  Molecules present a versatile platform for quantum information science, and
are candidates for sensing and computation applications. Robust spin-optical
interfaces are key to harnessing the quantum resources of materials. To date,
carbon-based candidates have been non-luminescent, which prevents optical
read-out. Here we report the first organic molecules displaying both efficient
luminescence and near-unity generation yield of high-spin multiplicity excited
states. This is achieved by designing an energy resonance between emissive
doublet and triplet levels, here on covalently coupled
tris(2,4,6-trichlorophenyl) methyl-carbazole radicals (TTM-1Cz) and anthracene.
We observe the doublet photoexcitation delocalise onto the linked acene within
a few picoseconds and subsequently evolve to a pure high spin state (quartet
for monoradicals, quintet for biradical) of mixed radical-triplet character
near 1.8 eV. These high-spin states are coherently addressable with microwaves
even at 295 K, with optical read-out enabled by intersystem crossing to
emissive states. Furthermore, for the biradical, on return to the ground state
the previously uncorrelated radical spins either side of the anthracene show
strong spin correlation. Our approach simultaneously supports a high efficiency
of initialisation, spin manipulations and light-based read-out at room
temperature. The integration of luminescence and high-spin states creates an
organic materials platform for emerging quantum technologies.
