Chromism mechanisms

Anils. A large number of anils have been reported to be photo-chromic in the solid state as typified by salicylidene-2-chloroaniline shown in Figure 4 (21). Early explanations for the mechanism of photo-chromism in the anils involved aggregation and crystal lattice interactions (22-26). However, recent investigations by Cohen et al. (21,27,28) show that isolated molecules in glassy solutions also exhibit photochromic activity, thus ruling out such specific solid-state interactions as prerequisite to photochromism. 

The presence of the ortho-hydroxy group is essential for the observation of both types of chromisms, and the mechanism involves intramolecular proton transfer via a six-membered-ring transition state, producing enol-keto tautomeric species, with the spectra of the keto forms showing a bathochromic shift (Scheme 16). 

In the case of Mannich bases (250) showing thermo- (and photo-)chromism, the proposed mechanism is quite similar123,124 (Scheme 23). 

Photochromlsm.—As reported last year, two publications 320> 321 on the photo-chromism of bianthrones present rather conflicting evidence for the mechanism of formation and decay of intermediates in this system. A study of the compounds (17), (18), and (19) has concentrated on the role of triplets in the... 

In addition to the polythiophenes, polydiacetylene and polysilane systems, for example, have also proven to be partieularly interesting from the point of view of ehromism [88-93]. Detailed studies of polydiacetylene derivatives [94] indicate that a single chain mechanism for the rod (ordered)-eoil(disordered) transition is the major cause of chromism. 

Mechanism of the Piezochromism of Hexaphenylhiimidazole. Toro Hayashi and Koko Maeda (Ochanomizu Univ., Tokyo). Bull Chem. Soc. Japan 38 (4), 685-6 (1965) (Eng.). Hexaphenylbmni-dazole exhibits photochromism, thermo-chromism, and piezochromism. The color change is effected by grinding or by pressure (10-50 kg./cm. ). In each instance, the color is due to the triphenyUmida-zolyl radical. 

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