Chromophores azobenzene

Table 1. Dipole moments, charge transfers, and molecular hyperpolarizabiUties for azobenzene chromophores, calculated using CNDO/S method ...

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

Kang, E.-H. Bu, T. Jin, R Sun, J. Yang, Y. Shen, J. 2007. Layer-by-layer deposited organic/inorganic hybrid multilayer films containing noncentrosymmetrically orientated azobenzene chromophores. Langmuir 23 7594-7601. 

Scheme 1. Ammonium amphiphile having an azobenzene chromophore.

The crystal structure can be considered as a structure regularly stacked with bimolecular layers along the a-axis. Within the bimolecular layer, two molecules related by inversion symmetry face each other in the tail-to-tail fashion with their molecular axes inclined by about 26° to the bilayer surface. This inclination enables the head-to-tail arrangement of azobenzene chromophores as expected from the spectroscopic study. 

Spectral shift (in wave number Av) of the azobenzene chromophore caused by intermolecular interaction can be estimated by using Kasha s equation (2), which is a function of the transition moment (n), distance between dipoles (r), and number of interacting molecules in bilayer assemblics(V). 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

For example, PPI dendrimers functionalized with azobenzene chromophores appended with aliphatic side chains assemble into large spherical vesicles in water below pH 8 (Fig. 11.14 Tsuda et al. 2000). 

Similar generation-dependent aggregation behavior was reported for a series of dendrons having a phosphate focal group and functionalized at the periphery with azobenzene chromophores in water (Zhang et al. 2007). In this system, the stability... 

Because of some aggregation (H aggregation) of the azobenzene chromophores, the collapse pressure increased with higher azobenzene/palmitoyl ratios. However, only the palmitoyl functionalized dendrimer exhibited a change in surface area upon irradiation with 365-nm light. The lack of photoinduced surface area changes... 

Fig. 6. Left schematic potential energy curves explaining the absorption transients seen in the newly formed trans-absorption bands. The strain between the peptide part and the azobenzene chromophore delays the transition to the relaxed ground state and causes the blue-shift observed as a function of time. Right View of the two phases of peptide motion occurring after the initial isomerzation of the azobenzene chromophore.

Here, it is worthwhile to note the difference in the response times of the dissolution and phase separation processes. This is important from the view point of energy conversion efficiency. At 19.5 °C, which is very close to Tc of the polymer solution with all trans azobenzene chromophores, the isomerization of a small number of chromophores, in other words, a small number of photons, was enough to raise Tc above 19.5 °C. Therefore, the transmittance increase took place immediately by irradiation for a very short time. The polymer chain was efficiently expanded by a small number of photons. 

PNIPAM containing pendant triphenylmethane leuconitrile groups also showed photostimulated phase transition from the phase separated state to the homogeneous state [13]. The triphenylmethane leuconitrile group is known to change the polarity more pronouncedly than the azobenzene chromophore by the ionic photodissociation of C-CN bond [14]. 

Table H Optical Spectra of Dibutylaminostilbene and Azobenzene Chromophores...

The azo-modified, elastin-like polypeptide XIV illustrated in Scheme 9 exhibits a so-called inverse temperature transition" that is, the compound gives cross-linked gels that remain swollen in water at temperature below 25 °C but deswell and contract upon a rise of temperature. The trans-cis photoisomerization of the azo units, obtained through alternating irradiation at 350 and 450 nm, permits photomodulation of the inverse temperature transition.[S9] The result indicates that attachment of a small proportion of azobenzene chromophores is sufficient to render inverse temperature transition of elastin-like polypeptides photoresponsive, and provides a route to protein-based polymeric materials capable of photomechanical transduction. 

---