Polymerization and photochromism of ammonium molybdate in porous glass

Abstract

Modification of porous glass (PG) plates is carried out by impregnation with aqueous solutions of ammonium molybdate (NH₄)₂MoO₄ with subsequent removal of water at 120°C. A long-wavelength shift of absorption spectra upon accumulation of the salt in PG indicates polymerization of MoO₄^2- anions at low concentrations of the encapsulated salt. Photochromism manifests itself as the anionic forms in PG become larger. UV irradiation of the modified plates causes enhancement of continuous absorption in the visible range. The proposed mechanism of photoreduction of the polianions in PG involves the removal of oxygen atoms from the bridging–Mo–O–Mo–bonds and stabilization of the colored forms by means of conjugation of the electrons released from the 4d-levels of pentavalent molybdenum.