Photochromism and Thermochromism

J.C. Crano and R.J. Guglielmetti (Eds.), Organic Photochromic and Thermochromic Compounds, Volume 1, Main Photochromic Families, Plenum Press, New York, 1999. 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

Photochromism and thermochromism derived from proton tautomerism. 244... 

Following the pioneering work of Hirshberg,[421 the photochromism of spiropyrans has been extensively studied.[43] The photochromic and thermochromic behavior of this class of compounds is due to the interconversion of the closed spiropyran form to the open merocyanine form (Scheme 15). UV irradiation of the closed form 28 results in ring-opening to the zwitterionic form 29, which reverts to the closed form either thermally or on irradiation with visible light. 

This survey of organic photochromic and thermochromic compounds focuses on the main families that are involved in existing commercial applications, such as variable optical transmission materials (ophthalmic glasses and lenses), or in potential uses such as optical storage (optical disks or memories). 

B.-Ya. Simkin, V. I. Minkin, and L. E. Nivorozhkin, Photochromic and thermochromic spirans. IX. Prediction of the stabilities of spiropyran structures and the electronic absorption spectra of their photocolored and thermocolored isomers, Chem. Heterocycl. Cpds., 1978, 948-959. 

Malatesta 43 investigated the photochromism and thermochromism of spirooxazines in cationic (CTAB, hexadecyltrimethylammonium bromide) and nonionic (TX-100) micellar solutions, and in sodium bis-2-ethylhexylsulfosuccinate (AOT) toluene-water inverted micelles. The thermo- and photocolorability increased in TX-100 and CTAB micelles and decreased in inverted micelles. 

G. Favaro, F. Ortica, and V. Malatesta, Photochromism and thermochromism of spiro(indolinoox-azines) in normal and reversed micelles, J. Chem. Soc., Faraday Trans., 91(22), 4099 410.3 (1995). 

K. Kaftory, Photochromic and thermochromic compounds. I. Structures of(E) and (Z) isomers of 2-isopropylidene-3-[l-(2-methyl-5-phenyl)-3-thienylethylidene] succinic anhydride and the photoproduct 7,7a-dihydro-4,7,7,7a-tetramethyl-2-phenylbenzo[6]1hiophene 5,6-dicarboxylic anhydride (P), Acta Cryst., C40, 1015-1019 (1984). 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

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