Oblique Unit Cell Research Articles

Lattice Symmetry-Guided Charge Transport in 2D Supramolecular Polymers Promotes Triplet Formation.

Singlet-to-triplet intersystem crossing (ISC) in organic molecules is intimately connected with their geometries: by modifying the molecular shape, symmetry selection rules pertaining to spin-orbit coupling can be partially relieved, leading to extra matrix elements for increased ISC. As an analog to this molecular design concept, the study finds that the lattice symmetry of supramolecular polymers also defines their triplet formation efficiencies. A supramolecular polymer self-assembled from weakly interacting molecules is considered. Its 2D oblique unit cell effectively renders it as a coplanar array of 1D molecular columns weakly bound to each other. Using momentum-resolved photoluminescence imaging in combination with Monte Carlo simulations, the study found that photogenerated charge carriers in the supramolecular polymer predominantly recombine as spin-uncorrelated carrier pairs through inter-column charge transfer states. This lattice-defined recombination pathway leads to a substantial triplet formation efficiency (≈60%) in the supramolecular polymer. These findings suggest that lattice symmetry of micro-/macroscopic structures relying on intermolecular interactions can be strategized for controlled triplet formation.

The Effect of Acyl Chain Position on the 2D Monolayer Formation of Monoacyl-sn-Glycerol at the Air/Water Interface: Quantum Chemical Modeling

This paper deals with the results of quantum chemical modeling of the monoacyl-sn-glycerol 2D cluster formation at the air/water interface using a semi-empirical PM3 method. The impact of the 2 or 3 positions of the acyl substituent on the thermodynamics of the monolayer formation is assessed for surfactants with an acyl substituent CnH2n+1COO chain length of n = 6–17 carbon atoms. The calculation shows a significant change in the spontaneous clusterization threshold for isomeric compounds, which differs only in the position of the acyl substituent with respect to the glycerol backbone. This change is almost equal to substituent shortening by approximately two methylene fragments. At the same time, the geometric parameters of the unit cell for resulting monolayers are not affected so drastically. The 2D films in question possess an oblique or orthorhombic unit cell with parameters for 2 and 3-monoacyl-sn-glycerol monolayers, as follows: a = 4.91 Å and 4.82 Å and b = 5.00 Å and 4.92 Å, with hydrocarbon chains tilted at t = 23.0° and 23.5°. The calculated results are in accordance with existing experimental data obtained using grazing incidence X-ray diffraction measurements and the π-A isotherm technique.

Crash-worthiness studies on multistage stiffened honeycomb core sandwich structures under dynamic impact loads

The high-velocity impact performance of honeycomb sandwich structures is enhanced by adopting multi-stage structures with varying cell wall thickness, and cell wall inclination, after preserving the structural mass. The performance of the enhanced structures is simulated considering bullet penetration tests. The penetration is observed to be reduced by 4.82% in case of two stage sandwich structures, as compared to the single stage sandwich structures. Furthermore, the reduction in penetration is estimated to be up to 11.34% when the cell wall thickness is increased from 0.05 mm to 0.15 mm. Moreover, honeycomb cores are stiffened using ribs in two different configurations, within the available space. The improvement in crash-worthiness is estimated by measuring the penetration of the single and two stage hybrid sandwich structures. The penetration distance of stiffened single and two stage structures is found to be reduced by 28.4% and 31.97%, respectively, as compared to the corresponding structures without stiffeners. On the other hand, inclination of the cell walls at an angle to the normal of the face plates is found to enhance the stiffness, and hence the penetration resistance. The honeycomb sandwich structure with oblique unit cells is estimated to dissipate three times more energy as compared to a regular structure. As a result, the impactor is unable to pierce through the sandwich structure. Therefore, two stage hybrid sandwich structures with oblique hexagonal unit cells are recommended for energy absorption in impact applications.

Two- and Three-Dimensional Physical-Chemical Characterization of CER[AP]: A Study of Stereochemistry and Chain Symmetry.

