Azobenzenes and related compounds

Chemists will be familiar with the bonding in these materials but other readers may not be. This bonding is best envisaged in the following way. 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

Many other papers dealing with azobenzene derivatives deposited by the LB technique have been published during the past few years, though, for the most part, they have only an indirect relevance to the main theme of this book. A brief selection of these papers is given here. Tanaka et al. 

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Song, X., Perlstein, J.,and Whitten, D.G., Supramolecular aggregates of azobenzene phospholipids and related compounds in bilayer assembUes and other microheterogeneous media structure, properties and photoreactivity, /. Am. Chem. Soc., 119,9144,1997. 

Full papers have appeared on the formation and reactivity of the compounds ML(CNR)2 (M = Ni, Pd, Pt L = Oz, azobenzene, olefin, diazo-fluorene, acetylene) (231-237) (see also Sections IV,D,2 and V,D). Complexes of the type Ni(olefin)(CNBu )2 have been prepared for a large range of olefins (234, 237). The isocyanide stretching frequencies have been measured and related to the electron-withdrawing properties of the olefin. Other unsaturated molecules such as imines, diazenes, ketones, nitroso compounds, and acetylenes have been similarly studied. The effect of substituent change has been found to be cumulative and an empirical relationship has been developed to predict v(NC) (237). 

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). 

Although the activation energies of aminoazobenzene-type compounds (Ea between 75 and 88 kj moH) are not very different from those of azobenzene-type molecules, therm Z -> E isomerization of aminoazobenzene-type molecules is in general much faster than that of the azobenzene-type compounds. Conventional flash experiments are necessary to monitor the changes. The half-life of the Z-form of dimethyl-aminoazobenzene in toluene at 298 K is 220 s. A Linear Free Energy Rehitionship and Hammett relation is established, which includes azobenzene- and aminoazobenzene type compounds.A linear In vs, n, the Taft parameter of solvent polarity, is also observed. The dependence of the isomerization rate on pressure is weak In most solvents, it increases less than 35% at 2100 bar, AV -1.65 ml moLL Methanol is exceptional, with AV = -17 ml... 

Pressure dependence was thoroughly investigated by Asano and his group. It turns out that the partial volumes of the Z-forms of 4-dimethylamino-4 nitorazobenzene and related molecules are ca. 250 cm moP in all solvents. Those of the E-forms are smaller and solvent-dependent. Thermal isomerization rates are weakly dependent on pressure in nonpolar solvents, but contrary to azobenzene- and aminoazobenzene-type compounds, they are strongly dependent in polar solvents in hexane 10%, in acetone 475% for 2100 bar (AV = -0.7 and -25.3 em mol, respectively). This has implications for the discussion of the mechanism of isomerization (Section 1.6). 

Two such closely related compounds as cis and trans isomers of azobenzene have been separated by exploiting the difference in polarities of the two isomers. It has already been mentioned earlier that as the polarity of a sample component increases, its retention time on an LSC column also increases. Thus, cis-azobenzene (I), which is the more polar one of the two azobenzenes, is retained on the chromatographic column longer than trans-isomer (II) whereby separation of the two geometrical isomers results. 

Niunerous retinoids are now known and these cannot be represented by a single generic structure. Generic stucture types I, II, and III, however, represent the structure of most retinoids synthesized to date Figure LI). Compounds represented by structure type I include isomers and closely related analogues of retinoic acid (1), acitretin (2) and its ethyl ester (etretinate), and mono-aromatic heteroarotinoids such as (6), (15)-(17), and (54)-(57). Retinoids type II include substituted naphthalene, stilbene, azobenzene, and diaromatic amides, and other hetero-substituted analogues... 

Asakawa M, et al. 1999. Photoactive azobenzene containing supramolecular complexes and related interlocked molecular compounds. Chem Eur J 5(3) 860 875. 

The relation between the type of photochromic compound and its effectiveness to induce a phase transition is a point of interest. When unsubstituted azobenzene is added to 5CB, the phase transition is not induced even after prolonged irradiation. BMAB is by itself liquid crystalline whereas azobenzene is not. It seems to be essential for a triggering photochromic compound to have effective interactions with the host liquid crystal. 

Azo compounds are systematically addressed as diaZenes. This has to be borne in mind when one conducts a literature research. The azo (diazene) group is isosteric with the ethene group stilbene and azobenzene have many related features, but they also possess relevant different properties that make azobenzenes superior for use as photo-switches. 

The formation of ortho palladium products from a-aryl nitrogen derivatives and palladium salts is well known. Complexes formed from azobenzene, Schiff bases, tertiary benzylamines and oximes readily undergo insertion of CO into the metal-carbon bond to give, after work-up, a variety of heterocyclic compounds. Unfortunately, such reactions use expensive palladium salts in stoichiometric quantities. However, a number of related reactions have been shown to proceed in the presence of only catalytic quantities of palladium. Isoindolinones, for example, can be synthesized in good yield by reaction of o-bromoaminoalkylbenzenes with CO (100 C, 1 bar) in the presence of catalytic amounts of Pd(OAc)2, PPha and Bu"3N (equation 56). °... 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

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