Photochromic properties of cationic merocyanine dyes. Thermal stability ofthe spiropyran form produced by irradiation with visible light

A series of novel cationic merocyanine dyes were prepared by condensation o f 1,2,3,3-tetramethylindolium derivatives and 3-formyl-4H-1-benzothiopyran or -benzopyran derivatives, and their photochromic properties were investig ated, especially focusing on the substituent effect in the indolium moiety. These dyes were converted into the spiropyran (SP) form so as to be colorl ess by irradiation with visible light, although, left under darkness, the c olored merocyanine (MC) form was reproduced. The thermal coloration was aff ected by two factors: the substituent in the indolium moiety and the counte r anion. An electron-donating substituent such as a methoxy group at the 5 position effectively retarded the thermal coloration, whereas an electron-w ithdrawing group such as a nitro group accelerated it. These substituent ef fects afforded good linearity for the Hammett rule with para sigma paramete rs, indicating that decrease of the electron density in the indoline moiety of the SP form facilitated heterolytic cleavage of the bond between the sp iro carbon and the oxygen of the pyran ring.