well characterized in water and alcohols. The fluorescence anisotropy and photophysical parameters of Ox1 in water and alcohols are less known, but the symmetry properties allows us to obtain analytical expressions for the rotational correlation times and for the local friction. Even when the bulk viscosity of the gel increases, there is a minor restriction for the rotation of the fluorescent probes, indicating that the gel is composed of a rigid network with solvent-filled cavities of volumes larger than 500-600 Angstrom(3). The similarity of the values of 5 for both probes is a strong validation for the model used for Ox1 and allows Ox1 to be used as an alternative to CV for the method.