Azobenzene Systems

organometallic ferrocene polymers (Liu et al., 1997), dendrimers (Junge and McGrath, 1997 Mekelburger et al., 1993), and even conjugated polydiacetylenes (Sukwattanasinitt et al., 1998), polyacetylenes (Teraguchi and Masuda, 

Rubner, 2000) (in the case of weak polyelectrolytes) of the assembly solution, the polyelectrolyte chain conformation is modified, and hence the resulting film architecture is tuned. For instance, one can control thickness (Losche et al., 1998 Dubas and Schlenoff, 1999), permeability (Rmaile and Schlenoff, 2003), morphology (Antipov et al., 2003 McAloney et al., 2003 Mendelsohn et al., 2000), and density (Dragan et al., 2003). Recently the technique has been modified to assemble the alternate layers using a spin-coater, which reduces the assembly times and adsorption solution volumes considerably (Chiarelli et al., 2001 Cho et al., 2001 Lee et al., 2003, 2001). 

As a film preparation technique, this method has numerous advantages. The adsorption of the polymers is quasithermodynamic, with the chains adsorbing into a local minimum, which makes the films stable against many defects (dewetting, pinhole formation, etc.). Importantly, the technique is not limited to flat surfaces any geometry that can be immersed in solution (or have solution flowed through) 

Many research groups have investigated the possibility of incorporating optically responsive azobenzene chromophores into the versatile PEM structures (examples presented in Fig. 1.8), including Advincula (Advincula et al., 2001, 2003 Advincula, 2002 Ishikawa et al., 2002), Kumar and Tripathy (Lee et al., 2000 Balasubramanian et al., 1998), Tieke (Ziegler et al., 2002 Toutianoush et al., 1999 Toutianoush and Tieke, 1998 Saremi and Tieke, 1998), 

---

Liu Z F, Loo B FI, Flashimoto K and Fu]ishima A A 1991 Novel photoeleotroehemioal hybrid one-way prooess observed in the azobenzene system J. Electroanal. Chem. Interfaclal Electrochem. 297 133-44... 

Another interesting variation on the linkage holding together two benzocrowns was reported by Shinkai and coworkers In their case, the benzo groups on the crowns were also part of a photolabile azobenzene system. The azo-linked bis-crown was prepared as follows (see Eq. 3.34). 4 -Nitrobenzo-l 5-crown-5 was reductively dimerized... 

This paper summarizes the theoretical analysis of some new molecules with methylsulfonyl group as the electron acceptor group, describes the syntheses of new stilbene and azobenzene systems, and presents the measurements of their optical spectra, ground-state dipole-moments, and molecular hyperpolarizability coefficients, p. We compare theoretical and experimental results and comment on the potential usefulness of these chromophores as components for NLO materials. The incorporation of sulfonyl-containing chromophores into polymers, and the NLO properties of the resulting materials, will be discussed in our forthcoming paper (9). 

Fig. 5.23 Dendrimer with push/pull 7i-azobenzene system (schematic)...

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Yesodha SK, Pillai CKS, Tsutsumi N. 2004. Stable polymeric materials for nonlinear optics a review based on azobenzene systems. Prog Polym Sci 29 45 74. 

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. 

In contrast to triphenylmethane leuco derivatives, the backward reaction of azobenzene systems was photoreversible. Irradiation with light having wavelengths between 330 and 380 nm resulted in the trans —s- cis isomerization while a temperature increase or irradiation with light having wavelengths of more than 420 nm induced the reverse cis —> trans isomerization reaction. 

Additionally, the bending direction of azobenzene systems could be controlled by the orientation of the photoactive moieties [170]. While in a homogeneous alignment (parallel to the surface) the films bent away from the light source, in a homeotropic alignment (perpendicular to the surface) the films bent towards the irradiation direction [171]. When exposed to UV-light, the surface of the homogeneous film contracted and the load on the films increased while the surface of the homeotropic film expanded and the load on the film decreased. 

The photoinduced properties of azobenzene systems are strongly related to their polymeric architecture. The way in which azobenzene units are bound to the polymeric chain and the nature, either amorphous or liquid crystalline, of the final material strongly affects the magnitude and stability of the photoinduced response (Natansohn and Rochon,2002). 

The mechanism of trans-cis photoisomerization in azobenzenes proceed either via rotation about the azo bond with the mpture of the tt-character under n-n excitation, or via inversion mechanism under n-jT excitation, where the iT-bond remains intact. Both mechanisms are shown in Figure 9.3. On the other hand, thermally induced reversible cis-trans back reaction proceeds through a rotational mechanism. The trans-cis photoconversion is almost completely reversible in picoseconds without any side reactions, whereas thermal cis trans conversion usually has timescales ranging from nano- to miUiseconds, or even hours, depending upon the substituent and the local environment A variety of azobenzene systems have been designed and synthesized, including cyclodextrins [40-42], admantanes [43], polycyclics [44], bacteriorodophsin [45], and crown ethers [46]. 

---