Structure of zone-cast HBC-C12H25 films

The structure of a thin zone-cast film of the hexa-n-dodecyl-substituted hexa-peri-benzocoronene (HBC) has been investigated using grazing incidence X-ray diffraction. A model with an orthorhombic unit cell containing two molecules accounts well for the observations. The molecules are arranged in a "herringbone" structure resembling the packing observed for unsubstituted HBC. The molecular disk planes are oriented perpendicularly to the substrate, rotated by approximately 39 degrees about the film normal. The relatively long side chains of dodecyl were found to be in an ordered interdigitated state. The aliphatic side chains and the aromatic HBC-cores segregate to form regular vertical domains spanning the film thickness. For in-plane rocking scans a discrete orientation distribution is observed with peaks at regular angle intervals. We interpret this as a grain boundary effect induced by alkyl chain stacking faults.     

Ordered arrays of organometallic iridium complexes with long alkyl chains on graphite

Iridium(III) fac-tris(2-phenylpyridine) fac-[Ir(ppy)3] complexes equipped with long alkyl chains were prepared to examine their capability to form organized arrays on the surface of highly oriented pyrolytic graphite (HOPG). The molecules form lamellar arrays at the 1-phenyloctane/HOPG interface. From the analysis of the STM images, it was concluded that the molecules align with alkyl chains being interdigitated. Similar lamellar arrays were also obtained at the air/HOPG interface upon drop-casting of toluene solutions. The lamellar structure at the molecular level leads to rectangular two-dimensional crystalline domains a few hundred nanometers long (nanoslips). Infrared external reflection spectroscopy suggested that the adsorbed alkyl chains adopt the trans-zigzag conformation in the nanoslip, although the orientations of the zigzag plane of the alkyl groups are mixed. Cyclic voltammetry indicates fast electron transfer between the adsorbed molecules and the substrate and significant intermolecular electronic interactions. It was found that annealing at high temperatures is an effective method to prepare ordered assemblies more than a few micrometer scale (microslips). The orientations of the nanoslips prepared from the racemic mixture exhibited an apparent 12-fold symmetry, while its optically active enantiomer resulted in more irregular domains with a six-fold symmetry, implying an important role of chirality on packing at the molecular level and on the orientation of the domains at larger scales. When drop-cast from more concentrated solutions than a few hundreds of micromolar, multilayers were obtained, in which the alkyl chains in the molecules are more or less perpendicular to the surface. This structure can be transformed into the nanoslips upon standing.     

Self-assembly of amphiphilic hexapyridinium cations at the air/water interface and on HOPG surfaces.

Mono- and multilayers of a novel amphiphilic hexapyridinium cation with six eicosyl chains (3) are spread at the air/water interface as well as on highly ordered pyrolytic graphite (HOPG). On water, the monolayer of 3 is investigated by recording surface pressure/area and surface potential/area isotherms, and by Brewster angle microscopy (BAM). Self-organized tubular micelles with an internal edge-on orientation of molecules form at the air/water interface at low surface pressure whereas multilayers are present at high surface pressure, after a phase transition. Packing motifs suggesting a tubular arrangement of the constituting molecules were gleaned from atomic force microscopy (AFM) investigations of Langmuir-Blodgett (LB) monolayers being transferred on HOPG at different surface pressures. These LB film structures are compared to the self-assembled monolayer (SAM) of 3 formed via adsorption from a supersaturated solution, which is studied by scanning tunnelling microscopy (STM). On HOPG the SAM of 3 consists of nanorods with a highly ordered edge-on packing of the aromatic rings and an arrangement of alkyl chains which resembles the packing of molecules at the air/water interface at low surface pressure. Additional details of the molecular packing were gleaned from single-crystal X-ray structure analysis of the hexapyridinium model compound 2b, which possesses methyl instead of eicosyl residues.     

The crystalline and liquid crystalline structures of benzyl-tri-octadecylammonium bromide complete the puzzle: how do Group VA halide salts with one-four long n-alkyl chains pack?

