Pulse-Shape Discrimination in High-Symmetry Organic Scintillators

In this work, we report the targeted structural modification of luminescent organic molecules to impart changes to the corresponding molecular and crystallographic symmetries. Fast neutron/gamma pulse-shape discrimination (PSD) has been characterized in high-symmetry organic crystals composed of chromophores that do not intrinsically exhibit PSD. These findings are rationalized in the context of second-rank tensor properties, which are symmetry-dependent factors that control key quantities such as the transport mobility, light yield anisotropy, refractive index, strain, and thermal expansion coefficient. Density-functional theory calculations confirm the role of crystallographic packing and symmetry upon the magnitude of exchange interactions between triplet excited states, as relevant to triplet-triplet annihilation and PSD.