  From organic electronics to biological systems, understanding the role of
intermolecular interactions between spin pairs is a key challenge. Here we show
how such pairs can be selectively addressed with combined spin and optical
sensitivity. We demonstrate this for bound pairs of spin-triplet excitations
formed by singlet fission, with direct applicability across a wide range of
synthetic and biological systems. We show that the site-sensitivity of exchange
coupling allows distinct triplet pairs to be resonantly addressed at different
magnetic fields, tuning them between optically bright singlet (S=0) and dark
triplet, quintet (S=1,2) configurations: this induces narrow holes in a broad
optical emission spectrum, uncovering exchange-specific luminescence. Using
fields up to 60 T, we identify three distinct triplet-pair sites, with exchange
couplings varying over an order of magnitude (0.3-5 meV), each with its own
luminescence spectrum, coexisting in a single material. Our results reveal how
site-selectivity can be achieved for organic spin pairs in a broad range of
systems.
