Polarity/polarization azobenzene dyes

Figure 10 Reorientation of an azobenzene dye by photoisomerization with polarized light. Molecular representations of the lruns and cis forms of an azobenzene are represented by straight and bent shapes to illustrate how a trans-cis-trans cycle can effect a reorientation of the traru-azobenzene. If the inducing hght is polarized in the horizontal direction, the vertically oriented molecule will be unable to absorb the hght, and this orientation will therefore be photostable.

The input polarization dependence of p- and s-polarized SH light was measured to elucidate the molecular orientation of L and ML species. The results can be fitted well to the theory and the relative values of three non-zero elements of the surface susceptibility [63-65], which determine the molecular orientation, are obtained from the fitting. Since hyperpolarizability of azobenzene dyes is known to be dominated by a single element which is an element of the hyperpolarizability along the tc-jt moment direction [52], the tilt angle 0 can be estimated from the relative values of the surface susceptibility. 

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. ... 

Materials with non-linear optical (NLO) properties are of particular interest to the telecommunications industry and specific to the technology of photonics [150]. The impregnation of polymeric materials with organic, azobenzene dyes is a possible route to the preparation of these materials. Poling techniques are required to avoid the polar molecules aligning antiparallel to each other, which avoids centro-symmetric arrangements [151]. A thorough review of the preparation and assessment of polymeric materials with azo-dyes for non-hnear optics has recently been prepared by Yesodha et al. [152]. 

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. 

These unexacting requirements make the simplest unsulphonated azo structures, often monoazo types, quite acceptable [80]. Typical of the least polar members of this class are Cl Solvent Yellow 2 (4-68), Cl Solvent Orange 1 (4.69) and Cl Solvent Red 17 (4.70). Simple azo structures carrying sulphonamide, sulphone or carboxylate ester groups are used where a somewhat more polar, less soluble dye is needed. Simple disazo compounds (4-amino-azobenzene— 2-naphthol, for example) are used as red solvent dyes. Probably the only structural feature worthy of note in this class is the occasional adoption of structures carrying long alkyl chains to enhance solubility, as in the case of the disazo dye Cl Solvent Yellow 107 (4.71). 

A closely related phenomenon induced by linearly polarized light was found independently by Gibbons et al., who employed a polyimide (PI) film doped with azobenzene molecules as a dichroic dye and showed that the direction of homogeneous... 

Coplanarity in 2,6-dimethylazobenzenes56 and its effect on the efficiency of transmission of electronic effects from one ring to the other was studied by Byrne and Happer using 13C chemical shifts. The methyl groups have a substantial effect on the planarity of the system, but, surprisingly, a loss of planarity has relatively little effect on the efficiency of transmission of both polar and resonance effects between the two rings. H and 13C NMR studies of substituent effects on the molecular planarity was performed by Koleva et al.51 Jirman58 was interested in H and 13C NMR spectra (Tables 5 and 6) of 2,2 -disubstituted azobenzenes (16) and 2,2 -azopyridine (17) suitable for preparation of metallized azo dyes. 

The prototype molecule for donor/acceptor substitution is 4-dimethyl-amino-4 -nitroazobenzene. Here, the n- n band is shifted far to the red due to the charge transfer character of the transition. The band has few vibrational features, and its energy is influenced by the polarity of the solvent. The weak n Tt band cannot be seen under the intense it —> it band (Figure 1.13). Most commercial azo dyes are pseudo-stilbenes rather than azobenzene-type molecules. 

Besides azobenzene, spirobenzopyran, and salicylidene derivatives, other photochromic dyes have been used in LBK films. For example, anthocyanine dyes 5 (see Figure 6.5) have been used to obtain monolayers that change the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

Thermal ablation. The SRG is normally 7t-shifted in relation to the interference pattern, and is the result of the application of a very intense pulse of laser light or a high-power continuous wave (cw) laser. The SRGs cannot be optically erased, but rather only thermally erased. In some experiments, some polarization dependence was observed, which cannot be explained with present knowledge of the field. The dye does not need to be azobenzene for such SRGs (see Grzybowski et... 

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