The stratum corneum represents the first skin barrier against chemical and physical damage. These unique properties are based on its peculiar lipid composition with ceramides (CERs) as the main protagonists. In this study, the structural and chemical properties of the α-OH phytosphingosine [AP] CER class have been investigated. α-OH CERs are present in the stratum corneum in their d-forms; however, in most model systems the diastereomer mixture with the synthetically produced l-form is used. The d-form is well-known to form a hydrogen bonding network that helps to reduce the permeability of the lipid matrix, while the l-form does not show any hydrogen bonding network formation. In this paper, 2D (monolayers) and 3D (aqueous dispersions) models have been used to thoroughly study the physical-chemical behaviors of CER[AP] diastereomers taking into account how the symmetry of the chain pattern influences the behavior of the molecules. The chains of both diastereomers arrange in an oblique unit cell, but only the d-CER[AP] forms a supramolecular lattice (subgel phase) in both model systems. Interestingly, the chain pattern does not play any role in structure formation since the hydrogen bonding network dictates the packing properties. The 1:1 mixture of the diastereomers phase separates into two domains: one is composed of practically pure d-form and the other one is composed of a mixture of the l-form with a certain amount of d-form molecules.

Efficacious symmetry-adapted atomic displacement method for lattice dynamical studies

Small displacement methods have been successfully used to calculate the lattice dynamical properties of crystals. It involves displacing atoms by a small amount in order to calculate the induced forces on all atoms in a supercell for the computation of force constants. Even though these methods are widely in use, to our knowledge, there is no systematic discussion of optimal displacement directions from the crystal’s symmetry point of view nor a rigorous error analysis of such methods. Based on the group theory and point group symmetry of a crystal, we propose displacement directions, with an equivalent concept of the group of k, deduced directly in the Cartesian coordinates rather than the usual fractional coordinates, that maintain the theoretical maximum for the triple product V spanned by the three displacements to avoid possible severe roundoff errors. The proposed displacement directions are generated from a minimal set of irreducible atomic displacements that keep the required independent force calculations to a minimum. We find the error in the calculated force constants explicitly depends on the inverse of V and inaccuracy of the forces. Test systems such as Si, graphene, and orthorhombic Sb2S3 are used to illustrate the method. Our symmetry-adapted atomic displacement method is shown to be very robust in treating low-symmetry cells with a large ‘aspect ratio’ due to huge differences in lattice parameters, use of a large vacuum height, or a very oblique unit cell due to unconventional choice of primitive lattice vectors. It is expected that our atomic displacement strategy can be used to address higher-order interatomic interactions to achieve good accuracy and efficiency.

Two Dimensional Oblique Molecular Packing within a Model Peptide Ribbon Aggregate

A10K (A=alanine, K=lysine) model peptides self‐assemble into ribbon‐like β‐sheet aggregates. Here, we report an X‐ray diffraction investigation on a flow‐aligned dispersion of these self‐assembly structures. The two‐dimensional wide‐angle X‐ray scattering pattern suggests that peptide pack in a two‐dimensional oblique lattice, essentially identical to the crystalline packing of polyalanine, An (for n>4). One side of the oblique unit cell, corresponding to the anti‐parallel β‐sheet, is oriented along the ribbon's axis. Together with recently published small angle X‐ray scattering data of the same system, this work thus yields a detailed description of the self‐assembled ribbon aggregates, down to the molecular length scale. Notably, our results highlight the importance of the crystalline peptide packing within its self‐assembly aggregates, which is often neglected.

Structure and Nonequilibrium Heat‐Transfer of a Physisorbed Molecular Layer on Graphene

AbstractThe structure of a physisorbed sub‐monolayer of 1,2‐bis(4‐pyridyl)ethylene (bpe) on epitaxial graphene is investigated by low‐energy electron diffraction and scanning tunneling microscopy. Additionally, nonequilibrium heat‐transfer between bpe and the surface is studied by ultrafast low‐energy electron diffraction. Bpe arranges in an oblique unit cell which is not commensurate with the substrate. Six different rotational and/or mirror domains, in which the molecular unit cell is rotated by 28 ± 0.1° with respect to the graphene surface, are identified. The molecules are weakly physisorbed, as evidenced by the fact that they readily desorb at room temperature. At liquid nitrogen temperature, however, the layers are stable and time‐resolved experiments can be performed. The temperature changes of the molecules and the surface can be measured independently through the Debye–Waller factor of their individual diffraction features. Thus, the heat flow between bpe and the surface can be monitored on a picosecond timescale. The time‐resolved measurements, in combination with model simulations, show the existence of three relevant thermal barriers between the different layers. The thermal boundary resistance between the molecular layer and graphene is found to be 2 ± 1 × 10−8 K m2 W−1.

