Solid-State Photochromism of Salts of Cationic Spiropyran with Various Anions: A Correlation between Reaction Cavity Volumes and Reactivity

The salts of cationic spiropyran with various anions ([PSP]X; X = I, NO3, SCN, BF4, ClO4, PF6, CF3BF3, OTf, and BPh4) were prepared, and their crystalline-state photochromism and crystal structures were investigated. Four salts (X = I, NO3, SCN, and BPh4) exhibited photochromism in the crystalline state. In all the crystals, the cations and anions are alternately arranged to form mixed-stacked columns, with the exception of [PSP]BPh o which has a grid-like arrangement of the anion. Most of the salts exhibit a disorder of cations. For all the salts, the reaction cavity between the pyridinium ring of the cation and the adjacent anion was determined based on their crystal structures. The reaction cavity volumes in the photochromic salts were larger than those in other salts (>8.9 angstrom(3)). These results demonstrate that the crystalline-state photochromism depends on the size of the reaction cavity necessary for structural isomerization. Additionally, shear stress also induces the isomerization of the cations of all the salts in the solid state.