FLUORESCENCE-SPECTRA OF PYRENE AND HUMIDITY EFFECTS ON THEM DURING THE SOL-TO-GEL-TO-XEROGEL TRANSITIONS OF MIXED SILICON ALUMINUM ALKOXIDE SYSTEMS

The fluorescence spectra of pyrene of four sol systems comprising silicon and aluminum alkoxides(Si: Al = 99.4: 0.6 ; 93.6: 6.4; 85.9: 14. 1; and 50: 50) have been observed during sol-to-gel-to-xerogel transitions under at relatively low concentration, on the order of 10(-4) mol dm-3, uncatalyzed, and constant temperature (300 K) conditions. The influence of relative humidity on the relative intensities of pyrene monomer and excimer-like fluorescence in the four xerogel states has also been observed. In three of the four systems studied (all but Si: Al = 99.4: 0.6), fluorescence spectra revealed that excimer concentration increased with time and reached a maximum near 55 days after which it decreased. From our data, it is apparent that the ratio of the intensity of the excimer-like fluorescence to that of the monomer fluorescence increases with relative humidity.