Crystal Phase Transition and Solid-State Photoisomerization of Benzyl (<i>Z</i>,<i>Z</i>)-Muconate Polymorphs Studied by Direct Observation of Crystal Structure Change

We investigated the polymorph structures of benzyl (Z,Z)-muconate crystals (ZZ-Bn-α, β, and γ) and their EZ photoisomerization behavior in order to clarify the EZ isomerization mechanism of diene compounds in the solid state. First, the thermodynamics and crystal phase transition of the polymorphs were investigated by differential scanning calorimetry and X-ray diffraction. A single-crystal-to-single-crystal transition was observed during the change in the polymorph structures from the ZZ-Bn-β to the α form. We also observed in situ the migration of the boundary between the different polymorph domains during the phase transition of the ZZ-Bn-γ to the α form by optical microscopy. We then investigated the change in the crystal structure of ZZ-Bn during the EZ photoisomerization in the solid state. The photoisomerization of ZZ-Bn to the corresponding EE isomer (EE-Bn) proceeded according to a bicycle-pedal model via a single-crystal-to-single-crystal topochemical reaction mechanism, which was directly confirmed by an X-ray single-crystal structure analysis. During the photoirradiation of the polymorph crystals of ZZ-Bn-α and β, a structural strain was accumulated in the crystal lattice and finally caused a phase transition accompanied by a change in the space group from P21/c to P21, resulting in a stable crystal structure identical to that of the recrystallized EE-Bn. On the other hand, the ZZ-Bn-γ crystals became amorphous during the photoirradiation because of the greater molecular motions in the crystals. Thus, we have demonstrated the validity of the in situ single-crystal structure analysis for investigating the EZ photoisomerization of diene compounds in the solid state.