Effect of Noncovalent Interactions on the Intersystem Crossing Behavior in Charge-Transfer Cocrystals of 3,5-Dinitrobromobenzene

The strength of the noncovalent interactions between donor and acceptor in charge-transfer (CT) cocrystals plays a significant role in controlling their optical behavior. Herein, we demonstrate the predominant influence of CT interactions between polarized π-systems on the intersystem crossing (ISC) between singlet and triplet states of three organic cocrystals. The cocrystals 1–3 are formed by the combination of an electron-deficient molecule such as 3,5-dintrobromobenzene (DNBB) with electron-rich molecules such as 4,4′-bipyridylethylene, carbazole, and 3,6-dibromocarbazole, respectively. The DNBB found to favor C–H···X hydrogen bonds over halogen bonds (Br···X) which was rationalized through electrostatic potential (ESP) of atomic sites of the molecules. The crystal structure analyses of cocrystals reveal that 1 exhibits infinite segregated stacks (−D–D–D– and −A–A–A−) and discrete mixed stacks (A–D–D–A) of donors and acceptors. On the other hand, 2 exhibits infinite mixed stacks (−D–A–D–A−) and 3 exhibits an offset fashion of infinite mixed stacks (−D–A–D–A−) of donors and acceptors, respectively. The predominant CT-stacking interactions affected the nature of electronic transitions and eventually the rate of ISC followed by phosphorescence behavior of cocrystals. The DNBB and cocrystal 1 found to exhibit phosphorescence but not 2 and 3. The optical behavior of DNBB and its binary cocrystals was rationalized by the time-dependent density functional theory (TDDFT) studies, spin–orbit coupling constant, and observed stacking interactions in the crystals. This work demonstrates the significance of nitro-aromatics in the design of solid state organic phosphorescent materials.