We elucidate the photophysical deactivation pathways of a novel perylenediimide−iridium-complex dyad (PDI−iridium), suitable for deep-red electroluminescent devices, in a joint experimental and quantum-chemical approach. Excitation of the PDI mainly decays via PDI fluorescence (55%), although a considerable part is deactivated via the PDI triplet by efficient spin−orbit coupling activated by the close-by iridium atom. Upon irradiation of the Ir-complex moiety, the phosphorescence usually observed for iridium complexes is efficiently quenched by triplet−triplet transfer to the PDI triplet, as demonstrated by transient absorption spectroscopy. The study reveals the importance of molecular orbital level control on the design of molecular dyads, which can be well-predicted at a quantum-chemical level.