Conformational tuning of magnetic interactions in coupled nanographenes

TL;DR

This study explores how twisting nanographene molecules like phenalenyl affects their magnetic interactions, combining experimental STM-IETS measurements with theoretical calculations to propose methods for controlling these interactions.

Contribution

It demonstrates the ability to tune magnetic exchange interactions in phenalenyl dimers by varying their dihedral angles through surface chemistry and functionalization strategies.

Findings

Exchange interactions can be modulated by molecular twist angles.

Theoretical models align with experimental STM-IETS data.

Strategies for inducing specific twist angles are proposed.

Abstract

Phenalenyl (C$_{13}$H$_9$) is an open-shell spin-$1/2$ nanographene. Using scanning tunneling microscopy (STM) inelastic electron tunneling spectroscopy (IETS), covalently-bonded phenalenyl dimers have been shown to feature conductance steps associated with singlet-triplet excitations of a spin-$1/2$ dimer with antiferromagnetic exchange. Here, we address the possibility of tuning the magnitude of the exchange interactions by varying the dihedral angle between the two molecules within a dimer. Theoretical methods, ranging from density functional theory calculations to many-body model Hamiltonians solved within different levels of approximation, are used to explain STM-IETS measurements of twisted phenalenyl dimers on a h-BN/Rh(111) surface. By means of first-principles calculations, we also propose strategies to induce sizable twist angles in surface-adsorbed phenalenyl dimers via functional groups, including a photoswitchable scheme. This work paves the way toward tuning magnetic couplings in carbon-based spin chains and two-dimensional lattices.