Anion-Mediated Charge-Transfer Assemblies Composed of Resorcin[4]Arene and Viologen Salts

Supramolecular assemblies of a concaved aromatic host as an electron donor (D) and a cationic heterocyclic guest as an electron acceptor (A) are reported. These molecular units were noncovalently bound to afford charge-transfer (CT) complexes in solution and a CT cocrystal, displaying drastic color change from colorless to orange. The binding motif of the assemblies in the solution state was analyzed by NMR spectroscopic analysis and visualized by X-ray crystallographic analysis. Theoretical calculations for the assemblies revealed electronic communication between the D and A fragments, proving their CT characteristics. The CT complexes were stabilized by not only CT and cation-π interactions, but also Coulomb interaction between the cationic acceptor unit and its anion supported by a hydrogen bond with a hydroxy group in the aromatic host, resulted in enhancement of orbital overlapping of the D-A fragments. Owing to the strong D-A interaction in the assemblies, the CT complexes undergo ultrafast charge recombination on a femtosecond timescale after photoexcitation of the CT transition, as confirmed by transient absorption spectroscopy. These results demonstrate that understanding of assembly motif and its noncovalent interactions can lead to create optical materials manipulating electron transition dynamics.