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 12+ 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 22+ which has been characterized by UV-visible, NMR and mass spectrometry. The 1H-NMR spectrum of 22+ revealed that the acceptor and donor units of 12+ form an intramolecular CT complex inside the cavity of CB[8], which was also supported by a molecular modeling study. Another D-A molecule (32+), 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 44+ exclusively. In 44+, two molecules of 32+ 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.