Tuning the Spin Interaction in Non-planar Organic Diradicals Through Mechanical Manipulation

Kavli Affiliate: Herre S. J. Van Der Zant

| First 5 Authors: Alessio Vegliante, Saleta Fernandez, Ricardo Ortiz, Manuel Vilas-Varela, Thomas Baum

| Summary:

Open-shell polycyclic aromatic hydrocarbons (PAHs) represent promising
building blocks for carbon-based functional magnetic materials. Their magnetic
properties stem from the presence of unpaired electrons localized in radical
states of $pi$ character. Consequently, these materials are inclined to
exhibit spin delocalization, form extended collective states, and respond to
the flexibility of the molecular backbones. However, they are also highly
reactive, requiring structural strategies to protect the radical states from
reacting with the environment. Here, we demonstrate that the open-shell ground
state of the diradical 2-OS survives on a Au(111) substrate as a global singlet
formed by two unpaired electrons with anti-parallel spins coupled through a
conformational dependent interaction. The 2-OS molecule is a protected
derivative of the Chichibabin’s diradical, featuring a non-planar geometry that
destabilizes the closed-shell quinoidal structure. Using scanning tunneling
microscopy (STM), we localized the two interacting spins at the molecular
edges, and detected an excited triplet state a few millielectronvolts above the
singlet ground state. Mean-field Hubbard simulations reveal that the exchange
coupling between the two spins strongly depends on the torsional angles between
the different molecular moieties, suggesting the possibility of influencing the
molecule’s magnetic state through structural changes. This was demonstrated
here using the STM tip to manipulate the molecular conformation, while
simultaneously detecting changes in the spin excitation spectrum. Our work
suggests the potential of these PAHs for a new class of all-carbon
spin-crossover materials.

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