Azobis benzocrown 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 ... 

S. Shinkai, K. Shigematsu, M. Sato and O. Manabe, Photoresponsible crown ethers. Part 6. Ion transport mediated by photoinduced cis-trans interconversion of azobis-(benzocrown ether), J. Chem. Soc., Perkin Trans. I, 1982, 2735-2739 J. Anzai, H. Sasaki, A. Ueno and T. Osa, Photo-induced potential changes across poly(vinyl chloride)-crown ether membranes, J. Chem. Soc., Chem. Commun., 1983, 1045 J. Anzai, H. Sasaki, K. Shimokawa, A. Ueno and T. Osa, Photocontrol of alkali metal ion permeability through the poly(vinyl chloride)/crown ether membranes, Nippon Kagaku Zasshi, 1984, 338. 

The cis-isomers of azobis(benzocrown ether)s, 4 and 5, can form intramolecular 1 2 metal/crown sandwich-type complexes with Rb " and Cs ". Examination of Table I reveals that the cis-to-trans isomerization from these sandwich-type complexes requires extra free energies of activation by 0.6-1.7 kcal mol"l. The energies should be consumed to disrupt the interaction between the metal and the crown ethers. On the other hand, the rate inhibition was not observed for 6 which has methylene spacers between the benzenes and the crown rings. Conceivably, the geometrical change induced by the isomerization of the N=N bond may be absorbed by the rotational freedom of these methylene groups. Thus, the effective inhibition occurs only in the rigid cis-isomers. 

The transport of ions mediated by the c i s — t rans interconversion of the photo responsive azobis(benzocrown ethers)... 

---