Formation of Head-to-Tail and Side-by-Side Aggregates of Photochromic Spiropyrans in Bilayer Membrane

Article abstract of DOI:10.1021/j100372a077

Aggregation phenomenon and thermal isomerization behaviors of photochromic spiropyran (SP) compounds in dioctadecyldimethylammonium (2C18N⁺2C1) bilayer membrane are reported.Spiropyran compounds having two alkyl chains form J-aggregate (head-to-tail) or H-aggregate (side-by-side) in the bilayer when they are converted to the photoinduced merocyanine form by UV light (PMC).The type of the aggregate depends on the chemical structure of this photochromic molecule.One type of PMC forms J-aggregate (J-PMC) at the mixing molar ratio of spiropyran to 2C18N⁺2C1 (R) ofmore than 0.01, and the other type forms H-aggregate (H-PMC) at R larger than 0.02.It is suggested in comparison with other isotropic media that an orientation effect of the anisotropic environment formed by the bilayer is essential for the controlled aggregate formation.J-PMC is stable in the bilayer regardless of the crystal <-> liquid crystal phase change, and H-PMC, on the other hand, is formed only in the crystal (gel) phase.Aggregation retards the rate of the thermal isomerization (PMC-SP) to large extents, depending on the aggregation type.This paper reports, for the first time, the aggregation phenomenon of photochromic spiropyran compounds in the bilayer matrix.