Understanding Odd−Even Effects in Organic Self-Assembled Monolayers

Azaylide-based gemini amphiphiles displaying unique self-assembling behavior via an even-odd effect of alkyl linker chain length

Herein, we report a straightforward synthesis of azaylide-based gemini amphiphiles using bis(diphenylphosphino)alkanes via the Staudinger reaction. The prepared gemini amphiphiles exhibited an even-odd effect in their self-assembly behavior depending on the length of the alkyl linkers. Furthermore, the assembled micelles had high host capability toward hydrophobic guests in water.

Odd-Even Alkyl Chain Effects on the Structure and Charge Carrier Transport of Two-Dimensional Sn-Based Perovskite Semiconductors

Oscillations in the chemical or physical properties of materials, composed of an odd or even number of connected repeating methylene units, are a well-known phenomenon in organic chemistry and materials science. So far, such behavior has not been reported for the important class of materials, perovskite semiconductors. This work reports a distinct odd-even oscillation of the molecular structure and charge carrier transport properties of phenylalkylammonium two-dimensional (2D) Sn-based perovskites in which the alkyl chains in the phenylalkylammonium cations contain varying odd and even carbon numbers. Density functional theory calculations and grazing-incidence wide-angle X-ray scattering characterization reveal that perovskites with organic ligands containing an alkyl chain with an odd number of carbon atoms display a disordered crystal lattice and tilted inorganic octahedra accompanied by reduced mobilities. In contrast, perovskites with cations of an even number of carbon atoms in the alkyl chain form more ordered crystal structures, resulting in improved charge carrier mobilities. Our findings disclose the importance of minor changes in the molecular conformation of organic cations have an effect on morphology, photophysical properties, and charge carrier transport of 2D layered perovskites, showcasing alkyl chain engineering of organic cations to control key properties, of layered perovskite semiconductors.

Effects of chain-chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2-6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even-odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°-79°) and a smaller CH2 tilt angle (56°-68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain-chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain-chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.

Anodic voltage performance of conducting polymer-functionalized boron nitride nanosheets: a DFT assessment

The search for low-diffusion barriers and high-capacity anode materials is considered a key step in boosting the efficiency of metal-ion batteries. Herein, we investigate the impact of a series of conducting polymers (CPs), namely, polyacetylene (PA), polypyrrole (PP), poly-p-phenylene (PPPh), and polythiophene (PT), on enhancing the material design and anodic performance of boron nitride nanosheet (BNNS)-based Li-ion and Na-ion batteries. For this purpose, first principle DFT simulations, utilizing both clustered and periodic models, are systematically performed to assess the stability of such nanostructures and their electronic behavior as potential anodic materials. It is revealed that frontier molecular orbitals calculated for BNNSs are stabilized upon association with the series of CPs, resulting in a reduction in the energy gaps of CP-BNNSs by nearly 50%, which in turn improves the charge transfer properties and cell reaction kinetics. A remarkable improvement in the cell voltage is predicted for PP and PT functionalized BNNSs, reaching approximately 3.5 V for Li+ and 3.0 V for Na+ ions. The outcome of the study emphasizes the influence of the size of metal ions, whether mono- or di-valent, and the nature of adsorbed conducting polymers. Manipulating the electronic features of boron nitride nanostructured surfaces through non-covalent functionalization with conducting polymers could pave the way for the design of highly efficient energy storage anodic CP-BNNS-based systems.

Selective Cleavage of α-Olefins to Produce Acetylene and Hydrogen

Acetylene production from mixed α-olefins emerges as a potentially green and energy-efficient approach with significant scientific value in the selective cleavage of C-C bonds. On the Pd(100) surface, it is experimentally revealed that C2 to C4 α-olefins undergo selective thermal cleavage to form surface acetylene and hydrogen. The high selectivity toward acetylene is attributed to the 4-fold hollow sites which are adept at severing the terminal double bonds in α-olefins to produce acetylene. A challenge arises, however, because acetylene tends to stay at the Pd(100) surface. By using the surface alloying methodology with alien Au, the surface Pd d-band center has been successfully shifted away from the Fermi level to release surface-generated acetylene from α-olefins as a gaseous product. Our study actually provides a technological strategy to economically produce acetylene and hydrogen from α-olefins.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Static charge is an ionic molecular fragment

What is static charge? Despite the long history of research, the identity of static charge and mechanism by which static is generated by contact electrification are still unknown. Investigations are challenging due to the complexity of surfaces. This study involves the molecular-scale analysis of contact electrification using highly well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. The strong correlation between charge generation and molecular fragments due to their signature odd-even effects further shows that contact electrification is based on cleavage of covalent bonds and transfer of ionic molecular fragments. Static charge is thus an alkyl carbocation; in general, it is an ionic molecular fragment. This mechanism based on cleavage of covalent bonds is applicable to general types of insulating materials, such as covalently bonded polymers. The odd-even effect of charging caused by the difference of only one atom explains the highly sensitive nature of contact electrification.

