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Kaliappan R, Ling Y, Kaifer AE, Ramamurthy V: Sulfonatocalix [8] arene as a potential reaction cavity: photo- and electro-active dicationic guests arrest conformational equilibrium. Langmuir. 2009 Aug 18;25(16):8982-92.
Smaller members of water-soluble sulfonated calixarenes have been extensively explored in the context of host-guest complexation, supramolecular chemistry, and potential sensors. However, larger members especially eight-membered calixarene (CA [8]) has received much less attention because of its floppy nature and tendency to exist as a mixture of conformational isomers. Our continued interest in identifying molecules with an internal cavity as reaction vessels has led us to examine the host-guest complexation of CA [8] with photoactive bispyridyl ethylenes. We find that 4,4'-bispyridyl ethylene and 3,3'-bispyridyl ethylene upon complexation to CA [8] arrest the conformational equilibrium and force the latter to adopt a single conformation in solution. During complexation, bispyridyl ethylenes are protonated by the sulfonic acid groups of CA [8]. The host-guest complex is stabilized via an electrostatic interaction between the cationic bispyridyl ethylenes and anionic sulfonated calix [8] arene, and we propose the complex to have an inverted capsular structure. This model is also consistent with the electrochemical behavior of 4,4'-dimethylviologen included within CA [8]. Rigidification of bispyridyl ethylenes by the host has a consequence on the excited-state chemistry of the former. Generally, prevalent geometric isomerization of bispyridyl ethylenes are prevented by CA [8] upon complexation. |
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