Crystal and Solution Structures of Photochromic Spirobenzothiopyran. First Full Characterization of the Meta-Stable Colored Species

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Abstract
Full elucidation for stable, colorless, and meta-stable colored structures of a new spirobenzothiopyran has been achieved both in the solid state and in a solution. 1‘,3‘,3‘-Trimethyl-6-nitrospiro[(2H)-1-benzothiopyran-2,2‘-indoline] with an ester group as a substituent at the 8-position of 1-sp shows photochromism. The blue-green colored species resulted from UV irradiation (365 nm) in one minute and spontaneously bleaches within one minute in acetone and methanol at 27 °C. UV exposure (365 nm) of 1-sp in methanol for 3 h at room temperature results in the growth of deep blue needlelike single crystals of the open form of spirobenzothiopyran, photomerocyanine 1-pmc, whose structures in the solid state and in solution were obtained unambiguously. The X-ray structural analysis of 1-pmc revealed the molecular structure of the zwitterionic photomerocyanine with s-trans,s-trans conformation. 1-pmc is soluble to polar solvents and thermally returns to 1-sp. In the DMSO solution, 1-pmc is found to return slowly to 1-sp (1-pmc gave only 18% of 1-sp for 30 min at 22 °C). The detailed NMR studies in DMSO-d6 including COSY and NOE techniques as well as isotope labeling of the compound showed the structure with s-trans,s-cis conformation in a solution.

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