Odd-even effects in aryl-substituted alkanethiolate SAMs: nonsymmetrical attachment of aryl unit and its impact on the SAM structure

Aryl-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) exhibit typically so-called odd-even effects, viz. systematic variations in the film structure, packing density, and molecular inclination depending on the parity of the number of the methylene units in the alkyl linker, n. As an exception to this rule, ATs carrying an anthracen-2-yl group (Ant-n) as tail group were reported to have different behavior due the non-symmetric attachment of the anthracene unit to the AT linker, providing additional degree of freedom for the molecular organization and allowing for partial compensation of the odd-even effects. In this context, the structure of SAMs formed by adsorption of anthracene-substituted ATs (Ant-n; n = 1-6) at room temperature on Au(111) substrate was investigated by high-resolution scanning tunnelling microscopy (STM). Most of these SAMs exhibit a coexistence of two different ordered phases, some of which are common for either n = odd or n = even while other vary over the series, showing a broad variety of different structures. The average packing density of the Ant-n SAMs, derived from the analysis of the STM data, varies by 7.5-10% depending on the parity of n, being, as expected, higher for n = odd. The respective extent of the odd-even effects is noticeably lower than that usually observed for other aryl-substituted monolayers (∼25%), which goes in line with the previous findings and emphasizes the impact of the non-symmetric attachment of the aromatic unit.

Side group dependent room temperature crystallization-induced phosphorescence of benzil based all organic phosphors

In this work, we report alkoxy substituted benzil based all organic room temperature phosphors which showed crystallization induced phosphorescence (CIP). Nine title compounds were prepared with various alkyl lengths (OCnH2n+1: n = 8-16) and the effect of alkyl side group length on the phosphorescence performance was investigated, as compared to p-anisil. It was found that both phosphorescence quantum yield and lifetime increased concomitantly as the alkyl length increased up to nonyloxy (BZL-OC9). Further increase in the carbon number caused the phosphorescence performance to deteriorate due to greater conformational freedom of the side groups. An incredible quantum yield of 70% was achieved for BZL-OC9. A promising finding is that the increased quantum yield was accompanied by the increase in the lifetime relative to p-anisil, which has been historically challenging. Single crystallography coupled with UV-Vis spectroscopy revealed that a higher level of intermolecular π-π interactions was observed from p-anisil while more alkyl interactions with less intermolecular π-orbital overlap were found for BZL-OC8. As a result, molecular rigidification with less phosphorescence quenching was achieved for BZL-OC8 leading to enhanced performance. A precipitation study on a dichloromethane solution as a function of the content of MeOH (poor solvent) proved that the emission of the BZL-OCn system is truly aggregation-induced. The current work demonstrates that strategic side group engineering could be a promising approach to developing high-performance all organic phosphors as well as improving the properties of existing phosphors.

Protein Adsorption on Mixed Self-Assembled Monolayers: Influence of Chain Length and Terminal Group

Mixed self-assembled monolayers (SAMs) are often used as highly tunable substrates for biomedical and biosensing applications. It is well documented, however, that mixed SAMs can be highly disordered at the molecular level and do not pack as closely or homogeneously as single-component SAMs, particularly when the chain lengths and head groups of the SAM thiol components are significantly different. In this study, we explore the impact of SAM structure and mixing ratio (-OH and -CH3 termini) on the weak physisorption behavior of bovine serum albumin (BSA), which adsorbs more readily to hydrophobic, methyl-terminated SAMs. Our results suggest that once the mixture includes 50% or more of the methyl terminus, the mixing ratio alone is a relatively good predictor of adsorption, regardless of the relative chain lengths of the thiols used in the mixture. This trend persists at any mixing ratio for SAMs where methyl- and hydroxyl-terminated groups are the same length or where the hydroxyl-terminated thiol is longer. The only variance observed is at low mixing ratios (<50% methyl-terminated) for a mixed SAM where the methyl-terminated component has a longer chain length. Relative protein adsorption increases on these mixtures, perhaps due to the disordered exposure of the excess alkane backbone. Taken together, however, we do not find significant evidence that varying chain lengths for mixed SAMs prepared on polycrystalline substrates and analyzed in air have an outsized influence on nanoscopic adsorption behavior, despite molecular-level disorder in the SAM itself.

