Azobenzene compounds

Rigid and fully conjugated azobenzene compounds (AZOs) have been incorporated into Hg based LAJs both in the trans and the cis form, as schematized in Fig. 14a, b [106, 107]. The measured currents are reported in Fig. 14c. Significantly, the currents flowing though the junction were also measured under in situ alternated irradiations at /. 370 nm and X = 450 nm. Under these condition the current... 

Unfortunately, 7 is a poor solvent for donor-acceptor compounds, and a maximum of 1 wt % concentration was achieved for 1. Limited solubility is a common problem with host-guest systems we found however, that the solubility of donor-acceptor stilbene and azobenzene compounds can be improved considerably by die attachment of bulky side-groups such as butyl or allyl to the chromophores. In this way, loading levels above 15 wt % were possible for certain compounds. On the other hand, simple cyclic side-groups such as pyrrolidino (12) led to poorly soluble materials (Figure 1). 

Figure 5. Absorption spectra for substituted azobenzene compounds.

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

Figure 6.11 shows the activity of an artificial enzyme can be controlled based on the phase behavior of a lipid bilayer. The catalytic site for hydrolysis was supplied by a monoalkyl azobenzene compound with a histidine residue which was buried in the hydrophobic environment of a hpid bilayer matrix formed using a dialkyl ammonium salt. Azobenzene compound association depended on the state of the matrix bilayer. The azobenzene catalyst aggregated into clusters when the bilayer matrix was in a gel state. In contrast, the azobenzene derivative can be dispersed into the liquid crystalhne phase of the bilayer matrix above its phase transition temperature. This bilayer-type artificial enzyme catalyzed the hydrolysis of a Z-phenylalanine p-nitrophenyl ester. The activation energy for this reaction in the gel state is twice as large as that observed in the hquid crystalline state. The clustering of the catalysts upon phase separation suppress their catalytic activity, probably due to the disadvantageous electrostatic environment around the catalysts and the suppressed substrate diffusion. This activity control is unique to such molecular assembhes. 

Up to now, photochemical studies are still confined to the reactions of simple azobenzene compounds, such as those derived by coupling a phenol or aromatic amine with a diazotised aniline. Azobenzene is isoelectronic with stilbene, its derivatives undergo facile trans cis photo isomerisation reactions in a manner analogous to that of stilbene. Azo dyes which possess a hydroxyl or amino substitutent ortho or para to the azo group... 

The oxidation of 4-aminoazobenzene with the UHP-acetic acid system was also investigated and the corresponding tri-azobenzene compound was formed. It appears as though the oxidative coupling reaction of the amino group preferentially occurs in contrast to the oxidation of the azo-moiety to produce the corresponding azoxy compound. Further experiments are needed to determine the facility of the azoxy formation by varying the stoichiometry of the oxidation... 

The C=C groups show no special features in the IR spectra of Au complexes 62 (item C-4 in Table 2), pointing to the lack of conjugation of these groups with the Au atom. See also Section IV.D.l below. IR spectroscopy was used to elucidate the structure of gold complexes with azobenzene compounds (165-167), including formation of N-metal and C-metal bonds. The resonance Raman spectra of bihrubin (168) and its complexes with Cu(II), Ag(I) and Au(III) show that the complexes have different structures, due to the different ionic charge and ionic radius of the metal ions . 

Azobenzene systems represent very attractive phototriggers in Ch LCs, thanks to their resistance to photofatigue, the simplicity of the molecules, and the ease of modification of their molecular structures. This chapter first describes helical twisting ability of chiral azobenzene compounds by focusing the structural effects on HTP as well as photochemical change in HTP, and then phototuning of helical structures of Ch LCs through the trans-cis photoisomerization of the chiral azobenzene compounds for applications to optical devices. 

Figure 10.3. Changes in helical pitch as a function of the chiral azobenzene compounds (azo-1, azo-2, and azo-3) in E44 before (o) and after (A) UV irradiation at 40°C.

To clarify the effect of the trans-cis photoisomerization of the chiral azobenzene compounds on the change in HTP, the change in the hehcal pitches of Ch LCs containing nonchiral azobenzene compound, p-azo-7-dl (Fig. 10.7), which was derived with racemic alcohol, was examined. Ch LCs were prepared by adding R811 and chiral or nonchiral azobenzene compounds with the same structure, p-azo-7-dl or p-azo-7, in E44. In the case of the (R811/p-azo-7/E44) sample, the helical pitch was increased by the trans-cis photoisomerization of... 

