Photo-decoloration Process of the Reverse Photochromic Spirans

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Abstract
The unstable colorless form (A′) of the reverse photochromic spiran (SP-3) (1,3,3-trimethyl-5-methoxyspiro[indoline-2,2′-benzopyran]-8′-carboxylic acid) was obtained in a stable state by vacuum evaporation (10−5 Torr) on a KBr substrate. It was confirmed by IR spectral analysis that reverse photochromism occurred between the colored (B′; merocyanine-like structure) and the colorless forms (A′; spiropyran). A colorless transient intermediate (X) was observed in the photo-decoloration process of the (SP-3). X was transformed into both A′ and B′. Thus the photo-decoloration efficiency (φB′→A′) depends on the ratio of the conversion from X to A′. State (B′) is stabilized in a polar or proton donative solvent. The life time of X is greatly influenced by the solvent as well as the stability of B′. The photo-decoloration efficiency (φB′→A′) of the spirans is effectively improved by substituting SP-2 with (methoxyl group) at 5-position. Whether X is observable or not depends on the substituent and its position. The reversibility of photochromic cycle of the spirans was restricted by the photobleaching of B′ by UV irradiation. It was observed that the photo-bleaching of B′ is accelerated in a degassed solution.