Azobenzene-attached complexes

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... 

Supramolecular chemistry typically involves the complexes based on molecular recognition, which include crown ethers, catenates, rotaxanes, etc. Azobenzene derivatives are often used as structural elements in such supramolecular assembly. Already in 1980 (Shinkai et ah, 1980), azobenzene moiety had been introduced into crown ether. Other recent examples include azobenzene-cyclodextrin complexes (Callari et ah, 2006), azobenzene-attached nanotube-cyclodextrin complexes (Descalzo et al., 2006), and molecular recognition complexes with DNA (Haruta et al., 2008). Numerous examples of photoswitch-able azobenzene supramolecular systems in solutions can be found (Yagai et al., 2005 Balzani et al., 2002). But there are no investigations in these systems concerning photoorientation and mass transport. 

Multi-mode molecular switching properties and functions of azo-conju-gated terpyridine complexes of transition metals have been studied [47-49]. The dependences of the photoisomerization behavior on the metal center and its oxidation state were investigated using azobenzene-attached terpyridine complexes of four metals Fe, Co, Ru, and Rh. As for the Fe and Ru complexes, 60-62, photoisomerization is totally inhibited when UV light irradiation excites the n -n band of the azobenzene moiety in the trans form due to the occurrence of energy transfer from the azobenzene unit to the complex unit [50-53]. 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

Several problems occurring in polyelectrolyte complexes can be circumvented by attaching the azobenzene inoiety covalently to a pplymer chain. Seki and Ichimiura used poly(vinylalcohol) as a hydrophilic backbone and attached azobenzene fatty acids 32 (n = 5,10) to this backbone. 

Sonication of 68 or 69 in the presence of transition metal ions produced coordi-natively polymerized bilayer membranes (CPBMs) as schematically illustrated in 70 26.27.130 Upon complexation with the transition metal ions, stability of bilayer membranes was remarkably improved. Amphiphiles 68 and 69 form complexes with transition metal ions with 1 1 stoichiometry. The pronounced enhancement of the stability of bilayer membranes suggests that each metal ion of the resulting CPBM is bound to two nitrogen atoms of the two adjacent azobenzene moieties as indicated by 70. Some metal ions of the CPBMs may be attached to one azobenzene unit. Even if metal ions are singly attached to the azo ligand, a polymeric cluster of metal ions bound to the dihydroxyazobenzene ligand is obtained upon sonication of the amphiphile with transition metal ions. 

Increasingly, CD-based molecular devices are being attached to surfaces to both increase their versatility and to extend their physical dimensions to the extent that they are often referred to as nanomachines. The first example shown in Figure 35 is based on the silica MCM-41 nanoparticle to which is attached an array of azobenzene derivatives in the E form, or stalks, adjacent to the many pores which characterize the nanoparticle as represented for a single pore by 134. In water, the dye Rhodamine B, 133, freely exchanges in and out of the pore. However, when p-CD subsequently complex the stalks egress of Rhodamine B... 

Figure 35 Scheme for the entrapment of Rhodamine B, 133, in a pore of a MCM-41 nanoparticle, 134, through capping the pore by complexation of /3-CD on azobenzene derivatives attached to the nanoparticle surface, 135 (Steps 1 and 2). The Rhodamine B is subsequently released by uncapping the pore through photoisomerization of the azobenzene unit from the to Z form and decomplexation of /3-CD from 136 (Step 3). 

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