Spiropyran thermochromic

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). 

Spiropyrans, and spirooxazines, better known for their photochromic behaviour (see sections 1.22 and 1.23), also exhibit thermochromism. The ring opening to produce the highly coloured merocyanine form is induced by heating either the solid or... 

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). 

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. 

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. 

F. P. Shvartsman and V. A. Krongauz, Quasi-liquid crystals of thermochromic spiropyrans. A material intermediate between supercooled liquids and mesophases, J. Phys. Chem., 88, 6448-6453 (1984). 

The previously mentioned photochromic spiropyrans also exhibit thermochromic activity upon melting [56-58], The melts are usually purple, red, or blue [19] and the color change is reversible however, the stability of the spiropyran/merocyanine composition is low at the thermochromic temperature transition range. As a consequence, the useful thermochromic lifetime of the spiropyrans is relatively short. Other molecular types that exhibit both photochromic and thermochromic activity are the indenone oxides, anils, spirooxazines, and chromemes. 

Bertelson RC. Spiropyrans In Crano JC, Guglielmetti RJ, Eds. Organic Photochromic and Thermochromic Compounds. Main Photochromic Families. Vol. Vol. 1. New York Plenum Press, 1999 11-83. 

I. Cabrera, V. Krongauz, and H. Ringsdorf, Photo- and thermochromic liquid crystal polymers with spiropyran groups, Mol. Cryst. Liq. Cryst. 755, 221-230 (1988). 

V S. Marevtsev, L. L. Zaichenko, V D. Ermakova, S. I. Beshenko, V. A. Linskii, A. T. Gradyushko, and M. I. Cherkashin, Effect of electron-donating and electron-accepting substituents on photo- and thermochromic properties of indolino-spiropyrans, Bull. Acad. Sci. USSR, Chem. Sci. 29, 1591— 1596 (1980) Chem, Abstr. 94, 29901 (1980). 

S. M. Aldoshin, V. A. Lokshin, A. N. Rezonov, N. V. Volbushko, N. Shelepin, M. I. Knyazhanskii, L. O. Atovmyan, and V. I. Minkin, Structures and photo- and thermochromic properties of spiropyrans of the 2-oxaindan series with polycondensed chromene fragments, Chem, Heterocycl. Compd. 1987, 614-624 Chem. Abstr. 108, 131516 (1987). 

As distinct from photochromic spiropyrans and spirooxazines, [2//J-chro-menes do not contain a heterocyclic fragment such as an indoline nucleus. Consequently, in such compounds there are no specific n-a orbital interactions that elongate and weaken the C, iro—O bond, the ruptures of this bond in the excited and ground states being responsible for the photo- or thermochromic properties of spiropyrans and spirooxazines as well as [2// -chromcncs. 

B. Y. Simkin and V. I. Minkin. Photo- and thermochromic spiropyrans. XI. Calculation of relative stability of spiropyran valency tautomers, Khim. Geterotsikl. Soedin. 1980, 177-184. 

The thermochromism of spiropyrans, discovered in 1926, has been extensively studied. Nearly every known compound of this class leads to deep color on melting (generally red, purple, or blue). However, heating solutions of spiropyrans also causes coloration. Day1 in his review gave a table reporting essentially thermochromic spiropyrans of the indoline and spirobipyran series. Bertelson6 summarized the main spectroscopic and physicochemical data obtained up to 1971. Thermochromic properties of spirooxazines have been more recently reported.7... 

During the past decade, new thermochromic spiropyrans have been described, particularly by Russian teams. For example, spiropyrans of the 2-oxaindane or azaindanone series with polycondensed chromene fragments (1-8), exhibiting photo- and thermochromic properties, have been synthesized by Minkin and co-workers.19,20... 

Krasieva and co-workers21 mentioned the thermochromic behavior of spiropyrans of the dithiolane series (9, 10). These data confirm that the annellation of the benzopyran moiety favors the thermochromic properties of this class of compounds. 

Although the thermochromic properties ofmonospiropyrans (with one pyran ring) have been described extensively, bis-spiropyrans with various structures have also been studied. For bis-spiropyrans in which the merocyanine fragments are... 

With compounds exhibiting negative thermochromism (i.e the colored form is more stable than the colorless form), the concentration of the merocyanine form is high enough to allow the relative concentration of each species to be measured by 1H - NMR spectroscopy. Using this technique, Nishimura et al46 have estimated the equilibrium constant for the spiropyran 54 to be 2.83 at 25°C in DMSO-ck as solvent. 

During the past 12 years (the period covered by this review), very few new thcrmochromic molecular systems have been reported. The main families of compounds have continued to be the major focus of interest, with only a shift in the balance between them for example, spiropyrans have been less studied (in terms of development of new compounds) while interest in anils has increased. There has also been a shift away from work essentially devoted to the description of new compounds, as, in recent years, the attention of authors has mainly been directed toward thermodynamic, kinetic, structural, and mechanistic aspects. For this purpose, a large variety of techniques, including not only spectroscopic techniques (UV-visible, NMR) but also X-ray diffraction and theoretical calculations, have been employed, leading to a better knowledge of the fundamental aspect of the thermochromism of organic compounds. 

S. R Makarov, B. Ya. Simkin, and V. L Minkin, Photo- and thermochromic spirans. 17. Theoretical prediction of barriers of thermal reactions involving valence isomerization of spiropyrans, Chem. Hetervcycl. Compd. 1988, 140-145. 

J. Zhou, Y. Li, Y. Tang, F. Zhao, and X. Song, A new method forthe determination of thermochromic equilibrium constant of indoline spiropyran, Wuli HuaxueXuebao 11, 97-100 (1995). 

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