We reported the synthesis, characterization, and self-assembly of two new regioisomeric π-conjugated chromophores (p-DBPy and d-DBPy) based on pyridyl-flanked diketopyrrolopyrrole (DPP), where the nitrogen atom in the pyridyl was proximal or distal to the central DPP unit. Their solid powders and solutions displayed different colors, especially for p-DBPy, which could be attributed to the conversion of molecular conformation and different intermolecular aggregations. UV–vis spectra demonstrated that the possible intermolecular hydrogen bonding existed in the solution of p-DBPy. Of particular interest is that the morphology of solid powders and thin films for each molecule investigated by SEM and AFM, respectively, were dramatically different owing to different intermolecular interactions and different crystal growth speed. Their spectroscopic and electrochemical properties turned out to be strongly dependent on the orientation of the pyridyl group. DFT calculations were performed to determine the optimized molecular conformation and molecular 3D charge density. At the liquid–solid interface, as visualized by STM, d-DBPy formed a 2D fishbone-like adlayer with linear molecular conformation by the molecule–substrate van der Waals force; however, p-DBPy self-assembled into a fish-scale-like pattern with flexed molecular conformation resulting from intra- and intermolecular hydrogen bonds. Compared with the 2D adlayers, the π–π stacking is another important driving force to determine the morphology of the films. The results demonstrated that understanding the effect of pyridyl orientation along the conjugated core on the molecular conformation and self-assembly is of critical importance for fabricating desired films with special morphology for high-performance organic electronic devices.

Global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by concentration modulation. Scanning tunneling microscopy investigations reveal that a heterochiral supramolecular double rosette-like structure was fabricated for the first time via hydrogen bond interactions with achiral 1-octanoic acid under low concentrations. When the solution concentration was increased, the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern was observed. Interestingly, these two metastable structures ultimately could transform into a stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only an achiral octamer arrangement could be obtained, owing to the changes of intermolecular hydrogen bonding, van der Waals force, and dipole–dipole interactions. The present results provided an important impetus for the induction and control of polymorphous chiral structural transformation through modulated solution concentration of achiral 1-octanoic acid.

Two-dimensional supramolecular assemblies of a series of 2,7-bis(10-n-alkoxycarbonyl-decyloxy)-9-fluorenone derivatives (BAF-Cn, n = 1, 3–6) consisting of polar fluorenone moieties and ester alkoxy chains were investigated by scanning tunneling microscopy on highly oriented pyrolytic graphite surfaces. The chain-length effect was observed in the self-assembly of BAF-Cn. Self-assembly of BAF-C1 was composed of a linear I pattern, where the side chains adopted a fully interdigitated arrangement. As the length of side chains increased, the coexistence of a linear I pattern and a cyclic pattern for the self-assembly of BAF-C3 was observed. Upon increasing the length of the alkoxy chain even further (n = 4–6), another linear II structure was observed in the BAF-Cn monolayer, in which the side chains in adjacent rows were arranged in a tail-to-tail configuration. It is reasonable to conclude that not only the van der Waals forces but also the dipole–dipole interactions from both the fluorenone cores and the ester alkoxy chains play critical roles in the self-assemblies of BAF-Cn. Our work provides detailed insights into the effect of intermolecular dipole–dipole and van der Waals interactions on the monolayer morphology of fluorenone derivatives.

We design a bifunctional molecule (5-bromo-2-hexadecyloxy-benzoic acid, 5-BHBA) with a bromine atom and a carboxyl group and its two-dimensional self-assembly is experimentally and theoretically investigated by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The supramolecular self-organization of 5-BHBA in two different solvents (1-octanoic acid and n-hexadecane) at the liquid–solid interface at different solution concentrations is obviously different due to the cooperative and competitive intermolecular halogen and hydrogen bonds. Three kinds of nanoarchitectures composed of dimers, trimers and tetramers are formed at the 1-octanoic acid/graphite interface based on –COOH⋯HOOC–, triangular CO⋯Br⋯H–C, –Br⋯O(H), Br⋯Br, and O⋯H interactions. Furthermore, by using n-hexadecane as the solvent, two kinds of self-assembled linear patterns can be observed due to the coadsorption, in which the dimers are formed by intermolecular –COOH⋯HOOC– hydrogen bonds. The molecule–solvent and solvent–solvent van der Waals force and intermolecular hydrogen bonds dominate the formation of coadsorbed patterns. We propose that the cooperative and competitive halogen and hydrogen bonds are related to the polarity of the solvent and the type of molecule–solvent interaction. The intermolecular binding energy of different dimers and their stability are supported by theoretical calculations. The result provides a new and innovative insight to induce the 2D self-assembled nanostructures by halogen and hydrogen bonds at the liquid–solid interface.