Photomodulation of Two-Dimensional Self-Assembly of Azobenzene–Hexa-peri-hexabenzocoronene–Azobenzene Triads

Achieving exquisite control over self-assembly of functional polycyclic aromatic hydrocarbons (PAH) and nanographene (NG) is essential for their exploitation as active elements in (nano)technological applications. In the framework of our effort to leverage their functional complexity, we designed and synthesized two hexa-peri-hexabenzocoronene (HBC) triads, pAHA and oAHA, decorated with two light-responsive azobenzene moieties at the pseudo-para and ortho positions, respectively. Their photoisomerization in solution is demonstrated by UV–vis absorption. 1H NMR measurements of oAHA suggested 23% of Z-form can be obtained at a photostationary state with UV irradiation (366 nm). Scanning tunneling microscopy imaging revealed that the self-assembly of pAHA and oAHA at the solid–liquid interface between highly oriented pyrolytic graphite (HOPG) and their solution in 1,2,4-trichlorobenzene can be modulated upon light irradiation. This is in contrast to our previous work using HBC bearing a single azobenzene moiety, which did not show such photomodulation of the self-assembled structure. Upon E-Z isomerization both pAHA and oAHA displayed an increased packing density on the surface of graphite. Moreover, pAHA revealed a change of self-assembled pattern from an oblique unit cell to a dimer row rectangular crystal lattice whereas the assembly of oAHA retained a dimer row structure before and after light irradiation, yet with a modification of the inter-row molecular orientation. Molecular mechanics/molecular dynamics simulations validated the self-assembly patterns of pAHA and oAHA, comprising azobenzenes in their Z-forms. These results pave the way toward use of suitably functionalized large PAHs, as well as NGs, to develop photoswitchable devices.

Functional group selective STM Imaging in self-assembled monolayers: Benzeneselenol on Au(111)

Benzeneselenol (PSe) self-assembled monolayers (SAMs) formed on Au(111) substrate by the immersion procedure with an immersion time of 24h and 4 weeks were studied by high-resolution scanning tunneling microscopy (STM). The short molecular rows, which have been previously attributed to irregular translational domains, were found to be regularly repeated within a single domain in the SAMs fabricated upon the immersion for 4 weeks, forming adlayer structure with a very large unit cell. This structure could be assigned as a (27×5) superlattice (α phase) containing 36 molecules in the oblique unit cell. This phase coexisted with a different phase having a commensurate (83×4) superstructure (β phase) containing 28 protrusions per rectangular unit cell. Analysis of the STM images suggested that each PSe molecule in the β phase was imaged not as one but as a pair of protrusions, which were attributed to the benzene ring and the selenium headgroup of the PSe molecule. At the given molecular length, the spacing between the protrusions defined the molecular tilt, allowing us to derive the orientation of the SAM constituents directly from the STM image.

Self-Assembled Monolayers of Oligophenylenecarboxylic Acids on Silver Formed at the Liquid-Solid Interface.

A series of para-oligophenylene mono- and dicarboxylic acids (R-(C6H4)nCOOH, n = 1-3, R = H,COOH) was studied. Adsorbed on Au(111)/mica modified by an underpotential deposited bilayer of Ag, the self-assembled monolayers (SAMs) were analyzed by near-edge X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy. In all cases SAMs are formed with molecules adopting an upright orientation and anchored to the substrate by a carboxylate. Except benzoic acid, all SAMs could be imaged at molecular resolution, which revealed highly crystalline layers with a dense molecular packing. The structures of the SAMs are described by a rectangular (5 × √3) unit cell for the prevailing phase of the monocarboxylic acids and an oblique ([Formula: see text]) unit cell for the dicarboxylic acids, thus evidencing a pronounced influence of the second COOH moiety on the SAM structure. Density functional theory calculations suggest that hydrogen bonding between the SAM-terminating COOH moieties accounts for the difference. Contrasting other classes of SAMs, the systems studied here are determined by intermolecular interactions whereas molecule-substrate interactions play a secondary role. Thus, eliminating problems arising from the mismatch between the molecular and the substrate lattices, coordinatively bonded carboxylic acids on silver should provide considerable flexibility in the design of SAM structures.