Pathologic polyglutamine aggregation begins with a self-poisoning polymer crystal

A long-standing goal of amyloid research has been to characterize the structural basis of the rate-determining nucleating event. However, the ephemeral nature of nucleation has made this goal unachievable with existing biochemistry, structural biology, and computational approaches. Here, we addressed that limitation for polyglutamine (polyQ), a polypeptide sequence that causes Huntington's and other amyloid-associated neurodegenerative diseases when its length exceeds a characteristic threshold. To identify essential features of the polyQ amyloid nucleus, we used a direct intracellular reporter of self-association to quantify frequencies of amyloid appearance as a function of concentration, conformational templates, and rational polyQ sequence permutations. We found that nucleation of pathologically expanded polyQ involves segments of three glutamine (Q) residues at every other position. We demonstrate using molecular simulations that this pattern encodes a four-stranded steric zipper with interdigitated Q side chains. Once formed, the zipper poisoned its own growth by engaging naive polypeptides on orthogonal faces, in a fashion characteristic of polymer crystals with intramolecular nuclei. We further show that self-poisoning can be exploited to block amyloid formation, by genetically oligomerizing polyQ prior to nucleation. By uncovering the physical nature of the rate-limiting event for polyQ aggregation in cells, our findings elucidate the molecular etiology of polyQ diseases.

Degradable and Recyclable Polyesters from Multiple Chain Length Bio- and Waste-Sourceable Monomers

Monomers sourced from waste or biomass are often mixtures of different chain lengths; e.g. catalytic oxidation of polyethylene waste yields mixtures of dicarboxylic acids (DCAs). Yet, polyesters synthesized from such monomer mixtures have rarely been studied. We report polyesters based on multiple linear aliphatic DCAs, present in chain length distributions that vary in their centers and ranges. We demonstrate that these materials can adopt high-density polyethylene-like solid state structures, and are ductile (e.g. Et 610 MPa), allowing for injection molding, or film and fiber extrusion. Melting and crystallization points of the polyesters show no odd-even effects as dipoles cannot favorably align in the crystal, similar to traditional odd carbon numbered, long-chain DCA polyesters. Biodegradation studies of 13 C-labelled polyesters in soil reveal rapid mineralization, and depolymerization by methanolysis indicates suitability for closed-loop recycling.

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

XFEL Microcrystallography of Self-Assembling Silver n-Alkanethiolates

New synthetic hybrid materials and their increasing complexity have placed growing demands on crystal growth for single-crystal X-ray diffraction analysis. Unfortunately, not all chemical systems are conducive to the isolation of single crystals for traditional characterization. Here, small-molecule serial femtosecond crystallography (smSFX) at atomic resolution (0.833 Å) is employed to characterize microcrystalline silver n-alkanethiolates with various alkyl chain lengths at X-ray free electron laser facilities, resolving long-standing controversies regarding the atomic connectivity and odd-even effects of layer stacking. smSFX provides high-quality crystal structures directly from the powder of the true unknowns, a capability that is particularly useful for systems having notoriously small or defective crystals. We present crystal structures of silver n-butanethiolate (C4), silver n-hexanethiolate (C6), and silver n-nonanethiolate (C9). We show that an odd-even effect originates from the orientation of the terminal methyl group and its role in packing efficiency. We also propose a secondary odd-even effect involving multiple mosaic blocks in the crystals containing even-numbered chains, identified by selected-area electron diffraction measurements. We conclude with a discussion of the merits of the synthetic preparation for the preparation of microdiffraction specimens and compare the long-range order in these crystals to that of self-assembled monolayers.

Two-dimensional self-assemblies of azobenzene derivatives: effects of methyl substitution of azobenzene core and alkyl chain length

Elucidating the correlation between the molecular arrangement and physical properties of organic compounds is critical to facilitating the development of advanced functional materials. X-ray structural analyses are generally performed to clarify this relationship. Several attempts have been made to ascertain the links between three-dimensional (3D) crystals and their two-dimensional (2D) structures, which can be revealed by scanning tunnelling microscopy (STM) at the molecular level. Thus, 2D self-assemblies of a series of azobenzene derivatives were investigated in this study, and the effects of methyl substitution of the azobenzene core and alkyl chain length on the 2D molecular arrangements at the solid/liquid interface were revealed. Three types of azobenzene derivatives were prepared; these contained azobenzene (Az), 3-methyl azobenzene (MAz), or 3,3'-dimethyl azobenzene (DAz) as cores and alkyloxy chains of different lengths (C8-13) at their 4,4' positions. The 2D structures of the Az and DAz compounds were found to be modulated owing to the odd-even effect of the alkyl chains in a specific chain-length range; this effect was only weakly exhibited by the MAz compounds. This result suggests that only the methyl-group substitution of the azobenzene core significantly affected the 2D structures. The 2D structural features have been discussed in terms of molecular conformation, as well as their correlation with the photo-melting behaviour of the azobenzene derivatives, particularly the MAz compounds.

