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

Abstract

A series of novel cationic merocyanine dyes were prepared by condensation of 1,2,3,3-tetramethylindolium derivatives and 3-formyl-4H-1-benzothiopyran or -benzopyran derivatives, and their photochromic properties were investigated, especially focusing on the substituent effect in the indolium moiety. These dyes were converted into the spiropyran (SP) form so as to be colorless by irradiation with visible light, although, left under darkness, the colored merocyanine (MC) form was reproduced. The thermal coloration was affected by two factors: the substituent in the indolium moiety and the counter anion. An electron-donating substituent such as a methoxy group at the 5 position effectively retarded the thermal coloration, whereas an electron-withdrawing group such as a nitro group accelerated it. These substituent effects afforded good linearity for the Hammett rule with para σ parameters, indicating that decrease of the electron density in the indoline moiety of the SP form facilitated heterolytic cleavage of the bond between the spiro carbon and the oxygen of the pyran ring.

References

1. M. Irie, Mol. Cryst. Liq. Cryst., 1993, 227, 263; Denshi Shashin Gakkaishi, 1996, 35, 344.
2. T. Sakagami, in Chemistry of Organic Photochromism, Kagaku Sosetsu, No. 28, Chem. Soc. Jpn., Gakkai Shuppan Center, Tokyo, 1996, p. 149 and references therein.
3. (a) R. Blonder, I. Willner and A. F. Bückmann, J. Am. Chem. Soc., 1998, 120, 9335; (b) I. Willner, Acc. Chem. Res., 1997, 30, 347; (c) K. Kimura, T. Yamashita and M. Yokoyama, J. Phys. Chem., 1992, 96, 5614; (d) S. L. Gilat, S. H. Kawai and J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1993, 1439.
4. For a review R. Guglielmetti, in Photochromism. Molecules and Systems, Studies in Organic Chemistry, eds. H. Durr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990, p. 314 and references therein.
5. E. Ando, M. Miyazaki, K. Morimoto, H. Nakahara and K. Fukuda, Thin Solid Films, 1985, 133, 21; H. Tomioka and T. Itoh, J. Chem. Soc., Chem. Commun., 1991, 532; A. Miyata, Y. Unuma and Y. Higashigaki, Bull. Chem. Soc. Jpn., 1991, 64, 1719; J. Hibino, M. Suziki and Y. Kishimoto, Nihon Kagaku Kaishi, 1992, 1125.
6. T. Tamaki and K. Ichimura, J. Chem. Soc., Chem. Commun., 1989, 1477.
7. A. Miyashita, M. Hirano, S. Nakano and H. Nohira, J. Mater. Chem., 1993, 3, 221; A. Miyashita, S. Nakano, M. Hirano and H. Nohira, Chem. Lett., 1993, 501.
8. S. Watanabe, H. Nakazumi, S. Kado, K. Maeda and T. Kitao, J. Chem. Res., 1990, (S)50; (M)546; S. Watanabe, H. Nakazumi, K. Maeda and T. Kitao, J. Chem. Soc., Chem. Commun., 1990, 421.
9. H. Nakazumi, K. Maeda, S. Yagi and T. Kitao, J. Chem. Soc., Chem. Commun., 1992, 1188.
10. Plotting the σ⁺ constant afforded large deviation from linearity. This may be because the positive charge is located not on the nitrogen atom in the indoline ring but on the benzothiopyran or benzopyran moiety. For Hammett-type analyses of the thermal decoloration for typical indolinospiropyrans, see R. C. Bertelson, in Photochromism, ed. G. H. Brown, Wiley-Interscience, New York, 1971, p. 49.
11. C. H. Chen and G. A. Reynolds, J. Org. Chem., 1979, 44, 3144.
12. A. Nohara, T. Umetani and Y. Sanno, Tetrahedron, 1974, 30, 3553.
13. N. P. Vasilenko, F. A. Mikhalenko and J. I. Rozhinsky, Dyes Pigments, 1981, 2, 231.
14. H. Nakazumi, T. Endo, H. Sonoda and T. Kitao, J. Heterocycl. Chem., 1985, 22, 821.
15. For practical purposes, the decreasing rate of the SP form was monitored.