Self-Assembled Nanostructures And Formation Mechanism Of Thienophenanthrene Derivatives Controlled By Intermolecular Weak Bonding At The Liquid/Solid Interface

As a building block core,we used thienophenanthrene(TP),a symmetric planar triangle because 1)its planarity and rigidity are advantageous to form coplanar assembly;2)it can be introduced different substituents at three vertex positions;3)it had been applied in polymer light-emitting devices and solar cells due to its photoelectric property.In addition,the introduction of two alkyl chains could stabilize the adlayers by the interdigitation of side chains and the molecule-substrate interactions at the liquid-solid interface.Therefore,we designed and synthesized a series of thienophenanthrene derivatives(TPs),and deeply investigated the self-assembled monolayers on the HOPG surface.By introducing the halogen(bromine and iodine)substituents,and changing the number and position of substituent groups,we found the types of halogen contacts present a variety of morphologies,and the self-assembled patterns show vastly different extents of polymorphism.In addition to the intermolecular interactions,the molecule–solvent interaction is also a key factor that can drastically modify intermolecular binding.We focus on understanding the competition of the delicate interplay between interchain van der Waals(vdW)interactions and intermolecular halogen bonding,as well as the solvent and solution concentration effects on the formation of polymorphism self-assembled structures.The main research works and innovative results have been summarized as follows:(1)We present the first case of the competition of halogen bonding and van der Waals interaction on the self-assembly of 6,9-DDTD at the aliphatic acid/graphite interface using STM.It was found that four different nanostructures could be formed by adjusting the 6,9-DDTD concentration.The surface phases show clear concentration dependence: in the saturated solution,the alternate and dislocated linear patterns show phase separate on the surface.In the concentration range,the alternated pattern disappeared gradually and the dislocated linear and tetramer patterns coexist,in which the percentage of tetramer pattern increases gradually.In the low concentration,only the linear pattern was formed resulting from the coadsorption of DDTD and 1-octanoic acid.In addition,the voltage pulse applied to the STM tip could induce the structural transformation.Our systemic study illustrates that the competition of interchain van der Waals interactions and Br···Br halogen bond as well as Br···H hydrogen bond,along with solvents coadsorption determine the polymorphous structures,which is also demonstrated through a combination of STM and density functional theory(DFT)calculations.The present work provides a molecular insight into the 2D assembly of thienophenanthrene derivative based on halogen bonding.(2)Halogen bonding with highly specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces.Herein,we first report the orientation of ester substituent for thienophenanthrene derivatives(5,10-DBTD and 5,10-DITD)affect positive charge distribution of halogens,thus determining the formation of halogen bond(XB)by scanning tunneling microscopy and density(STM)function theory(DFT).The system presented here mainly includes hetero-halogen X···O=C and X···S XBs,H···Br and H···O hydrogen bonds(HBs),where the directionality and strength of XB and HB by varying the halogen substituent determine the molecular arrangement.This study provides a detailed understanding of the role of ester orientation,concentration,and solvent effects on the formation of halogen bonds and proves relevant for the identifying of the multiple halogen bonding in supramolecular chemistry.(3)The photophysical properties of two π-conjugated thiophene-based thienophenanthrene derivatives(5,10-and 6,9-DBTD)were reported,and their self-assembled monolayers in aliphatic solvents under different concentrations were investigated by scanning tunneling microscopy at liquid/graphite surfaces.The STM results revealed that the self-assembled structures of two compounds show strongly solvent and concentration dependence.When aliphatic hydrocarbon was used as the solvent,6,9-DBTD produces almost one stable coassembled linear structure.However,the self-assembly of 5,10-DBTD shows structural diversity and exhibits concentration effect.At low solution concentrations coadsorbed patterns are formed with the aliphatic solvent molecules,whereas at high concentrations 5,10-DBTD favors the formation of dense polymorphous structures.All ordered self-assembled adlayers critically depend on not only the weak interchain van der Waals interactions,but the halogen bonds and hydrogen bonds.Density functional theory calculations unravel that the coadsorption of aliphatic solvent molecules could change the Br atom contact modes,in which the type of Br···X(O S and Br)interactions from type-II or hetero-halogen X···O=C and X···S XBs to type-I,which are useful tools to dissect and explain the formation of halogen bond mechanism.(4)In this work,we synthesized two thienophenanthrene derivatives(TP-Ben and TP-Nap)containing 1-ethynyl-4-(hexadecyloxy)-benzene or 2-ethynyl-6-(hexadecyloxy)-naphthalene units,which are V-shaped molecules with different aromatic units and identical long side alkoxy chains.The photo-physical properties of TP-Ben and TP-Nap were reported,and their self-assembled monolayers under different concentrations were investigated by scanning tunneling microscopy at liquid/graphite surfaces.A systematic study revealed that TP-Ben molecules could form a wave-line pattern,and TP-Nap molecules construct a linear pattern.The observed patterns were the most stable configurations,and stabilized by intermolecular hydrogen bonds,dipole-dipole interactions as well as vdW interactions.The solution concentration and solvents have no effect on the formation of the self-assembled configuration.