Photochroyic Glasses: Properties, Applications & Mechanisms - I. Introduction

A photochromic material is one which undergoes a change of color when exposed to light and reverts to its original color when the light is removed. Review articles carrying extensive bibliographies have been written by Brown and Shaw (1) and by Schwab and Bertelson (2). Organics. Schwab and Bertelson divide the reactions in different organic photochromic materials into some half-dozen categories; Windsor (3) reduces these to the following three main classes of general interest, based on how they work. 1. Stereoisomers. Absorption of light breaks one of the chemical bonds in a. ring molecule, thus allowing the molecule to unwind and form a different geometrical arrangement. The reverse process is a reforming of the bond. Examples of this class are the spiropyrans and the anils. 2. Dyes. A triphenyl methane dye, for example, is oxidized by energetic light; the absorption characteristics of the positive ion so formed are different from those of the original electrically neutral benzene rings. 3. Triplet States. In the class of polynuclear aromatic hydrocarbons, ground-state molecules are excited first to a singlet state by irradiation, and then go, via the lowest triplet state, to an excited triplet state. Visible light is absorbed in the triplet-triplet transition.