Systematical Investigation of Chain Length Effect on the Melting Point of a Series of Bifunctional Anthraquinone Derivatives via X-ray Diffraction and Scanning Tunneling Microscopy

摘要

Introducing a continuously changing alkyl chain is effective to modify the conjugated molecule, which is widely used in studying the diversity of two-dimensional (2D) self-assembly and the performance of three-dimensional (3D) bulk materials. In this study, a series of bifunctional anthraquinone derivatives (A-OCn, n = 3-18) are synthesized to investigate the interrelation between the chain length and the melting point by differential scanning calorimetry (DSC). It is observed that the A-OCn (n = 1 and 4) crystallizes in monoclinic and the A-OCn, (n = 3, 5, 6) crystallizes in triclinic by single-crystal X-ray diffraction (XRD). With the help of scanning tunneling microscopy (STM), the A-OCn, (n = 7-18) is found to exhibit an odd-even alternation in their self-assembled structures. The theoretical calculations for crystal structures and the packing density analyses for self-assembly indicate that the hydrogen bond strength gradually reduces with the increase of the chain length, while the van der Waals interaction is opposite. Namely, the hydrogen bonds and the van der Waals interactions coregulate the variation of melting points. In general, this work provides an understanding at the molecular level on how alkyl chain length regulates the melting point, and we believe that it will have implications for the optimization of future organic materials.