Photochromic transformations

Valence tautomerism is responsible for the photochromic transformation exhibited by the xanthenone (143). This phenomenon is a type of dynamic isomerism in which the primary change is a shift in the position of the valence bonds as illustrated in Scheme 16 (68JOC3469). 

Starting from the early works devoted to this subject, it is generally accepted that the explanation for the photochromic transformations of the... 

The photochromic transformation and the observed spectral changes or changes in physical or chemical behavior are related to the modifications of the geometry of the system and its electronic distribution. This will be seen in the examples given below. 

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. 

Photochromic transformations of 1,4-naphthoquinone were discovered after the introduction of a methyl substituent in the peri position for suppression of its photochemical reactivity toward intermolecular hydrogen abstraction." The mechanism of photochromism is based on reversible photoenolization (Scheme... 

The well-known photochromic transformations of anthraquinones are closely associated with the photoinduced migration of hydrogen, acyl, or aryl groups. Although photochromism of these compounds fits the reaction shown in Scheme 9, the processes of photochromic transformation exhibit some features related to the nature of the photoreactive state and details of the mechanism of the photochromic transformations. 

As in the case of 5-methyl-1,4-naphthoquinone, photochromic transformations of 1-alkylanthraquinone derivatives represent a thermally reversible photoenolization which was first demonstrated for l-methyl-9,10-anthraquinone (II, R1= CH3, R2 = R3 = H) (Scheme 11).16... 

The study of photochromic transformations of 1-methylanthraquinone and its deuterium analog by means of laser photolysis over a wide temperature range30... 

Scheme 13) and 9-acetoxy-l,4-anthraquinone (IIA, R1 = OCOCH3, R2= R3 = H) (Scheme 14) in a glassy matrix at 77 k. 19,21,34 It was found that photochromic transformations occurred only in acetoxyanthraquinones with electron-donating substituents (amino and methoxy groups) and also in unsubstituted 9-acetoxy-l,4-anthraquinone. However, unsubstituted I-acetoxy-9,10-anthraquinone and its derivatives with the substituents in 2-,4-,5- and 8-positions did not exhibit photochro-mism (Table 7.2).21,28,34... 

It has been found11,13,15,19 50 that photochromic transformations of aryloxyanthraquinones depend on the position and the electron character of substituents in the anthraquinone cycle (Scheme 15). The structure of the photoinduced form was supported by the results of an IR spectral study4... 

Tables 7.3 and 7.4 show that unsubstituted and most of the substituted 1-phenoxy-9,10-anthraquinones exhibit photochromic properties. The majority of compounds with mono- and dialkylamino substituents shows no photochromic transformations but undergoes irreversible photochemical conversion.5,8 The photo-chromism is also absent in the case of l-phenoxy-8-hydroxy-anthraquinone. It is suggested31 that, unlike 1-naphthoxyanthraquinone,32 the structural isomer of 1-phenoxy-anthraquinone arises from the rotameric isomer.

In the case of l-phenoxy-4-acetoxy anthraquinone (II, R 1 = OC6H5, R2 = OCOCH3), the photoinduced migrations of phenyl and acetyl groups occur simultaneously (Scheme 16).19 The study of photochromic transformations of this compound by the laser photolysis method22 showed that the processes include the phototransfer of either phenyl or acetyl groups as well as subsequent thermal transfer of either acetyl or phenyl groups, respectively. 

A similar mechanism of photochromic transformation is suggested for 1-phenoxy-4-hydroxy-9,10-anthraquinone.19 The only difference is that the photo-chromic transformations can be observed at 77 K because they occur with high rate and with a very low activation energy. 

Photochromic transformations that are similar to these of the above anthraqui-nones are also characteristic for anthrapyridones and anthrapyridines. The character of spectral changes (Table 7.5) and the similarity of photochromic transformations to the conversion processes for the acyl derivatives of 1-phenoxy-4-aminoanthraquinone suggest an analogous mechanism for 6-phcnoxy-A-methy-lanthrapyridone (IIB1) (Scheme 17).37 Anthrapyridones of another type (IIB2) show similar photochromic properties. 

Anthrapyridine (IIC, R1 = OCH3, R2= CH3) manifests photochromic transformations in chloroform and ethanol, in contrast to the phenoxy derivatives of anthrapyridone. 39 This difference is associated with effective photoinduced phototropic transformations of anthraquinones in methanol to the lactime form (Scheme 18), which is rapidly isomerized to the initial compound. 

Among the synthesized pyrazoloanthrones, the compounds with phenoxy groups in 5 - and 7-positions are light-sensitive compounds, but photochromic transformations are observed for 7-phenoxy derivatives of pyrazoloanthrone only (Table 7.7). 

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... 

Similar to 1-phenoxyanthraquinones, photochromism is absent in 6-dimcthv-lamino and 6-phenylamino derivatives of ll-phenoxy-5,12-naphthacenequinone. Photochromic properties are absent for ll-phenoxyphenol-6-amino-5,12-naphtha-cenequinone because of steric hindrances to forming the transient spirocomplex (see Scheme 9) during photochromic transformation of this compound. 

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. 

Closer examination of the mechanism of photochromic transformations in phenoxynaphthacenequinones by pica- (25 ps) and nanosecond (8 ns) photoexcitation showed that photoisomerization of these compounds, as in photochromic anthraquinones, is an adiabatic reaction that proceeds through the triplet state of the initial form 54 ... 