Herein, the photophysical properties of two π-conjugated thienophenanthrene derivatives (6,9- and 5,10-DBTD) are reported. Their self-assembled monolayers in aliphatic hydrocarbon solvents under different concentrations were investigated by scanning tunneling microscopy on a graphite surface. The STM results revealed that the self-assembled structures of the two geometrical isomers exhibited absolutely different behaviors. At the aliphatic solvent/graphite interface, 6,9-DBTD produced almost a single stable coassembled linear structure, except for that with n-tridecane as the solvent. However, the self-assembly of 5,10-DBTD showed structural diversity, and it presented a gradient variety through increasing the chain length of the aliphatic solvents as well as the solution concentration. All ordered self-assembled adlayers critically depend on not only the interchain van der Waals (vdW) interactions, but also on multiple intermolecular interactions, including Br⋯OC and Br⋯S hetero-halogen bonds, homo-Br⋯Br interactions, and H⋯Br and H⋯O hydrogen bonds. We proposed that the cooperation and competition of the intermolecular interactions involving a Br atom and interchain vdW forces induce this structural variety. Density functional theory calculations support to unravel the different elementary structural units based on halogen bonds and hydrogen bonds and were useful tools to dissect and explain the formation mechanism.

The self-assembly of 2,7-bis(decyloxy)-9-fluorenone on highly oriented pyrolytic graphite is investigated at the solid/gas interface by scanning tunneling microscopy, 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 identified to be bound with weak intermolecular C(sp2)–H···O═C hydrogen bonds. As discerned by its position in the fluorenone moiety, the C(sp2)–H group with larger chemical shift value and stronger acidity displays a higher priority when involving in the formation of a C(sp2)–H···O hydrogen bond. Owing to the high density of C(sp2)–H donors, various hydrogen-bonding configurations arise, further leading to the occurrence of structural polymorphism. Besides, intermolecular van der Waals interactions as well as the dipole–dipole interactions act as the complementary roles to stabilize the whole monolayer. The underlying mechanism is further confirmed by the density functional theory calculations.

The effects of the position and number of bromine substituents on the self-assembled patterns of phenanthrene derivatives by changing multiple weak intermolecular interactions were investigated at the 1-octanoic acid/graphite interface at different concentrations by scanning tunneling microscopy. Two Br substituted DBHP molecules (2,7-DBHP, 3,6-DBHP) and BHP without a Br group formed a linear lamellar pattern by the van der Waals interactions between the alkoxyl chains in each lamella at high concentrations, which forces the phenanthrene derivatives to self-organize in a π–π stacked edge-on conformation. On decreasing the solution concentration, owing to the molecule–molecule van der Waals force and Br⋯Br halogen bonds or the molecule–solvent cooperative Br⋯O (CO) hydrogen and Br⋯HO–hydrogen bonds, 2,7-DBHP molecules were found to form two kinds of network structures, whereas 3,6-DBHP molecules formed only a zigzag pattern due to the intermolecular Br⋯Br van der Waals type interactions. One bromine substituted phenanthrene derivative (3-DBHP) formed a dislocated linear pattern by two C–H⋯Br hydrogen bonds in each dimer. These observations revealed that an important modification of the position and number of halogen substituents might dramatically change the self-assembly behaviors by different intermolecular interactions including Br⋯Br and Br⋯O halogen bonding, Br⋯Br van der Waals type interactions, and H⋯Br hydrogen bonding. DFT calculations were explored to unravel how slightly tuning the molecular structure defines the geometry of a 2D self-assembled nanoarchitecture through the different elementary structural units having Br⋯Br and Br⋯H interactions.

The self-assembled behaviors of two fluorenone derivatives, 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 limitation of expression of structural polymorphism. In addition, the concentration-dependent polymorphism as well as the relative phase transition is discussed in terms of the stability and packing density of different polymorphs. Furthermore, the different self-assembled behaviors of BHUF molecules in these two solvents at lower concentrations are associated with the different energy gain upon solvent coadsorption. The investigation provides a simple and alternative strategy to construct the structural polymorphs by utilizing multiple hydrogen bonds at the liquid–solid interface.

