HOMO Level Pinning in Molecular Junctions: Joint Theoretical and Experimental Evidence.

A central issue in molecular electronics in order to build functional devices is to assess whether changes in the electronic structure of isolated compounds by chemical derivatization are retained once the molecules are inserted into molecular junctions. Recent theoretical studies have suggested that this is not always the case due to the occurrence of pinning effects making the alignment of the transporting levels insensitive to the changes in the electronic structure of the isolated systems. We explore here this phenomenon by investigating at both the experimental and theoretical levels the I/ V characteristics of molecular junctions incorporating three different three-ring phenylene ethynylene derivatives designed to exhibit a significant variation of the HOMO level in the isolated state. At the theoretical level, our NEGF/DFT calculations performed on junctions including the three compounds show that, whereas the HOMO of the molecules varies by 0.61 eV in the isolated state, their alignment with respect to the Fermi level of the gold electrodes in the junction is very similar (within 0.1 eV). At the experimental level, the SAMs made of the three compounds have been contacted by a conducting AFM probe to measure their I/ V characteristics. The alignment of the HOMO with respect to the Fermi level of the gold electrodes has been deduced by fitting the I/ V curves, using a model based on a single-level description (Newns-Anderson model). The extracted values are found to be very similar for the three derivatives, in full consistency with the theoretical predictions, thus providing clear evidence for a HOMO level pinning effect.

Potential of polymethacrylate pseudo crown ethers as solid state polymer electrolytes.

The association of kinetic studies, DFT calculations and 1H-7Li NMR analyses allowed the control of the cyclo-ATRP of PEG9DMA and the production of polymethacrylate pseudo crown-ethers of various molar masses. Their potential to act as a solid-state polymer electrolyte in Li-ion batteries has been highlighted and may come from the supramolecular organization of the cyclo-PEG forming a Li+ diffusion channel.

Estimation of π-π Electronic Couplings from Current Measurements.

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

Correlation between the shape of the ion mobility signals and the stepwise folding process of polylactide ions.

In the field of polymer characterization, the use of ion mobility mass spectrometry (IMMS) remains mainly devoted to the temporal separation of cationized oligomers according to their charge states, molecular masses and macromolecular architectures in order to probe the presence of different structures. When analyzing multiply charged polymer ions by IMMS, the most striking feature is the observation of breaking points in the evolution of the average collision cross sections with the number of monomer units. Those breaking points are associated to the folding of the polymer chain around the cationizing agents. Here, we scrutinize the shape of the arrival time distribution (ATD) of polylactide ions and associate the broadening as well as the loss of symmetry of the ATD signals to the coexistence of different populations of ions attributed to the transition from opened to folded stable structures. The observation of distinct distributions reveals the absence of folded/extended structure interconversion on the ion mobility time scale (1-10 ms) and then on the lifetime of ions within the mass spectrometer at room temperature. In order to obtain information on the possible interconversion between the different observed populations upon ion activation, we performed IM-IM-MS experiments (tandem ion mobility measurements). To do so, mobility-selected ions were activated by collisions before a second mobility measurement. Interestingly, the conversion by collisional activation from a globular structure into a (partially) extended structure, i.e. the gas phase unfolding of the ions, was not observed in the energetic regime available with the used experimental setup. The absence of folded/extended interconversion, even upon collisional activation, points to the fact that the polylactide ions are 'frozen' in their specific 3D structure during the desolvation/ionization electrospray processes. Copyright © 2017 John Wiley & Sons, Ltd.

Probing the interaction between 2,2'-bithiophene-5-carboxylic acid and TiO2 by photoelectron spectroscopy: A joint experimental and theoretical study.

The interaction between 2,2'-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO2 single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO2 (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO2 valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted.

The role of curvature in Diels-Alder functionalization of carbon-based materials.

We have estimated theoretically the impact of curvature on the free energies of activation and reaction associated with Diels-Alder reactions on carbon-based materials. Significant reduction is observed for both energy values with increasing curvature for core-functionalization, while the opposite trend prevails for edge-functionalization, as further supported by SEM/fluorescence measurements.

Linchpin dienes: key building-blocks in the synthesis of polyenic frameworks.

A myriad of biologically active products incorporates polyenic frameworks. Among the syntheses developed to access these moieties, metal-catalyzed cross-couplings emerged as reactions of choice especially due to their high stereoselectivity. Particularly, the use of bifunctionalized compounds (linchpin reagents) allows a fast, modular and efficient access to polyenic chains. In this review, we will focus on the preparation of bifunctionalized dienes and on their utilization in the synthesis of polyenes.

Work function shifts of a zinc oxide surface upon deposition of self-assembled monolayers: a theoretical insight.

