Quinones photochromism

At present, the photochromism of quinones is explained by the reversible photoinduced para-ana-quinoid transformations due to photochemical migration of different hydrogen, aryl, and acyl groups (Scheme 1). In this chapter the results of the studies on quinone photochromism have been systematized with regard to recent advances in this field. 

Analysis of experimental and theoretical data on quinone photochromism shows that their photochromic transformations are caused by photoinduced para-ana-quinone reconstruction of molecules. Photochromic changes involve triplet states of the initial and photoinduced forms and, probably, the intermediate a-spirocomplex. 

Studies of the efficiency of quinone photochromism showed that the introduction of electron-donor substituents in the quinone cycle as well as the bulky substituents in the migratory group reduced the efficiency of the photoinduced para- ana-quinone rearrangement. At the same time, the introduction of electron-donor substituents in the migratory phenoxy group was favorable for photochromic transformations. On the contrary, the introduction of electron-acceptor substituents decreased the efficiency of the phototransformations. These experimental data agree well with the known concept of the photochromic transformations of these compounds as reversible intramolecular photoinduced substitutions. 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

Burnham, K. S. Schuster, G. B. A search for chiral photochromic optical triggers for liquid crystals photoracemization of l,l -binaphthylpyran through a transient biaryl quinone methide intermediate. J. Am. Chem. Soc. 1998, 120, 12619-12625. 

The only very slight photochromic behaviour obtained upon irradiation of bleached aspen CTMP may be due to the presence of syringyl instead of guaiacyl units in hardwood lignin. These structures are less able to form orr/to-quinones during irradiation than softwood pulps (8). 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

Chapter 7 (Photochromic quinones). The mechanism of the photochromic reaction of certain quinones involves proton or group transfer (Scheme 7). 

In chapter 7, special emphasis has been placed on the synthesis of representative polycyclic quinones and their photochromic behavior, including the spectral, kinetic, and fatigue characteristics of such systems. Potential applications are focused on recording and multiplication of images, optical memories, and gradation masking. 

At present, a number of photochromic quinones have been synthesized, including derivatives of naphthoquinone (I), substituted anthraquinones (II, IIA-IID) and naphthacenequinones (IIIA—IIIF), pentacenequinones (IV), and phtha-loylpyrenes (V) (Figure 7.1). The family of photochromic anthraquinones involves... 

A number of photochromic phenoxynaphthacenequinones were obtained using the initially synthesized 6-substituted ll-phenoxynaphthacene-5,12-quinones (IIIA) (Scheme 4).46 6,11-Dichloro-naphthacenequinone was the initial compound for the synthesis of these 6-chloro-ll-phenoxynaphthacenequinone compounds. 6-Chloro-1 l-tosylaminonaphthacene-5,12-quinone was obtained by interaction with phenox-ide potassium in DMSO and /j-tolucncsulfamidc in nitrobenzene, respectively. The latter compound was hydrolyzed in sulfuric acid to form 6-chloro-l 1-aminonaphtha-cene-5,12-quinone. The treatment of 6-chloro-l l-phenoxynaphthacene-5,12-quinone and 6-chloro-l l-aminonaphtacene-5,12-quinone with potassium phenoxide gave rise to 6-tosylamino- and 6-amino-ll-phenoxynaphthacene-5,12-quinones... 

Photochromism of quinones is caused by the reversible photoisomerization of the para quinone structure to the ana quinone structure as a result of photo- and thermoinduced intramolecular substitution according to the associative mechanism with participation of the 8-complex (Scheme 9).5... 

In addition to the photoinduced transfer of the phenyl group, photoinduced migration of an acyl group or a hydrogen atom is rendered possible in photochromic quinones. The mechanism of the photochromic transformation of quinones depends on the structure of the initial compounds. 

Photochromism of 2-chloro-6-phenoxy- and 2-chloro-8-phenoxyanthrapyri-dines indicates that the presence of the fixed quinone structure is not needed for the existence of the photoinduced arylotropy of these compounds.38... 

Unlike phenoxyanthraquinones, the photochromism of phenoxynaphthacene-quinones is characterized by a long lifetime of the photoinduced ana form. This property allows one to perform a more detailed study of the mechanism of the photochromic transformations (Scheme 20).5... 

Photochromic transformations from the para- to ana-quinone structure of these compounds occur through intermediate photoproducts in the triplet state.51 The experimental evidence of the influence of oxygen in solution and the viscosity of solvents on the lifetime of these photoproducts supports this statement. It was found that both initial and photoinduced forms have lower triplet levels of the mi -type. 

Reversibility of the spectral changes under irradiation and the photostability of the compound without the phenoxy substituent suggested a photochromic para-ana-quinone transformation of 9-phenoxy-pyrone (9-phenoxynaphthacene [12,11 -bc]-2,8-pyrandione) (Table 7.10, Scheme 22).48... 

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