Comparative effects of TCNB and TBrPA on configuration and electronic transitions in anthracyl-chalcone cocrystals

The electronic characteristics of cocrystal-formers can influence the molecular packing and photophysical properties of multi-component cocrystals. Two charge-transfer (CT) cocrystals were obtained from a fluorescent parent molecule (P), (2E)-3-(9-Anthracenyl)-1-(3,4-dichlorophenyl)-2-propen-1-one (APO), using 1,2,4,5-Benzenetetracarbonitrile (TCNB) and Tetrabromophthalic anhydride (TBrPA) as cocrystal-formers (F), were grown through π … π and n … π stacking interactions. The two cocrystal-formers, TCNB and TBrPA, were associated with different configurations of the parent molecule and distinct cocrystal packing modes. TCNB, with stronger electron-accepting capability, formed a P … F … P stacking mode with ( Z )-APO, whereas TBrPA formed a P … F … P … F stacking mode with ( E )-APO. Compared to the individual parent molecule, two cocrystals displayed broadened UV absorption band extending into the red region, alongside pronounced redshifts in fluorescence emission peaks with enhanced quantum yields and fluorescence lifetimes. Density functional theory (DFT) calculations indicated that TCNB acted as the electron acceptor in the APO-TCNB cocrystal (ATC), whereas TBrPA acted as the electron acceptor in the APO-TBrPA cocrystal (ATB). The introduction of TCNB resulted in the π→π* transition mode of cocrystal ATC, while the introduction of TBrPA led to the mixed transition mode of n→π* and π→π* in cocrystal ATB. The variation of bandgap and transition mode in two cocrystals was attributed to excited-state transition dipole moment variations driven by CT effect, enabling optical tuning. This comparative study revealed that TCNB and TBrPA induced distinct electronic transition modes and photophysical responses in the resulting cocrystals, providing case-specific insights into the structure-property relationships of CT cocrystals.