Low molecular weight azobenzene

Advincula RC, Fells E, Park M K. 2001. Molecularly ordered low molecular weight azobenzene dyes and polycation alternate multilayer films aggregation, layer order, and photoalignment. Chem Mater 13(9) 2870 2878. 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

Here the quantum yield of a small number of azobenzene residues built into the backbone of a polyamide was compared with the quantum yield of an analogous low molecular weight azobenzene derivative in a series of polyamide - DMSO mixtures. As expected, the quantum yield for the azobenzene residues in the polymer backbones was very small in systems containing a high polymer concentration, since here conformational transitions of the polymer chains would necessarily be cooperative, while the photoisomerization of the small molecule was relatively insensitive to the polymer concentration. However, in highly dilute solution there was absolutely no difference between the photoisomerization efficiency of azobenzene residues built into the polymer chains and the small azobenzene derivatives. This identity in the behavior of the polymer and its analog in dilute solution shows that the isomerization of azobenzenes in the chain backbone can be accomplished without a simultaneous hindered rotation in another portion of the chain. 

The number of chromophores in the LBK film can be enhanced by-mixing the poly(glutamate) (37) with a low molecular wei t azobenzene dye. Both chromophores, the covalently bound and the admixed dye, can be isomerized in LBK films by irradiation with UV light. The trans to cis isomerization reduces significantly the interaction between the bound and the admixed chromophores, and phase separation occurs. Upon repeated isomerization cycles, the low molecular weight dye is expelled from the LBK film and forms crystals on top of the film. This can be easily detected by polarized light microscopy. ... 

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