Sulfonatocalix[8]arene as a Potential Reaction Cavity: Photo- and Electro-active Dicationic Guests Arrest Conformational Equilibrium

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 LIS to examine the host-guest complexation of CA[8] with photoactive bispyridyl ethylenes. We find that 4,4'-bispyridyl ethylene and 3,3'-bispyridvl 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,ire 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 all 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,I consequence on the excited-state chemistry of the former. Generally, prevalent geometric isomerization of bispyridyl ethylenes are prevented by CA[8] upon complexation.