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.

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.

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.

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.

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.

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

This review focuses on the preparation of dienic linchpin reagents and on their use in the synthesis of polyenic frameworks.

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

We have investigated at the DFT level the way the work function of ZnO is affected upon deposition of self-assembled monolayers made of 4-tert-butylpyridine and various benzoic acids.

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.

Influence of Structural Organization On Optical and Transport Properties in Organic Materials

Correlated quantum-chemical calculations performed on supermolecular structures, i.e., on clusters made of several oligomer chains in interaction, provide insight into structure-property relationships in well-organized molecular films. This supermolecular approach is applied to crystalline phases of two prototypical conjugated oligomers and shows that variations in chain packing can lead to dramatically different optical (Davydov) splittings and carrier mobilities. Optimal chain organizations for various types of devices are discussed.

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.

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.

Characterization of the molecular parameters determining charge transport in anthradithiophene

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene.

Organic semiconductors: a theoretical characterization of the basic parameters governing charge transport

Organic semiconductors based on pi-conjugated oligomers and polymers constitute the active elements in new generations of plastic (opto)electronic devices. The performance of these devices depends largely on the efficiency of the charge-transport processes; at the microscopic level, one of the major parameters governing the transport properties is the amplitude of the electronic transfer integrals between adjacent oligomer or polymer chains. Here, quantum-chemical calculations are performed on model systems to address the way transfer integrals between adjacent chains are affected by the nature and relative positions of the interacting units. Compounds under investigation include oligothienylenes, hexabenzocoronene, oligoacenes, and perylene. It is shown that the amplitude of the transfer integrals is extremely sensitive to the molecular packing. Interestingly, in contrast to conventional wisdom, specific arrangements can lead to electron mobilities that are larger than hole mobilities, which is, for instance, the case of perylene.

Photoinduced electron-transfer processes along molecular wires based on phenylenevinylene oligomers: a quantum-chemical insight

Quantum-chemical techniques are applied to model the mechanisms of photoinduced charge transfer from a pi-electron donating group (tetracene, D) to a pi-electron-acceptor moiety (pyromellitimide, A) separated by a bridge of increasing size (p-phenylenevinylene oligomers, B). Correlated Hartree-Fock semiempirical approaches are exploited to calculate the four main parameters controlling the transfer rate (k(RP)) in the framework of Marcus-Jortner-Levich's formalism: (i) the electronic coupling between the initial and final states; (ii) and (iii) the internal and external reorganization energy terms; and (iv) the variation of the free Gibbs energy. The charge transfer is shown to proceed in these compounds through two competing mechanisms, coherent (superexchange) versus incoherent (bridge-mediated) pathways. While superexchange is the dominant mechanism for short bridges, incoherent transfer through hopping along the phenylene vinylene segment takes over in longer chains (for ca. three phenylenevinylene repeat units). The influence of the chemical structure of the pi-conjugated phenylenevinylene bridge on the electronic properties and the rate of charge transfer is also investigated.

Negative differential resistance in phenylene ethynylene oligomers

The origin of the sharp peak profile (i.e., negative differential resistance, NDR) observed in the I/V curves of three-ring phenylene ethynylene oligomers is a topic of major current interest. Here, quantum-chemical calculations are performed to analyze the evolution of the one-electron structure of an unsubstituted three-ring oligomer under the influence of a static electric field (which models the driving voltage applied in the experiments). The results indicate that the rotation of the central ring of the oligomer induces resonant tunneling processes over a limited voltage range. This can thus be responsible for the NDR signature observed experimentally.

Negative differential resistance behavior in conjugated molecular wires incorporating spacers: a quantum-chemical description

Recent experimental studies have demonstrated that single molecules or a small number of self-assembled molecules can perform the basic functions of traditional electronic components, such as wires and diodes. In particular, molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunneling diodes (RTDs), whose I/V characteristics reveal a Negative Differential Resistance (NDR) behavior (i.e., a negative slope in the I/V curve). Here, quantum-chemical calculations are used to describe on a qualitative basis the mechanism leading to NDR in polyphenylene-based molecular wires incorporating saturated spacers. This description is based on the characterization of the evolution of the wire electronic structure as a function of a static electric field applied along the molecular axis, which simulates the driving voltage between the two electrodes in the RTD devices. We illustrate that the main parameters controlling the NDR behavior can be modulated through molecular engineering of the wires.

Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: joint experimental and theoretical investigation of physical properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.

Scaling law for second-order hyperpolarizability in poly(triacetylene) molecular wires

Poly(triacetylenes) are rodlike molecules with electrons delocalized over a one-dimensional path. We show that they exhibit a power-law dependence of the second-order hyperpolarizability gamma on the monomer unit n for short molecules and a smooth saturation toward a linear increase in longer molecules. The power law of gamma?n(a) with a approximately 2.5 from dengenerate four-wave mixing and third-harmonic generation measurements is in good agreement with quantum-chemical calculations. The critical conjugation length for saturation in the three cases is shown to be approximately 60 carbon-carbon bonds, which indicates the upper boundary for the electron delocalization in such a one-dimensional molecular wire.