Modeling Powder X-ray Diffraction Patterns of the Clay Minerals Society Kaolinite Standards: KGa-1, KGa-1b, and KGa-2

AbstractThree kaolinite reference samples identified as KGa-1, KGa-1b, and KGa-2 from the Source Clays Repository of The Clay Mineral Society (CMS) are used widely in diverse fields, but the defect structures have still not been determined with certainty. To solve this problem, powder diffraction patterns of the KGa-1, KGa-1b, and KGa-2 samples were modeled. In a kaolinite layer among three symmetrically independent octahedral sites named as A, B, and C and separated from each other by b/3 along the b parameter, the A and B sites are occupied by Al cations, whereas, the C sites located along the long diagonal of the oblique kaolinite unit cell are vacant. The layer displacement vectors t1 and t2 are related by a pseudo-mirror plane from defect-free 1Tc kaolinite enantiomorphs, whereas, the random interstratification within individual kaolinite crystallites creates right-hand and left-hand layer sub-sequences producing structural disorder. A third layer displacement vector, t0, located along the long diagonal of the oblique layer unit cell that contains the vacant octahedral site and coincides with the layer pseudo-mirror plane may exist. Thus, a structural model should be defined by the probability of t1, t2, and t0 layer displacement translations Wt1, Wt2, and Wt0, respectively, determined by simulated experimental X-ray diffraction (XRD) patterns. X-ray diffraction patterns were calculated for structures with a given content of randomly interstratified displacement vectors, and other XRD patterns were calculated for a physical mixture of crystallites having contrasting structural order with only C-vacant layers. The samples differ from each other by the content of high- and low-ordered phases referred to as HOK and LOK. The HOK phase has an almost defect-free structure in which 97% of the layer pairs are related by just the layer displacement vector t1 and only 3% of the layer pairs form the enantiomorphic fragments. In contrast, the LOK phases in the KGa-1, KGa-1b, and KGa-2 samples differ from HOK phases by the occurrence probabilities for the t1, t2, and t0 layer displacements. In addition, the LOK phases contain stacking faults that displace adjacent layers in arbitrary lengths and directions. Low XRD profile factors (Rp = 8-11%) support the defect structure models. Additional structural defects and previously published models are discussed.

Oxygen Adsorption on the Fe(110) Surface: The Old System – New Structures

Adsorption of oxygen on the (110) surface of epitaxial iron films on tungsten (110) was studied using low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), and Auger electron spectroscopy within an exposure range of 0–300 Langmuir (L). Selected oxygen adsorption structures on Fe(110) reported in the literature were critically compared and revised in reference to the present study. The initial adsorption of 1/4 oxygen monolayer resulting in the commonly observed (2 × 2) structure was followed by a structure that was frequently termed as (3 × 1). Its complex LEED pattern was ultimately resolved and interpreted as originating from two structural domains of a large oblique unit cell (eight times larger than the substrate unit cell) and 3/8 oxygen coverage. A new (3 × 2) structure was identified at a coverage of 2/3. The domain interpretation of last two structures was verified by LEEM and confirmed by density functional theory calculations. The onset of oxygen–iron bonding formation was recognized by the change in the symmetry of the LEED pattern and the shape of the iron AES signal. Finally, the formation of an iron oxide FeO(111) monolayer was evidenced at the oxygen exposure of ∼300 L.

Quantum chemical approach to the description of the amphiphile clusterization at the air/liquid and liquid/liquid interfaces with phase nature accounting. II. 1-( n -Alkyl)-imidazoles at the air/water interface

Abstract In the framework of the model for surfactant clusterization description at the air/liquid and liquid/liquid interfaces proposed earlier (Vysotsky et al. J. Phys. Chem. B, 2015, 119 (7), 3281–3296) and based on the quantum chemical semi-empirical PM3 method the structural and thermodynamic parameters of the formation of 1-( N -alkyl)-imidazole monolayers with alkyl chain lengths of 6–16 carbon atoms at the air/water interface are calculated. The model allows the calculation of the thermodynamic parameters (enthalpy, entropy and the Gibbs’ energy) of monomers, clusters and monolayers at the interface between two liquids or between the air and liquid phases. To obtain such results one needs to calculate the corresponding thermodynamic parameters for both adjacent phases within the semi-empirical PM3 method (in the frameworks of the COSMО procedure), then to calculate the increments from the different types of the pairwise interactions and, with the help of the additive scheme, to calculate the required parameters. It has been shown that 1-( n -alkyl)-imidazole forms monolayers with hexagonal and oblique unit cells normal conditions. The parameters of the unit cell are calculated for each of these monolayers: 4.6 Ǻ and 7.2 Ǻ (the angle γ between the sides of the unit cell is equal to 62°) for the hexagonal unit cells and 4.6 Ǻ and 4.6 Ǻ ( γ = 80°) for the oblique unit cells. Enthalpy, entropy and Gibbs’ energy for small clusters (dimers–hexamers), representing the structural elements of the investigated monolayers, are calculated. The probable way of the clusterization process is proposed on the basis of these parameters. In the framework of the model the thermodynamic functions of the monolayer formation are calculated. These results indicate that for both types of the monolayers (with oblique and hexagonal unit cells) spontaneous clusterization is possible when the alkyl chain length is ≥14–15 carbon atoms.

