Butterfly crown ethers

A series of azobis(benzocrown ethers) called butterfly crown ethers such as 9 and 10 were synthesized [11-15]. The photoresponsive molecular motion is... 

Shinkai et al.111-151 synthesized a series of azobis(benzocrown ethers) called butterfly crown ethers , of which compounds 9 and 10 are examples. Their photoresponsive molecular motion resembles that of a flying butterfly. It was found that the proportion of their Z forms at the photostationary state increases remarkably with increasing concentration of Rb+ and Cs+, which interact with two crown rings in a 1 2 sandwich fashion. This is clearly due to the bridge effect of the metal cations with the two crowns, results that support the view that the Z forms make an intramolecular 1 2 complex with these metal cations. As expected, the Z forms extracted alkali metal cations with large ion radii more efficiently than did the corresponding E forms. In particular, the photoirradiation effect on 9 is quite remarkable for example, ( )-9 (n= 2) extracts Na+ 5.6 times more efficiently than (Z)-9 (n= 2), whereas (Z)-9(n= 2) extracts K+ 42.5 times more efficiently than ( )-9(n= 2). l ... 

Figure 45 (a) Photoswitable K+-ion binding of butterfly crown ethers, (b) Schematic representation of light-driven K+-ion transport. 

Multisite crown ethers (30) and (31) are polymacrocycles. These molecules are potentially like cryptands in view of the possibilities for forming inclusion-like species. The photoresponsive crowns provide an excellent example of this aspect, and consist of two crown ethers, as in (30a and 30b), attached via a photosensitive azo linkage. This molecule undergoes reversible isomerization (likened to a butterfly motion), shown in equation (13). The cis form gives a stable 1 1 cation ligand complex with the larger alkali cations (actually a 1 2 cation crown ratio). Concentrations of (30b) in solutions are thus noted to be enhanced by the addition of these cations.100,101 Other multisite crowns have been prepared from diphenyl- and triphenyl-methane dyes, e.g. (31).102... 

S. Shinkai, T. Nakaji, T. Ogawa, K. Shigematsu, O. Manabe, Photoresponsive Crown Ether 2. Photocontrol of Ion Extraction and Ion-Transport by a Bis(Crown Ether) with a Butterfly-Like Motion , J. Am. Chem. Soc., 103, 111 (1981)... 

Photochemical butterfly-like E — Z photoisomerization of a bis(crown ether) azobenzene derivative 354 was found to be thermally reversible and the stereoisomers exhibit unique contrasting behaviour in the presence of metal ions.1108 The concentration of the Z-isomer in the photostationary state was noticeably enhanced by the addition of K+, Rb + or Cs +, because the corresponding Z-complex achieved a stable sandwich geometry (Scheme 6.162). As a result, the cations could be selectively extracted by the Z-derivative from an aqueous phase to an organic solvent (o-dichlorobenzene), whereas no complexation (i.e. no transfer) took place in the case of the E-isomer. 

Shinkai et al. reported photoresponsive crown ethers (Figure 54) comprising two 15-crown-5 molecules linked by an azobenzene unit, which exhibited a butterfly-like motion. They found that the rate of the thermal isomerization was suppressed by added alkali metal cations and different alkali metal cations could be extracted efficiently by the different conformations of the molecules with high selectivity. These studies demonstrate that in principle ion extraction and ion transport through a liquid membrane can be controlled by light. Although this is not a mechanical molecular machine, it is considered as the prototype of early molecular machines and... 

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