Chromism

Expt. no. Combination of solvents Mixing ratio (w/w) Refractive index Color change with pressure Before After Piezo- chromism... 

Kaye and Chang38 found that the highly syndiotactic PU had a larger hypo-chromism than the less syndiotactic ones in aqueous media. The authors found interesting results on the thermal melting of PU-PA complexes (see Fig. 4). The melting... 

In this chapter, photoelectrochemical control of size and color of silver nanoparticles, i.e., multicolor photo-chromism [1], is described. Silver nanoparticles are deposited on UV-irradiated Ti02 by photocatal5dic means [2]. Size of the nanoparticles can be roughly controlled in the photocatalytic deposition process. However, it is rather important that this method provides nanoparticles with broadly distributed sizes. The deposited silver nanoparticles are able to be dissolved partially and reduced in size by plasmon-induced photoelectrochemical oxidation in the presence of an appropriate electron acceptor such as oxygen. If a monochromatic visible light is used, only the particles that are resonant with the light are dissolved. That is, size-selective dissolution is possible [3]. This is the principle of the multicolor photochromism. 

Scheme 10.3 General scheme for reversible photo chromism...

It is important, however, to realize that whilst many types of chemical species exhibit electro-chromism, only those with favorable electrochromic performance parameters1 are potentially useful in commercial applications. Thus, most applications require electrochromic materials with a high contrast ratio, coloration efficiency, cycle life, and write-erase efficiency.1 Some performance parameters are application dependent displays need low response times, whereas smart windows can tolerate response times of up to several minutes. 

M. Leclerc and K. Faid, Conformation-induced chromism in conjugated polymers, in Handbook of Conducting Polymers, T.A. Skotheim, R.L. Elsenbaumer, and J.R. Reynolds, Eds., Marcel Dekker, New York, 1998, pp. 695-706. 

Other Chromisms lono-, Piezo-, Electro-, and Magnetochromism 609... 

Complex 11 formed good Langmuir-Blodgett (LB) films, which could be transferred to hydrophilic and hydrophobic quartz surfaces. On the former, the UV Amax was found to be 322 nm, and on the latter, 306 nm. This remarkable chromism originates in the chain extension induced by hydrogen bonding with the hydrophilic surface. In... 

Figure 18 UV chromisms of 87 dependent on (a) molecular weight and (b) solvent.206 Reprinted with permission from Saxena, A. Fujiki, M. Naito, M. Okoshi, K. Kwak, G. Macromolecules 2004, 37, 5873-5879, 2004 American Chemical Society.

Most polymers do not form crystals suitable for single crystal X-ray diffraction, so powder or film methods are usually employed. X-ray and LJV data recorded at various temperatures provide the detailed information required to correlate conformational and electronic properties, since the former is sensitive to the inter- and intrachain packing, and the latter is sensitive to the conformation. DSC provides further evidence for any phase transitions. Detailed studies have been performed by Winokur and West,260 261 who reported a comparison of the polymorphism, structure, and chromism in poly(di- -octylsilylene), (Si- -Oct2), 89, and poly(di- -dccylsilylcnc)(Si- -Dcc2) , 90. These investigations will be described in detail for the useful insights into polysilane structures that they afford. 

Okada S, Peng S, Spevak W, Charych DH (1998) Color and chromism of polydiacetylene vesicles. Acc Chem Res 31 229-239... 

Weak coupling leads to minor alterations of the absorption spectrum (hypo-chromism or hyperchromism, Davidov splitting of certain vibronic bands). 

Buncel E. and Rajagopal S. (1990) Solvato-chromism and Solvent Polarity Scales, Acc. Chem. Res. 23, 226—231. 

R. H. Sprague, L. P. Smyth and G. L. Lewis Color and constitution. I. Halo-chromism of anhydronium bases related to the cyanine dyes. J. Amer. chem. Soc. 62, 1117 (1940). 

Conformational transitions and electron redistribntion on pressure results in changes of the ion-radical properties and this is essential for optoelectronic devices. Pressure is the thrust distributed over a surface. The most important results of compression are reduction of molecular volumes and conformational changes of organic componnds. Spectral changes of solids are also termed piezo-chromism. Let ns consider several relevant examples of piezochromism. 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

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). 

Solvatochromism can be defined as the phenomenon whereby a compound changes colonr, either by a change in the absorption or emission spectra of the molecnle, when dissolved in different solvents. It is one of the oldest of the chromisms, having been described as long ago as 1878, but nowadays it is usual to extend the concept of the solvent to inclnde solids, micelles and films. A textbook published recently covers in detail the theoretical aspects of solvatochromism. Consequently in this section, the theoretical aspects will be dealt with only briefly before moving onto the practical applications of the topic, which have increased noticeably in the last decade. ... 

Many of the other chromisms that have been described are irreversible and much less important in terms of commercial applications. However, they do display interesting behaviours, which could well prove to be of value in the future. 

The chemistry of spiropyrans has been reviewed (48CRV(43)509) whilst a review on thermochromism deals with spiropyrans and dixanthylenes (711) (63CRV65). The photo-chromism of spiropyrans has also been discussed (B-71MI22405). 

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. 

---