Connecting the Phase State and Volatility of Dicarboxylic Acids at Elevated Temperatures

The partitioning of semivolatile organic molecules between condensed phases and the vapor phase has broad application across a range of scientific disciplines, with significant impacts in atmospheric chemistry for regulating the evolving composition of aerosol particles. Vapor partitioning depends on the molecular interactions and phase state of the condensed material and shows a well-established dependence on temperature. The phase state of solid organic material is not always well-defined, and many examples can be found for the formation of amorphous subcooled liquid states rather than crystalline solids. This can lead to significant changes to vapor equilibrium processes by modifying the thermodynamics and kinetics of evaporation. Here, we explore the influence of phase state on the evaporation dynamics of a series of straight-chain dicarboxylic acids across a range of above-ambient temperatures. These molecules show an odd/even alteration in some of their properties based on the number of carbon atoms that may be connected to their phase state under dry conditions. Using a newly developed linear-quadrupole electrodynamic balance, we levitate single particles containing the sample and expose them to dry conditions across a range of temperatures (ambient to ∼350 K). Using the rate of evaporation measured from the change in the size or relative mass, we derive the vapor pressure and enthalpy of vaporization. Light scattering data allows for unambiguous identification of the phase of the particles (crystal vs amorphous) allowing the vapor equilibrium properties to be attributed to a particular state. This work highlights a new experimental method for characterizing vapor pressures of low volatility substances and extends the temperature range of available data for the vapor pressure of terminal dicarboxylic acids. These measurements show that crystalline and subcooled liquid states persist at elevated temperatures and provide a direct comparison between subcooled and crystal phases under the same experimental conditions.

Turning Nonselective Inhibitors of Type I Protein Arginine Methyltransferases into Potent and Selective Inhibitors of Protein Arginine Methyltransferase 4 through a Deconstruction-Reconstruction and Fragment-Growing Approach

Protein arginine methyltransferases (PRMTs) are important therapeutic targets, playing a crucial role in the regulation of many cellular processes and being linked to many diseases. Yet, there is still much to be understood regarding their functions and the biological pathways in which they are involved, as well as on the structural requirements that could drive the development of selective modulators of PRMT activity. Here we report a deconstruction–reconstruction approach that, starting from a series of type I PRMT inhibitors previously identified by us, allowed for the identification of potent and selective inhibitors of PRMT4, which regardless of the low cell permeability show an evident reduction of arginine methylation levels in MCF7 cells and a marked reduction of proliferation. We also report crystal structures with various PRMTs supporting the observed specificity and selectivity.

Entropy of Branching Out: Linear versus Branched Alkylthiols Ligands on CdSe Nanocrystals

Surface ligands of semiconductor nanocrystals (NCs) play key roles in determining their colloidal stability and physicochemical properties and are thus enablers also for the NCs flexible manipulation toward numerous applications. Attention is usually paid to the ligand binding group, while the impact of the ligand chain backbone structure is less discussed. Using isothermal titration calorimetry (ITC), we studied the effect of structural changes in the ligand chain on the thermodynamics of the exchange reaction for oleate coated CdSe NCs, comparing linear and branched alkylthiols. The investigated alkylthiol ligands differed in their backbone length, branching position, and branching group length. Compared to linear ligands, lower exothermicity and entropy loss were observed for an exchange with branched ligands, due to steric hindrance in ligand packing, thereby justifying their previous classification as "entropic ligands". Mean-field calculations for ligand binding demonstrate the contribution to the overall entropy originating from ligand conformational entropy, which is diminished upon binding mainly by packing of NC-bound ligands. Model calculations and the experimental ITC data both point to an interplay between the branching position and the backbone length in determining the entropic nature of the branched ligand. Our findings suggest that the most entropic ligand should be a short, branched ligand with short branching group located toward the middle of the ligand chain. The insights provided by this work also contribute to a future smarter NC surface design, which is an essential tool for their implementation in diverse applications.

Removal of Thiol-SAM on a Gold Surface for Re-Use of an Interdigitated Chain-Shaped Electrode

The self-assembled monolayer (SAM) is the most common organic assembly utilized for the formation of the monolayers of alkane-thiolates on gold electrode, resulting in a wide range of applications for the modified SAM on gold in various research areas. This study examined the desorption of a SAM that was developed on the gold surface of an interdigitated chain-shaped electrode (the ICE, a unique electrode design, was fabricated by our group) with the goal of determining the most efficient strategy of SAM removal for the ICE to be re-used. A simple and proficient solution-based cleaning procedure was applied for the removal of a SAM on the gold surface of the ICE by using a sodium borohydride solution within short-term treatment, resulting in efficiency for the recovery of the originally electrochemical characteristic of ICE of 90.3%. The re-use of ICE after the removal process was confirmed by the successful re-deposition of a SAM onto the electrode surface, resulting in the high efficiency percentage of 90.1% for the reusability of ICE with the SAM modification. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used as tools to investigate the changes in the electrode interface at each stage of the SAM removal and the electrode recycling. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were employed, being powerful spectrum techniques, for the characterization of the bonding structure and chemical state of the bare ICE and the modified ICE at each treatment step. Based on the comprehensive discussion of analytical chemistry from the obtained EIS and CV data in this study, we confirmed and proved the effectiveness of this promising method for the removal of a SAM from the ICE and the re-use of ICE in the field of material deposition, with the aims of saving money, improving experimental handling, and protecting the environment.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: Effect of the alkyl chain substitution position

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronics and molecular devices. In this study, we investigated the effect...

