Organic Molecular Films Research Articles

Discovery of Crystallizable Organic Semiconductors with Machine Learning.

Crystalline organic semiconductors are known to have improved charge carrier mobility and exciton diffusion length in comparison to their amorphous counterparts. Certain organic molecular thin films can be transitioned from initially prepared amorphous layers to large-scale crystalline films via abrupt thermal annealing. Ideally, these films crystallize as platelets with long-range-ordered domains on the scale of tens to hundreds of microns. However, other organic molecular thin films may instead crystallize as spherulites or resist crystallization entirely. Organic molecules that have the capability of transforming into a platelet morphology feature both high melting point (Tm) and crystallization driving force (ΔGc). In this work, we employed machine learning (ML) to identify candidate organic materials with the potential to crystallize into platelets by estimating the aforementioned thermal properties. Six organic molecules identified by the ML algorithm were experimentally evaluated; three crystallized as platelets, one crystallized as a spherulite, and two resisted thin film crystallization. These results demonstrate a successful application of ML in the scope of predicting thermal properties of organic molecules and reinforce the principles of Tm and ΔGc as metrics that aid in predicting the crystallization behavior of organic thin films.

Creation of Quasi‐DNA Origami at the Air/Water Interface Using the Collapsed Mechanism of Langmuir Monolayer

AbstractIt has achieved the preparation of quasi‐deoxyribonucleic acid (DNA) origami using the techniques of organized molecular films. After the adsorption of DNA from the subphase onto a Langmuir monolayer of polymers containing diamino‐s‐triazine rings, the monolayer collapses owing to high surface pressure, resulting in the formation of a three‐dimensional folded structure of DNA. A homopolymer featuring a diamino‐s‐triazine ring and a copolymer containing 20 % long alkyl chains are used to form the Langmuir monolayer. A‐T base pairs of salmon testis‐derived DNA from the subphase are hydrogen‐bonded and adsorbed onto each Langmuir monolayer. Following the collapse of the Langmuir monolayer, the film height almost doubles, indicating the formation of a bilayer structure consisting of a bioadsorbed polymer monolayer, especially evident in copolymer‐collapsed films. During this period, the bioconjugated bilayer structure adopts a mesoscopic linear morphology within the plane. In this quasi‐DNA origami structure, the chirality in the DNA molecule is maintained, as evidenced by the circular dichroism (CD) spectra displaying a positive Cotton effect. These results correspond to the proposal of a new DNA morphology control technology.

Photoluminescence lifetime engineering via organic resonant films with molecular aggregates

Abstract Manipulating the spontaneous emission rate of fluorophores is vital in creating bright incoherent illumination for optical sensing and imaging, as well as fast single-photon sources for quantum technology applications. This can be done via increasing the Purcell effect by using non-monolithic optical nanocavities; however, achieving the desired performance is challenging due to difficulties in fabrication, precise positioning, and frequency tuning of cavity-emitter coupling. Here, we demonstrate a simple approach to achieve a wavelength-dependent photoluminescence (PL) lifetime modification using monolithic organic molecular aggregates films. These single monolithic organic films are designed to have a Lorentzian dispersion, including epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) spectral regions with increased and decreased photonic density of states, respectively. This dispersion leads to enhanced and depressed PL decay rates at different wavelengths. Both time-resolved photoluminescence (TRPL) and fluorescence lifetime imaging microscopy (FLIM) measurements are implemented to verify the validity of this approach. This approach offers a promising way to design dual-functional optical sources for a variety of applications, including bioimaging, sensing, data communications, and quantum photonics applications.

Interfacial electronic properties of hybrid interfaces

A detailed understanding of interfacial electronic properties of organic–inorganic hybrid junctions is crucial for their technological applications. In case of organic–inorganic hybrid junctions information about interfacial charge transfer, band bending and work function is crucial for efficient charge carrier movement across interfaces. In this context, investigation of filled and empty starts near Fermi level is relevant in molecular electronics to know electron affinity, and charge injection barriers. A multi-technique approach using ultraviolet photoemission and Inverse photoemission spectroscopy provides quantitative information about frontier occupied and unoccupied states near Fermi level, thereby providing crucial information about energy level alignment. Inverse photoemission spectroscopy technique is particularly helpful for the study of the electronic properties at metal–molecule interfaces and thus provides information about transport gap for conjugated organic molecules. Organic molecular beam epitaxy has been extensively utilized to deposit conjugated organic molecular thin films on various oxide, transition metal dichalcogenides and metallic substrates. Novel approaches have been investigated explored in order to control the interfacial energy level alignment by introducing intermediate electron donor and electron acceptor organic molecular layers across interfaces to tune the work function of electrodes. This review summarizes recent advances in exploiting combined approach using photoemission and inverse photoemission spectroscopy for the investigation of electronic properties of conjugated organic molecular interfaces.

