| Research Abstract |
The absortion, fluorescence, and fluorescence-excitation spectra of some aromatic photoprobe molecules(l-naphthol, benzoqunolines, rhodamine VB, fluorescein, and pyrene)have been observed along with the progress of the sol-gel reaction of SI-AI and Si-Ti mixed alkoxides. The spectral changes during the sol-gel process reflected the changes of the polarity, microviscosity, and other chemical and physical properties around the photoprobes of the reaction specimes. In a case of small fraction of AI, gelation occurred relatively slow and polarity and microviscosity around the photoprobe molecules increased relatively fast. On the other hand, the reverse chemical and physical phenomena are observed in a case of large fraction of AI. In a case of acid catalyst, hydrolysis proceeded fast and the rate determing reaction is condensation step. On the other hand, in a case of basic catalyst, the condensation proceeds fast and the rate determining step is hydrolysis. Observation of the fluorescence spectra of aromatic molecules is a useful photophysical and photochemical probe for studying the sol-gel-xerogel transition of metal alkoxides on a molecular level.
The thermal stability, similar to the photostability, of aromatic molecules encapsulated in xerogel state prepared by the sol-gel method is one of the important point for the utility of functional materials. We found that the encapsulated rhodamine B and fluorescein molecules have increased thermal stability than in air circumstance. This enhancement can be sttributed to the prevention of the permeation of oxygen into the pores of the prepared silica gel.
Excimer-like fluorescence of pyrene during the sol-gel reaction disappeared before gelation, but the fluorescence intensity became strong in encapsulated state into pores of the Si-Ti and SI-AI. This striking phenomenon can be attributable to the adsorption of two molecules at Ti site and 1 : 1 complex formation between aromatic molecules and AI.
|
