Crystal Structures During Photoisomerization

Powder X-ray diffraction has verified that the isomerization occurs via a crys-tal-to-crystal reaction process, and that the diffraction profiles of the crystals after photoirradiation consist of overlapped patterns of diffraction due to the crystals of 10 and 11. This indicates that the crystal domains of each isomer exist simultaneously in the crystals accompanied by crystal phase separation during the photoisomerization. Single crystal structure analysis has disclosed that the crystals of 11 as the photoproduct have a symmetry different from that of the starting crystals of 10 (Fig. 14). 

The (Z,Z)-isomers provided either the corresponding dimers or (E,E)-isomers during UV irradiation. During the photoreaction of (E,E)-isomers, some crystals provided the dimer and others showed no reaction. Neither (Z,Z)- nor (E,Z)-isomers were formed because the photoisomerization of muconates occurs via a one-way reaction mechanism. On the basis of the results of single crystal structure analysis, various molecular packing structures were observed, depending on the number and... 

The alkyl turn isomerization is very favorable in the crystalline-state reaction. To explain the movement of the 4-cb group, the reaction cavity for 4-cb in the crystal structure before irradiation is shown in Fig. 6.22a. The reaction cavity after irradiation, which includes both of 4-cb and 1-cb, is shown in Fig. 6.22b. Since the two cavities are very similar, it is reasonable to assume that the reaction cavity does not change to a great extent during the photoisomerization. 

Fig. 5.15 Schematical illustration of photoisomerization of molecular motor, a Molecular structure of chiral motor 21. b Polygonal texture of a LC film doped with 1 wt% chiral motor 21. c Glass rod rotating on the LC during irradiation with ultraviolet light. Frames 1-4 (from l ) were taken at 15 s intervals and show clockwise rotations of 28° (Irame 2), 141° (frame 3) and 226° (Same 4) of the rod relative to the position in fiame 1. Scale bars, 50 pm. d Surface structure of the liquid-crystal film (atomic force microscopy image 15 pm ). Reproduced with permission from [103]. Copyright 2006 Nature Publishing Group...

---