Insight from Molecular Packing: Charge Transfer and Emission Modulation through Cocrystal Strategies

Charge transfer (CT), occurring from donor to acceptor involving pi electrons, plays a significant role in regulating the emission of materials. Especially, in multicomponent systems, the CT degree depends on not only the electron rich-deficient abilities of components but also the molecular packing. Herein, three novel cocrystals (BPPa-2I, BPPa-3I, and BPPa-TCNB) based on BPPa, 2I (diiodo-tetrafluorobenzene), 3I (triiodo-trifluorobenzene), and TCNB (1,2,4,5-tetracyanobenzene) are fabricated through halogen bonding (XB) and CT interactions. The three cocrystals exhibit red-shifted emissions from 470 to 505 nm to 530 nm, covering blue via green to yellow, which are attributed to the enhanced CT degrees. On the basis of crystal structural analysis, the effective distance between two components in molecular packing is responsible for the different CT degrees; that is, a fully face-to-face stacking between donor and acceptor can improve the CT degree more effectively. The present study paves an easy way to construct a "packing-CT degree-emission" relationship.