Tuning\nthe Spin Interaction in Nonplanar Organic Diradicals\nthrough Mechanical Manipulation

Open-shell polycyclic\naromatic hydrocarbons (PAHs) represent promising\nbuilding blocks for carbon-based functional magnetic materials. Their\nmagnetic properties stem from the presence of unpaired electrons localized\nin radical states of π character. Consequently, these materials\nare inclined to exhibit spin delocalization, form extended collective\nstates, and respond to the flexibility of the molecular backbones.\nHowever, they are also highly reactive, requiring structural strategies\nto protect the radical states from reacting with the environment.\nHere, we demonstrate that the open-shell ground state of the diradical <b>2-OS</b> survives on a Au(111) substrate as a global singlet formed\nby two unpaired electrons with antiparallel spins coupled through\na conformational-dependent interaction. The <b>2-OS</b> molecule\nis a “protected” derivative of the Chichibabin’s\ndiradical, featuring a nonplanar geometry that destabilizes the closed-shell\nquinoidal structure. Using scanning tunneling microscopy (STM), we\nlocalized the two interacting spins at the molecular edges, and detected\nan excited triplet state a few millielectronvolts above the singlet\nground state. Mean-field Hubbard simulations reveal that the exchange\ncoupling between the two spins strongly depends on the torsional angles\nbetween the different molecular moieties, suggesting the possibility\nof influencing the molecule’s magnetic state through structural\nchanges. This was demonstrated here using the STM tip to manipulate\nthe molecular conformation, while simultaneously detecting changes\nin the spin excitation spectrum. Our work suggests the potential of\nthese PAHs as all-carbon spin-crossover materials.