Supramolecular structures of halogenated oligothiophenes on the Si(111)-√3 ×√3-Ag surface

We have studied the adsorption of brominated tetrathienoanthracene (TBTTA) molecules onto the Si(111)-√3×√3-Ag surface at room temperature. The two-dimensional √3 silver adlayer acts to passivate the silicon surface and provides a high-mobility template for TBTTA adsorption. Scanning tunneling microscopy (STM) images reveal that at low coverage, the molecules readily migrate to step edges and defects in the √3 overlayer. With increasing coverage, the molecules eventually form compact supramolecular structures. In terms of the hexagonal √3 lattice vectors (a√3 and b√3), the oblique unit cell of these structures can be defined by lattice vectors am=3a√3+2b√3, and bm=−a√3+b√3. The structures are quite fragile and can decompose under repeated STM imaging. This is particularly true at higher bias and suggests an electric field-induced dissociation in these instances. With increasing molecular dose, the size and stability of the structures increases. At higher coverage, the spatial extent of the supramolecular structures is often limited by defects in the underlying √3 layer. Our results suggest that the √3-Ag surface provides a relatively inert substrate for the adsorption of TBTTA molecules, and that the supramolecular structures are held together by relatively weak intermolecular forces.

Crucial Role of the Double Bond Isomerism in the Steroid B-Ring on the Membrane Properties of Sterols. Grazing Incidence X-Ray Diffraction and Brewster Angle Microscopy Studies.

Three cholesterol precursors-desmosterol, zymosterol, and lanosterol-were comprehensively characterized in monolayers formed at the air/water interface. The studies were based on registration of the surface pressure (π)-area (A) isotherms complemented with in situ analysis performed with application of modern physicochemical techniques: grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy (BAM). In this approach we were interested in the correlation between molecular structures of the studied sterols found in the cholesterol biosynthetic pathway and their membrane properties. Our results revealed that only desmosterol behaves in Langmuir monolayers comparably to cholesterol, the molecules of which arrange in the monolayers into a hexagonal lattice, while the two remaining sterols possess extremely different properties. We found that molecules of both zymosterol and lanosterol are organized on the water surface in the two-dimensional oblique unit cells despite the fact that they are oriented perpendicular to the monolayer plane. The comparison of chemical structures of the investigated sterols leads to the conclusion that the only structural motive that can be responsible for such unusual behavior is the double bond in the B sterol ring, which is located in desmosterol in a different position from in the other two sterols. This issue, which was neglected in the scientific literature, seems to have crucial importance for sterol activity in biomembranes. We showed that this structural modification in sterol molecules is directly responsible for their adaptation to proper functioning in biomembranes.

Quantization of the Molecular Tilt Angle of Amphiphile Monolayers at the Air/Water Interface