Halogen bonding with high 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 how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. 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 identification of multiple halogen bonding in supramolecular chemistry.

Understanding the formation and structural transition of the two-dimensional chirality of self-assembly is a subject which still gains significant interest in surface or interface chirality studies. Here, we present the solvent-induced chiral structural transition of a 2-hydroxy-7-pentadecyloxy-9-fluorenone (HPF) molecules’ self-assembled adlayer through coassembly with achiral aliphatic solvents under different concentrations. Polymorphic chiral patterns are obtained at low concentrations of aliphatic solvents with different chain lengths. The HPF molecules form coassembled structures with these solvents through van der Waals interactions. At the same time, at high concentrations, HPF molecules uniformly form a nonchiral multimer structure without coadsorbed aliphatic solvent molecules. What is interesting is that these structures under different concentrations will finally change into a zigzag structure, which is the thermodynamically most stable configuration. Especially when using n-hexadecane as the solvent, the adlayer shows perfect steric matching due to the close chain length of HPF and n-hexadecane, which can maximize the molecule–solvent interactions. Thus, HPF molecules in n-hexadecane exhibit the most diversiform configuration. The distinct concentration-dependence has proven that the solvent molecules can act as a coadsorbed component through van der Waals interactions rather than simply a dispersant and further result in the probability and stability of chiral self-assembled monolayers by subtle tuning of the solvent–molecule and solvent–substrate interactions. This result provides a simple and alternative strategy to construct the 2D chiral assembled monolayer.

In this present work, a scanning tunneling microscope (STM) operated under ambient conditions was utilized to probe the self-assembly behavior of 2,7-bis-nonyloxy-9-fluorenone (F–OC9) at the liquid–solid (l/s) interface. On the highly oriented pyrolytic graphite (HOPG) surface, two-dimensional (2D) polymorphism with diversity of intermolecular dipole interactions induced by solvent was found. Solvents ranged from hydrophilic solvating properties with high polarity, such as viscous alkylated acids, to nonpolar alkylated aromatics and alkanes. 1-Octanol and dichloromethane were used to detect the assembly of F–OC9 at the gas–solid (g/s) interface. The opto-electronic properties of F–OC9 were determined by UV-vis and fluorescence spectroscopy in solution. Our results showed that there were tremendous solvent-dependent self-assemblies in 2D ordering for the surface-confined target molecules. When a homologous series of alkanoic acids ranging from heptanoic to nonanoic acid were employed as solvents, the self-assembled monolayer evolved from low-density coadsorbed linear lamellae to a semi-circle-like pattern at relatively high concentrations, which was proven to be the thermodynamic state as it was the sole phase observed at the g/s interface after the evaporation of solvent. Moreover, by increasing the chain length of the alkylated acids, the weight of the carboxylic group, also being the group responsible for the dielectric properties, diminished from heptanoic to nonanoic acid, which could make the easier/earlier appearance of a linear coadsorption effect. However, this was not the case for nonpolar 1-phenyloctane and n-tetradecane: no concentration effect was detected. It showed a strong tendency to aggregate to generate coexistence of separate domains of different phases due to the fast nucleation sites. Furthermore, thermodynamic calculations indicated that the stable structural coexistence of the fluorenone derivative was attributed to synergistic intermolecular dipole–dipole and van der Waals (vdWs) forces at l/s interface. It is believed that the results are of significance to the fields of solvent induced polymorphism assembly and surface science.

Global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by concentration modulation. Scanning tunneling microscopy investigations reveal that a heterochiral supramolecular double rosette-like structure was fabricated for the first time via hydrogen bond interactions with achiral 1-octanoic acid under low concentrations. When the solution concentration was increased, the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern was observed. Interestingly, these two metastable structures ultimately could transform into a stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only an achiral octamer arrangement could be obtained, owing to the changes of intermolecular hydrogen bonding, van der Waals force, and dipole–dipole interactions. The present results provided an important impetus for the induction and control of polymorphous chiral structural transformation through modulated solution concentration of achiral 1-octanoic acid.

The supramolecular patterns of a thienophenanthrene derivative could be switched among dissimilar polymorphs with different halogen-bond densities by solution concentration, which is demonstrated through a combination of STM and density functional theory (DFT) calculations.

