Azobenzenes motion

Harada J, Ogawa K (2001) Invisible but common motion in organic crystals a pedal motion in stilbenes and azobenzenes. J Am Chem Soc 123 10884-10888... 

Free cw-azobenzene, excited at 480 nm displays a biexponential decay of the excited state Si with time constants of 0.1 ps and 0.9 ps. Here the ultrafast kinetic component dominates the absorption change (it contains 90 % of the whole amplitude). A direct interpretation would relate the fast component to a free isomerizational motion, where the most direct reaction path on the Si and So potential energy surface is used without disturbance. The slower process may be assigned to a less direct motion due to hindrance by the surrounding solvent molecules. This interpretation is supported by the observation of the absorption changes in the APB and AMPB peptides. Here both reaction parts are slowed down by a factor of 2 - 3 and both show similar amplitudes The peptide molecules hinder the motion of the azobenzene switch and slow down considerably the initial kinetics. However, in all samples the transition to the ground state is finished within a few picoseconds. 

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.

The initial motion of the light triggered switch, the isomerization of azobenzene, is ultrafast and occurs on the timescale of 200 fs and 2 ps. 

Another system constructed based on the unique coordination property of Cu is a molecular photo-electro transducer , which works in a cyclic manner totally powered by light irradiation [60]. The UV/blue light controlled repetitive motion of azobenzene moieties in 6,6/-bis(4"-tolylazo)-4,4/-bis(4-ferf-bulylphcnyl)-2,2 -bipyridine, 74, causes reciprocal Cu1 translocation between two coordination environments in complex 75, resulting in pumping of the redox potential of Cu1 (Scheme 3). Therefore, UV/blue light information can be successfully transformed into an electrode potential change and... 

Azobenzenes have been utilized to measure the free volume in polymers and the speed of polymeric segmental motion [42, 43], Azobenzenes that are covalently bonded to a polymer backbone may influence various properties of the macromolecule. Photoisomerization of such substances will cause changes in wettability [44], viscosity [45], solubility [46], membrane properties [47], and swelling properties [48]. 

Cojocariu C, Rochon P. (2004) Light-induced motions in azobenzene-containing polymers. Pure Appl Chem 76 1479-1497... 

Two studies have been reported on azobenzenes 2, 2 5 4 jp case of the sterically hindered 0,0,0substituted azobenzines the effect of exciting lower M n, tt ) and higher (ir, ir ) states has been studied25. in cyclodextrin inclusion complexes of azobenzene there is partial blockage of rotational motion about the N=N bond25. ... 

By means of comparison, in related experiments on PI-3 a and PI-3 b, the mean absorbance returned to its initial value within two minutes after removal of the irradiation light. Clearly, the polymer rigidity intrinsic to the donor-embedded samples must affect the rate of isomerization of the cis azobenzene derivative, suggesting that motion of the chromophore and the polymer backbone are somehow coupled. 

Natansohn, A., Rochon, P., Meng, X., Barrett, C., Buffeteau, X, Bonenfant, S., and Pezolet, M. Molecular addressing Selective photoinduced cooperative motion of polar ester groups in copolymers containing azobenzene groups. Macromolecules 31, 1155 (1998). 

To summarize, there are three types of photoinduced motions at the molecular level, at the nanometer, or domain, level, and at the micrometer (macroscopic) level. All are the result of photoinduced isomerization of the azobenzene groups. One interesting direction of our research was to try to exploit these phenomena for their photonic applications. We have demonstrated, at least as proof of principle, a few possible photonic functions for the new materials we synthesized. Some of these are summarized in publications they will also be reviewed here. 

To our knowledge, this was the first instance when cooperative motion was demonstrated in amorphous polymers below Tg, although its effects had been briefly mentioned before. Cooperative motion is well known in liquid crystalline and crystalline polymers, especially in the ordered phases, and it is caused by the thermodynamic tendency to maintain order. Below Tg, it was believed that there is not enough mobility to allow like groups to move together. This belief was similar to the belief that liquid crystallinity was necessary to move azobenzene groups under illumination. These are only two examples of how our research led to a revision in the understanding of polyrner physical chemistry. 

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