The impact of the length of alkyl chain on the behavior of benzyl alcohol homologues - the interplay between dispersive and hydrogen bond interactions

In this work, we examined the effect of the length of alkyl chain attached to the benzene ring on the self-assembling phenomena for a series of phenyl alcohol (PhA) derivatives, from phenylmethanol (benzyl alcohol) to 7-phenyl-1-heptanol, by means of X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, and Broadband Dielectric Spectroscopy (BDS) methods. XRD data in the reciprocal and real spaces showed a gradual increase in the local order with the elongation of the alkyl chain. However, the position and full width at half maximum of the main diffraction peak exhibited a non-systematic behavior. To better understand this fact, PhAs were subjected to FTIR spectroscopic studies. These investigations revealed that the association degree and the activation energy of dissociation increase as the alkyl chain length grows. On the other hand, BDS data showed a non-monotonic variation in the Kirkwood correlation factor with increasing length of the alkyl chain, indicating a competition between interactions of the non-polar and polar parts of the molecules in the studied PhAs. Finally, it was also found that the molar surface entropy for PhAs increases with the number of methylene groups, approaching values reported for alkanes, which indicates suppression of the surface order for PhAs with a long alkyl chain. This variability of the various parameters as a function of the length of the side chain shows that the interplay between soft interactions has a strong impact on the local structure and intra and intermolecular dynamics of the studied PhAs.

Mesomorphic Behavior of Symmetric Azomethine Dimers Containing Different Chromophore Groups

A series of new azomethine dimers was synthesized by the condensation reaction of flexible bis-benzaldehydes with four aromatic amines containing phenyl, naphthyl, anthracene and pyrene groups. Their right structure was confirmed by FTIR and ¹H-NMR spectroscopy and their thermal properties were investigated by thermogravimetric analysis, differential scanning calorimetry and polarized light optical microscopy. A view on their photophysical behavior was gained by UV-vis and photoluminescence spectroscopy. The dimers containing pyrene and anthracene presented liquid crystalline behavior, while the other dimers were crystalline compounds. Two dimers containing pyrene moieties formed mesomorphic glasses and had intense luminescence, promising properties for applications in building optoelectronic devices.

Heat Conduction Performance over a Poly(ethylene glycol) Self-Assembled Monolayer/Water Interface: A Molecular Dynamics Study

This study examined the interfacial heat condition between a poly(ethylene glycol) (PEG) self-assembled monolayer (SAM) and water using molecular dynamics simulation. It was found that the PEG SAM has higher thermal boundary conductance (TBC) than the traditionally used alkane-based SAM. The TBC conditionally varied with the length of the PEG molecules, where interfacial thermal resistance was a key factor. Our results reveal that the TBC of the PEG SAM/water interface is greatly influenced by its structural properties rather than the matching of vibrational properties between the SAM terminal and water. The structural analysis shows that the water structure around the terminal oxygen atom of the SAM plays a vital role in controlling the TBC. In this study, the concept of free volume has also been exploited, and the result suggests that the reduction of the free volume fraction accommodates a higher TBC. The model was precisely validated against experimental data by calculating the tilt angle and dihedral angle of the PEG SAM, the persistence length of the PEG chain in the water medium, and the sulfur position of the PEG SAM headgroup on the gold surface using quantitative scanning transmission electron microscopy image simulation.

Structure-Property Relationships in Bithiophenes with Hydrogen-Bonded Substituents

The use of crystal engineering to control the supramolecular arrangement of π-conjugated molecules in the solid-state is of considerable interest for the development of novel organic electronic materials. In this study, we investigated the effect of combining of two types of supramolecular interaction with different geometric requirements, amide hydrogen bonding and π-interactions, on the π-overlap between calamitic π-conjugated cores. To this end, we prepared two series of bithiophene diesters and diamides with methylene, ethylene, or propylene spacers between the bithiophene core and the functional groups in their terminal substituents. The hydrogen-bonded bithiophene diamides showed significantly denser packing of the bithiophene cores than the diesters and other known α,ω-disubstituted bithiophenes. The bithiophene packing density reach a maximum in the bithiophene diamide with an ethylene spacer, which had the smallest longitudinal bithiophene displacement and infinite 1D arrays of electronically conjugated, parallel, and almost linear N-H⋅⋅⋅O=C hydrogen bonds. The synergistic hydrogen bonding and π-interactions were attributed to the favorable conformation mechanics of the ethylene spacer and resulted in H-type spectroscopic aggregates in solid-state absorption spectroscopy. These results demonstrate that the optoelectronic properties of π-conjugated materials in the solid-state may be tailored systematically by side-chain engineering, and hence that this approach has significant potential for the design of organic and polymer semiconductors.

