Crystal Engineering Approach to Forming Cocrystals of Enaminones: Structural Insights into Enhanced Solid-State Luminescence

The formation of cocrystals offers an effective design strategy for constructing organic luminescent materials with tailored emission features. Nevertheless, challenges in crafting molecular architectures and controlling intermolecular interactions continue to impede further advancement. In this study, we explore the expanded potential of the enaminone scaffold beyond its conventional medicinal applications by designing three mixed-stacked charge transfer cocrystals (CTCs). These CTCs are constructed using enaminone-based compounds (D1, D2, and D3) as donors and 1,2,4,5-tetracyanobenzene (TCNB) as the acceptor. All the CTCs exhibit significant red-shifts in their emission with identical emission wavelength at 626 nm, exceeding 100 nm compared to their precursors. Each of the three cocrystals features π···π mixed stacking structures with robust hydrogen bonds. The existence of CT interactions in the cocrystals was corroborated by various analytical techniques. The luminescence tuning is elucidated through an analysis of crystal packing, intermolecular interactions, and Frontier orbital distribution from DFT calculations, confirming that the CT-induced effect can switch optical properties via π···π interactions. We believe our findings contribute to the advancement of our knowledge of the relationship between molecular fluorophore stacking and luminescence features in CTCs, offering valuable insights for modulating luminescence properties.