Controlling and unraveling structural polymorphism has received special attention in 2D self-assembled monolayers. In this work, we investigated the steric matching and solution concentration controlled structural variety in the self-assembly of 2,7-bis(n-alkoxy)-9-fluorenone (F–OCn) at the n-tetradecane and n-tridecane/graphite interface under different concentrations, respectively. Scanning tunneling microscopy (STM) revealed that the coadsorbed adlayers of F–OCn and solvents (n = 12 to 16) were formed and exhibited concentration dependent 2D phases due to the steric matching. The self-assembled monolayer of F–OCn (n = 12 to 16) evolved from a low-density coadsorbed linear lamellar packing, which was formed at low concentrations, to higher-density patterns at relatively high concentrations. F–OC14 exhibited a complex structural variety, in which a systematic trend of decrease in the molecular density per unit cell with decreasing concentration was obtained. Except for F–OCn (n = 13, 15, 17), the zigzag structure showing the linear lamella with dimers was observed. Systematic experiments revealed that the self-assembly of F–OCn was chain-length dependent. The results provide insight into the structural variety exhibited by a series of organic molecules and furnish important guidelines to control the morphology by changing the solution concentration.

Studying two-dimensional (2D) and three-dimensional (3D) crystallization in tandem is a powerful way to acquire a deep understanding of molecular self-assembly. X-ray crystallography results indicate that N-[6-(fluoren-9-ylideneamino)hexyl]fluoren-9-imine (C1), N-[12-(fluoren-9-ylideneamino)dodecyl]fluoren-9-imine (C2), and co-crystal of naphthalene-1,5-diamine and 9-fluorenone (C3) are single-, poly- and co-crystals, respectively. Furthermore, the self-assembled structures of these three kinds of crystals (C1, C2 and C3) at the 1-phenyloctane/HOPG interface are investigated using scanning tunneling microscopy under ambient conditions. The C1 molecule, with a short chain, is lying flat on the substrate with a close packing phase, which is the same in its 3D crystal structure. The C2 molecule, bearing a longer chain, forms two types of linear structures, which are stable enough to endure continuous tip scanning. In Type I, the C2 molecules lie flat on the substrate to form a linear zigzag pattern, while in Type II one of the fluorene cores in each C2 molecule adopts an edge-on arrangement and interlocks with the adjacent fluorene core in one lamella. In the co-crystal C3, naphthalene-1,5-diamine and 9-fluorenone arrange perpendicular to the HOPG surface in a herringbone pattern via hydrogen bonds and π–π interactions. The lying or standing orientation of the three kinds of crystals show that the functional groups tethered to the middle spacer can modulate the motifs of self-assembly in the 2D and 3D crystallization. Furthermore, it also highlights that physical adsorption on the HOPG surface is not only controlled by the adsorbate–substrate interactions but also by the size and shape of the adsorbates.

Fluorenone derivatives (F–OCn) with various lengths of peripheral alkyl chains (with carbon numbers of n = 12–18) were synthesized, and their self-assembled adlayers were investigated in solvents with different polarities and functionalities by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface. The chain-length effect on the self-assembly of F–OCeven was observed in 1-phenyloctane. With the shortening of the side chain, the self-assembled pattern changed from a dense- and loose-packed structure to a pliers-like structure. Self-assembly of F–OCodd showed a uniform lamellar pattern. An even–odd effect was observed resulting from the direction of the end methyl group in the alkyl chain unit. Furthermore, when the samples using dichloromethane as solvent were observed within 3 h, a less ordered lamellar structure appeared in most cases. The pliers-like pattern was observed for self-assembly of F–OC16 and F–OC14. However, F–OC17 formed a zigzag structure. Observation of the odd–even and chain-length effects on the self-assembled adlayers might provide an analytical method for examining the structural and chemical homogeneities.