Mesoscopic 2D molecular self-assembly on an insulator

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly—where organic molecules act as building blocks and where surfaces play the role of supporting templates—allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Surface morphology and molecular aggregation state of the organized molecular films of cyclic moiety-containing polyguanamine derivatives with a flexible linker moiety

Surface morphology and molecular aggregation state of the organized molecular films of cyclic moiety-containing polyguanamine derivatives with a flexible linker moiety

Characterization of Molecular Arrangement of Long‐chain Ferrocenyl Derivatives Having Asymmetric Carbon by Method of Organized Molecular Films and Formation of Its Helical Nanofibers

AbstractFormation of organized molecular film and helical nanofibers of ferrocene derivatives having R‐12‐hydroxystearyl chains with asymmetric carbon was investigated. The 12‐hydroxystearyl chain, which induces “thixotropy” due to the formation and breakage of intermolecular hydrogen bonds, was introduced into the ferrocenyl unit. The structure of this derivative as a monolayer on the water surface and its Langmuir‐Blodgett (LB) film properties were analyzed. The monolayer showed a two‐dimensional phase transition, corresponding to the formation of fibrous morphology. In the LB multilayers, intermolecular hydrogen bonding between hydroxyl groups at the 12‐position of the chain and crystalline packing was observed. Raman mapping revealed the difference in surface morphology of each derivative depending on the number of 12‐hydroxystearyl chains. The derivative having the single chain showed a right‐handed helical morphology in the film and this behavior is similar to those having two and three chains. In circular dichroism spectra, the film showed a positive Cotton effect. Since the helical chains easily entangle with each other to form a sponge‐like organization, it was expected that the solvent uptake was the origin of gelation induction to its contact solvent.

KMap.py: A Python program for simulation and data analysis in photoemission tomography

Ultra-violet photoemission spectroscopy is a widely-used experimental technique to investigate the valence electronic structure of surfaces and interfaces. When detecting the intensity of the emitted electrons not only as a function of their kinetic energy, but also depending on their emission angle, as is done in angle-resolved photoemission spectroscopy (ARPES), extremely rich information about the electronic structure of the investigated sample can be extracted. For organic molecules adsorbed as well-oriented ultra-thin films on metallic surfaces, ARPES has evolved into a technique called photoemission tomography (PT). By approximating the final state of the photoemitted electron as a free electron, PT uses the angular dependence of the photocurrent, a so-called momentum map or k-map, and interprets it as the Fourier transform of the initial state’s molecular orbital, thereby gaining insights into the geometric and electronic structure of organic/metal interfaces.In this contribution, we present kMap.py which is a Python program that enables the user, via a PyQt-based graphical user interface, to simulate photoemission momentum maps of molecular orbitals and to perform a one-to-one comparison between simulation and experiment. Based on the plane wave approximation for the final state, simulated momentum maps are computed numerically from a fast Fourier transform (FFT) of real space molecular orbital distributions, which are used as program input and taken from density functional calculations. The program allows the user to vary a number of simulation parameters, such as the final state kinetic energy, the molecular orientation or the polarization state of the incident light field. Moreover, also experimental photoemission data can be loaded into the program, enabling a direct visual comparison as well as an automatic optimization procedure to determine structural parameters of the molecules or weights of molecular orbitals contributions. With an increasing number of experimental groups employing photoemission tomography to study molecular adsorbate layers, we expect kMap.py to serve as a helpful analysis software to further extend the applicability of PT. Program summaryProgram Title:kMap.pyCPC Library link to program files:https://doi.org/10.17632/tnrm9jcccc.1Developer’s respository link:https://github.com/brands-d/kMap/Code Ocean capsule:https://codeocean.com/capsule/5788845Licensing provisions: GPLv3Programming language: Python 3.xNature of problem: Photoemission tomography (PT) has evolved as a powerful experimental method to investigate the electronic and geometric structure of organic molecular films [1]. It is based on valence band angle-resolved photoemission spectroscopy and seeks an interpretation of the angular dependence of the photocurrent, a so-called momentum map, from a given initial state in terms of the spatial structure of molecular orbitals. For this purpose, PT heavily relies on a simulation platform which is capable of efficiently predicting momentum maps for a variety of organic molecules, which allows for a convenient way of treating the effect of molecular orientations, and which also accounts for other experimental parameters such as the geometrical setup and nature of the incident photon source. Thereby, PT has been used to determine molecular geometries, gain insight into the nature of the surface chemistry, unambiguously determine the orbital energy ordering in molecular homo- and heterostructures and even reconstruct the orbitals of adsorbed molecules [1–4].Solution method:kMap.py is a Python program that enables the user, via a PyQt-based graphical user interface, to simulate photoemission momentum maps of molecular orbitals and to perform a one-to-one comparison between simulation and experiment. Based on the plane wave approximation for the final state, simulated momentum maps are computed numerically from a fast Fourier transform (FFT) of real space molecular orbital distributions [2] which are used as program input and which are usually obtained from density functional calculations. The user can vary a number of simulation parameters such as the final state kinetic energy, the molecular orientation or the polarization state of the incident light field. Moreover, also experimental photoemission data can be loaded into the program, enabling a direct visual comparison as well as an automatic optimization procedure to minimize the difference between simulated and measured momentum maps. Thereby, structural parameters of the molecules [2] and the weights of molecular orbitals to experimentally observed emission features can be determined [3].