We have investigated at the theoretical Density Functional Theory level the way the work function of zinc oxide layers is affected upon deposition of self-assembled monolayers (SAMs). 4-tert-Butylpyridine (4TBP) and various benzoic acids (BA) were adsorbed on the apolar (101[combining macron]0) ZnO and used as probe systems to assess the influence of several molecular parameters. For the benzoid acids, we have investigated the impact of changing the nature of the terminal group (H, CN, OCH3) and the binding mode of the carboxylic acid (monodentate versus bidentate) on the apolar (101[combining macron]0) surface. For each system, we have quantified the contribution from the molecular core and the anchoring group as well as of the degree of surface reconstruction on the work function shift. For the benzoic acids, the structural reorganization of the surface induces a negative shift of the work function by about 0.3 ± 0.15 eV depending on the nature of the binding mode, irrespective of the nature of the terminal function. The bond-dipole potential strongly contributes to the modification of the work function, with values in the range +1.2 to +2.0 eV. In the case of 4TBP, we further characterized the influence of the degree of coverage and of co-adsorbed species (H, OH, and water molecules) on the ZnO/SAM electronic properties as well as the influence of the ZnO surface polarity by considering several models of the polar (0001) ZnO surface. The introduction of water molecules in the (un)dissociated form at full coverage on the non-polar surface only reduces the work function by 0.3-0.4 eV compared to a reference system without co-adsorbed species. Regarding the polar surface, the work function is also significantly reduced upon deposition of a single 4BTP molecule (from -1.44 eV to -1.73 eV for our model structures), with a shift similar in direction and magnitude compared to the non-polar surfaces.

Combining mass spectrometry diagnostic and density functional theory calculations for a better understanding of the plasma polymerization of ethyl lactate.

The focus of this work is on the growth mechanism of ethyl lactate-based plasma polymer film (ELPPF) that could be used as barrier coatings. In such an application, the ester density of the plasma polymer has to be controlled to tune the degradation rate of the material. Our strategy consists of correlating the plasma chemistry evaluated by RGA mass spectrometry and understanding, via DFT calculations, the chemistry of the synthesized thin films. The theoretical calculations helped us to understand the plasma chemistry in plasma ON and OFF conditions. From these data it is unambiguously shown that the signal m/z 75 can directly be correlated with the precursor density in the plasma phase. The combination of XPS and chemical derivatization experiments reveal that the ester content in the ELPFF can be tailored from 2 to 18 at. % by decreasing the RF power, which is perfectly correlated with the evolution of the plasma chemistry. Our results also highlight that the ELPPF chemistry, especially the ester content, is affected by the plasma mode of operation (continuous or pulsed discharge, at similar injected mean power) for similar ester content in the plasma. This could be related to different energy conditions at the interface of the growing films that could affect the sticking coefficient of the ester-bearing fragments.

Exploring the energy landscape of the charge transport levels in organic semiconductors at the molecular scale.

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

Methodological aspects of the quantum-chemical description of interface dipoles at tetrathiafulvalene∕tetracyanoquinodimethane interfaces.

The formation of dipoles at interfaces between organic semiconductors is expected to play a significant role in the operation of organic-based devices, though the electronic processes at their origin have still to be clearly elucidated. Quantum-chemical calculations can prove very useful to shed light on such electronic interfacial phenomena provided that a suitable theoretical approach is used. In this context, we have performed calculations on small vertical stacks of TTF-TCNQ molecules, first at the CAS-MRCI level to validate the use of single-determinantal approaches, then at the MP2 level set as a benchmark. Various density functional theory (DFT) functionals have then been applied to larger stacks, showing that long-range corrected functionals are required to reproduce MP2 results taken as benchmark. Finally, the use of periodic boundary conditions at the DFT level points to the huge impact of depolarization effects between adjacent stacks.

Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au: a joint experimental and theoretical study.

Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Φ(Au)) when chemisorbed on its surface. In particular we show that the Φ(Au) can be modulated with external stimuli by controlling the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Φ(trans)-Φ(cis)). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Molecular packing and charge transport parameters in crystalline organic semiconductors from first-principles calculations.

The generation of mobile charges and their transport across organic layers are commonly the most critical steps affecting the performance of organic-based electronic devices. Charge-transport properties are often described by quantum-chemical calculations which, however, face a challenge when the nanostructure of the material has to be concomitantly addressed together with electronic aspects. We tackle here this challenging task by applying dispersion-corrected Density Functional Theory methods, which allow us not only to give an insight into the molecular packing but also to accurately extract key molecular parameters governing charge transport. When applied to a set of functionalized (chlorinated) tetracene molecules, our approach yields the expected molecular packing, which has motivated its use to predict the packing of other fluorinated or brominated derivatives which are not yet synthesized. The charge mobilities have been calculated on the basis of the determined packing motifs and exhibit significant differences among the derivatives. This work paves the way towards the development of a computational protocol that could be implemented not only for idealized packing motifs or known crystallographic structures but also for self-organizing materials as well as supramolecular and host-guest interactions.

Theoretical characterization of the structural and hole transport dynamics in liquid-crystalline phthalocyanine stacks.

