Exclusive formation of 1 : 1 and 2 : 2 complexes between cucurbit[8]uril and electron donor-acceptor molecules induced by host-stabilized charge-transfer interactions

The exclusive formation of 1:1 and 2:2 host-guest complexes between cucurbit[8]uril (CB[8]) and electron donor-acceptor (D-A) molecules by host-stabilized charge-transfer (CT) interactions is reported. Reaction of CB[8] with D-A molecule 1(2+) containing dipyridyliumylethylene as an electron-acceptor unit and 2,6-dihydroxynaphthalene as an electron-donor unit, which are connected by a but-2-enyl linker, resulted in the exclusive formation of 1:1 complex 2(2+) which has been characterized by UV-visible, NMR and mass spectrometry. The H-1-NMR spectrum of 2(2+) revealed that the acceptor and donor units of 1(2+) form an intramolecular CT complex inside the cavity of CB[8], which was also supported by a molecular modeling study. Another D-A molecule (3(2+)), which contains a rigid 1,4-xylyl linker to connect a dipyridyliumylethylene unit and 2-hydroxynaphthalene unit reacts with CB[8] to form 2:2 complex 4(4+) exclusively. In 4(4+), two molecules of 3(2+) form a CT complex with each other while the donor and acceptor units of one guest molecule interact with the acceptor and donor units, respectively, of the other inside the cavities of two CB[8] molecules.