Macrocycles Composed of Biphenylene and Butadiyne Units with Antiaromatic Character

Abstract

Herein, we report the synthesis and characterization of π-conjugated macrocycles, with diameters over 2.4 nanometers, composed of biphenylene and butadiyne units. Specifically, biphenylene-2,7-diyl–butadiyne-1,4-diyl (BB) macrocycles were synthesized via the intermolecular Hay coupling reaction of a 2,7-diethynylbiphenylene derivative. The BB macrocycles exhibited absorption bands extending up to ~600 nm and emitted red luminescence. Moreover, scanning tunneling microscopy observations revealed that the BB macrocycles formed self-assembled molecular networks at the liquid–graphite interface, showing higher tunneling efficiency. The BB macrocycles exhibited high energy levels of the highest occupied molecular orbitals and narrow energy gaps due to the elongated π-electron conjugation through the biphenylene units. Theoretically predicted responses to an external magnetic field using quantum chemical calculations of BB macrocycle models revealed reinforced antiaromatic character at four-membered rings. The present information will advance the structural design and property tuning of macrocyclic compounds with antiaromatic character.