Xinrui Miao

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.

A robust superhydrophobic PDMS@ZnSn(OH)6 coating with under-oil self-cleaning and flame retardancy

Some drawbacks such as lack of durability, requirement for high-cost fluorosilane modification, and complicated fabrication processes have limited the practical applications of superhydrophobic coatings for several years. In this work, we provide a novel method to fabricate a robust superhydrophobic coating without pre-modification by pressing ZnSn(OH)6 (ZHS) particles into polydimethylsiloxane (PDMS) solution, and then curing. PDMS used in this work is not only a low surface energy material, but also an adhesive to enhance the bonding force between particles and substrates. The PDMS@ZHS coating (ZHS particles coated on PDMS) is also superhydrophobic when it is immersed into dodecane, and shows excellent self-cleaning in air and oil environments. After undergoing serious chemical corrosion and mechanical abrasion, the superhydrophobicity of the coating remained well. For different kinds of substrates, this coating can be covered and repels water as well. Owing to the excellent flame-retardancy of ZHS particles, PDMS@ZHS coated paper displays excellent flame retardancy in limiting oxygen index (LOI), vertical flame, and cone calorimeter tests. We believe that this simple, environmentally friendly, and versatile fabrication of the PDMS@ZHS coating has excellent real-life applications.

Two-dimensional self-assembly of a porphyrin-polypyridyl ruthenium(II) hybrid on hopg surface through metal-ligand interactions.

The synthesis and self-assembly behavior of porphyrin-polypyridyl ruthenium(II) hybrid, which consists of a flexible alkyl chain attached with two conjugated moieties is described. The electronic absorption spectrum and emission spectra show that the [C(8)-TPP-(ip)Ru(phen)(2)](ClO(4))(2), abbreviated as (C(8)ip)TPPC has optical properties. Scanning tunneling microscopy (STM) studies found that the pi-pi interaction and metal-ligand interaction allow (C(8)ip)TPPC to form self-assembled structure and have an edge-on orientation on the highly oriented pyrolytic graphite (HOPG) surface. The multidentate structure in (C(8)ip)TPPC molecules act as linkers between the molecules and form metal-ligand coordination, which forces the assembly process in the direction of stable columnar arrays. In addition, although the sample was stored for two months in ambient conditions, STM experiments showed that the order of (C(8)ip)TPPC self-assembly only slightly decreased which indicates that the self-assembled monolayer is stable. This work demonstrates that introducing a metal-ligand in the porphyrin-polypyridyl compound is a useful strategy to obtain novel surface assemblies.

One-Step Fabrication of Non-Fluorinated Transparent Super-Repellent Surfaces with Tunable Wettability Functioning in Both Air and Oil

In this paper, we have developed a one-step thermal treatment of polydimethylsiloxane (PDMS) liquid to create transparent super-repellent surface (TSS) and super-repellent powder. They are super-repellent toward water and ethylene glycol. During the one-step thermal treatment, PDMS soot is generated and deposited onto a glass slide (GS) surface to fabricate the TSS without fluorosilane modification. The facilely obtained TSS presented superhydrophobicity and self-cleaning property when immersed into very low surface tension oils such as petroleum ether, hexadecane, peanut oil, and dodecane. Interestingly, by controlling heating time and temperature, wettability of the treated GS surface and the remained white powder can be regulated. The mechanism of tunable wettability was revealed and analyzed by investigating the variations of surface morphology and chemical composition. More importantly, TSS was able to repel highly corrosive aqua regia and saturated NaOH solutions. TSS maintained excellent superhydrophobicity even after chemical and mechanical damages. This simple thermal deposition method was also applicable for other thermally stable substrates to achieve super-repellency, which are believed to find very promising applications.

Robust and thermal-healing superhydrophobic surfaces by spin-coating of polydimethylsiloxane

HYPOTHESIS Superhydrophobic surfaces easily lose their excellent water-repellency after damages, which limit their broad applications in practice. Thus, the fabrication of superhydrophobic surfaces with excellent durability and thermal healing should be taken into consideration. EXPERIMENTS In this work, robust superhydrophobic surfaces with thermal healing were successfully fabricated by spin-coating method. To achieve superhydrophobicity, cost-less and fluoride-free polydimethylsiloxane (PDMS) was spin-coated on rough aluminum substrates. FINDINGS After being spin-coated for one cycle, the superhydrophobic PDMS coated hierarchical aluminum (PDMS-H-Al) surfaces showed excellent tolerance to various chemical and mechanical damages in lab, and outdoor damages for 90days. When the PDMS-H-Al surfaces underwent severe damages such as oil contamination (peanut oil with high boiling point) or sandpaper abrasion (500g of force for 60cm), their superhydrophobicity would lose. Interestingly, through a heating process, cyclic oligomers generating from the partially decomposed PDMS acted as low-surface-energy substance on the damaged rough surfaces, leading to the recovery of superhydrophobicity. The relationship between the spin-coating cycles and surface wettability was also investigated. This paper provides a facile, fluoride-free and efficient method to fabricate superhydrophobic surfaces with thermal healing.

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.

Highly efficient separation of surfactant stabilized water-in-oil emulsion based on surface energy gradient and flame retardancy

HYPOTHESIS Surface energy gradient would generate an imbalance force to drive tiny water droplets in dry air from the hydrophilic bumps to superhydrophobic domains, which has found on the Stenocara beetles back. EXPERIMENTS Inspired by this phenomenon, we introduced a pristine superhydrophilic filter paper on the lower surface energy superhydrophobic filter paper. ZnSn(OH)6 particles and polydimethylsiloxane were mixed to prepare the superhydrophobic coating, and the coating was spray-coated on the poly(dialkyldimethylammonium chloride) covered filter paper to separate the span 80 stabilized water-in-isooctane emulsion. A pristine filter paper was added on the superhydrophobic filter paper to fabricate another membrane for separation. FINDINGS The results revealed that with a pristine filter paper, the membrane performed higher efficiency and more recyclability, and it could separate the emulsions with higher surfactant concentrations. The stabilized water droplets passed the superamphiphilic surface, and hindered by the superhydrophobic surface, generating a surface energy gradient for better separation. In addition, the superhydrophobic membrane could be protected from fire to some degree due to the introduced ZnSn(OH)6 particles with excellent flame retardancy. This easy and efficient approach via simply bringing in pristine superhydrophilic membrane has great potential applications for water-in-oil emulsion separation or oil purification.

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.