  High-spin molecules allow for bottom-up qubit design and are promising
platforms for magnetic sensing and quantum information science. Optical
addressability of molecular electron spins has also been proposed in first-row
transition metal complexes via optically-detected magnetic resonance (ODMR)
mechanisms analogous to the diamond-NV colour centre. However, significantly
less progress has been made on the front of metal-free molecules, which can
deliver lower costs and milder environmental impacts. At present, most
luminescent open-shell organic molecules are $\pi$-diradicals, but such systems
often suffer from poor ground-state open-shell characters necessary to realise
a stable ground-state molecular qubit. In this work, we use alternancy symmetry
to selectively minimise radical-radical interactions in the ground state,
generating $\pi$-systems with high diradical characters. We call them m-dimers,
referencing the need to covalently link two benzylic radicals at their meta
carbon atoms for the desired symmetry. Through a detailed electronic structure
analysis, we find that the excited states of alternant hydrocarbon m-diradicals
contain important symmetries that can be used to construct ODMR mechanisms
leading to ground-state spin polarisation. The molecular parameters are set in
the context of a tris(2,4,6-trichlorophenyl)methyl (TTM) radical dimer
covalently tethered at the meta position, demonstrating the feasibility of
alternant m-diradicals as molecular colour centres.
