Quinonoid radial π-conjugation

Abstract

Radially π-conjugated macrocycles with mixed aromatic and quinonoid units are considered. As a function of including an increasing number of aromatic units into a ring-like nanohoop with quinonoid units, a transition occurs where the HOMO and LUMO levels cross, leading to a topological transition described for the first time. Such transitions have been seen before in ethynylene-linked oligoacene polymers as a function of the acene size on gold surfaces and in various π-conjugated polymers as a function of external strain, but not in small molecular nanohoops or any other zero-dimensional system. Near the level crossing, the HOMO–LUMO gap becomes very small, offering novel photophysical properties while maintaining extensive delocalization. The open shell character of the rings changes continuously as the composition is gradually changed, switching from a singlet ground state to a triplet, providing a zero-dimensional analogy to topological transitions between a non-trivial to a trivial phase as observed in linear one-dimensional conjugated polymers. The spins of the triplet are localized near the two aromatic-quinonoid connections.