Isomerization of the azo group

The first example of facile reversible trans-to-cis isomerization of the azo group through a combination of photoirradiation and a redox cycle has been achieved in an azobenzene-attached tris(bipyridine)cobalt system [56-58]. This combination makes possible both forward and backward isomerization in response to irradiation from a single light source (Scheme 1). The Co11 complex, 70-2BF4, with trans-azobenzene moieties affords the cis form in... 

A shown above, the eonformational ehanges are eonfined to the isomerization of the azo group. 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

Na—Np isomerism Perhaps the most immediately obvious source of isomerism in metal complexes of tridentate azo compounds is the involvement of different nitrogen atoms of the azo group in coordination to the metal. This is not relevant in the case of symmetrical azo compounds but two structurally isomeric metal complexes (79) and (80) are conceivable for unsymmetrical azo compounds. These have been designated by Pfitzner78 as Na and Np isomers. 

N-a, 3 isomerism arises from the fact that only one nitrogen atom of the azo group can act as third ligand. In the case of metallizable unsymmetrical azo compounds two isomers can be discriminated in a 1 1 metal complex [34] and three isomers in a symmetrical 1 2 metal complex [35] (Figure 2.10). 

Structuration is not completely elucidated. It is well known that essentially for the surface structuration is the trans-cis isomerization process of the azo-groups, under UV-VIS irradiation. As a consequence, a very careful study regarding the pho-tochromic behavior of the azo-polysiloxanes was effectuated. The photochromic response at the light stimuli was evaluated in different conditions trans-cis isomerization under UV irradiation cis-trans relaxation in the presence of the natural visible light cis-trans relaxation only thermally activated (in the dark). It must be underlined that there are no reported studies in the literature concerning the cis-trans relaxation process in the presence of natural visible light, a very important aspect in our opinion regarding the elucidation of SRG formation mechanism. 

A major feature of the azo group is its capability to isomerize this is the property used widely in photoresponsive organic thin films. The E-Z photoisomerization of azobenzene was detected by Flartley, who created the Z-form by irradiation of E-azobenzene. Generally, the E-forms of azo compounds are more stable than the Z-forms. The parent E-azobenzene is by ca. 50 kj moT the more thermodynamically stable isomer. Isomerization is the main photoreaction of most aromatic azo compounds. Other thermal- and photoreactions lose the competition with isomerization. 

These studies confirm that the BEM groups move together with the DRIM groups, and the kinetics of the process depends on copolymer composition. While the photoorientation and relaxation of the azo groups followed the already established biexponential behavior, the BEM groups photo-induced orientation could be described by a single exponential, and they did not relax. This suggests that the cis-trans isomerization that happens when the pump beam is turned off is the most important relaxation process in all polymer films. 

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