The self-assembly of azobenzene derivatives (CnAzCOOH) with various lengths of peripheral alkyl chains (with carbon number of n = 8, 10, 12, 14, 16) were observed by scanning tunneling microscopy on highly oriented pyrolytic graphite (HOPG) surface. The effect of van der Waals interactions and the intermolecular hydrogen bonding on the two-dimensional self-assembly was systematically studied. No alkyl-chain length effect was observed according to the STM images. All kinds of CnAzCOOH adopting the same pattern self-assembled on the HOPG surface, suggesting the formation of the two-dimensional structures was dominated by the hydrogen bonding of the functional groups. It could be found that two CnAzCOOH molecules formed a hydrogen-bonded dimer with “head-to-head” fashion as expected; however, the dimers organized themselves in the form of relative complex lamellae. Three dimers as a group arranged side by side and formed a well-defined stripe with periodic dislocations due to the registry mechanism of the alkyl chain with the underlying HOPG surface. The hydrogen bonds between the adjacent dimers in one lamella were formed and dominated the self-assembled pattern.Highlights► Self-assembly of azobenzene derivatives with various lengths of alkyl chains were observed by STM. ► No alkyl-chain length effect was observed according to the STM images. ► CnAzCOOH molecules formed hydrogen-bonded dimers with “head-to-head” fashion. ► Kinked structure was driven by the intermolecular multiple hydrogen bonding interactions.

Two-dimensional self-assembly of 2,7-ditridecyloxy-9-fluorenone (F-OC13) is investigated by scanning tunneling microscopy (STM) in solvents with different polarities and functional groups on a high oriented pyrolytic graphite surface. The STM images reveal that the self-assembly of F-OC13 is strongly solvent-dependent. 1-Phenyloctane can coadsorb on the self-assembly of F-OC13, and the structural transformation of the adlayer from the linear structure to alternate lamella can be observed with the decrease of the concentration. At the 1-octanol/HOPG interface, only a well-ordered linear pattern is obtained. The intermolecular hydrogen bonding between the 1-octanoic acid and the F-OC13 molecule is responsive for the formation of butterfly configuration. When n-tridecane or n-tetradecane is used as solvent, a regular alternate pattern is formed under high concentrations, and a coadsorbed lamellar structure is observed under low concentrations. Furthermore, when the sample with use of the methanol, dichloromethane, or toluene as solvent is observed within one hour, a denser-packed structure appears. After the sample is placed more than three hours, in methanol and dichloromethane, a regular alternate pattern is formed corresponding to the result using n-tridecane or n-tetradecane as a solvent under high concentration. In toluene, the alternated pattern is similar with that in 1-phenyloctane at low concentration. The solvent induced self-assembly polymorphism is discussed in terms of factors of the polarity of the F-OC13 molecule and the nature of the solvent. The results provide a new objective to fabricate and control molecular nanopatterns based on the polar group in the molecule.

The effects of the position and number of bromine substituents on the self-assembled patterns of phenanthrene derivatives by changing multiple weak intermolecular interactions were investigated at the 1-octanoic acid/graphite interface at different concentrations by scanning tunneling microscopy. Two Br substituted DBHP molecules (2,7-DBHP, 3,6-DBHP) and BHP without a Br group formed a linear lamellar pattern by the van der Waals interactions between the alkoxyl chains in each lamella at high concentrations, which forces the phenanthrene derivatives to self-organize in a π–π stacked edge-on conformation. On decreasing the solution concentration, owing to the molecule–molecule van der Waals force and Br⋯Br halogen bonds or the molecule–solvent cooperative Br⋯O (CO) hydrogen and Br⋯HO–hydrogen bonds, 2,7-DBHP molecules were found to form two kinds of network structures, whereas 3,6-DBHP molecules formed only a zigzag pattern due to the intermolecular Br⋯Br van der Waals type interactions. One bromine substituted phenanthrene derivative (3-DBHP) formed a dislocated linear pattern by two C–H⋯Br hydrogen bonds in each dimer. These observations revealed that an important modification of the position and number of halogen substituents might dramatically change the self-assembly behaviors by different intermolecular interactions including Br⋯Br and Br⋯O halogen bonding, Br⋯Br van der Waals type interactions, and H⋯Br hydrogen bonding. DFT calculations were explored to unravel how slightly tuning the molecular structure defines the geometry of a 2D self-assembled nanoarchitecture through the different elementary structural units having Br⋯Br and Br⋯H interactions.

