Isomerization of azobenzene

Some simple azo compounds, because of restricted rotation about the ( N=N ) double bond, are capable of exhibiting geometrical isomerism. The geometrical isomerism of azobenzene, the simplest aromatic azo compound which may be considered as the parent system on which the structures of most azo colorants are based, is illustrated in Figure 3.1. The compound is only weakly coloured because it absorbs mainly in the UV region giving a 2m.lx value of 320 nm in solution in ethanol, a feature which may be attributed to the absence of auxochromes (see Chapter 2). 

Figure 3.1 The photo-induced geometrical isomerism of azobenzene...

Erasing of the image can be achieved by switching the photoirradiation to 525 nm to induce cis - trans isomerization of azobenzene. Since the absorbance of the cis isomer at 525 nm is weak, it takes a longer period than the image recording process. 

Yabe A, Kawabata Y, Niino H, Tanaka M, Ouchi A, Takahashi H, Tamura S, Tagaki W, Nakahara H, Fukuda K. Cis-trans isomerization of azobenzenes included as guests in Langmuir-Blodgett films of amphiphilic beta-cyclodextrin. Chem Lett 1988 l-4. 

The initial motion of the light triggered switch, the isomerization of azobenzene, is ultrafast and occurs on the timescale of 200 fs and 2 ps. 

Effect of /3-cyclodc trin on cis trans isomerization of azobenzenes was studied by Sanchez and de Rossi [26], It was found that the cis-trans thermal isomerization of / -Mcthvl Red, o-Methyl Red and Methyl Orange is inhibited by fi-CD at constant pH. The isomerization rate decreases 4, 8, and 1.67 times, respectively, in a solution containing 0.01 M /J-CD. This effect was attributed to the formation of inclusion complexes hindering rotation of the -N=N- bond. Isomerization of Methyl Yellow and naphthalene-l-azo-[4 -(dimethylamino)benzene] requiring mixed organic-aqueous... 

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. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

Fischer, G., and E. Fischer Photosensitized Isomerization of Azobenzene. To be published. 

Another photochemically based gating system is the one by Brinker and coworkers in which the cis-trans isomerization of azobenzene was applied.69 (Triethoxysilylpropylureido)azobenzene was co-condensed with TEOS by evaporation-induced self-assembly to yield a functionalized mesoporous silica thin film. The propylureidoazobenzene groups inside of the mesopores could then be iso-merized from their stable elongated trans structure to the more compact cis geometry upon UV irradiation. The trans geometry could be recovered by thermal treatment. The authors suggested that the results of the transitions from trans to cis could lead to an increase in pore size of approximately 0.7 nm and an increase of the polarity from 0 to 3 D. This could be applied to regulation of ion transport however, they did not confirm such expectations.69... 

Allosteric control of a ligand-gated ion channel75 and light-controlled expulsion of molecules from mesostructured silica nanoparticles,76 both based on the photo-induced isomerization of azobenzene, have also been reported. 

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

A similar ds-selective inclusion was observed for 4,4 -dimethylazobenzene. Although the cis-trans isomerization of azobenzene by UV irradiation is a well-known process, 100% conversion from trans to cis at a photostationary state is normally impossible. In the presence of cage 2, however, only the cis isomer is trapped by the cage via the formation of the stable tennis-ball like S4 dimer, pushing the cis-trans photo-equilibration overwhelmingly toward the ds-isomer [26]. 

Figure 17d shows the switching behavior of the LB film of APT(8-6), which is very different from the two cases above. The conductivity increases with the trans-to-cis isomerization of azobenzene as in the case of APT(8-12), and the increase in conductivity is about 6% in the photostationary state. The conductivity stays, however, unchanged with the cis-to-trans isomerization. Further irradiation with UV light causes the conductivity to show another increase. This stepwise behavior is observed about 30 times. The increment per cycle decreases with an increasing number of cycles and the conductivity tends to become constant in a cycle repeated more than 30 times. 

One of the most frequently exploited photochromic processes is the cis trans isomerization of azobenzene. Irradiation of the more stable trans configuration with 360 nm ultraviolet light induces conversion to the cLs isomer, which is stable for long periods in the absence of heat or light. Exposure to moderate heat or irradiation in the visible spectrum induces the transformation back to the trans species. 

