Photochromic reaction

Cis-Trans Isomerizations. Experiment [6] studies the photochromic properties of frans-dibenzoylethylene. In this case, the highly conjugated bright-yellow trans diastereomer is rapidly isomerized under intense sunlamp visible radiation, via excited electronic states, to the colorless cis aUcene. A tt electron is promoted to an anti-bonding tt molecular orbital, which destroys the tt bond, and thus permits facile rotation about the remaining o- bond and formation of the ds alkene  

A good example of photochromic behavior is the highly colored cis- and frans-azobenzene. In this case the transition is promoted by ultraviolet 

Tautomerism, A number of proton and valence tautomers are subject to photo-chemical induction. One example is 2-methylbenzophenone (I), a colorless compound that can be photochemically induced to tautomerize to a system with extended conjugation. The tautomer (II) is a yellow material that will revert to the colorless form under thermal conditions  

Homolysis and Heterolysis of Bonds. Photochromic homolysis has been observed with materials such as 2,3,4,4-tetrachloro-l-(4H)-naphthalenone (III)  

Excitation leading to heterolysis and zwitterion formation has been observed in numerous spiropyrans, as shown here for IV. These compounds undergo ring opening to yield a zwitterion (V)  

A recent study by laser flash photolysis showed that a triplet state is not involved in the coloration mechanism of spirooxazines when in solution. A similar conclusion was reported, noting that the photocoloration occurs only in the excited singlet state because of the independence of the reaction to the presence of oxygen.29 

Aramaki et al.30 examined the photochromic reactions of spirooxazines by picosecond time-resolved Raman spectroscopy. Vibrational resonance Raman spectra of the merocyanine isomer(s) recorded over a 50-ps-1.5-ns interval did not change. This indicated that the open ring opening to form a stable merocyanine isomer or the distribution of isomers31 was complete within 50 ps and that the isomer(s) distribution remained unchanged for at least 1.5 ns. 

Resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) has proven to be a useful technique for investigating the molecular structures of transient species.33 In nonpolar and polar solvents, CARS spectra of spirooxazine derivatives indicated the existence of two similar isomeric species. 

Malatesta35 proposed the key intermediate product (compound 28) of the oxidative degradation of photochromic spirooxazines. These species may result from the photochromic irreversible degradation of the spirooxazines even under conditions of partial or total absence of oxidation as, for example, in polymers coated with thin films of barrier agents such as SiO 2, SiOxCy, AI2O3, and MgO. 

Malatesta et al.36 disclosed that spirooxazines react easily in their open merocyanine (MC) forms with free radicals to give deeply colored, reduced, free-radical adducts (FRA) that are devoid of photochromic activity. The radicals attack the C5I=C6 double bond of MC and yield stable, deeply colored, free-radical adducts (compound 29) that can no longer close back to the corresponding spiro form. The adducts absorb in the 510-560-nm region and are characterized by high molar absorptivities. 

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Photochemistry. The most important photochemical processes that proceed from the excited state are geometrical isomerization and photochromic reactions. 

The reverse reaction, the photochemical ring opening of sphopyranes (22b), takes place by absorption ia the short-wave uv region of the spectmm and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is ia the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colorless to a colored solution (87,88). These photochromic reactions can be used for technical appHcations (89). 

Schematically, the photochromic reaction can be stated by the simple equation 1 ...

Another important concept in the discussion of photochromic systems is fatigue. Fatigue is defined as a loss in photochromic activity as a result of the presence of side reactions that deplete the concentration of A and/or B, or lead to the formation of products that inhibit the photochemical formation of B. The inhibition can result from quenching of the excited state of A or screening of active light. Fatigue, therefore, is caused by the absence of total reversibihty within the photochromic reaction (eq. 2). 

The organic photochromic systems that have been studied are numerous and it is helpful to classify them into a few categories by way of the general mechanism of the photochromic reaction in each category. 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980 s he invented a new class of photochromic molecules - diaryl-ethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives. 