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Chirality in supramolecular self-assembled monolayers of achiral molecules on graphite: formation of enantiomorphous domains from arachidic anhydride

The molecular arrangement and chirality of the self-assembled arachidic anhydride monolayer on graphite were investigated using scanning tunneling microscopy (STM). This molecule has two identical alkyl chains, linked by an anhydride group in the middle. In its extended form, one alkyl chain is shifted, with respect to the other, along the molecular backbone. Upon adsorption on graphite, this achiral anhydride spontaneously forms two types of homogeneous domains (denoted as m and m') with mirror symmetry. The angle from the molecular chain to the row-packing direction is 98.0 degrees +/- 0.5 degrees and 82.0 degrees +/- 0.5 degrees for domains m and m', respectively. Domain m is the mirror image of m'. The molecular arrangement of this self-assembled monolayer shows that domains m and m' are two-dimensional enantiomers with opposite chiralities. This new molecular packing motif is confirmed by line-profile analyses along the molecule-chain and the row-packing directions. This finding demonstrates the spontaneous formation of highly ordered homogeneous enantiomorphous domains on graphite resulting only from weak van der Waals forces between the achiral arachidic anhydride molecules.     

Langmuir-Blodgett films as aligning layers for homeotropic alignment of liquid crystal molecules

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Direct Observation of Alkyl Chain Interdigitation in Conjugated Polyquarterthiophene Self‐Organized on Graphite Surfaces

A solution processible polymer—poly(3,3‴‐didodecylquaterthiophene) (PQT‐12) is investigated at the liquid/solid interface using the scanning tunneling microscopy (STM). Two‐dimensional ordered films made up of self‐assembled domains, with dimensions of 100 nm × 50 nm adsorbed on highly oriented pyrolytic graphite (HOPG) were formed. These domains consist of parallel lamellar polymer chains, with the alkyl chains forming interdigitated structures, along with U‐shaped and closed ring segments of the polymer chains. A polymer chain packing model is proposed herein, which attempts to propose a correlation between the packing of long chains and charge mobilities. These STM results could help in understanding the relationship between the extended conjugation and molecular organization of the PQT‐12 chains.

     

β-联碳酰基类衍生物有序自组装膜的STM研究

在大气条件下, 利用扫描隧道显微镜研究了四个β-联碳酰基类衍生物在高定向裂解石墨(HOPG)表面的自组装结构. 研究分子的结构中均包含π电子共轭体系和烷基链. 实验研究了分子结构对自组装结构的影响, 并利用分子结构的变化实现了自组装膜结构的调控. 结果表明, 在甲苯溶剂中制备的这些自组装结构均长程有序, 分子间氢键和偶极相互作用是影响自组装膜结构变化的重要因素.     

取代烷基侧链长度对二维纳米结构的影响

合成了一系列烷基取代的间苯三酚衍生物,并在大气条件下用扫描隧道显微镜研究了它们在高定向裂解石墨表面的吸附和组装行为.实验结果表明,这些自组装分子具有条状结构特征.在链长较短的分子图像中,两条平行的烷氧基链肩并肩地排列在苯环的一侧,另一条烷氧基链则排列在苯环的另一侧,链与链之间彼此相互交错排列形成均一的烷基条带.当链长增加时,这种高稳定性和密排结构遭到破坏,出现单个分子和分子对共存的组装结构.这是由于烷基链与烷基链之间以及烷基链与基底之间的作用力共同决定的.通过调控分子烷基链的长度可以得到不同的表面二维纳米结构.     

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.     

Self-assembly of small polycyclic aromatic hydrocarbons on graphite: a combined scanning tunneling microscopy and theoretical approach

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to high- and low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.     