The Missing Link in the Magnetism of Hybrid Cobalt Layered Hydroxides: The Odd-Even Effect of the Organic Spacer

A dramatic change in the magnetic behaviour, which solely depends on the parity of the organic linker molecules, has been found in a family of layered Co^II hydroxides covalently functionalized with dicarboxylic molecules. These layered hybrid materials have been synthesized at room temperature using a one-pot procedure through the epoxide route. While hybrids connected by odd alkyl chains exhibit coercive fields (H_c ) below ca. 3500 Oe and show spontaneous magnetization at temperatures (T_M ) below 20 K, hybrids functionalized with even alkyl chains behave as hard magnets with H_c >5500 Oe and display a T_M higher than 55 K. This intriguing behaviour was studied by density functional theory with the incorporation of a Hubbard term (DFT+U) calculations, unveiling the structural subtleties underlying this observation. Indeed, the different molecular orientation exhibited by the even/odd alkyl chains, and the orientation of the covalently linked carboxylic groups modify the intensity of the magnetic coupling of both octahedral and tetrahedral in-plane sublattices, thus strongly affecting the magnetic properties of the hybrid. These findings offer an outstanding level of tuning in the molecular design of hybrid magnetic materials based on layered hydroxides.

Olefin-Bridged Bidentate Adsorbates for Generating Self-Assembled Monolayers on Gold

A series of custom-designed olefin-bridged bidentate adsorbates composed of an olefin group linking symmetrical hydrocarbon moieties of varying chain lengths was synthesized and used for the preparation of self-assembled monolayers (SAMs) on gold. The structures of the adsorbates are in the form Z-[CH₃(CH₂)_m]₂(C═C)[CH₂SH]₂ (OBCnSH) where m = 12-15 and n = m + 3 (OBC15SH, OBC16SH, OBC17SH, and OBC18SH). The influence of the olefin linker on the structural and interfacial properties of the SAMs was investigated and compared to SAMs formed from analogous n-alkanethiols. Characterization techniques included ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation-infrared reflection-adsorption spectroscopy (PM-IRRAS), and contact angle measurements. The OBCnSH SAMs exhibited ellipsometric thicknesses that were similar to their monodentate counterparts, suggesting that the new olefin-bridged adsorbates pack similarly to the monodentate analogs. Characterization by PM-IRRAS revealed that the OBCnSH SAMs were as conformationally ordered as those derived from the reference n-alkanethiols with the exception of the adsorbate with the shortest chain length OBC15SH, which exhibited low coverage and a liquid-like structure. Unlike the SAMs derived from the n-alkanethiols, the OBCnSH SAMs failed to exhibit "odd-even" effects. However, the OBCnSH SAMs displayed similar hexadecane contact angles as their n-alkanethiol counterparts with the exception of OBC15SH, which exhibited markedly diminished hexadecane contact angles. The similar structural and interfacial properties of the OBCnSH SAMs, when compared to analogous n-alkanethiol SAMs, render the molecular architecture of the olefin-bridged dithiol as a robust platform for the synthesis of adsorbates with two chemically distinct tailgroups for use in the preparation and study of phase-incompatible "conflicted" interfaces.

Effects of Coformer and Polymer on Particle Surface Solution-Mediated Phase Transformation of Cocrystals in Aqueous Media

The purpose of the present study was to investigate the effect of the coformer difference on particle surface solution-mediated phase transformation (PS-SMPT) during cocrystal particle dissolution in aqueous media in the absence and presence of polymers. SMPT can occur either in the bulk phase or at the particle surface because drug molecules can be supersaturated at the dissolving cocrystal surface, as well as in the bulk phase. Previously, bulk phase SMPT has been primarily investigated in formulation development. However, little is known about the effects of coformers and polymers on PS-SMPT of cocrystals. In this study, six carbamazepine (CBZ) cocrystals were used as model cocrystals (malonic acid (MAL), succinic acid (SUC), glutaric acid (GLA), adipic acid (ADP), saccharin (SAC), and nicotinamide (NCT); nonsink dissolution tests were performed with or without a precipitation inhibitor (hydroxypropyl methylcellulose (HPMC)) at pH 6.5. The residual particles were analyzed by powder X-ray diffraction, differential scanning calorimetry, polarized light microscopy (PLM), and scanning electron microscopy. Real-time PLM was used to directly observe rapid PS-SMPT. In the absence of HPMC, supersaturation was not observed in the bulk phase for all cocrystals. All cocrystals rapidly transformed to CBZ dihydrate aggregates via PS-SMPT (mostly within 1 min). In contrast, in the presence of 0.1% HPMC, supersaturation was observed for CBZ-SUC, CBZ-ADP, CBZ-SAC, and CBZ-NCT but not for CBZ-MAL and CBZ-GLA. The cocrystals with lower solubility coformers tended to induce higher supersaturation in the bulk phase. The PS-SMPT of CBZ-SUC, CBZ-ADP, and CBZ-SAC was slowed down by HPMC. By suppressing PS-SMPT, the cocrystals exhibited its supersaturation potential, depending on the properties of each coformer. To take advantage of the supersaturation potential of cocrystals to improve oral drug absorption, it is important to suppress particle surface SMPT in addition to bulk phase SMPT.