Matsuda K, Yamaguchi H, Sakano T, Ikeda M, Tanifuji N, Irie M (2008) Conductance photoswitching of diarylethene gold nanoparticle network induced by photochromic reaction. J Phys Chem C 112(43) 17005-17010... 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

As mentioned in sections 1.2.2.2 and 1.2.3.2, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzo and naphthopyrans. Consequently, spectral data collected from the literature is only comparable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any other one listed in this and related sections of this chapter. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (2. ) of the coloured state, the change in optical density (absorbance) on exposure to the xenon light source (AOD) and the fade rate which is the time in seconds for the AOD to return to half of its equilibrium value. 

Compounds which exhibit photochromism are classified according to the mode of their photochromic reaction, because changes in colour can arise as a result of heterolytic cleavage, homolytic cleavage, isomerization, tautomerization and oxidation. 

Derivatives of 2,3-epoxycyclopentadienone also undergo heterolytic cleavage. The coloured species is believed to be a pyrylium ion (134), illustrated in Scheme 11 by the photochromic reaction of 2,3-diphenyIindenone oxide (133) (21CB2327). 

We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23]... 

Figure 7.43 Photochromic reaction (left) and the spectra of a fulgide (right)...

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

Photochemistry. The most important photochemical processes that proceed from the excited state arc geometrical isomerization and photochromic reactions. Pholoisomerization of polymethines is a reversible Irans-cis transfer. The cis-isomer absorbs at longer wavelength with a smaller intensity than the trans-isnmer. 

Compound 311 underwent a thermally irreversible photochromic reaction in solution (08JCS(CC)335). Upon alternate irradiation with UV and visible light, a hexane solution of 311 reversibly changed its color from colorless to bluish purple due to the isomerization between the open- and closed-ring isomers, 311 and 312. 

The absorption spectrum at 0° has a maximum at 600 nm. Upon rotation of the direction of the incident polarized light by as much as 90°, the absorption intensity decreases. The anisotropy of the absorption spectra reflects the regular orientation of the photogenerated closed-ring isomers and indicates that the photochromic reaction occurred in the single-crystalline phase. The blue color disappeared by irradiation with visible light a > 480 nm). Anthracene-substituted derivatives also showed photochromic properties (01JPC(A)1741). 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

Some diarylethene derivatives that possess strongly electron-withdrawing substituents deviate from the general ruleJ5,23 The closed-ring isomers of 12b and 13b, possessing dicyanoethylene substituents, reverted to the open-ring isomers in 3.3 min and 186 min, respectively, at 60 °C. The dithienylethenes 14b, with pyridinium ion substituents, and 15b, with formyl residues, also underwent thermally reversible photochromic reactions. 

The specific rotation values of 11E and its UV pss at the sodium D-line (589 nm [a]D) in toluene were -572° and -186°, respectively hence markedly different. This phenomenon was reproduced in PMMA films. Because irradiation at the sodium D-line wavelength does not induce photochromic reactions, measurement of optical... 

The inclusion complexation of spiropyrans in cyclodextrins has also been explored as a means to control photochromic reactions.1591 Distinct differences in complexation of sulfonic acid-modified spiropyrans to various cyclodextrins were observed and the closed spiropyran form bound to (3-cyclodextrin was stable towards photochemical ring-opening. 

Fig. 3 Photochromic reactions of poly(L-glutamic acid) containing azobenzene units in the side chains (III).

The photochromic reactions are illustrated in Figure 3. At room temperature in the dark, all the azo groups are in the trans configuration, which is planar and fully conjugated. Irradiation at 350 nm produces isomerization to the cis isomer, which is not planar and is less conjugated, for steric reasons. Indeed, the two benzene rings are forced to occupy skewed planes. The back-reaction to the trans form is achieved by irradiation at 450 nm or simply by keeping the samples in darkness. 

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