Azobenzene conformational change

Saito, T. Kobayashi, T. Conformational Change in Azobenzene in Photoisomerization Process Studied with Chirp-Controlled sub-lO-fs Pulses.]. Phys. Chem. A 2002, 106, 9436-9441. 

Photochemomechanical systems have also been studied using gels and photochemical reactions such as photochromism of spiropyrans [20], trans-ds transition of azobenzene [21] and photo-dissociation of triphenylmethane leuco cyanide [22]. It is an attractive approach to utilize a molecular level conformational change. 

Photochemical switching of the electrical properties of conductive Langmuir-Blodgett films results from photoinduced conformational changes produced by an azobenzene unit [8.253]. 

Investigation of light-induced conformational changes has also been extended to solid films of azobenzene-containing poly(L-aspartate)s, but no conformational change was induced by photoisomerization of the azobenzene units. This was probably due to the limited mobility of the polypeptide chains in the films. 57 ... 

Photoisomerization of an azobenzene function located in a complex molecule is often accompanied by conformational changes. This approach has again been employed in the construction of photoresponsive crown ethers, a topic which has been the subject of a recent review. Cylindrical ionophores in which two diaza-crown ethers are linked by two photoresponsive azobenzene groups change their ability to bind polymethylene-diammonium salts on irradi-... 

The third mechanism (3) is the simplest one. When trans-cis photoisomerizable chromophores are incorporated into the polymer backbone, the photoinduced configuration change of the chromophores is expected to induce a conformation change of the polymer chain. Azobenzene (4) is the most widely used as the trans-cis photoisomerizable photoreceptor molecule. It undergoes isomerization from the trans to the cis form under ultraviolet irradiation, while the cis form can return to the trans form either thermally or photochemically [11]. 

Photostimulated conformation changes observed for polypeptides with pendant azobenzene residues may also be classified as the mechanism (5) [23, 24]. 

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The photostimulated conformation change of polystyrene with pendant azobenzene groups in cyclohexane has also been studied by the time-resolved light scattering technique [26]. 

The conformation change of polyamide with pendant azobenzene grou (6) in aqueous solution also induced a change in pH [14]. Ultraviolet irradiation caused the pH of an aqueous solution of the polyamide to decrease and visible irradiation returned the pH to the initial value. 

A comparison of the photochemical behavior of azobenzene residues In the backbone and the side chains of polymers Is of special Interest since the short lifetime of the excited state makes It more difficult for the chain to undergo long-range conformational changes favoring the trans-cls Isomerization process. 

As revealed distinctively by experiments on the photoinduced regulation of in-plane alignment by silica plates whose surfaces were modified with azobenzenes,148-151 the azimuthal reorientation of liquid crystals was achieved by the uniaxial conformational change of command molecules localized at the topmost substrate surface. This situation was confirmed for polymer films by the results obtained with the following systems. The first consisted of the chemical modification of PVA thin films. Careful treatment of PVA films with azobenzene acid... 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

Fig. 8A-C. Molecular switches based on the binding ability of crown ethers. A A photo-switchable binding event based on conformational change upon azobenzene isomerization. B A photoswitchable binding event based on both conformational and electrostatic changes. C A redox-switchable calix[4]arene-crown ether...

Fig. 11A-E. Some switchable units for supramolecular chemistry. A A chemically switchable guest 25 based on an amine. B A photoswitchable host for saccharides based on conformational change. C A pH-switchable guest for cucubituril. D A redox-switchable cyclophane. E A photoswitchable azobenzene-based guest for the cyclophane 22...

Azobenzenes buried at different levels with respect to the monolayer surface have been studied and the electrochemical behaviour of the films was clearly dominated by the length of the alkane chain above the level of the azobenzene group. This behaviour was attributed to the steric inhibition of the conformational change of the azobenzene moiety... 

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