Molecular Origin of the Odd-Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH₃(CH₂)_nSAu, n = 6-19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level, by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit dependencies on both the length (n) and parity (n_odd or n_even) of the polymethylene chain. The peak positions of the CH₂ stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of n_odd have a greater degree of structural gauche defects than SAMs of n_even. The peak intensities and positions of the CH₃ stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH₃. The correlations between the different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects on the electrical characteristics of tunnel junctions.

Odd-even alternations in helical propensity of a homologous series of hydrocarbons

Odd and even homologues of some n-alkane-based systems are known to exhibit notably different trends in solid-state properties; a well-known illustration is the zigzag plot of their melting point versus chain length. Odd-even effects in the solid state often arise from intermolecular interactions that involve fully extended molecules. These effects have also been observed in less condensed phases, such as self-assembled monolayers; however, the origins of these effects in such systems can be difficult to determine. Here we combined NMR and computational analysis to show that all-syn contiguously methyl-substituted hydrocarbons, with chain lengths from C6 to C11, exhibit a dramatic odd-even effect in helical propensity. The even- and odd-numbered hydrocarbons populate regular and less-controlled helical conformations, respectively. This knowledge will guide the design of helical hydrocarbons as rigid scaffolds or as hydrophobic components in soft materials.

Annealing effect for self-assembled monolayers formed from terphenylethanethiol on Au(111)

Formation of several different structural phases and desorption took place from a standing-up phase at an annealing temperature of 473 K.

Controlling Supramolecular Chirality in Peptide-π-Peptide Networks by Variation of the Alkyl Spacer Length

Self-assembled supramolecular organic materials with π-functionalities are of great interest because of their applications as biocompatible nanoelectronics. A detailed understanding of molecular parameters to modulate the formation of hierarchical structures can inform design principles for materials with engineered optical and electronic properties. In this work, we combine molecular-level characterization techniques with all-atom molecular simulations to investigate the subtle relationship between the chemical structure of peptide-π-peptide molecules and the emergent supramolecular chirality of their spontaneously self-assembled nanoaggregates. We demonstrate through circular dichroism measurements that we can modulate the chirality by incorporating alkyl spacers of various lengths in between the peptides and thienylene-phenylene π-system chromophores: even numbers of alkyl carbons in the spacer units (0, 2) induce M-type helical character whereas odd numbers (1, 3) induce P-type. Corroborating molecular dynamics simulations and explicating machine learning analysis techniques identify hydrogen bonding and hydrophobic packing to be the principal discriminants of the observed chirality switches. Our results present a molecular-level design rule to engineer chirality into optically and electronically active nanoaggregates of these peptidic building blocks by exploiting systematic variations in the alkyl spacer length.

bola-Type Ala-Ala Dipeptides: Odd-Even Effect in Molecular Packing Structures

A series of bola-type Ala-Ala dipeptides with different alkylene bridges (n = 10-15) were synthesized. In methanol, the molecules with even-numbered carbon bridges self-assembled into twisted nanoribbons, while those with odd-numbered carbon bridges self-assembled into straight belts. The morphology displays a pronounced odd-even dependence upon the number of carbons (n) in the connecting alkylene bridge. The circular dichroism spectra of the self-assemblies showed that molecules with even- and odd-numbered carbon bridges stacked in different structures. FT-IR spectra indicated that the dipeptides with even-numbered carbon bridges formed hydrogen bonds between the amide group and carboxyl ester group, while those with odd-numbered carbon bridges formed hydrogen bonds only between the amide groups. X-ray diffraction patterns revealed that molecules with odd- and even-numbered carbon bridges stacked in monoclinic and triclinic structures, respectively.

A simple vaporous probe with atomic-scale sensitivity to structural ordering and orientation of molecular assembly

Understanding the structural ordering and orientation of interfacial molecular assemblies requires an insight into the penetration depth of the probe molecules which determines the interfacial reactivity. In contrast to the conventional liquid probe-based contact angle measurement in which penetration depth is complicated by the liquid cohesive interaction, we report here a new approach that features a simple combination of vaporous hexane, which involves only van der Waals interaction, and quartz crystal microbalance operated at the third harmonic resonance, which is sensitive to sub-monolayer (0.2%) adsorption. Using this combination, we demonstrated the ability of probing the structural ordering and orientation of the self-assembled monolayers with a sensitivity from penetrating the top portion of the monolayers to interacting with the very top atomic structure at the interface. The determination of the dependence of the adsorption energy of vaporous hexane on the penetration depth in the molecular assembly allowed us to further reveal the atomic-scale origin of the odd-even oscillation, which is also substantiated by density functional theory calculations. The findings have broader implications for designing interfacial reactivities of molecular assemblies with atomic-scale depth precision.

Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

The odd-even effect in luminescent [Eu₂ (L)₃ (H₂ O)_x ]⋅y(H₂ O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2-6 and y=0-4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand-metal coordination via the mixed mode of bridge-chelate coordination, except for the Eu³⁺ -oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd-even effect on the 4 f-4 f luminescence intensity parameters (Ω₂ and Ω₄ ) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( α * ) were simulated by a ghost atom located at the middle of each ligand chain. The values of α * were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( Q L n L n ) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.

Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH₂)_nBPh (BPh = biphenyl, n = 1-7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle of the alkyl chain is fixed at 30° and independent of n. For values of n = 1-3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on self-assembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

The supramolecular structure and van der Waals interactions affect the electronic structure of ferrocenyl-alkanethiolate SAMs on gold and silver electrodes

Understanding the influence of structural properties on the electronic structure will pave the way for optimization of charge transport properties of SAM devices. In this study, we systematically investigate the supramolecular and electronic structures of ferrocene (Fc) terminated alkanethiolate (SC _n Fc) SAMs on both Au and Ag substrates with n = 1-15 by using a combination of synchrotron based near edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), and density functional theory (DFT) calculations. Odd-even effects in the supramolecular structure persist over the entire range of n = 1-15, which, in turn, explain the odd-even effects in the onset energy of the highest occupied molecular (HOMO) orbital. The orientation of the Fc moieties and the strength of Fc-substrate coupling, which both depend on n, affects the work function (WF). The variation of WF shows an odd-even effect in the weak electrode-Fc coupling regime for n ≥ 8, whereas the odd-even effect diminishes for n < 8 due to hybridization between Fc and the electrode (n < 3) or van der Waals (vdW) interactions between Fc and the electrode (n = 3-7). These results confirm that subtle changes in the supramolecular structure of the SAMs cause significant electronic changes that have a large influence on device properties.

Polypentenamer Renaissance: Challenges and Opportunities

This Viewpoint highlights the viability and increasing variety of functionalized polypentenamers as unique and valuable materials created through enthalpy-driven ring-opening metathesis polymerization (ROMP) of low ring strain cyclopentene monomers. The terms "low ring strain" and "enthalpy-driven" are typically conflicting ideologies for successful ROMP; however, these monomers possess a heightened sensitivity to reaction conditions, which may be leveraged in a number of ways to provide performance elastomers with good yield and precise functional topologies. Over the last several years, a rekindled interest in these systems has led to a renaissance of research aimed at improving their synthesis and exploring their potential. Their chemistry, applications, and future outlook are discussed.

Photoisomerization of azobenzene-substituted alkanethiolates on Au(111) substrates in the context of work function variation: the effect of structure and packing density

Photoisomerization of a series of custom-designed, azobenzene-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) substrates was studied in the context of work function variation, using Kelvin probe measurements as a transduction technique. These SAMs featured variable packing density (by ∼14%; due to the odd-even effects) and, as an option, were additionally decorated with the electron donating/withdrawing -CH3 and -CF3 tail group, respectively, which induce additional dipole moments. The efficiency of photoisomerization and the respective extent of work function variation (ΔΦ) were found to be quite low and independent of the packing density in the SAMs, within the given odd-even packing density variation. They could only be increased, up to ca. 40 meV for ΔΦ, by mixing the azobenzene-substituted ATs with shorter "matrix" molecules, which were introduced for a partial release of the sterical constraints. The ΔΦ values for the SAMs decorated with the -CH3 and -CF3 tail groups were found to be lower than those for the monolayers without such a decoration, which correlated well with the theoretical estimates for the change of the dipole moment of the relevant molecules upon the photoisomerization.

Computational and experimental approach to understanding the structural interplay of self-assembled end-terminated alkanethiolates on gold surfaces

Molecular organization dictates phases, stability and subsequent electronic structure of self-assembled monolayers. With appropriate density functionals, ab initio molecular dynamics (AIMD) simulations predicted and elucidated experimental orientations.

A perspective on the magnetism of alkanethiol-coated gold thin films

Intriguing ferromagnetic behaviour has been reported in gold thin films — a diamagnetic material in the bulk — wherein large magnetic moments and uncommon anisotropy are often hallmark features. The tuning of the electronic and magnetic properties by the presence of molecular self-assembled monolayers has been proposed. In this work, we present the study of the magnetism of a wide collection of alkanethiols of differing chain lengths coated on Au. We find no or only very weak magnetism, casting doubt on the universality and reproducibility of this phenomenon.