Effect of the electron donating group on the excited-state electronic nature and epsilon-near-zero properties of curcuminoid-borondifluoride dyes

Epsilon-near-zero (ENZ) properties have been reported in organic molecular films. In particular, cyanine and squaraine films have been shown to exhibit ENZ properties in the visible spectral region with a strong 3rd order nonlinear optical response near the ENZ spectral region. Noting both cyanine and squaraine belong to the polymethine family, a series of six curcuminoid borondifluoride (Curc) derivatives were developed to examine whether such a polymethine character is positively correlated with the ENZ property of the organic films. Those Curc derivatives possess a Donor–Acceptor–Donor (D–A–D) architecture with acceptor, AcacBF2, located at the molecular center. The backbone of Curc is designed such that the donor strength can be tuned to transit between charge transfer (CT) and polymethine character. This balance between CT and polymethine character of the Curc series is examined based on the Lippert–Mataga plot. As donor strength in the D–A–D structure increases, CT character is less marked resulting in a more dominant polymethine character. The structural and optical properties of the Curc films with a thickness in the order of 30 nm were examined to correlate the polymethine character with the ENZ response. The results obtained in isotropic Curc thin films demonstrate that an increase of polymethine character associated with a stronger donor strength leads to an appearance/enhancement of the ENZ property in the visible spectrum range from 500 to 670 nm. Overall, this study provides useful guidelines to engineer new organic materials showing ENZ properties in a desired spectral range.

Probing the Growth of Organic Molecular Films Embedded between Cobalt and Iron Electrodes: Ferromagnetic Nuclear Resonance Approach

AbstractPhysical properties of magnetic nanostructures and devices strongly depend on the morphological characteristics of their various components. This is especially true and becomes particularly complex in hybrid nanostructures, where soft organic molecules are at the vicinity of ferromagnetic metallic films. The supramolecular architecture of molecular films embedded between Fe and Co layers is investigated by ferromagnetic nuclear resonance (FNR). With such sample architecture, the presence of pin holes in the organic layers is detected by FNR contributions in a specific spectral range. The methodology that is developed allows the probing of the continuity and the packing of zinc tetraphenyl porphyrin (ZnTPP) molecular films between the Co and Fe films. The experimental results suggest that, regardless of the nature of the ferromagnetic underlayer, at least 15 monolayers of ZnTPP are necessary to form continuous and pin‐hole free molecular films. In addition, quantitative analyses show that ZnTPP layers exhibit distinct morphologies that are dependent on the nature of the ferromagnetic metallic underlayer.

Multiscale molecular simulations of the morphological evolution of small-molecule organic solar cells during the vacuum codeposition process

