Building hybrid luminescent materials: Understanding stacking modes in aggregate structure to emission modulation

Two charge transfer (CT) complexes A1 and A2 with tunable emissions were explored by diluting anthracene functionalized stilbene as donors with 1,2,4,5-tetracyanobenzene (TCNB) as the acceptor unit. The effect of substituent tailoring on the supramolecular packing and optical properties of the two CT products was accomplished by various solid-state characterization and theoretical calculations. It was found that these binary CT cocrystals exhibit two types of alternatively sandwiched face-to-face patterns (A1 (DADA), A2 (DAD··DAD), and red-shifted emissions relative to their monomers through π-π interactions accompanied by hydrogen bonds as indicated by the structural analysis. The two crystalline aggregates feature molar ratios of 1:1 or 1:2 and molecular packing-dependent emissions, induced by different dihedral angles attributed to the substituent effect, thereby leading to diverse π-π interactions and charge transfer effects respectively, which further results in tunable photophysical properties. These results suggest that the marriage of substituent engineering and cocrystal engineering is a promising molecular design strategy that will help to understand the relationship between emission property and molecular packing with the aim to guide further the development of novel organic solid luminescent materials.