Synthesis of a Novel Photochromic Spirothiopyranobenzopyrylium Dye

A novel indolinospirothiopyran dye, 2-N,N-dimethylaminoethenyl-(1″,3″,3″-trimethylindoline-2″-spiro-2′-2′H-1′-thiopyrano)[5′,6′-c]benzo[b]pyrylium iodide (3a), was prepared by condensation of 1,2,3,3-tetramethylindolium iodide (9) with 2-N,N-dimethylaminoethenyl-3-formyl-4H-1-benzopyran-4-thione (8), and its structure was confirmed by X-ray crystallography. The spirothiopyran dye possesses its absorption band in a visible region (λ_max = 499 nm) in the ring-closed form. Upon irradiation with visible light (390 < λ < 480 nm), the ring-opened form having an absorption band at the longer wavelength was formed. In the thermal decay of the ring-opened form, the original spectrum was recovered with a few isosbestic points, indicating the reversible isomerization of 3a between the spirothiopyran form and the ring-opened form.

photochromism - indorinospirothiopyran - spirothiopyranobenzopyrylium - X-ray crystallographic analysis - photomerocyanine analog - photochemistry - dye - thione