Synthesis and spectral and photochromic properties of 7,7′-disulfoperinaphthothioindigo

Conditions for the preparation of individual 2-oxonaphtho1,8-bc]thiophene-6-sulfonic acid were found, and 7,7-disulfoperinaphthothioindigo (I) was synthesized. The rate of photochemical isomerization of I is higher in dimethylformamide than in water, while the rate of thermal isomerization is lower by a factor of 40 in DMF than in water. A change in the pH of an aqueous solution of I, enriched with the cis form, over the range 1–9 does not affect the rate constant of thermal cis -trans isomerization; at pH 11.2 and 11.6, the constant increases sharply. The solutions are unstable at 11.6