Effects of Ionizing Radiation on Halogen-Bonded Dipyridyl-Naphthalenediimide Cocrystals

Abstract

Cocrystals are promising modular materials that can contain aromatics as coformers with the ability to fluoresce upon radiation exposure for use in scintillation and dosimetry. The materials must be able to endure significant exposures to ionizing radiation, and there is currently a minimal understanding of atomistic criteria to enhance the structural stabilities of organic materials for such applications. The current study examines four cocrystals with a common molecular component as a naphthalene backbone-dipyridyl-naphthalenediimide (NDI) that interact with the halogen-bond (XB) donors I2, diiodobenzene (DIB), and diiodotetrafluorobenzene (DITFB). Powder X-ray diffraction was used to assess changes in crystallinity upon gamma (γ) irradiation and was combined with density functional theory calculations that provide atomistic-level differences in bond lengths, packing, and electrostatic energy surfaces. The presence of the aromatic groups did not affect the structural integrity of cocrystals; rather, the combination of both stronger primary and secondary interactions involving the XB systems ((NDI)·(X-donor), where X-donor = I2, DIB, or DITFB) supported an increase in structural integrity. The results provide likely trends (involving factors such as aromaticity, secondary interactions, and packing) that impact the design of multicomponent scintillators and/or radiation shielding materials. Importantly, this study found that aromaticity is not necessary to increase structural stability; rather, the primary and secondary interactions that hold the organic molecules together are of importance.