Smectic A phases with strings of interdigitated molecules in swallow tailed compounds

X-ray diffraction studies are reported of smectic A phases exhibited by swallow-tailed compounds. The special molecular shape consisting of regions having differing space filling requirements results in antiparallel packing of the molecules; this influences the conformations of the alkyl chains. The layer spacings show unusually good agreement with the molecular lengths and this is the case for molecules with long and short aliphatic chains. We have observed also an additional incommensurable structural element which we believe to be unique for non-polar, low molar mass compounds. The X-ray diffraction patterns provide evidence for the existence of strings of interdigitated molecules.     

Chiral expression at the solid-liquid interface: a joint experimental and theoretical study of the self-assembly of chiral porphyrins on graphite

The chiral organization of an enantiopure functional molecule on an achiral surface has been studied with the aim of understanding the influence of stereogenic centers on the self-assembly in two dimensions. A chiral tetra meso-amidophenyl-substituted porphyrin containing long hydrophobic tails at the periphery of the conjugated pi-electron system was prepared for this purpose. Scanning tunneling microscopy (STM) images of the compound at the graphite-heptanol interface reveal a chiral arrangement of the molecules, with the porphyrin rows tilted by 13 degrees with respect to the normal to the graphite axes. In terms of molecular modeling, a combination of molecular dynamics simulations on systems constrained by periodic boundary conditions and on unconstrained large molecular aggregates has been applied to reach a quantitative interpretation on both the density of the layer and its orientation with respect to the graphite surface. The results show clearly that (i) the methyl groups of the stereogenic point toward the graphite surface and (ii) the porphyrin molecules self-assemble into an interdigitated structure where the alkyl chains align along one of the graphite axes and the porphyrin cores are slightly shifted with respect to one another. The direction of this shift, which defines the chirality of the monolayer, is set by the chirality of the stereogenic centers. Such an arrangement results in the formation of a dense chiral monolayer that is further stabilized by hydrogen bonding with protic solvents.     

Molecular structure and dynamics in monolayers of long chain alkanes and alkyl-derivatives

Long chain alkanes (C₃₄H₇₀ and C₅₀H₁₀₂), a fatty acid (C₁₇H₃₅COOH) and an alkyl-substituted triiodobenzoate (I₃H₂C₆COOC₁₈H₃₇) have been adsorbed at the interface between organic solutions and the basal plane of graphite. In-situ scanning tunneling microscopy (STM) has been employed to investigate their structure and dynamics on the scale of 10 pm and 1 ms or longer. All adsorbates form two-dimensional polycrystals. The molecules tend to organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The n-alkane chains pack in a lattice commensurate with the graphite lattice and the carbon skeleton planes approximately perpendicular to the substrate. Due to the additional space required by a carboxyl end group the alkyl lattice in the fatty acid is incommensurate with the substrate and the carbon skeleton planes lie approximately parallel to the surface. In the triiodobenzoate the headgroup takes the space of about two alkyl chains resulting in an interdigitated packing.     

Highly Ordered Columnar Structures from Hexa-peri-hexabenzocoronenes—Synthesis,X-ray Diffraction,and Solid-State Heteronuclear Multiple-Quantum NMR Investigations

Improved long-range ordering in the columnar mesophase of hexa(para-n-dodecylphenyl)hexabenzocoronene 1 has been achieved by inserting phenyl rings between the extended aromatic core of hexabenzocoronene and the alkyl side chains, which are needed to form liquid crystalline phases. The long-range hexagonal order of the columns is demonstrated by X-ray scattering, while the improved packing of the aromatic cores within the columns and the molecular mobility is probed by a newly developed heteronuclear multiple-quantum MAS NMR technique.     

