Bao Zha

Hydrogen-Bonding-Induced Polymorphous Phase Transitions in 2D Organic Nanostructures

The 2D self-assembly of various 2-hydroxy-7-alkoxy-9-fluorenone (HAF) molecules has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface. A systematic study revealed that HAF molecules with different numbers of carbon atoms in their alkoxy chains could form two or three different kinds of nanostructures, that is, less-ordered, flower-like, and zig-zag patterns, owing to the formation of different types of intermolecular hydrogen bonds. The observed structural transition was found to be driven by molecular thermodynamics, surface diffusion, and the voltage pulse that was applied to the STM tip. The zig-zag pattern was the most stable of these configurations. An odd-even effect on the flower-like structure, as induced by the odd and even number of carbon atoms in the side chain, was observed by STM. The influence of the odd-even effect on the melting point has a close relationship with the molecular self-assembled pattern. Our results are significant for understanding the influence of hydrogen-bonding interactions on the dominant adsorption behavior on the surface and provide a new visual approach for observing the influence of the odd-even effect on the phase transition.

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly

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.

Structural transition control between dipole–dipole and hydrogen bonds induced chirality and achirality

Nano-fabrication is an issue which has gained extensive attention in molecular engineering. Thus, we have probed intensively surface-based 2D self-assembly of 2-hydroxyanthraquinone (2-HA) derivatives by scanning tunneling microscopy (STM). During the STM process, two interesting nanostructures resembling closely Chinese knots and wheat were identified, thus they were denoted as Knot-like and Wheat-like patterns for legibility. Moreover, careful observation suggests that the Knot-like structure is chiral while the Wheat-like structure is achiral. Systematic analysis indicates that these two arrangements are mainly dominated by synergistic forces of dipole–dipole and hydrogen bonding interactions. To the best of our knowledge, the dipole induced chirality and achirality have been rarely reported, and the synergistic forces of dipole–dipole and hydrogen bonding interactions on dominating 2D assembly have never been proposed. In addition, structural transition between the Knot-like and Wheat-like configurations can be regulated by concentration and solvent as the alkyl chain length changes. Note that the phase transformation is in most cases incomplete. A summary of surface coverage for 2-HA-OCn (n = 12, 14, 16, 18, 20) molecules shows the general trend that a Knot-like structure is preferred in polar solvents and under low concentration, while a Wheat-like structure takes priority in nonpolar solvents and under high concentration. Besides, 2-HA-OCn (n = 11, 13, 15, 17) molecules adopted a Wheat-like pattern which differs from the Wheat-like pattern in the relative orientation of adjacent ribbons, attributed to a minimum of steric repulsion between the interdigitated alkyl chains. This study presents efficient strategies for the manipulation of chiral and achiral nanostructures, and the results are believed to be of significance to the fields of 2D self-assembly and interface science.

Two-dimensional self-assembly of single-, poly- and co-crystals at the liquid/solid interface

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.

Side chain position, length and odd/even effects on the 2D self-assembly of mono-substituted anthraquinone derivatives at the liquid/solid interface

The formation of self-assembled adlayers of 1-hydroxyanthraquinone (1-HA) and 2-hydroxyanthraquinone (2-HA) derivatives with various side chain length were investigated using scanning tunneling microscopy for the purpose of determining the influence of chemical structure on 2D molecular arrangement in a self-assembly process. Different structures labeled as Linear I, Linear II, Linear III, Linear IV and Z-like were presented based on their packing modes. Weak O⋯H–C hydrogen bonds existing between adjacent anthraquinone moieties are the key forces driving the formation of ribbon A, A′, B and C, which are the basic rows of the self-assembled structures. The emergence of odd or even numbers of carbon atoms in the alkyl chain inducing structural diversity is an indication that one of the driving forces for 1-HA-OCn (n = 15, 16) and 2-HA-OCn (n = 12, 14–16) molecules to assemble into ordered 2D nanostructures is the van der Waals interactions between interdigitated alkyl chains. 1-HA-OC16 and 1-HA-OC15 exhibited lamellar structures packed in Linear I and Linear II fashions. 2-HA-OC15 and 2-HA-OC16 adopted Linear III structures and Z-like packing modes. Moreover, when the number of carbon atoms in the side chain of 2-HA-OCn molecules was decreased to 12, the self-assembled pattern could present a Linear IV phase. Notably, 2-HA-OC14 showed the coexistence of Z-like and Linear IV phases. Systematic experiments revealed that a better understanding of the alkyl chain position, length and odd/even effects on 2D self-assembly would shed light on better control of assembly patterns and the design of new molecular materials.

Solvent-dependent self-assembly of 4,7-dibromo-5,6-bis(octyloxy)benzo[c][1,2,5] thiadiazole on graphite surface by scanning tunneling microscopy

Solvent effect on self-assembly of 4,7-dibromo-5,6-bis(octyloxy)benzo[c][1,2,5] thiadiazole (DBT) on a highly oriented graphite (HOPG) surface was investigated by scanning tunneling microscopy (STM) by using 1-phenyloctane, 1-octanoic acid, and 1-octanol as the solvents. Two different patternswere obtained in 1-phenyloctane and 1-octanoic acid, suggesting that the self-assembly of DBT was solvent dependent. At the 1-phenyloctane/HOPG interface, a linear structure was revealed due to the intermolecular halogen bonding. When 1-octanoic acid and 1-octanol are used as the solvents, the coadsorption of solvent molecules resulting from the hydrogen bonding between DBT and solvent made an important contribution to the formation of a lamellar structure. The results demonstrate that solvents could affect the molecular self-assembly according to the variational intermolecular interactions.

Effects of alkyl chain number and position on 2D self-assemblies

Utilizing the effects of alkyl chain number and position is an efficient strategy for the fabrication and regulation of diverse self-assembly structures. Based on self-assembling behavior, we explored these two effects with regard to the self-assembly mechanism and thermal, spectral, and crystal analysis. Eight types of anthraquinone derivatives were used, with eight different self-assembly configurations observed at the 1-octanoic acid/HOPG interface, namely six linear structures, one zigzag structure, and one dimer-linear structures. As these anthraquinone derivatives possess different molecular symmetries, which can affect the molecular dipole, the formation mechanism of these self-assembled networks was investigated with regard to dipole–dipole interactions. Furthermore, density functional theory calculations showed that an increasing alkyl chain number led to a rise in intermolecular van der Waals interactions. However, despite the variation in alkyl chain number and position, the strength of hydrogen bonds between the anthraquinone cores was particularly dependent on the assembled structure. This work provides a comprehensive method for fabricating self-assemblies at liquid/solid interfaces in 2D crystal engineering, and we believe it will be of significance toward studying the effects of alkyl chain number and position on structural diversity in supramolecular chemistry.