The mesoscale morphologies of organic small molecular films fabricated via vacuum deposition processes are critical to the performance of small-molecule solar cells and organic light-emitting diodes. In the present study, the morphological evolution of the active layer of DPDCPB:${\mathrm{C}}_{70}$ small-molecule solar cells during vacuum codeposition processes was revealed by a series of GPU-accelerated coarse-grained molecular-dynamics simulations. The ${\mathrm{C}}_{70}$ and DPDCPB molecules were coarsened into ellipsoids and bonded ellipsoids, respectively. The interactions between ellipsoids were described by the Gay-Berne formulation and were parametrized to reproduce potential energy surfaces from all-atom atomistic simulations using a genetic algorithm. Due to the significantly reduced overall degrees of freedom, this coarse-grained scheme allowed us to simulate the vacuum codeposition processes and monitor the morphological evolution of systems with system length scales compatible with those of the experiments (\ensuremath{\sim}30 nm). Our simulations indicate that the film morphologies are closely correlated with the DPDCPB:${\mathrm{C}}_{70}$ blending ratio. High ${\mathrm{C}}_{70}$ concentration leads to a rough film surface, which is in accordance with experimental observations and can be attributed to the strong self-aggregation behavior of ${\mathrm{C}}_{70}$ molecules. The morphological property analysis indicates that the rough film surface has an almost negligible impact on the DPDCPB/${\mathrm{C}}_{70}$ domain percolations, and the device with the optimal deposition ratio should give the most balanced hole/electron transfer in respective DPDCPB/${\mathrm{C}}_{70}$ domains. The present study demonstrates that by using the ellipsoid-based coarse-grained model, it is possible to study the morphological evolution of small-molecule organic thin film during vacuum deposition processes with molecular scale details, which can provide valuable insights for experimental teams to further optimize device fabrication protocols for the next generation of organic optoelectronic devices.

Collapse Mechanism in Few-Layer MoS2Langmuir Films

Recent advances in the liquid-phase exfoliation enabled large-scale production of two-dimensional (2D) materials, including few-layer graphene and transition metal dichalcogenides. The exfoliated flakes of 2D materials allow cost-effective deposition of continuous films for various applications ranging from optoelectronics to lubrication technology. The self-assembly of 2D materials on water subphase and subsequent transfer of such a Langmuir film onto a solid substrate offers an unprecedented layer quality in terms of spatial homogeneity as it proceeds in thermodynamic equilibrium. However, while the formation of conventional organic molecular Langmuir films has been widely studied, the application of the Langmuir technique to rigid inorganic 2D materials is still rather unexplored. Here, we study the underlying mechanism behind the formation and collapse at the critical surface pressure of the Langmuir film composed of few-layer MoS2 flakes. The in situ wide-angle X-ray scattering measured in real time and other supportive techniques applied ex situ after the film transfer onto a Si/SiO2 substrate were employed. We identify all principal compression stages up to the Langmuir monolayer collapse and beyond, relying on the texture, surface pressure, and elastic modulus temporal evolution. The results obtained and the conclusions drawn can be extended to a large family of the inorganic Langmuir films of other 2D materials to optimize the deposition process for envisaged application.

2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film.

We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF4). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF4 at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis.

Fabrication of organo-modified carbon nanotube with excellent heat resistance and preparation of its polymer-based nanocomposite by simple melt compounding

Crystalline polymer/single-walled carbon nanotube (SWCNT) nanocomposites were prepared using organo-modified SWCNTs with surface modification by long-chain phosphonic acid derivatives having bidentate bonds, resulting in excellent heat resistance properties. Surface modification with a long-chain phosphonate is non-desorbing, has a high desorption suppression temperature and can improve miscibility with matrix polymers by changing the other end of the functional group in various ways. In addition, since organo-SWCNTs showed amphiphilic properties when introducing a modified chain after hydrophilization, their structure and morphology could be characterized by the method of organized molecular films. The utilization of phosphonic acid derivatives containing fluorocarbon chains endowed SWCNTs with remarkable cohesive properties in phase-separable crystalline fluoropolymers to achieve polymer-based nanocomposite formation. Fluorinated SWCNTs could be easily introduced into fluoropolymers via a simple melt-compounding method. The crystalline fluoropolymer/fluorocarbon-modified SWCNT nanocomposite showed clear improvement in crystallization temperature compared with the matrix polymer. Simultaneously with the improvement in the crystallization temperature, it was confirmed that the crystallite size was increased, the lamella was thickened, the mechanical properties were improved, and the pyrolysis temperature was increased. These enhancements of the physical properties seem to be the result of the interaction between the modified-chain end and the polymer ends, and it was predicted that heterogeneous nucleation was induced by the nucleation effect.

Arrangement and electronic properties of cobalt phthalocyanine molecules on B-Si(111)- 3×3 R30∘

The molecular arrangement and the interfacial electronic properties of cobalt phthalocyanine (CoPc) on the deactivated B-Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\phantom{\rule{0.16em}{0ex}}R{30}^{\ensuremath{\circ}}$ surface are analyzed using scanning tunneling microscopy and spectroscopy as well as photoemission studies. Our data demonstrate that for low coverages of CoPc the molecules lie flat with the ${p}_{z}$ orbital of the Si surface atom forming a hybrid state with the ${d}_{{z}^{2}}$ orbital of the transition metal. This hybridization is observed in a broadening of the corresponding Si $2p$ core level photoemission spectra and in an additional contribution to the valence band spectra. Furthermore, this additional hybridization state is detected in the tunneling spectra. For higher CoPc coverages, the CoPc molecules are tilted with respect to the Si surface forming highly ordered organic molecular films. The spectroscopic data of the thick film demonstrates that the electronic properties resemble those of pure CoPc.

Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films.

The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure-characterized by sharp grain boundaries and overlapping domains-to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure-property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics.

Properties of the Langmuir and Langmuir\u2013Blodgett monolayers of cholesterol-cyclosporine A on water and polymer support

The paper deals with the cholesterol–cyclosporine A (Chol–CsA) monolayers at the air/water interface investigated using the Langmuir trough coupled with the Brewster’s angle microscopy. The compressed films were transferred onto the PEEK polymer support by means of the Langmuir–Blodgett technique. To improve molecules adhesion and organization the PEEK surface was treated with air plasma before thin films deposition. The obtained surfaces were characterized by means of atomic force microscope (AFM). Then, the wettability of the supported monolayers was determined by the contact angle measurements. Finally, the surface free energy and its components were evaluated from the theoretical approach proposed by van Oss et al. The obtained results reveal correlation between properties of the Langmuir monolayers at the air/water interface and those of the Langmuir–Blodgett films on PEEK. This was found to be helpful for understanding the wettability of organized molecular films on the polymer support as far as biocompatibility improve is concerned. The preparation of films with defined polarity and various compositions is an important step in the development of polymer surfaces with increased biofunctionality. It is believed that the results presented in this paper can be exploited in the in vivo studies.

Study of energy level alignment at weakly interacting small organic molecular thin film interfaces: The validity of classical model from inorganics

In this study, we discussed the validity of an inorganic semiconductor-based model on energy level alignment at weakly interacting substrate-organic film interfaces, in which the influence of Gaussian distributed density of states on injection barrier based on the thermal equilibrium condition after contacted with substrates was quantitatively investigated. A “Z”-like shape of the energy level alignment has been quantitatively obtained: a slope “S = 1” is clearly given, when the substrate work function is located approximately in between the threshold of ionization energy (IEth) and electron affinity (EAth); on the other hand, “S ∼ 0” is given while the substrate work function is lower than EAth or larger than IEth. Although different calculated results compared with experimental data can also be found, in which the calculated minimal injection barrier is ∼0.2 eV smaller than experimental data, this simplified practical method can be easily applied to calculate the device-related electrostatic potential inside dielectric films.

Ultrahigh-vacuum organic molecular-beam deposition system for in situ growth and characterization.

A compact ultrahigh-vacuum molecular-beam deposition system has been developed for the in situ synthesis of organic thin films and multilayers. The system incorporates all the features (heater, thickness monitor, evaporators) necessary for controlled organic thin-film growth. It can be used independently, or it can be docked to the in situ growth system and transferred to other instruments of the PGM beamline, thus allowing extensive film preparation and characterization. A manipulator dedicated to specimen preparation and organic-film deposition with temperature control between 200 K and ∼800 K has been developed. The design and performance of the system are reported with emphasis on a novel solution of masks developed to achieve position-dependent film deposition. To demonstrate the enhanced capabilities of the PGM beamline in the growth and in the characterization of electronic-structure studies of organic molecular films and their heterostructures through synchrotron-based spectroscopies, this paper presents some preliminary results of a study of Fe-phthalocyanine growth on Si substrates and on in situ prepared La0.67Sr0.33MnO3 buffer layers on SrTiO3 single crystal.

Micromechanical Redox Actuation by Self-Assembled Monolayers of Ferrocenylalkanethiolates: Evens Push More Than Odds.

Microcantilever transducers can be valuable tools for the investigation of physicochemical processes in organized molecular films. Gold-coated cantilevers are used here to investigate the electrochemomechanicsof redox-active self-assembled monolayers (SAMs) of ferrocenylalkanethiolates (Fc(CH2) nS) of different alkyl chain lengths. A significant odd-even effect is observed in the surface stress and cantilever movement generated by the oxidation of the SAM-confined ferrocenes as the number of methylene units n in the SAM backbone is varied. We demonstrate that stronger alkyl chain-chain interactions are at the origin of the larger surface stresses generated by SAMs with an even versus odd n. The findings highlight the impact of subtle structural effects and weak van der Waals interactions on the mechanical actuation produced by redox reactions in self-assembled systems.