CHARGE-TRANSFER TRANSITIONS OF COPPER(II) IN DRYING SILICATE XEROGELS

Silica xerogels, containing copper nitrate hemipentahydrate, were prep ared from the hydrolysis and condensation of tetraethoxysilane. Coordi nation of the copper ion was monitored during gel drying by ultraviole t-visible (UV-vis) spectroscopy and electron spin resonance (ESR) spec troscopy. As the last of the interstitial water and ethanol left the S iO2 network, the coordination of copper changed. This was observed vis ually by a change in colour from blue to green and by a corresponding change in the spectra, both UV-vis and ESR. The primary difference det ected by UV-vis spectroscopy was the appearance of a strong absorption band at similar to 400 nm, which was attributed to a charge transfer transition between the central metal and a ligand.