2D Self-assembled Nanostructures And Formation Mechanism Of Fluorenone And Naphthalene Derivatives Controlled By Intermolecular Weak Bonding At The Surface/interface

We chose the fluorenone and naphthalene cores as the conjugated moieties because(1)various types of functionaities(especially hydrogen bonding groups)can be facilely introduced at the 2 and 7 positions on the fluorenone core;(2)the presence of carbonyl group on the fluorenone moiety not only can provide the dipole–dipole interactions but also can act as the hydrogen-bonding acceptor to form both intermolecular and molecule–solvent hydrogen bonds,which further contribute the presence of multiple intermolecular forces in the monolayer;(3)C(sp~2)–H groups on the fluorenone moiety are excellent donors to form the intermolecular complementary hydrogen bonds,which are not systematically investigated in the previous studies;(4)the alkyl chains can be easily introduced to enhance the van der Waals both in molecule–molecule and molecule–substrate interactions.By introducing multiple types of hydrogen-bonding functionalities,varying the types of solvents,adjusting the solute concentration,the structural polymorphism was observed.We mainly focus on the delicate role of intermolecular hydrogen bonds during the self-assembly process.Besides,other intermolecular forces such as dipole–dipole interactions,van der Waals interactions also play an important role.Especially,we further explored the correlationship between the 2D self-assembly and 3D crystal structure of a naphthalene derivative as to gain a deep insight of intermolecular weak C(sp~2)–H···N≡C hydrogen bonds.The main research works and innovative results are shown as follows:(1)Theself-assembledbehaviorsoftwofluorenonederivatives,2,7-bis((11-hydroxyundecyl)oxy)-9-fluorenone(BHUF)and 2,7-bis((10-carboxydecyl)oxy)-9-fluorenone(BCDF),were investigated at the liquid–solid interface by scanning tunneling microscopy.Two solvents,1-octanoic acid and 1-phenyloctane,were employed in consideration of their distinct polarity and solubility.It is observed that the BHUF molecules self-assemble into seven different polymorphs upon adsorption,while only two different polymorphs are observed in the BCDF monolayer.The theoretical calculation is performed to reveal the underlying mechanism.As compared to that of–C=O···HO–hydrogen bonds,the enhanced binding energy of intermolecular–C=O···HOOC–hydrogen bonds in the BCDF monolayer would dominate the intermolecular van der Waals(vdWs)interactions and the molecule–solvent interactions,thereby resulting in a limited expression of structural polymorphism.In addition,the concentration-dependent polymorphism and the relative phase transition are discussed in terms of the stability and packing density of different polymorphs.Furthermore,the different behaviors of BHUF molecules in these two solvents at lower concentrations are associated with the different energy gain upon solvent co-adsorption.The investigation provides a simple and alternative strategy to construct the structural polymorphs by utilizing multiple hydrogen bonds at the liquid–solid interface.(2)The self-assembly of 2,7-bis(decyloxy)-9-fluorenone(BDF)on highly oriented pyrolytic graphite(HOPG)is investigated at the solid/gas interface by scanning tunneling microscopy(STM),which allows us to determine the effect of its molecular structure on the formation of monolayer morphology.By varying the solution concentration in dichloromethane,seven types of supramolecular assemblies can be obtained.The concentration-dependent structural polymorphism is discussed in terms of thermodynamics.In particular,these different patterns are associated with weak intermolecular C(sp~2)-H···O=C hydrogen bonds.As discerned by its position in the fluorenone moiety,the C(sp~2)-H group with larger chemical shift value and stronger acidity displays a higher priority when involving in the formation of a C(sp~2)-H···O hydrogen bond.Owing to the high density of C(sp~2)-H donors,various hydrogen bonding configurations arise,further leading to the presence of structural polymorphism.Besides,intermolecular van der Waals interactions and the dipole-dipole interactions play the complementary roles to stabilize the whole monolayer.The underlying mechanism is further confirmed by the density functional theory(DFT)calculations.(3)Owing to a wide range of applications within the areas such as chiral sensors,enantiomeric resolution,and asymmetric catalysis,understanding chiral adsorption phenomena at the interface is thereby of great importance.In particular,the role of multiple hydrogen bonds in inducing chiral diversiform morphologies has never been systematically investigated.Herein,by delicate control of the volume ratio of 1-octanoic acid and 1-octanol asthemixedsolvent,aseriesofself-assemblednanostructuresof2-hydroxyl-7-pentadecyloxy-fluorenone(HPF)were sequentially fabricated,including the achiral denselypacked pattern,the chiral"6-2"pattern,the chiral alternate pattern,and the chiral double-rosette pattern.Eventually,those patterns would evolve into an achiral and thermodynamically favored zigzag pattern.Based on DFT calculations,we demonstrate that the stabilities of diversiform morphologies originate from different hydrogen bonding and molecular packing densities.In addition,quantum theory of atoms in molecule(QTAIM)analysis is further applied to interpret the nature of these hydrogen bonds.(4)Understanding the underlying mechanisms and the relationship between two-dimensional(2D)molecular self-assembly and three-dimensional(3D)crystal structure are of great importance.In the present study,the self-assembly behaviors of CEN molecule were systematically investigated in various types of solvents.Depending on the solvents,CEN molecule selectively self-assembled into the thermodynamically or kinetically favored pattern,that is,alternate and linear lamella structures.Through the combination of DFT and QTAIM analysis,it is found that the weak bifurcated C(sp~2)–H···N≡C hydrogen bonds between adjacent CEN molecules act as an important force in the stabilization of self-assembled monolayer.Moreover,rather than simply acting as a driving force in the self-assembled nanostructure,the intermolecular bifurcated C(sp~2)–H···N≡C hydrogen bonds also stabilize the dimer unit with similar structure according to the analysis of the 3D packing structure.The present study provides a deep insight of the role of intermolecular C(sp~2)–H···N≡C interactions in 2D and 3D crystal structures.