Photochromism in the Acid-Base Complex Crystals

The color fading processes after UV irradiation for 3-a and 3-b are shown in Fig. 7.14. Although the red color of 3-a disappeared after 30 s, that of 3-b returned to the original color after 10 min at room temperature. 

The crystal structures of 3, 3-a, and 3-b before irradiation were determined by X-rays. The two salicylideneaniUne molecules, 3, make a dimer using the 3-carboxyl group around an inversion center. The dihedral angle of the two phenyl rings is 26.72(6)°. The two hydrogen bonds are formed between 03 and 02 of 3 and N2 of 

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