Toward a Charged Homo[2]catenane Employing Diazaperopyrenium Homophilic Recognition

An octacationic diazaperopyrenium (DAPP²⁺)-based homo[2]catenane (DAPPHC⁸⁺), wherein no fewer than eight positive charges are associated within a mechanically interlocked molecule, has been produced in 30% yield under ambient conditions as a result of favorable homophilic interactions, reflecting a delicate balance between strong π-π interactions and the destabilizing penalty arising from Coulombic repulsions between DAPP²⁺ units. This DAPPHC⁸⁺ catenane is composed of two identical mechanically interlocked tetracationic cyclophanes, namely DAPPBox⁴⁺, each of which contains one DAPP²⁺ unit and one extended viologen (ExBIPY²⁺) unit, linked together by two p-xylylene bridges. The solid-state structure of the homo[2]catenane demonstrates how homophilic interactions play an important role in the formation of DAPPHC⁸⁺, in which the mean ring planes of the two DAPPBox⁴⁺ cyclophanes are oriented at about 60° with respect to each other, with a centroid-to-centroid separation of 3.7 Å between the mean planes of the outer ExBIPY²⁺ and inner DAPP²⁺ units, and 3.6 Å between the mean planes of the two inner DAPP²⁺ units. We show that irradiation of the DAPPHC⁸⁺ catenane at 330 nm in acetonitrile solution results in simultaneous energy and electron transfer. The latter occurs from the inner DAPP²⁺ dimer to the outer ExBIPY²⁺ unit, leading to the generation of a temporary charge-separated state within a rigid and robust homo[2]catenane. Compared to DAPPBox⁴⁺, both forward- and back-electron transfer in DAPPHC⁸⁺ occur with faster rates, owing to the closer proximity between the electron donor and acceptor in the homo[2]catenane than in the separated cyclophane.