Balancing the Heterocycles···Pyromellitic Dianhydride Interactions to Achieve Tunable Emission in Charge-Transfer Cocrystals

Cocrystal engineering focuses on adjustable noncovalent interactions (NCIs) and packing motifs through different functional molecular blocks to modulate physicochemical properties. NCIs are vital driving forces in the construction of organic cocrystal materials, such as π–π interactions, arene–perfluoroarene interactions, hydrogen bonds, and halogen bonds. Herein, pyromellitic dianhydride (PMDA) serves as the electron acceptor, and three heterocycle–acene-fused electron-rich donors, including 5H-benzo[4,5]thieno[3,2-c]carbazole (BTCZ), 5H-benzofuro[3,2-c]carbazole (BFCZ), and 5,12-dihydroindolo[3,2-a]carbazole (ICZ), are selected to react with PMDA to fabricate cocrystals. Single crystals of BTCZ-PMDA, BFCZ-PMDA, and ICZ-PMDA were grown via slow cooling and solvent evaporation, and their crystal structures, quantified NCIs, and photoluminescent properties were investigated. The results indicate that two types of π–π interactions along the π–π stacking direction show imbalanced interaction energies for BTCZ-PMDA and BFCZ-PMDA, while two π–π interaction energies are well balanced within ICZ-PMDA. Moreover, the red shift of the photoluminescent peak from 1.83 eV (BTCZ-PMDA) to 1.53 eV (ICZ-PMDA) can be ascribed to the balanced heterocycles···PMDA interactions and the overall enhancement of π–π and hydrogen bond interactions (N–H···O and C–H···O). Therefore, balancing the solid-state NCIs can offer a promising strategy to modulate fluorescence within charge-transfer cocrystals.