It is shown on the basis of quantum chemical calculations that the tilt angle of the alkyl chains with respect to the interface in the amphiphilic monolayers changes discontinuously and has a sequence of discrete values depending on the structural peculiarities of the intermolecular CH···HC interactions realized between the monolayer molecules. This is caused by the fact that the geometric parameters of the alkyl chains for amphiphile monomers and their aggregates obtained using quantum chemical calculations virtually coincide for different amphiphile types and are determined by the mutual disposition of the molecules in the cluster. Two quantum numbers for the angles φ and δ of the amphiphile alkyl chains with respect to the normal to the p- and q-directions of the two-dimensional film propagation (corresponding to minimum Gibbs energy of clusterization) define the value of the polar tilt angle, t, of the monolayer molecules. Their specific values depend on the size of the hydrophilic part of the molecule. The electronic structure and geometric parameters of the dimers were examined for six amphiphile types having increasing size of the hydrophilic part in the following sequence: saturated alcohols, carboxylic acids, amides of carboxylic acids, α-hydroxycarboxylic acids, α-amino acids, and N-acyl-substituted alanine. Quantum chemical calculations of the geometric structure of the considered dimers showed that the tangent of φ and δ angles depends linearly on the number of the lost CH···HC interactions due to sterical hindrances occurring because of the orientation of voluminous hydrophilic parts of molecules in the monolayer. The values of the increment and the free term in the obtained linear dependence of tan φ or tan δ agree well with those derived using correlation analysis of the values of considered angles for optimized dimer structures of the amphiphile type in the framework of the program package Mopac2000 (PM3 method). The calculated values of the molecular tilt angle in the monolayer having oblique unit cells agree well with the existing experimental data obtained by using grazing incidence X-ray diffraction.

Unraveling theLiNbO3X-cut surface by atomic force microscopy and density functional theory

The ${\text{LiNbO}}_{3}$(2$\overline{1}\overline{1}0$) surface, commonly referred to as X-cut, is investigated by means of atomic force microscopy and first-principles calculations. Atomically resolved atomic force microscopy images show geometrical patterns not compatible with truncated bulk terminations. Fast Fourier transformation of the real-space images shows an oblique surface unit cell with lattice parameters of $a=0.75\ifmmode\pm\else\textpm\fi{}0.02$ nm, $b=0.54\ifmmode\pm\else\textpm\fi{}0.02$ nm, and $\ensuremath{\alpha}=94.{8}^{\ensuremath{\circ}}$. Comparing these experimental results with the theoretical models of stable surface terminations provides clear evidence for the formation of a -${\text{Li}}_{12}$ termination. The LN X-cut is, thus, characterized by a nonstoichiometric and Li-enriched composition. An analysis of the surface electronic charge indicates that the atomic force microscopy contrast is governed by the charge accumulation at the oxygen ions.

Structural Characterization of a Model Gram-Negative Bacterial Surface Using Lipopolysaccharides from Rough Strains of Escherichia coli

Lipopolysaccharides (LPS) make up approximately 75% of the Gram-negative bacterial outer membrane (OM) surface, but because of the complexity of the molecule, there are very few model OMs that include LPS. The LPS molecule consists of lipid A, which anchors the LPS within the OM, a core polysaccharide region, and a variable O-antigen polysaccharide chain. In this work we used RcLPS (consisting of lipid A plus the first seven sugars of the core polysaccharide) from a rough strain of Escherichia coli to form stable monolayers of LPS at the air–liquid interface. The vertical structure RcLPS monolayers were characterized using neutron and X-ray reflectometry, while the lateral structure was investigated using grazing incidence X-ray diffraction and Brewster angle microscopy. It was found that RcLPS monolayers at surface pressures of 20 mN m–1 and above are resolved as hydrocarbon tails, an inner headgroup, and an outer headgroup of polysaccharide with increasing solvation from tails to outer headgroups. The lateral organization of the hydrocarbon lipid chains displays an oblique hexagonal unit cell at all surface pressures, with only the chain tilt angle changing with surface pressure. This is in contrast to lipid A, which displays hexagonal or, above 20 mN m–1, distorted hexagonal packing. This work provides the first complete structural analysis of a realistic E. coli OM surface model.

Two-Dimensional Order in Mercury-Supported Langmuir Films of Fatty Diacids

The structure of mercury-supported Langmuir films of dicarboxylic acid molecules with 13 ≤ n ≤ 22 carbons is studied by X-ray methods and surface tensiometry. The molecules lie surface-parallel, forming mono-, bi-, or trilayers, depending on coverage. All films exhibit a full 2D order of the same single-molecule oblique unit cell. In particular, the distinct odd-even structure difference of 3D crystals of the same molecules is not observed. The unit cell's width and angle show a small systematic decrease with n, while the length increases commensurately with the molecular length. These results show the films to consist of closely packed, extended, polymer-like chains of diacid molecules, bound by their carboxyl end groups. Evidence is presented for the inclusion of a single mercury atom in the carboxyl-carboxyl bond. The possible conformation of this bond and implications of the parity-independent structure are discussed.

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule–substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.