Azobenzene ligands

NLO materials (16 and 17) (Fig. 13) have been obtained from polyurethanes by the incorporation of sidechains with boron chromophores.37 The dihydroxy ligand of an azobenzene ligand containing a dimesityl boron acceptor was reacted in a polycondensation fashion with the diisocyanate groups of the polyurethanes to yield the desired polymers. Halogen displacement and transmetallation reactions have been utilized in the development of extended ir-conjugated systems of tri-9-anthrylborane with dendritic structures.38 In one (18) (Fig. 14) of the novel compounds, three identical... 

PHOTORESPONSIVE HYBRID SILICA MATERIALS CONTAINING AZOBENZENE LIGANDS... 

Most of the work on photoresponsive azobenzene-containing materials is based on polymer matrices. Numerous chemistries have been utilized to graft azobenzene ligands to various polymer chains. The azobenzene chromophores transfer light energy into conformational changes upon photoirradiation, which can be used to control chemical and physical properties of the materials, such as viscosity, conductivity, pH, solubility, wettability, permeability, transport properties, mechanical properties, and structural properties. 

Azobenzene ligands have also been incorporated into inorganic silica matrices. The robust inorganic silica framework does have some advantages in terms... 

Brinker and coworkers also prepared photoresponsive nanoporous silica particles using aerosol-assisted assembly, EISA, and surfactant-directed self-assembly (SDSA) techniques (Liu, 2004). The as-prepared photoresponsive nanocomposite particles were prepared so that the azobenzene ligands pointed toward the hydrophobic micellar interiors. After surfactant removal by solvent extraction, nanoporous particles with azobenzene ligands positioned on the pore surfaces were formed (Fig. 13.8). 

The azobenzene ligands incorporated into the mesoporous silica materials undergo photoisomerization upon light irradiation. Compared with the photoisomerization of the azobenzene ligands in solution, it exhibited some difference as described later. 

Brinker and coworkers also monitored the absorbance (A) at 355 nm (it it transition) in situ to ealeulate the kinetics of the ds trans isomerization of the azobenzene ligands contained in the nanocomposite film deposited on the ITO/ glass substrate in buffer solution (Liu, 2004). The first-order plot o ln Aao, Aq)/ (Aao—At)) vs. t was not perfectly linear in the entire region. The slope (rate constant, k) gradually decreases (t< 250 min) and remains constant (t> 250 min). The deviation from first-order kinetics is common for azobenzene ligands confined in sol-gel matrices or polymers (Bohm et al., 1996 Ueda et al., 1992). These data imply that the kinetics is composed of two parts, a fast one (ki) and a slow one ( 2). When t is small, the fast proeess is predominant. On the contrary, the slow process becomes predominant as time (i) increases. A double exponential equation was used to fit the data and evaluate ki and k2-... 

Here At and A -j are the absorbance at time t and infinite time. k and 2 are the rate constants of the fast and the slow processes. a and U2 represent the relative contributions of the fast and slow processes to the kinetics of the cis trans isomerization. The rate constants of the fast (ki) and the slow (k2) proeesses at 20°C are 1.7 X 10 " s and 1.7 x 10 s respectively. The rate constant of the thermal cis- trans isomerization of azobenzene derivatives in solution is usually on the order of 10 s Compared with this value, the rate of cis trans isomerization of azobenzene ligands incorporated into the nanocomposite films is faster for ki and is in the same order for k2, which is good for applications requiring fast switching between the two isomerization states. 

Brinker and coworkers proposed a physical explanation to the foregoing experimental results (Liu, 2004). In the photoresponsive nanoeomposites, a portion of the azobenzene ligands positioned on the mesopore surfaees has... 

The thermal cis- trans isomerization of the azobenzene ligands confined in the nanopores has a constant rate constant and exhibits faster isomerization than in solution with the exception of the bulky AzoG3 dendrimer. This result is consistent with Brinker and coworkers observation of azobenzene-modified nanoporous silica films and supports their two-rate-constant physical model. The nanoporous silica materials prepared by Brinker and coworkers have a cubic (BCC) pore structure. The azobenzene ligands positioned on the pore coimections have different local environments from those positioned on the spherical pore surfaces. Thus the azobenzene ligands isomerize at two different rates—fast and slow. The MCM-41 nanoporous materials prepared by Zink and coworkers have a hexagonal array pore structure in which all the azobenzene ligands positioned on the channel surfaces have the same local environment. Thus simple one-rate-constant first-order kinetics is sufficient to describe the isomerization process. 

Figure 13.17. Schematic drawing of the electrochemical cell (top) and mass transport of probe molecules through the photoresponsive nanocomposite membrane integrated on an ITO electrode [bottom). A shows slower diffusion through smaller pores with azobenzene ligands in their trans configuration. B shows rapid diffusion through larger pores with azobenzene ligands in their cis configuration. Source Liu et al., 2004. Reprinted with permission.

In Fig. 13.18, the normalized eurrent ratio of Brij 56-templated TSUA-modified nanocomposite films is 40%. However, in another experiment using P123-templated films that have a larger pore size (ca. 6.5 nm), the normalized current ratio is only 1.2%. Because the molecular length of azobenzene ligands located on the pore surfaces is 1.8 and 1.5 nm in the trans and cis forms, respectively, the optically triggered restriction in pore size is expected to have a diminished effect on transport for P123-templated films compared with the smaller pore size Brij 56-templated films. 

Macroscopic deformation is a collective behavior of the numerous azobenzene ligands incorporated into the material. It is much easier to detect experimentally than molecular-level deformation. Despite the difficulty in device fabrication and microscopic force measurement, rigorous studies of single-molecule optomechanical transduction using an individual polymer chain containing photorespon-sive azobenzene ligands in the backbone were reported. 

Figure 13.25. Photo and thermal isomerization of azobenzene ligands in the photoresponsive aerogel, (a) As-prepared, (b) after UV irradiation of sample (a) for 40 min, (c) after room light exposure of sample (b) for 60 min, (d) after heating sample (b) to 100°C for 10 min.

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