Synthesis, Photochromic Properties, and Crystal Structures of Salts Containing a Pyridinium-Fused Spiropyran: Positive and Negative Photochromism in the Solution and Solid State

Salts containing the merocyanine form of a pyridinium-fused spiropyran ([ 6′-MC ]X; X = I and PF 6 ) were prepared, and their crystal structures were determined. In addition, the photochromic properties of the salts were spectroscopically and kinetically investigated. In the solution state, the salts exhibited negative photochromism. Theoretical calculations revealed that the negative photochromism of the salt originates from the drastic stabilization of the merocyanine structure by electron delocalization of the pyridinium ring. Furthermore, the salts containing the merocyanine and spiropyran forms ([ 6′-MC ]I, [ 6′-MC ]PF 6, and [ 6′-SP ]PF 6 ) were obtained by recrystallization. The crystals of [ 6′-SP ]PF 6 exhibited positive photochromism in the solid state; however, no photochromism was observed in the [ 6′-MC ]X crystals.