Two-dimensional assemblies of banana-shaped liquid crystal molecules on HOPG surface

Two-dimensional assemblies of a series of banana-shaped liquid crystal molecules, 1,3-phenylene bis[4-(4-n-alkylphenyliminomethyl) benzoates] (P-n-PIMB, n = 12, 14, 18), are investigated on a highly oriented pyrolytic graphite (HOPG) surface by using scanning tunneling microscopy (STM) at a submolecular level in an ambient environment. Two types of molecular arrangements are observed in these self-assemblies with different stripe widths that resulted from the organization of the alkyl chains. The polarization of the P-n-PIMB monolayers is exactly compensated by the adjacent lamellae with opposite bent direction. Furthermore, a bilayer structure is found in the assembly of P-18-PIMB molecules. In comparison with a gemini surfactant molecule, 4,4'-di[4-(octadecylmethylamino)styryl]-1,1'-pentamethylenebispyridinium dibromide (OPD), P-n-PIMB molecules appear in more complicated packing arrangements. The results in this paper demonstrate the effects of the molecular structures on the adlayer arrangement and will be helpful in fabricating two-dimensional molecular structures on solid surfaces.     

Two-dimensional self-assembly of linear molecular rods at the liquid/solid interface

We report on the synthesis and scanning tunneling microscopy (STM) studies of a series of linear molecular rods (1-5) comprising different numbers and/or spatial arrangements of perfluorinated benzene and benzene subunits interlinked with diacetylenes in the para position and decorated with or without terminal dodecyl chains. The molecules organize themselves into well-ordered 2D crystal structures at the liquid/solid interface through intermolecular and molecule-substrate interactions. Whereas the molecules substituted by dodecyl chains form the lamellar structures with alternating rigid core rows and alkyl chain rows, the unsubstituted ones change the orientation of the rigid backbones with respect to the lamellar axis. The molecular arrangement is not influenced by fluoro substituents on any phenyl ring of the backbone, which suggests that the interactions between the π-conjugated backbones are dominated by close packing rather than by the dipole moments of the rods or fluorine-based intermolecular interactions.     

Coincidence of the molecular organization of beta-substituted oligothiophenes in two-dimensional layers and three-dimensional crystals

The molecular arrangements of three different alkyl-substituted oligothiophenes both in two-dimensional adsorbed layers at a substrate interface and in bulk three-dimensional crystals were studied. Scanning tunneling microscopy (STM) was used to investigate the ordering of the conjugated oligomers in two-dimensional layers adsorbed on graphite. These data were compared with the X-ray structure determinations of single crystals revealing the arrangement in the three-dimensional bulk material. Quaterthiophenes 1 and 2, bearing dodecyl and hexyl side chains, respectively, exhibit a lamella-type stacking of the conjugated backbone concomitant with an interlocking of the alkyl side chains both on the surface and in the crystal. In contrast, the arrangement of propyl-substituted quaterthiophene 3 is rather "herringbone-like" due to the reduced interactions of the shorter alkyl side chains. In all three cases, evidently, the two-dimensional ordering at the graphite surface is coincident with the molecular packing in one cross-section of the three-dimensional crystal.     

Synthesis of dehydrobenzo[18]annulene derivatives and formation of self-assembled monolayers: implications of core size on alkyl chain interdigitation

The synthesis of a series of dodecadehydrotribenzo[18]annulene ([18]DBA) derivatives is reported, together with their steady-state absorption and fluorescence properties. The main focus, though, is on the self-assembly of these compounds at the liquid-solid interface as investigated with scanning tunneling microscopy (STM), highlighting the effect of alkyl chain orientation and alkyl chain length on the molecular ordering. Owing to the large triangular pi-electron system of the [18]DBAs, two different types of alkyl chain orientation are observed. The observed changes in the monolayer networks upon elongation of the alkyl chains are attributed to the increased van der Waals interactions between molecules and substrate. The effect of the core size on the alkyl chain orientation and, as a result, the monolayer structure is discussed in relation to the results obtained previously for triangularly-shaped dehydrobenzo [12]annulene ([12]DBA) derivatives and triphenylene derivatives. A guideline for substituent spacing allowing control of molecular alignment for large planar pi-electron systems utilizing directional alkyl chain interdigitation is also discussed.     

Self-organized monolayer of meso-tetradodecylporphyrin coordinated to Au(111)

The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au(111), H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-C=N-) nitrogen atoms of the porphyrin macrocycle to Au(111).