Azobenzenes, light

CTABr + amphiphilic azobenzenes. Light and dark reactions are compared... 

CTABr -I- amphiphilic azobenzenes. Light and dark reactions are compared Aq. micelles affect excimer formation with change in geometry of binding... 

This facile reaction involves a modest change in the absorption of visible light, largely because of the visible absorption band of <7 -azobenzene [1080-16-6] having a larger extinction coefficient than azobenzene [17082-12-1]. Several studies have examined the physical property changes that occur upon photolysis of polymeric systems in which the azobenzene stmcture is part of the polymer backbone (17). 

Azobenzene [103-33-3] M 182.2, m 68", pK 2.48. Ordinary azobenzene is nearly all in the transform. It is partly converted into the cw-form on exposure to light [for isolation see Hartley J Chem Soc 633 1938, and for spectra of cis- and /ran5-azobenzenes, see Winkel and Siebert Chem Ber 74B 6707947]. trans-Azobenzene is obtained by chromatography on alumina using 1 4 benzene/heptane or pet ether, and crystd from EtOH (after refluxing for several hours) or hexane. All operations should be carried out in diffuse red light or in the dark. 

Solvent can alter a dye s color. One inteipretation is that light absorption moves an electron from one part of the molecule to another with a resulting change in overall polarity. Examine the HOMO and LUMO of azobenzene, 4-hydroxyazobenzene and 4-amino-4 -nitroazobenzene. Which, if any, of the molecules would be expected to change color in different solvents How does excitation change the polarity of these molecules Explain how you reached your conclusions. 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

To recover the free base, dissolve the hydrochloride in the minimum volume of boiling alcohol, add concentrated ammonia solution dropwise until a clear solution results and the blue colour has become light brown. Add water carefully imtil a cloudiness appears, warm on a water bath until the cloudiness just disappears, and allow to cool. Yellow crystals of p-amino-azobenzene separate on cooling. 

The compound exists normally as the trans or ( )-isomer 21a. This molecule is essentially planar both in the solid state and in solution, although in the gas phase there is evidence that it deviates from planarity. When irradiated with UY light, the ( )-isomer undergoes conversion substantially into the cis or (Z)-isomer 21b which may be isolated as a pure compound. In darkness, the (Z)-isomer reverts thermally to the (F)-isomer which is thermodynamically more stable because of reduced steric congestion. Some early disperse dyes, which were relatively simple azobenzene derivatives introduced commercially initially for application to cellulose acetate fibres, were found to be prone to photochromism (formerly referred to as phototropy), a reversible light-induced colour change. C. I. Disperse Red 1 (22) is an example of a dye which has been observed, under certain circumstances, to give rise to this phenomenon. 

Recently, Aida and Jiang [96] synthesized a series of dendrimers similar to those reported by McGrath and coworkers and reported photoisomerization of their core azobenzene units using infrared radiation. This new strategy holds immense potential for using dendrimers as light-harvesting matrices. 

Three azobenzeneophane-type crown ethers in which the 4,4 positions of azobenzene are joined by a polyoxyethylene chain have been synthesized (Shinkai, Minami, Kusano Manabe, 1983). On irradiation with UV light, the ( ) (or trans) form (198) is isomerized to the (Z) (or cis) isomer (199). The ( ) isomer may be regenerated by heating, or by irradiation with visible light the interconversion is completely reversible. 

Zhang C, Du MH, Cheng HP, Zhang XG, Roitberg AE, Krause JL (2004) Coherent electron transport through an azobenzene molecule a light-driven molecular switch. Phys Rev Lett 92(15) 158301... 

Pace G, Ferri V, Grave C, Elbing M, Zhamikov M, Major M, Rarnpi MA, Samori P (2007) Cooperative light-induced molecular movements of highly ordered azobenzene self-assembled monolayers. Proc Natl Acad Sci USA 104 9937-9942... 

Mativetsky JM, Pace G, Elbing M, Rampi MA, Mayor M, Samori P (2008) Azobenzenes as light-controlled molecular electronic switches in nanoscale metal-molecule-metal junctions. J Am Chem Soc 130 9192-9193... 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

Table 3.37 Light fastness of substituted azobenzenes on polyester [173]...

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4-18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]-... 

McGrath and Junge [36] reported a photoresponsive poly(aryl ether) dendrimer with azobenzene as the dendrimer core. These dendrimers exhibited reversible trans to cis photoisomerization by irradiation at 350 nm. The authors proposed the use of this type of dendrimer as novel photoswitchable transport vectors. This is based on the expected ability of dendrimers to encapsulate or eject small molecules reversibly upon light perturbation. 

---