Structural Integrities of Symmetric and Unsymmetric trans-Bis-pyridyl Ethylene Powders Exposed to Gamma Radiation: Packing and Electronic Considerations Assisted by Electron Diffraction.

Radiation detection (dosimetry) most commonly uses scintillating materials in a wide array of fields, ranging from energy to medicine. Scintillators must be able to not only fluoresce owing to the presence of a suitable chromophore but also withstand damage from radiation over prolonged periods of time. While it is inevitable that radiation will cause damage to the physical and chemical properties of materials, there is limited understanding of features within solid-state scintillators that afford increased structural integrity upon exposure to gamma (γ) radiation. Even fewer studies have evaluated both physical- and atomistic-level properties of organic solid-state materials. Previous work demonstrated cocrystalline materials afford radiation resistance in comparison to the single component counterparts, as realized by trans-1,2-bis(4-pyridyl)ethylene (4,4'-bpe). To support the rational design of radiation-resistant scintillators, we have examined all symmetric and unsymmetric isomers of trans-1-(n-pyridyl)2-(m-pyridyl)ethylene (n,m'-bpe, where n and/or m = 2, 3, or 4) solid-state crystalline materials. Experimental methods employed include single-crystal, powder, and electron diffraction as well as solid-state fluorimetry. Periodic density functional theory (DFT) calculations were used to understand the atomistic-level differences in bond lengths, bond orders, and packing. Electron diffraction was also utilized to determine the structure of a nanocrystalline sample. The results provide insights into possible trends involving factors such as molecular symmetry which provides radiation resistance as well as information for rationally designing single and multicomponent scintillators with the intent of minimizing changes upon γ-radiation exposure.