Salts
containing the merocyanine form of a pyridinium-fused spiropyran
([6′-MC]X; X = I and PF6) 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]PF6,
and [6′-SP]PF6) were obtained by recrystallization.
The crystals of [6′-SP]PF6 exhibited
positive photochromism in the solid state; however, no photochromism
was observed in the [6′-MC]X crystals.