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

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.

Negatively photochromic organic compounds: Exploring the dark side

“…118 The presence of the 8-carboxylic acid unit alone (46) in the benzopyran unit is sufficient to stabilise, by intramolecular H-bonding, the ring -opened species which then exhibits negative photochromism (Scheme 10). 119,120 Placement of the carboxylic acid group at C-6 resulted in no photochromic response unless the mixture was acidified with malonic acid. In contrast to these foregoing observations the 6-sulfonic acid derivative 47 ( During a study of the preparation of 6-styryl substituted spiropyrans the 6-formyl intermediate 55 ( Figure 16) was prepared as dark violet crystals, an ethanolic solution of which, with λ max at 530 nm, reversibly bleached upon irradiation with visible light.…”

Water‐Soluble Spiropyrans with Inverse Photochromism and Their Photoresponsive Electrostatic Self‐Assembly

“…Since the majority of spiropyrans are poorly water‐soluble, they have mainly been studied in non‐aqueous solvents and relatively little is known of their behavior in water. The few that are water‐soluble incorporate only a single ionic group such as a carboxylate, a quaternary nitrogen group, or a sulfonate group . In the latter case, the sulfonate group is almost always introduced by alkylation of the indoline nitrogen via a sultone .…”

Spontaneous Isomerization of a Hydroxynaphthalene-Containing Spiropyran in Polar Solvents Enhanced by Hydrogen Bonding Interactions

“…Interestingly, the MC absorption increases significantly by the addition of water: the absorbance increases with an increase in the water content of the water/DMSO mixtures, suggesting that water enhances the SP → MC isomerization, as observed for several spiropyrans. 19 − 22 As shown in Figure S11 ( Supporting Information ), dye 5 ( Scheme 2 ), which also shows solvent-driven spontaneous isomerization, 22 shows only a small increase in the MC absorbance in DMSO, and the absorbance is much lower than that of 1 even in a DMSO/water (5/5 v/v) mixture. These results suggest that 1 undergoes highly efficient SP → MC isomerization even bearing an electron-donating hydroxynaphthalene moiety.…”

“…“Solvent” is one of the most basic and easy-to-use stimuli triggering the SP ⇌ MC isomerization. Early reports describe that, as summarized in Scheme 2 , some spiropyran-bearing electron-withdrawing groups, such as carboxylic acid ( 2 ), 19 its methyl ester ( 3 ), 20 sulfonic acid ( 4 ), 21 and nitro ( 5 ) groups, 22 when dissolved in polar organic or aqueous media, are isomerized to the MC forms even in the dark condition; although they exist as the SP forms in nonpolar media. As shown in Scheme 3 a, in nonpolar media, the ground-state energy of the MC forms lies at a level higher than that of the SP form; 23 therefore, SP → MC isomerization does not occur.…”