Photoreversible Photographic Systems. VI. Reverse Photochromism of 1,3,3-Trimethylspiro[indoline-2,2′-benzopyran]-8′-carboxylic Acid

Abstract

Abstract Many derivatives of 1,3,3-trimethylspiro[indoline-2,2′-benzopyran] (1) exhibit photoreversible color changes in organic solvents. One of the derivatives 1,3,3-trimethylspiro[indoline-2,2′-benzopyran]-8′-carboxylic acid (2) shows “reverse photochromism” in polar solvents. The compound 2 exhibits a deep color in the polar solvent and the color is bleached by irradiation with either visible or ultraviolet light. The color reappears thermally as the irradiation is ceased. The thermal conversion rate from the colorless form to the colored one depends on the solvents. On the other hand, the isomer 1,3,3-trimethylspiro[indoline-2,2′-benzopyran]-6′-carboxylic acid shows neither normal nor reverse photochromism under usual conditions, but exhibits reverse photochromism when acid such as malonic acid is added into the solution. The compound 2 is assumed to have a merocyanine dye-like structure due to the intramolecular proton transfer.