Aromaticity controls the excited-state properties of host–guest complexes of nanohoops

Abstract

π-Conjugated organic molecules have exciting applications as materials for batteries, solar cells, light-emitting diodes, etc. Among these systems, antiaromatic compounds are of particular interest because of their smaller HOMO–LUMO energy gap compared to aromatic compounds. A small HOMO–LUMO gap is expected to facilitate charge transfer in the systems. Here we report the ground and excited-state properties of two model nanohoops that are nitrogen-doped analogs of recently synthesized [4]cyclodibenzopentalenes – tetramers of benzene-fused aromatic 1,4-dihydropyrrolo[3,2-b]pyrrole ([4]DHPP) and antiaromatic pyrrolo[3,2-b]pyrrole ([4]PP). Their complexes with C₆₀ fullerene show different behavior upon photoexcitation, depending on the degree of aromaticity. [4]DHPP acts as an electron donor, whereas [4]PP is a stronger electron acceptor than C₆₀. The ultrafast charge separation combined with the slow charge recombination that we found for [4]PP⊃C₆₀ indicates a long lifetime of the charge transfer state.