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In this work, we describe a method to control the relative proportion of prompt and delayed luminosity of organic-based scintillators via direct and exponential emission from an extrinsic triplet state. This approach involves the incorporation of triplet-harvesting heavy metal complexes in plastic scintillator matrices to convert intrinsically non-luminescent host states to highly emissive guest states. Measurements on these plastic scintillators indicate improved light yields over the undoped polymers and the ability to perform neutron/gamma particle-discrimination. A similar extent of molecular-level control is not possible in traditional organic materials due to complex decay kinetics and the absence of spectral information for the delayed triplet-derived emission. The materials described here address these limitations through efficient host-guest triplet harvesting, which enables particle discrimination according to conventional pulse-shape discrimination (PSD) and a previously unreported spectral-shape discrimination (SSD) scheme.