Few publications on the spectroscopic and isomerization properties of simple azo compounds have appeared in the last 15 years, as compared to the decades before. There is, however, one exception Ultrashort time-resolved spectroscopy of azobenzene and its relatives has opened new access to the dynamics following pico- and femtosecond excitation. The results are most relevant for the mechanisms of the photophysical and photochemical processes, which in azoaromatic compounds primarily are isomerizations. There is, however, a host of newer investigations into the isomerization of azobenzene and its family that are directed to applications in photoswitchable systems and devices. Some of them are relevant for the understanding of the parent molecules and therefore are included in this chapter. 

The thermal Z —> E isomerization of azobenzene has been widely used to determine free volume in polymers at room and temperatures as low as 4 K.90b9i Jhe thermal reaction is also important in the context of photo-response, as an information written or a signal or state produced by switching E to Z is slowly fading. However, the Z-lifetime is strongly modified by strain in the molecule Z-azobenzene in solution at room temperature has a half life of about 2 days the Z,E E,E isomerization in the [3.3] 4,4 )azo-benzenophane 9 has a half life of ca. 4 min. the [2.2] 4,4 )azobenzenophane 7 has a half life of ca. 15 seconds and in dibenzo[2.2][4.4 )-azobenzeno-phane 8 the life of the E,Z-isomer drops to 1 s. On the other hand, the Z,Z Z,E isomerization in these phanes is slowed down enormously Z,Z-7 lives 2.5 days, Z,Z-9 about 5 days, and Z,Z-10 about 1 year at room temperature. Activation energies are available in the publications. The Z,E E,E isomerization in most azobenzenophanes is very fast. However, in 2,19-Dioxo[3.3](3,3 )azobenzolophane 12, the Z,E-form is relatively stable, The remarkable differences in these and other structures are not due to different activation enthalpies but to different activation entropies. 

Isomerization of azobenzenes may also be sensitized by triplet sensitizers. Jones and Hammond and Fischer came up with different results 2% of Z-form versus 25% in the photi tationary state. A thorough study by Lemaire and coworkers showed that two triplet states at 195 and 180 kj mol in the E- and at 190 and ca. 140 kJ moT in the Z-isomer are involved in the reaction. According to Bortolus and Monti,sensitizers with high (>190 kJ moT triplet energy transfer their energy efficiently (diffusion controlled) to both E- and Z-azobenzene. Still, the isomerization yield is small— 9e- z = 0.015—in agreement with Jones and Hammond. On the other hand, the sensitized Z —> E isomerization has < -,e = 1-0. Azobenzenophane 9 also undergoes triplet-sensitized isomerization. ... 

FIGURE 1.14 The rotation and inversion mechanisms of isomerization of azobenzene. (Adapted from reference 7, by permission.)... 

Tung, C.-H., and Guan, J.-Q. (1996). Modification of photochemical reactivity by Nafion. Photocyclization and photochemical cis-trans isomerization of azobenzene. /. Org. Chem. 61, 9417-9421. 

Yamashita, S. (1961). Iodine-catalyzed cis-trans isomerization of azobenzene. Bull. Chem. Soc. Japan 24, 842-845. 

Asano, T, Yano, T., and Okada, T. (1982). Mechanistic study of thermal Z-E isomerization of azobenzenes by high-pressure kinetics. J. Am. Chem. Soc. 104, 49W-A904. 

Hong, J.-D., Park, E.-S., and Jung, B.-D. (1999). Studies on photochemical isomerization of azobenzene in self-assembled boloamphiphilic monolayers. Mol. Cryst. Liq. Cryst.. Sci. Technol. Sect. A, 327, 119. 

Asano, T., Okada, T, Shinkai, S., Shigematsu, K., Kosano, Y., and Manabe, O. (1981). Temperature and pressure dependence of thermal cis-to-trans isomerization of azobenzenes which evidence an inversion mechanism J. Am. Ghem. Soc. 103, 5161-5165. 

Gille, K., Knoll, H., and Quitsch, K, (1999). Rate constants of the thermal cis-trans isomerization of azobenzene dyes in solvents, acetone/water mixtures, and in microheteroge-neous surfactant solutions. Int.J. Ghem. Kinet. 31, 337-350. 

Kuriyama, Y., and Oishi, S. (1999). Mechanism of thermal isomerization of azobenzene in zeolite cavities. Chem. Lett. 1045-1046. 

The isomerization of azobenzenes is discussed in detail by Rau in Chapter 1. In this section, I shall recall the basic features of photoisomerization of... 

FIGIIRE 3.1 (Top) Trans<->cis isomerization of azobenzenes. (Bottom) Simplified model of the molecular states. 

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