Photochromism of Viologens

When viologens are embedded in appropriate polymer matrices, the viologen cation radicals are stabilized by the surrounding solid matrices through restriction of both air-oxidation and the thermal reverse-electron transfer. The resulting color changes in thin polymer matrix films can often be observed visibly upon exposure to ordinary sunlight or a steady-state irradiation.8 

---

This chapter relates to the photochromism of viologens and thionine dyes proceeding by electron transfer. The colored species spontaneously disappear in a liquid phase, because the rate of back electron transfer (dotted line in Figure 9.1) is rapid and is accelerated by the existence of oxygen in the system. If the compound is dispersed in a solid-state or layered system, the reversible reaction can be controlled. Since the color development via electron transfer is generally very rapid and the reverse electron transfer can also be carried out electrochemically,4 the combination of both photoreduction and electrooxidation will receive much attention for application in recording devices. 

Figure 9.4. Relationship between energy and reaction coordinate indicating photochromism of viologen in PVP matrix film.

Polymers incorportating viologen units in their main chains were also prepared (Figure 6 see Appendix for details of the procedure).46,47 The effects of counterions and additives on the photochromism of viologen ionene polymers... 

Some of the major developments involving photochromism will very likely utilize other phenomena, in conjunction with photochromism, resulting in useful applications. Some of the work discussed in the chapters of this book illustrate the concept of coupling photochromism with another phenomenon. For example, combining the photochromic electron transfer reaction of viologens with electrochemistry shows promise in the development of electronic applications (Chapter 9). 

This section relates to the reversible color change of viologens via a photoreduction mechanism principally in the solid state. The effect of the kind of viologens (bipyridinium rings and counteranions) and polymers on photochromism is discussed. The viologens and their abbreviation under discussion in this section are given in Scheme 3. 

Table 9.1 summarizes the effects of N-substitution and counterions on the photochromic behavior of viologens. The rate of electron transfer decreased withN-substitution as follows dibenzyl > benzyl, propyl > dipropyl and with the counteranion, Cl > Br-> I-> BF4T The order is consistent with the first dark redox... 

Since the developed color of viologens induced by photoreduction is vivid, and a variety of color species can be obtained by changing their chemical structure, it can be expected that there are several kinds of potential applications using photochromic viologens, especially an erasable-direct-read-after-write (EDRAW) media for recording devices with high storage density and reliability. 

H. Kamogawa and T. Ono, Redox photochromism in films of viologens, Chem. Mater. 1991, 1020-1023. 

M. Yamana and T. Kawata, Electrochromism of viologen derivatives, Nippon Kagaku Zasshi, 1977, 941 H. Miyata, Y. Sugahara, K. Kuroda and C. Kato, Synthesis of montmorillonite-viologen intercalation compounds and their photochromic behavior, J. Chem. Soc., Faraday Thins. /., 1987, 83, 1851. 

We have demonstrtaed that viologen dications 55 and bisviologen trications 56 (X = PF ) in polyvinylpyrrolidone films give blue radical-cations on irradiation being thermally reversible, thus showing clear photochromism of this system. ... 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

The peak shift due to aggregation is observed not only in LBK films containing azobenzene chromophores, but also for other chromophores with extended Ji-systems, such as viologen polymers. For monolayers of the poly(p-phenylene sulfonate) 9/ dioctadecyldimethylammonium bromide (DODA) complex, the peak shift due to aggregation results in a piezo-chromic effect—that is, upon compression of the monolayer, a significant shift of the poly(p-phenylene sulfate) A band is observed (see Figure 6.9). This photochromic effect has been shown to be based on the improved 7t-Jt interaction upon compression of the monolayer. ... 

This chapter, far from being exhaustive, is aimed at outlining the properties of the radical ions and of the free radicals derived from photochromic compounds independently of whether or not these paramagnetic species are involved in the photochromic process itself. In particular, it will deal with the EPR of dihydro-and dialkyldipyridyl radical cations (viologens) and of the radical anions of a variety of nitro-substituted spirofmdoline-benzopyrans], spiro[indoline-naphtho-pyrans], and spiro[indoline-naphthoxazines] as well as their triphenylgermyloxy nitroxides and with the triplet spectra observed upon irradiation of 2,2,4,4-tetrachloro-1 -keto-1,4-dihydronaphthalene. 

Photochromic materials have been widely used in textiles. " °°Typical organic photochromic compounds include azobenzenes, spiropyrans, sprroooxaztnes, viologens, fulgides, 1,4-dihydroxy anthraquinone and diarylethenes.They are based on different photochromisms, that is, isomerization of molecules, ionization of molecules and redox reaction of molecules. Azobenzenes, spiropyrans and viologens have been most widely used in textiles. 

Nanasawa has reported studies on new photochromic viologens such as 4,4 -bipyridine 51, 2,2 -bipyridine, and 1,10-phenanthroline based on electron transfer. The colorless viologens composed of diquaternary salts turn into a violet-blue radical-anion 52 by one-electron reduction - and are further reduced to the yellowish quinonoid 54 via the biradical 53. New results in crown ether containing viologenes, in the crystalline state as well as in polymer were reported recently. 

---