Photo and thermochromism

The initial product 38, from aqueous basic potassium hexacyanofer-rate(III) oxidation, was first formulated as the 4,4 -adduct and later corrected to the 2,2 -isomer on the basis of IR spectra, although it is still apparently given the 4,4 -structure in later papers. It is piezochromic, and it also rapidly isomerizes in solution in the cold, via the triarylimidazolyl radical, to the l,2 -diraer 39, which is both photo- and thermochromic. The method of choice for preparing compounds 39, which are important materials in photo- and thermocopying processes (Section IV,A,2), is to carry out the oxidation by using basic hexacyanoferrate(III) at room temperature in the presence of benzene. ... 

The dimer 43, which is piezo-, photo-, and thermochromic, is formed together with two 4//-imidazole products on hexacyanoferrate(IIl) oxidation of a p-phenylene-bis-1//-imidazole precursor. ... 

As already mentioned, most interesting are the properties of the aziridines connected with cleavage of the C-C bond of the sterically strained three-membered cycle, namely, thermo- and photoinduced cycloaddition reactions. Photo- and thermochromism inherent to these compounds are also linked with the breaking of this bond in the literature. 

Bernard Mark Heron was born in Workington, England in 1965. After graduation (GRSC) from Lancashire Polytechnic (Preston) in 1987 and a brief period in industry he obtained his PhD (CNAA) in Benzothiopyran Chemistry in 1992 under the supervision of Professor John Hepworth at the University of Central Lancashire. A postdoctoral fellowship in heterocyclic chemistry (1992-95) and an industrially funded lectureship at Central Lancashire (1995-98) were followed by appointment to a James Robinson Lectureship at the University of Hull (1998-2000). Dr. Heron was appointed as a senior lecturer (2000-present) in the Department of Colour and Polymer Chemistry at the University of Leeds. His research interests include the chemistry and applications of heterocyclic compounds, color chemistry, and organic photo- and thermochromic materials. 

B. S. Luk yanov, L. E. Nivorozhkin, and V. I. Minkin, Photo- and thermochromic spirans 18. [Note an earlier paper (ref. 183) is also numbered 18 in this series] Indolinospirochromenes with n-acceptor substituents in the 8 position, Chem. Heterocycl. Cpds., 1993, 152-154. 

B. Ya. Simkin, S. P. Makarov, V. I. Minkin, and V. A. Pichko, Photo- and thermochromic spirans. 18. Theoretical study of the mechanisms of the photocolorization and photodecolorization of 2H-pyrans and their structural analogs. Triplet state, Chem. Heterocycl. Cpds., 1991, 250-255. 

It has therefore been concluded that any further modification of the photo-chromic compound 1 should be associated with structural variations in its 2,3-dihydroperimidine moiety. In the course of subsequent studies, the spirocyclohex-adienones belonging to structural types 1-9 have been found to display photo- and thermochromic properties. 

The first representative of a new group of photo- and thermochromic compounds in which the naphthalene fragment in spiroperimidine (1) is changed to a quinoline moiety has been obtained by coupling 2.6-di-fert-butyl-l, 4-bcnzo-... 

There are many types of photochromic and thermochromic materials. Due to the introductive nature and consequent brevity of this work, only selected subject areas will be examined. This work is intended to introduce product designers, chemists, color matchers, and laboratory technicians to the special colorant area of photo-and thermochromics. 

This section describes the characteristics of the photo- and thermochromism derived from proton tautomerization in crystals, using the photo- and thermochromic behaviour of 2-alkylisophthalaldehydes, salicylideneanilines, and dinitrobenzylpyridines as typical examples. 

I. Cabrera, V. Krongauz, and H. Ringsdorf, Photo- and thermochromic liquid crystal polysilox-anes, Angew, Chem. Int. Ed. Engl. 26, 1178-1180 (1987). 

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). 

B. Y. Simkin, V. I. Minkin, and L. E. Nivorozhkin, Photo- and thermochromic spirans. IX. Predicting of stability of spiropyran structures and electron absorption spectra of their photo- and thermochromic isomers, Khim. Geterotsikl. Soedin. 1978, 1180-1192. 

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... 

Minkin and co-workers35 recently developed a series of novel photo- and thermochromic perimidine spirocyclohexadienones for which the mechanism involved a ring-chain tautomerism. For instance, for 48 (Scheme 6) the absorption maxima for the closed and open forms are, respectively, 390 and 584 nm (log e = 3.8) in octane as solvent. 

The molecular conformation and the possibility or impossibility of stack packing in the crystal have been shown to be responsible for photo- and thermochromism.60 Molecules for which the less favorable planar structure is stabilized by stack packing possess thermochromic properties. The strong inter-... 

On the other hand, in photochromic crystals, with no stack-packing, a nonadiabatic acoplanarization occurs following the phototransfer, leading to an absorption at wavelengths up to 540 nm. A comparison of the photo- and thermochromic conversions for salicylideneanilines is given in Figure 7. 

From semiempirical calculations by the CNDO/2 method, Aldoshin and Atovmyan63 calculated the relative energy of photo- and thermochromic salicy-lidenearylamines. In all cases, a twisted conformer is the most stable, but the energy gap between the conformer and the planar form is narrow (lOkJmol-1). 

Figure 7. Representation of photo- and thermochromic conversions of JV-salicylideneaniline.62...

The salicylidene-6-aminomethyldihydroabietates 173 and 174 (Table 4) are other examples of molecules exhibiting photo- and thermochromic behavior in spite of the direct conjugation of the imino nitrogen atom with a phenyl ring.75 76... 

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. 

S. M. Aldoshin, L. O. Atovmyan, and O. S. Filipenko, Structural aspects of photo- and thermochromic conversions of crystal salicylalarylimines, Proceedings of the 14th Conference on Applied Crystallorgraphy. Vol. 2, pp. 483 188 (1990). 

V. N. Komissarov, V A. Kharlanov, L. Yu. Ukhin, E. Yu. Bulgarevich, and VI. Minkin, Photo-and thermochromic Mannich bases. Derivatives of 2,6-di-fert-butylphenol and aromatic ortho-hydroxy aldehydes, Zh. Org. Khim. 28,513-517 (1992). 

Stabilized radicals, and so on [1]. An unprecedented example is the in situ observation of a light-induced radical pair produced in a crystal of 2,2 -di (p-chlorophenyl)-A,A, 5,5 -tetraphenyl-biimidazole (o-Cl-HABI) by X-ray diffraction, as shown in Scheme 7.1 [2], HABl derivatives are well known as the photo- and thermochromic compounds, the polymerization photo-initiators in imaging materials, and holographic photopolymers since it was prepared in 1960 [3-5], It has been proposed that the photolysis of the H ABI derivatives in benzene solution instantaneously produces a pair of lophyl (triarylimidazoyl) radicals. The lophyl radical has been proposed to be an initiator for the radical polymerization. Despite the intriguing properties of HABI derivatives, there is very few report [6]. 

V.A. Linskii, T. Gradyushuko and M.I. Cherkashin, Effect of electron donor and electron acceptor substituents on photo and thermochromic properties of indoline spiropyranes. Isv. Akad. Nauk. SSR, Ser. 

J. B. Flannery, Jr., Photo- and thermochromic transients from substituted... 

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