In-situ single-crystal X-ray diffraction analysis of photochromic dibenzobarrelene derivative

11,12-dibenzoyl-9,10-dihydro-9,10-dimethyl-9,10-ethenoanthracene 1a (Fig. 1) shows a reversible color change from pale-yellow to pink in the solid state by UV/Vis irradiation. We tried to observe the structure of the pink colored from 1a, by using in-situ single-crystal X-ray structure analysis, and make clear the reaction mechanism of the photochromism. X-ray single-crystal analyses before and after UV irradiation at 90 K shows not intermediates or products by the tri-π-methane rearrangement but disordered electron density around one of the two methyl groups at 9,10-position. This disorder disappeared with decay of its pink color at room temperature for several hours. This means that some kind of a motion or a reaction of methyl group will be involved in this photochromism. Diffuse reflectance spectroscopies of 1a after UV irradiation to the single-crystals and the powdered crystals show different UV/Vis spectra. This result means that the photoproducts have different structures and both of them turned back to 1a in solution immediately with disappearance of their colors. (author)