We design a bifunctional molecule (5-bromo-2-hexadecyloxy-benzoic acid, 5-BHBA) with a bromine atom and a carboxyl group and its two-dimensional self-assembly is experimentally and theoretically investigated by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The supramolecular self-organization of 5-BHBA in two different solvents (1-octanoic acid and n-hexadecane) at the liquid–solid interface at different solution concentrations is obviously different due to the cooperative and competitive intermolecular halogen and hydrogen bonds. Three kinds of nanoarchitectures composed of dimers, trimers and tetramers are formed at the 1-octanoic acid/graphite interface based on –COOH⋯HOOC–, triangular CO⋯Br⋯H–C, –Br⋯O(H), Br⋯Br, and O⋯H interactions. Furthermore, by using n-hexadecane as the solvent, two kinds of self-assembled linear patterns can be observed due to the coadsorption, in which the dimers are formed by intermolecular –COOH⋯HOOC– hydrogen bonds. The molecule–solvent and solvent–solvent van der Waals force and intermolecular hydrogen bonds dominate the formation of coadsorbed patterns. We propose that the cooperative and competitive halogen and hydrogen bonds are related to the polarity of the solvent and the type of molecule–solvent interaction. The intermolecular binding energy of different dimers and their stability are supported by theoretical calculations. The result provides a new and innovative insight to induce the 2D self-assembled nanostructures by halogen and hydrogen bonds at the liquid–solid interface.

Controlling and unraveling structural polymorphism has received special attention in 2D self-assembled monolayers. In this work, we investigated the steric matching and solution concentration controlled structural variety in the self-assembly of 2,7-bis(n-alkoxy)-9-fluorenone (F–OCn) at the n-tetradecane and n-tridecane/graphite interface under different concentrations, respectively. Scanning tunneling microscopy (STM) revealed that the coadsorbed adlayers of F–OCn and solvents (n = 12 to 16) were formed and exhibited concentration dependent 2D phases due to the steric matching. The self-assembled monolayer of F–OCn (n = 12 to 16) evolved from a low-density coadsorbed linear lamellar packing, which was formed at low concentrations, to higher-density patterns at relatively high concentrations. F–OC14 exhibited a complex structural variety, in which a systematic trend of decrease in the molecular density per unit cell with decreasing concentration was obtained. Except for F–OCn (n = 13, 15, 17), the zigzag structure showing the linear lamella with dimers was observed. Systematic experiments revealed that the self-assembly of F–OCn was chain-length dependent. The results provide insight into the structural variety exhibited by a series of organic molecules and furnish important guidelines to control the morphology by changing the solution concentration.

In this present work, a scanning tunneling microscope (STM) operated under ambient conditions was utilized to probe the self-assembly behavior of 2,7-bis-nonyloxy-9-fluorenone (F–OC9) at the liquid–solid (l/s) interface. On the highly oriented pyrolytic graphite (HOPG) surface, two-dimensional (2D) polymorphism with diversity of intermolecular dipole interactions induced by solvent was found. Solvents ranged from hydrophilic solvating properties with high polarity, such as viscous alkylated acids, to nonpolar alkylated aromatics and alkanes. 1-Octanol and dichloromethane were used to detect the assembly of F–OC9 at the gas–solid (g/s) interface. The opto-electronic properties of F–OC9 were determined by UV-vis and fluorescence spectroscopy in solution. Our results showed that there were tremendous solvent-dependent self-assemblies in 2D ordering for the surface-confined target molecules. When a homologous series of alkanoic acids ranging from heptanoic to nonanoic acid were employed as solvents, the self-assembled monolayer evolved from low-density coadsorbed linear lamellae to a semi-circle-like pattern at relatively high concentrations, which was proven to be the thermodynamic state as it was the sole phase observed at the g/s interface after the evaporation of solvent. Moreover, by increasing the chain length of the alkylated acids, the weight of the carboxylic group, also being the group responsible for the dielectric properties, diminished from heptanoic to nonanoic acid, which could make the easier/earlier appearance of a linear coadsorption effect. However, this was not the case for nonpolar 1-phenyloctane and n-tetradecane: no concentration effect was detected. It showed a strong tendency to aggregate to generate coexistence of separate domains of different phases due to the fast nucleation sites. Furthermore, thermodynamic calculations indicated that the stable structural coexistence of the fluorenone derivative was attributed to synergistic intermolecular dipole–dipole and van der Waals (vdWs) forces at l/s interface. It is believed that the results are of significance to the fields of solvent induced polymorphism assembly and surface science.

The supramolecular patterns of a thienophenanthrene derivative could be switched among dissimilar polymorphs with different halogen-bond densities by solution concentration, which is demonstrated through a combination of STM and density functional theory (DFT) calculations.