HTP of some chiral azobenzene compounds in E44 are given in Table 10.1. As described in the preceding section, HTPs of the chiral compounds increased as the distance between azobenzene group and chiral group became short. Therefore, the... 

Figure 10.7. Structures of chiral azobenzene compounds and a nonphotochromic chiral compound.

TABLE 10.1. HTPs [( x 108m mol g-E44), Experimental], the Length of the Long Axis, L, and the Diameter, D [(A), Measured from Molecular Orbital Calculation with PM3 Method], Data of Chiral Azobenzene Compounds... 

In binary Ch LC systems consisting of the chiral compound and the host nematic LC, the intermolecular interaction between the chiral azobenzene molecule and LC molecule contributes to HTP of the chiral azobenzene compounds. In this section, photoswitching behavior of some chiral azobenzene compounds (Fig. 10.7) is examined in terms of the influence of the chiral groups on the photochemical change in HTP. 

The photoisomerization ratios determined by comparing H NMR spectra of azobenzene compounds before and after UV irradiation are given in Table 10.3. The yields of cis form of all chiral azobenzene compounds at the photostationary state were 90% (Alam et al., 2007). Therefore, it is assumed that there is no effect of the yield of cis form at photostationary state on the change in HTP under UV light. 

The photochromic chiral azobenzene compounds, azo-1, azo-4, azo-5, azo-6, azo-7, and azo-10, only differ in their chiral substituents as can be seen in Fig. 10.7. Their HTPs were decreased upon UV irradiation because of the trans-cis photoisomerization of the chiral azobenzene molecules (Table 10.3). 

TABLE 10.3. Characterization of Chiral Azobenzene Compounds and Chiral... 

As mentioned in the preceding section, the anisotropy in the molecular shape influences significantly HTP of chiral compounds. Thus, to discuss the structural effect on the photochemical change in HTP, the molecular aspect ratio, LjD, was estimated with MOPAC/PM3 method. The molecular aspect ratios of the chiral azobenzene compounds in their isomeric states are given in Table 10.3. The data reveal that HTP of the chiral azobenzene compounds in both the isomeric states... 

This section discusses the photochemical switching of selective reflection, transparency, helical sense, and lasing through the phototuning of helical structure of the Ch LCs by photoisomerization of chiral azobenzene compounds. 

Alam MZ, Yoshioka T, Ogata T, Nonaka T, Kurihara S. 2007. The influence of molecular structure on helical twisting power of chiral azobenzene compounds. Liq Cryst 470 63 70. Bahr C, Escher C, Fliegner D, Heppke G, Molsen H. 1991. Behavior of helical pitch in cholesteric and chiral smectic C phases. Ber Bunsen Ges 95(10) 1233 1237. 

With this design, it is easy to understand that a concentration of azobenzene monomer before polymerization (>10wt%) is needed. Generally, azobenzene compounds have a Hmited solubility in LCs (<3%). To overcome this problem, we prepared a dimethacrylate azobenzene monomer that is Hquid crystaUine on its own (following structure), with the following phase transition temperatures (°C) on heating Cr 70 S 99 N 145 Iso. 

Figure 13.22. Bending responses as a function of time for silicon microcantilevers coated with (a) a monolayer of the trans-azobenzene compound on the gold surface, (b) a bare gold surface, (c) a monolayer of 1-dodecanethiol on the gold surface after exposure to a 365 nm UV light. Source From Ji et al., 2004. Reprinted with permission.

The creation of anisotropy is treated in some detail in Section 4.4, which deals with the trans-cis isomerization of azobenzene compounds. 

In recent years, optical dichroism and birefringence based on photo-induced trans-cis-trans isomerization of azobenzene groups has been observed with preoriented liquid-crystalline polymers [31-35] at temperatures above the glass transition temperature, and also with various amorphous polymers at temperatures well below the glass transition temperature. In the case of a polyimide (see Chart 5.7), a quasi-permanent orientation can be induced [36-38]. Here, the azobenzene groups are rather rigidly attached to the backbone and photoisomerization occurs at room temperature, i.e. 325 °C below the glass transition temperature, Tg = 350°C. This behavior is in accordance with the fact that the isomerization quantum yields of azobenzene compounds are very similar in solution and in polymer matrices 0 trans cis) 0,1 and 0(cis trans) 0.5. 

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