We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns.

Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy.

Partially deuterated 1,4-distyrylbenzene () is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of is investigated over a wide temperature range by (13)C, (2)H NMR as well as fluorescence spectroscopy. Simulations of the (2)H NMR spectral shapes reveal an overall wobble motion of in the channels with an amplitude of about 4 degrees at T = 220 K and 10 degrees at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 180 degrees flips of the terminal phenyl rings with a frequency of 10(6) Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol(-1) which imply a strong sterical hindrance by the surrounding PHTP channels. The experimental vibrational structure of the fluorescence excitation spectra of is analyzed in terms of small amplitude ring torsional motions, which provide information about the spatial constraints on by the surrounding PHTP host matrix. Combining the results from NMR and fluorescence spectroscopy as well as of time-dependent density functional calculations yields the complete potential surfaces of the phenyl ring torsions. These results, which suggest that intramolecular mobility of is only reduced but not completely suppressed by the matrix, are corroborated by MD simulations. Unrealistically high potential barriers for phenyl ring flips are obtained from MD simulations using rigid PHTP matrices which demonstrate the importance of large amplitude motions of the PHTP host lattice for the mobility of the guest molecules.

A theoretical view of unimolecular rectification.

The concept of single molecule rectifiers proposed in a theoretical work by Aviram and Ratner in 1974 was the starting point of the now vibrant field of molecular electronics. In the meantime, a built-in asymmetry in the conductance of molecular junctions has been reported at the experimental level. In this contribution, we present a theoretical comparison of three different types of unimolecular rectifiers: (i) systems where the donor and acceptor parts of the molecules are taken from charge-transfer salt components; (ii) zwitterionic systems and (iii) tour wires with nitro substituents. We conduct an analysis of the rectification mechanism in these three different types of asymmetric molecules on the basis of parameterized quantum chemical models as well as with a full non-equilibrium Green's function-density functional theory (NEGF-DFT) treatment of the current-voltage characteristics of the respective metal-molecule-metal junctions. We put a particular emphasis on the prediction of rectification ratios (RRs), which are crucial for the assessment of the technological usefulness of single molecule junctions as diodes. We also compare our results with values reported in the literature for other types of molecular rectification, where the essential asymmetry is not induced by the structure of the molecule alone but either by a difference in the electronic coupling of the molecule to the two electrodes or by attaching alkyl chains of different lengths to the central molecular moiety.

Charge hopping in organic semiconductors: influence of molecular parameters on macroscopic mobilities in model one-dimensional stacks.

We present a Monte Carlo approach to estimate how molecular parameters impact hopping rates and charge mobilities in organic pi-conjugated materials. Our goal is to help in establishing structure-properties relationships. As a first step, our approach is illustrated by considering a model system made of a one-dimensional array of pentacene molecules; we describe the variations of the electron-transfer rates and of the resulting charge mobilities as a function of electric field and of the presence of molecular disorder and traps. The results highlight that there is no direct relationship between the degree of spatial overlap among adjacent molecules and charge mobility.

Influence of copolymer interface orientation on the optical emission of polymeric semiconductor heterojunctions.

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Pathways for photoinduced charge separation and recombination at donor-acceptor heterojunctions: the case of oligophenylenevinylene-perylene bisimide complexes.

Semiempirical Hartree-Fock techniques have been applied to assess the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The corresponding rates have been estimated in the framework of the Marcus-Levich-Jortner formalism for different geometries of the complexes. The results indicate that dissociation pathways involving the lowest two charge transfer excited states contribute significantly to the dynamics of the whole process. The rates are found to be strongly sensitive to the relative position of the donor and acceptor units and can be rationalized in terms of symmetry arguments applied to relevant electronic levels.

Impact of the computational method on the geometric and electronic properties of oligo(phenylene vinylene) radical cations.

We report on a quantum-chemical study of the electronic and optical properties of unsubstituted oligo(phenylene vinylene) (OPV) radical cations. Our goal is to distinguish the impact of the choice of molecular geometry from the impact of the choice of quantum-chemical method, on the calculated optical transition energies. The geometry modifications upon ionization of the OPV chains are found to depend critically on the theoretical formalism: Hartree-Fock (HF) geometry optimizations lead to self-localization of the charged defects while pure density functional theory (DFT) results in a complete delocalization of the geometric modifications over the whole conjugated backbone. The electronic structure and vertical transition energy associated with the lowest excited state of the radical cations have been calculated at the post-Hartree-Fock level within a configuration interaction (HF-CI) scheme and using the time-dependent DFT (TD-DFT) formalism for different radical cation geometries. Interestingly, the changes in the calculated optical properties obtained when using different geometric structures are less important within a given method than the differences between methods for a given structure. The optical excitation is localized with HF-CI and delocalized with TD-DFT, almost irrespective of the molecular geometry; as a result, HF-CI excitation energies tend to saturate as the chain length increases, in contrast to the results from TD-DFT.