Nonaqueous Synthesis of a Highly Photochromic Tungstic Acid

Abstract

A photochromic tungstic acid has been synthesized in nonaqueous media for the first time. The action of bromine on powdered tungsten metal, in methanol, produced a highly photochromic tungstic acid, WO₃·0.75 H₂O, in 93% yield. The material was much more photochromic than the previously known WO₃·0.5 H₂O, reversibly passing from white to dark blue by the action of ultraviolet light. The mechanism of the synthesis involves the solvent, methanol, producing by-products of methyl formate and organic bromides. An adsorbed organic compound, such as a normal alcohol, appears necessary for photochromism, and the material may be dispersed in thin films of plastic without loss of activity.