From an insulating Zn-porphyrin metallacage to electrically conducting inclusion complexes featuring extended π-donor/acceptor stacks

Abstract

π-Donor/Acceptor charge-transfer (CT) interactions between redox-complementary π-systems often give rise to non-native optical and electronic properties that are beneficial for modern electronics and energy technologies. However, the formation of extended supramolecular π-donor/acceptor stacks capable of long-range charge transport requires ingenious design strategies that can help reinforce otherwise weak π-donor/acceptor noncovalent interactions. Herein, we demonstrate that a large tetragonal prismatic metal–organic cage (MOC2⁸⁺) having two parallel π-donor tetrakis(4-carboxyphenyl)-Zn-porphyrin (ZnTCPP) faces located ∼14 Å apart can accommodate up to three redox-complementary planar aromatic guests (either three π-acceptor guests or two π-acceptors surrounding one π-donor guest) between the ZnTCPP faces, forming extended π-donor/acceptor stacks. While empty MOC2⁸⁺ behaves as an insulator due to the lack of charge delocalization across its large cavity, its inclusion complexes saturated with π-acidic hexaazatriphenylene hexacarbonitrile (HATHCN) and hexacyanotriphenylene (HCTP) displayed noticeably higher electrical conductivity (8.7 × 10⁻⁶ and 1.3 × 10⁻⁶ S m⁻¹, respectively) owing to more facile charge transport through the π-donor/acceptor stacks composed of the π-acidic guests intercalated between the ZnTCPP faces. Thus, this work demonstrates that tetragonal prismatic metallacages with two parallel electroactive faces can facilitate the creation of extended π-donor/acceptor stacks by encapsulating redox-complementary planar guests, which in turn facilitates through-space charge delocalization, generating non-native electrical conductivity.