Optimal Compositions in the NDI:Pyrene Charge-Transfer Complexes Revealed by Thermal Analysis and Structural Characterizations

Supramolecular assemblies of charge-transfer (CT) complexes have been widely studied due to their diverse applications; the alternate stacking of the donor (D) and acceptor (A) shows great importance, owing to their unique physicochemical properties. Although knowing the precise composition and packing structure is crucial for the development of CT complexes, the optimal D/A ratio of a CT complex remains difficult to characterize in many cases. To identify the optimal D/A ratio of the unknown CT complexes, the phase behaviors of the binary mixtures composed of dihexyl naphthalene diimide (NDI) and pyrene (py) derivatives were analyzed by differential scanning calorimetry (DSC) and in situ temperature-dependent X-ray diffraction. New endothermic phase transitions were observed once the coassembly units formed the CT complexes in the bulk phase. The highest T_m for a given CT complex was found at the optimal D/A ratio. Moreover, the mesophases of the NDI/py CT complexes were also observed, demonstrating the complex and abundant self-assembly behaviors of the NDI/py mixtures. In addition to the common 1:1 alternate DA stacking, our single-crystal results also revealed the 2:1 DDA and the 1:2 DAA stacking in the 1-D supramolecular column of the NDI/py mixtures. The optimal D/A ratios for the supramolecular column given by the single-crystal results match with the results of the DSC analysis, verifying that the thermal analysis based on the colligative properties of the D/A mixtures is an effective tool in revealing the optimal compositions for the CT complexation. With a better understanding of the optimal D/A compositions, the structure-property relationship of the CT complexes could be better evaluated in the future.