  Optical control and read-out of the ground state spin structure has been
demonstrated for defect states in crystalline semiconductors, including the
diamond NV- center, and these are promising systems for quantum technologies.
Molecular organic semiconductors offer synthetic control of spin placement, in
contrast to current limitations in these crystalline systems. Here we report
the discovery of spin-optical addressability in a diradical molecule that
comprises two trityl radical groups coupled via a fluorene bridge. We
demonstrate the three important properties that enable operation as a
spin-photon interface: (i) triplet and singlet spin states show
photoluminescence peaked at 640 and 700 nm respectively; this allows easy
optical measurement of ground state spin. (ii) the ground state spin exchange
is small (~60 {\mu}eV) that allows preparation of ground state spin population.
This can be achieved by spin-selective excited state intersystem crossing, and
we report up to 8% microwave-driven contrast in photoluminescence. (iii) both
singlet and triplet manifolds have near-unity photoluminescence quantum yield,
which is in contrast to the near-zero quantum yields in prior reports of
molecular diradicals. Our results establish these tuneable open-shell organic
molecules as a platform to engineer tailor-made spin-optical interfaces.
