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

Abstract

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]­BPh4, 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 Å3). 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.

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

“…In the crystal, the PF 6 – anion was located below the pyridinium ring, and this ion pair was stacked in the [101] direction (Figure b). This ionic arrangement has been observed in related salts such as [ 5′-SP ]­X …”

“…This ionic arrangement has been observed in related salts such as [5′-SP]X. 19 Crystallization of [6′-SP]PF 6 in the SP form was caused by the use of the PF 6 − anion, which has weaker electrostatic interactions than I − . This resulted in destabilization of multivalent MC and formation of an equilibrium mixture of the MC and SP forms in solution.…”

“…To control the properties of salts by the photoisomerization of spiropyran accompanying the changes in polarity, methods including liquefaction and the crystalline-state isomerization of salts containing cationic spiropyran, 1-alkyl-3,3,5′,6′-tetramethylspiro­[indoline-2,2′-[2 H ]­pyran­[3,2- b ]­pyridin]-5′-ium ([ 5′-SP ] + , Figure a) can be used. These compounds exhibit positive photochromism regardless of the type of anion or substituent. − …”

Nitro–Nitrito Photoisomerization of Cationic Platinum(II) Complexes in the Solid State: Reactivity in Polymorphic Crystals and Glassy State

“…Correlation between the reactivity and reaction cavity has also been found for various reactions in molecular crystals. [22][23][24][25] In this study, to further elucidate the effect of the packing structure on the reaction, we synthesized salts of the Pt II complex with PF6 and (CF3SO2)2N anions (1-PF6 and 1-Tf2N, Figure 2). 1-PF6 formed three polymorphs and a pseudopolymorph, whereas 1-Tf2N was an ionic liquid exhibiting both crystalline and glassy states at low temperature.…”

Nitro–Nitrito Photoisomerization of Platinum(II) Complexes with Pt(NO2)42– and (FSO2)2N– Anions: Correlation between Isomerization Ratio and Reaction Cavity

“…Correlations between the reactivity of complexes and their reaction cavities have been found in various molecular crystals. [24][25][26][27]…”