Photo-and thermochromic properties of 1′,3′,3′-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride in the crystalline state

1′,3′,3′-Trimethyl-6-nitro-8-pyridiniomethylspiro2H-1]benzopyran-2,2′-indoline] chloride (1) was synthesized. Single crystals (crystal hydrates) of the open merocyanine form of compound 1 were obtained under the dark conditions, and their crystal structure was studied. On irradiation with visible light compound 1 crystallizes from propan-2-ol in the polycrystalline closed form. The spectral and photochemical properties of compound 1 in the polycrystalline state and the kinetic characteristics on thermal transformation of the closed form to the open form were studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1548–1553, September, 2006.