(Invited) Interplay Between π–Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers

Abstract

The field of carbon magnetism has gained an increased attention in view of the recent progress made in the synthesis and characterization of open-shell polycyclic aromatic hydrocarbons following a bottom-up synthetic approach[1,2]. Under this scenario, the comprehensive fabrication of magnetic porphyrinoid-based polymers emerges as a highly appealing field of research. However, the vast majority of the porphyrinoid species and polymers studied to date exhibit a closed-shell ground state[3].In this work, we introduce an exemplary approach toward the bottom-up fabrication of unprecedented magnetic porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents studied on Au(111) under ultra-high vacuum (UHV) conditions. The chemical structure of the polymer, formed by thermal-activated intra- and intermolecular oxidative ring closure reactions followed by controlled tip-induced hydrogen dissociation from the porphyrinoid units, have been clearly elucidated by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS), complemented by computational investigations reveals the open-shell character, i.e. antiferromagnetic singlet ground state (S=0) of the formed polymers, which display singlet−triplet inelastic excitations observed between spins of adjacent porphyrinoid units only along a specific π-conjugation pathway, thus revealing the interplay between π-conjugation and magnetic exchange. We envision that our approach can be a highly relevant step towards the on-surface synthesis of covalently linked 1D magnetic organic polymers with prospects in nanoscale spintronic devices.