The proposed mechanism of the photochromic transformations was supported by the character of the spectral changes (Table 7-10) and similarity of the photochromic transformations for these compounds to 1 l-phenoxy-6-aminonaphtha-cenequinone. 

Photochromic transformations of 8-phenoxy derivatives of pyrazolonaphthacenones56,57 were similar to phototransformations of the above-described photochromic pyrazoloanthrones (Scheme 23) (Table 7.11). Photochromism of these compounds depends on the character of a substituent in the heterocyclic fragment. 

It turned out that formerly synthesized l,3-dichloro-6-phenoxy-7,12-phtha-loylpyrene exhibited photochromic transformations in aprotic solvents (Scheme... 

The structural similarity of derivatives of phenoxynaphthacenequinone and phenoxyphthaloylpyrene and the photoinduced changes of absorption (Table 7.12) and IR spectra suggest an identical mechanism for photochromic transformations. 

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. 

Unlike derivatives of acetoanthraquinone, l-phenoxy-4-acetoxy-9,10-anthra-quinone exhibited photochromic transformations under irradiation that was absorbed both by the initial form and photoinduced forms. This effect was caused by the... 

The study of the efficiency of photochromic transformations for a series of aryloxy-substituted anthraquinones in polymer matrices showed that the quantum yields of phototransformations in a polymer were large and somewhat dependent on the compound s structure (Table 7.15).29... 

As in the case of the photochromic quinones discussed earlier, the kinetics of the photochromic transformations for phenoxypyrazoloanthrones were determined by the irradiation intensity that activated the initial or photoinduced form41,42... 

A comparative study of the efficiency of the photochromic transformations for a series of 6-acetyl-amino-ll-naphthacene-5,12-quinones (Table 7.8) showed that, as for 4 derivatives of 1-phenoxyanthraquinone, the light sensitivity of these compounds decreased with the increasing electron-donor capacity of substituents.46... 

The presence of the electron-donor substituents in the migration group of 11-aryloxy derivatives of 6-amino-5,12-naphthacenequinone was favorable for photochromic transformations.52 Aryloxynaphthacenequinones with electron-acceptor substituents in the phenoxy group were photoisomerized less efficiently. 

A comparative study of the photochromic transformations for 6-phenoxy-5,12-naphthacenequinone in toluene and polymer matrix showed that the value of the quantum yields was unchanged with a polymer binder in place of toluene. 61 Quantum yields for direct and back photoreactions were equal to cpB= 0.3 and (Pa = 0.005 under UV light and q>A = 0.005 under visible irradiation. 

The insertion of phenoxynaphthacenequinone in the polymeric chains led to an effective decrease in the rate of photochromic transformation.53 The phototransformation rate of this compound in polymers depended on the polymer nature as follows polysiloxane > poly(methyl methacrylate) > polystyrene. This rate was affected moderately by changing the concentration of the photochromic compounds, but it increased with the length of the chain in poly(methyl methacrylate) and... 

The photochromism of phenoxypyrone is similar to the photochromic transformations for phenoxynaphthacenequinones.48... 

The efficiencies of the photochromic transformations for phenoxypentacene-quinone and the derivatives of phenoxyphthaloylpyrene match the photocoloring and photobleaching efficiencies for the phenoxy derivatives of naphthacenequinone.57... 

An analysis of the kinetic characteristics of photochromic quinones showed that the photochromic transformations of methylnaphthoquinone, alkyl- and acyloxyan-thraquinones in solution at room temperature were observed in the microsecond range. As a consequence, these compounds may be of theoretical interest in the future. The photochromic aryloxyanthraquinones are characterized by a lifetime of the photoinduced form that reaches several minutes in solution and several hours in polymer films at room temperature, which makes them acceptable for a number of applications. In this regard, of special interest are photochromic naphthacenequi-... 

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. 

The specially synthesized l-arylcyanomethyl-9,10-anthraquinones with a long lifetime for the photoinduced form showed photodegradation in alcohols as well.36 It was found that despite the variable lifetime of the photoinduced form of the different compounds of this type, the dark reaction of its interaction with alcohol dominated over the hydrogen atom transfer, which was due to the photochromic process. The efficiency of the photochromic transformations depended on the nature of the... 

It was found39 that photochromic transformations of phenoxynaphthacenepyr-idones in chloroform were accompanied by the destruction of the photoinduced form. Photochromic phenoxypyrazolonaphthacenones also exhibited fatigue.56... 

N.P. Gritsan, V.A. Rogov, N.M. Bazhin, V.V Russkikh, and E.P. Fokin, Study of photochromic transformations for 1-methylanthraquinone and l-oxy-4-methylanthraquinone by method of low-temperature and flash photolysis, Teor. Eksp. Khim. 15, 290-296 (1979) (Russ.). 

N. P. Gritsan, V. A. Rogov, N. M. Bazhin, V. V. Russkikh, and E. P. Fokin, Influence of temperature, an environment and a substituent nature on the thermal stage of photochromic transformations of 1-methylanthraquinone derivatives, Izv. Akad. Nauk SSSR, Ser. Khim. NaukNl, 89-94 (1980) (Russ.). 

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