Chromism causes

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

Attempts to photoswitch the intramolecular magnetic interaction based on photo-chromism of diarylethenes have been overviewed. The switching between the disjoint and non-disjoint structures caused a change in the interaction between the separated spins. Photochromic diarylethene is one of the most favorable photoswitching units for magnetic interactions. This system has the possibility to be applied to the molecular-scale information processing system [73-77]. 

The UV/Vis absorption of solute molecules can be influenced not only by surrounding solvent molecules but also by other environments, e.g. embedment in solids, polymers, glasses, micelles, or proteins, as well as adsorption on solid surfaces. Therefore, the more general term peri-chromism (from the Greek peri = around and chroma = colour) has been recommended (Prof. E. M. Kosower, Tel Aviv/Israel, private communication to the author). Solvatochromic shifts caused by dye inclusion into protein interiors have been called enzymichromism [438]. 

Specific colour change phenomena are named using the suffix chromism and a prefix that describes the stimulus causing the change. Table 1 provides a list of phenomena and the stimulus involved, lonochromism, thermochromism, photochromism, and electro-chromism are the most extensively studied. The others listed are generally less well-established and have either experienced limited commercial exploitation or are essentially academic curiosities. Indeed, the list in the table is probably not exhaustive. 

Chromism is the ability of a material to change its colour according to a stimulus. Chromic materials might also be called chameleonic materials , due to their ability to adapt to their environment [34]. The types of chromism are defined after the different stimuli that cause their change, such as ... 

Typical examples are solvatochromism and thermochromism. The former chromism is observed upon dissolution of the polymers in solvent. This effect is caused by the localization of the electronic wave function as a result of the disorder introduced by the coiled (random) conformation [48]. These chromisms depend largely upon solvent quality. Choice of suitable side-groups on the polythiophene backbone brings about interesting chromism in the solid form as well. Copolymerization supplies further variations in the arrangement of the side-groups on the backbone and, hence, produees specific effects. 

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. 

Phenomenologically chromism is a color change in response to some external stimulus. The name will vary depending on the type of external stimulus. If color change results from polarity of a solvent, it is called solvatochromism. Photochromism is caused by light and electrochromism is caused by electric fields. Other stimuli such as heat, pressure, or magnetism provide the names thermochromism, piezochromism, and magnetochromism, respectively. 

These chromisms were first recognized for a series of polydiacetylene [39] and polysilane [40] compounds. In these systems, extent of the disorder can be mea-smed by deviation from the fully-stretched aM-trans backbone conformation [41]. In the case of the poly thiophene and its derivatives, the conformational disorder is caused by the distortion around the cr-bonding interconnecting the thiophene rings [42]. The TT-delocalization along the polythiophene backbone will be maximized when the polymer chains assume the fully-stretched S-anti form. This delocalization will be hampered by the ring distortion of any amount. 

Solvatochromism (12, 13). In solvatochromism the dissolution of a substance is accompanied by color formation, but the cause is not the formation of salts. On the other hand, if strongly colored salts are formed under the influence of mineral or Lewis acids, the process is called halo-chromism. The difference between the two phenomena is also apparent in the changes of the type of absorption spectra. In the case of halochromism absorption curves are radically changed (old bands disappear and new ones are formed), but in the case of solvatochromism the character of absorption curves is usually unchanged, and they are only shifted (as a whole) to shorter or longer wavelengths. Halochromism will be treated below. 

---