Azoaromatics azobenzenes

The spectroscopic and photochemical, especially isomerization, features of the azoaromatics warrant separation into three classes according to the relative energy of the lowest lying (n, c ) and (7C,7C ) states the azobenzene type, the aminoazobenzene type, and the pseudo-stilbene type. This determines the structure of this chapter. Section 1.2 provides basic information on... 

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. 

Azobenzene-type azoaromatics generally do not emit with reasonable quantum yields after excitation either to the (n,JC ) or to the states. 

In contrast to the polyurethane systems with chromophoric soft segments discussed above, in these polymers no photoisomerization of the incorporated azoaromatic group could be achieved in the solid state for both types of polyurethanes with MDI or TDI based hard segments. In solution, however, photoisomerization of the azochromophore occurs by exposure to light in the region of the maximum trans absorption and the same fraction of cis-isomers in the photostationary state could be reached for the chromophoric polyurethane as for a model compound of the azoaromatic hard segment unit, 4,4 -di(phenylurea)azobenzene. 

Azobenzene-type azoaromatics generally do not emit with reasonable quantum yields after excitation either to the (n,n ) or to the (Jt,n ) states. There are only very few reports, and in most of them powerful lasers that allow detection of quantum yields down to 5 10 are used. Struve reported a short-lived fluorescence of azobenzene in cyclohexane from the (n,Jt ) state (x = 25 ps) and even from the second excited (Jt,7C ) state (X = 5 ps), a finding that violates Kasha s rule, but gains support from the fact of the large (n,Jt ) - state separation. In a conference contribution, Hamai and... 

All azoaromatics, be they of the azobenzene, arainoazobenzene, or donor/acceptor pseudo-stilbene type, experience considerable spectral changes on protonation or complexation. Ortho metallation has the same effect.Usually the ic— ic band is red-shifted, possibly due to the localized charge at the N-atom. By the same token, the (n,Jt ) state is shifted to higher energies. Minor band shifts and intensity changes indicate double protonation of azobenzene. 

Supramolecular spherical assemblies of NPs with photoresponsive adhesion/dispersal behaviour were also obtained in a ternary system hierarchically combining the host-guest interaction of different types of CDs toward porphyrin and azobenzene. The inclusion complexation of an azobenzene modified water soluble porphyrin (1) with phthalo-cyanine-grafted permethyl (3-CDs (2) could be reversibly cross-linked to relatively larger nanospheres with naphthyl bridged bis(a-CD)s (3). The large spheres (12 -3) turned reversibly to small-sized particles (1 2) upon photoisomerization of the azoaromatic group in 1 (Fig. 13). 

Fig. 6. Photoinduced change in the degree of swelling of an azoaromatic polymer membrane in water at 25 °C Unit mole fraction of the azobenzene moiety in the polymer 0.024 ( ) in the dark, (O) under UV irradiation...

Angiolini and co-workers [52, 53] synthesized and investigated methacrylic polymers containing the side chain in the chiral cyclic (S)-3-hydroxypyrrolidine moiety interposed between the main chain and the trans azoaromatic chromophore, such as poly[(S)-3-methacryloyloxy-l-(4-azobenzene)pyrrolidine]... 

The exploitation of the ,Z-isomerization of the azoaromatic group for conformational problems was first studied in the 1960s. Goodman and Fabca synthesized copolypeptides of L-p-(phenylazo)phenylalanine with y-benzyl-L-glutamate. They found that photoisomerization of the azobenzene group in synthetic polypeptides resulted in changes in the ORD spectra due to changes in the conformation. 

Yokoyama, S., Nakahama, X, Otomo, A., and Mashiko, S., Preparation and assembled structure of dipolar dendrons based electron donor/acceptor azobenzene branching, Chem. Lett., 1137,1997. Goodman, M. and Falxa, M.L., Conformational aspects of polypeptide structure. XXIII. Photoisomerization of azoaromatic polypeptides,/. Am. Chem. Soc., 89, 3863,1967. 

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