Single‐crystalline photochromism of diarylethene dimers bridged by a spiro structure

New types of diarylethene dimers bridged by a spiro structure were synthesized. One of their dimers formed three kinds of polymorphic forms by recrystallization from different solvents: hexane (1a‐α), acetone (1a‐β), and acetonitrile (1a‐γ). In crystals of 1a‐α and 1a‐β the molecules are packed in solvent‐free crystal structures, whereas crystal 1a‐γ includes acetonitrile molecules in the crystal. The difference of significant photochromic reactivities in their polymorphic crystals was observed for photocoloration reactions. Crystals 1a‐β and 1a‐γ showed photochromism in the single‐crystal phase, but 1a‐α did not do so. Their photochromic reactivity was found to depend on the distance between the reactive carbon atoms by X‐ray crystallographic analysis. When the distance is less than 4.2 Å in the antiparallel conformation, the molecule can undergo photochromism in the crystal. On the other hand, crystals of bromo‐substituted diarylethene dimer (2a) cannot be suitable for X‐ray crystallographic analysis when it was recrystallized from hexane, acetone, and acetonitrile. However, it formed single crystal by recrystallization from p‐xylene, which showed photochromism in the crystalline phase. The edge‐to‐face aromatic interaction between p‐xylene molecules assisted the crystal growth of the diarylethene dimer. Upon irradiation with ultraviolet light, two kinds of the enantiomers, (M,SS)‐ and (P,RR)‐2b, are produced. Partial photobleaching reactions of either of the two enantiomers by irradiation with linearly polarized visible light were confirmed by polarized absorption spectroscopy.