Solid emission color tuning of organic charge transfer cocrystals based on planar π-conjugated donors and TCNB

Two luminescent Charge Transfer (CT) cocrystals involving planar phenanthrene derivatives namely, formyl phenanthrene (FP) and acetyl phenanthrene (AP) as donors (D) and 1,2,4,5-tetracyanobenzene (TCNB) as an acceptor (A) building block, are formed by molecular self-assembly. Detailed structural and spectroscopic measurements elucidated the mixed stack sequence DADAD in the CT cocrystals. The solid supramolecular architecture for both the cocrystals forms 2D sheet, supported by the extended network of C-H center dot center dot center dot O, and C-H center dot center dot center dot N hydrogen bonds as evidenced by the crystallographic observation. Interestingly, the two cocrystals display tunable emissions compared to the blue emissions of donor compounds, which correlate with the formation of excited CT state between the donor and acceptor motifs as a result of mixed stack orientation. The nature of the CT interactions in the two cocrystals was further explored by applying density functional theoretical (DFT) studies. Such a supramolecular cocrystal approach provides a facile platform towards the design of new luminescent two component CT complexes with desired functionalities.