Research On The Regulation Of Self-assembly Structure And Its Formation Mechanism By Molecular Structure Change

Structure–property relationships are the key to the design and development of crystal and supramolecular materials.For molecular crystals,this requires not only a thorough understanding of the interactions between molecules,but also the rational creation of molecular structures with the desired properties.For this purpose,the influence of molecular structure change on molecular stacking structure and weak intermolecular interaction was investigated by changing the size of substituent group and introducing functional groups,which are two common methods of molecular modification.The main results are summarized as follow:?1?We designed and synthesized a series of disubstituted anthraquinone derivatives?A-OC_n,n=3–18?with continuously varying carbon chain length.The crystal preparation becomes more and more difficult as the chain length increases.However,the longer the carbon chain is,the easier it is to form a stable self-assembly structure on the HOPG substrate.Therefore,XRD and STM were used to explore the two-dimensional stacking structure of A-OC_n?n=1,3–6?and A-OC_n?n=7–18?respectively.The crystal structures revealed that the special C?sp2?–H···O?sp3?hydrogen in A-OC₃ suggests that introducing a proper side chain length not only induces the changes of hydrogen bond strength but also the hydrogen bond sites.The analysis of the two-dimensional self-assembly structure revealed that the self-assembly structure presented an obvious odd-even effect with the change of side chain.?2?The influence of carbon chain length on melting point was further analyzed by DSC.The results show that the melting point decreases alternately with the increase of carbon chain length,and the lowest melting point is the at A-OC₈,and then the melting point increases slowly.Based on the three-dimensional stacking structure molecules,we analyze the change of the weak interaction between the molecules by means of the visualization methods Hirshfled surface energy framework analyses in combination with the intermolecular interaction energy decomposition.The results suggested that the main reason for the melting point decrease is the weakening of intermolecular hydrogen bond.According to the changes of two-dimensional self-assembly structure,stacking density and melting point,the reason for the slow rise in the melting point is that the growth of chain length not only stabilizes the stacking structure but also increases the van der Waals forces between alkyl chains.Namely,the melting point is dominated by van der Waals forces.?3?Dithienobenzodithiophene?DTBDT?as a promising polymer donor material plays a crucial role in improving the power conversion efficiency?PCE?of polymer solar cells.Recently,Li et al.reported that introducing halogen atoms on the DTBDT can improve the order of molecular packing and elevate crystallinity which eventually leads to high PCE.Non-covalent interactions are perceived to have caused the difference on crystallinity,but the packing structure of polymer is difficult to figure out.Herein,two DTBDT derivatives,H-DTBDT and Br-DTBDT were synthesized and their self-assembly behaviors are distinguishable from the degree of order.Density functional theory calculations and topology analysis of the electron density validated that Br···S interactions are responsible for the highly ordered self-assembled structure of Br-DTBDT.This study is expected to elucidate the influences of the bromine group on the self-assembly behavior and we think this results are meaningful to design high-performance organic semiconductor and functional supramolecular materials.