Intermolecular Screening Research Articles

Unravelling molecular interactions in uracil clusters by XPS measurements assisted by ab initio and tight-binding simulations

The C, N and O 1s XPS spectra of uracil clusters in the gas phase have been measured. A new bottom-up approach, which relies on computational simulations starting from the crystallographic structure of uracil, has been adopted to interpret the measured spectra. This approach sheds light on the different molecular interactions (H-bond, π-stacking, dispersion interactions) at work in the cluster and provides a good understanding of the observed XPS chemical shifts with respect to the isolated molecule in terms of intramolecular and intermolecular screening occurring after the core–hole ionization. The proposed bottom-up approach, reasonably expensive in terms of computational resources, has been validated by finite-temperature molecular dynamics simulations of clusters composed of up to fifty molecules.

Interplay between Solution Processing and Electronic Structure in Metal-Free Organic Magnets Based on a TEMPO Pentacene Derivative

We have used X-ray photoelectron spectroscopy (XPS) to describe the electronic structure of two newly synthesized derivatives of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) carrying the 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) radical as a substituent. XPS results underline the existing interplay between electronic structure and preparation methods. Our observations suggest a strong influence of the film processing on the shakeup satellite intensities of the N 1s core level spectra. Preparation methods, by tuning the different degree of intermolecular screening, may be use as a tool to influence the properties and the behavior of these materials as thin films, including their local magnetic-exchange coupling.

Modification of the PTCDA-Ag bond by forming a heteromolecular bilayer film

The understanding of the fundamental physical properties of metal-organic and organic-organic interfaces is crucial for improving the performance of organic electronic devices. This is particularly true for (multilayer) systems containing several molecular species due to their relevance for donor-acceptor systems. A prototypical heteromolecular bilayer system is copper-II-phthalocyanine (CuPc) on 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on Ag(111). In an earlier work we have reported a commensurate registry between both organic layers and an enhanced charge transfer from the Ag substrate into the organic bilayer film [Phys. Rev. Lett. 108, 106103 (2012)], which both indicate an unexpectedly strong intermolecular interaction across the organic-organic interface. Here we present new details regarding electronic and geometric structure for the same system. In particular, we provide evidence that the enhanced charge transfer from the substrate into the organic bilayer does not involve CuPc electronic states, hence, there is no significant charge transfer into the second organic layer. Furthermore, we report vertical bonding distances revealing a shortening of the PTCDA-Ag(111) distance upon CuPc adsorption. Thus, electronic and geometric properties (charge transfer and bonding distance, respectively) both indicate a strengthening of the PTCDA-Ag(111) bond upon CuPc adsorption. We explain these findings---in particular the correlation between CuPc adsorption and increased charge transfer into PTCDA---in a model involving an intermolecular screening mechanism.

Intermolecular reaction screening as a tool for reaction evaluation.

Synthetic organic chemistry underpins many scientific disciplines. The development of new synthetic methods proceeds with the ultimate intention of providing access to novel structural motifs or providing safer, increasingly efficient, or more economical chemical reactions. To facilitate the identification and application of new methods in solving real synthetic problems, this Account will highlight the benefits of providing a fuller picture of both the scope and limitations of new reactions, with a primary focus on the evaluation of functional group tolerance and stability of a reaction using intermolecular screens. This Account will begin with a discussion on reaction evaluation, specifically considering the suitability of a given reaction for application in target-oriented synthesis. A comparison of desirable and essential criteria when choosing a reaction is given, and a short discussion on the value of negative and qualitative data is provided. The concept of intermolecular reaction screening will be introduced, and a direct comparison with a traditional substrate scope highlights the benefits and limitations of each and thus the complementary nature of these approaches. In recent years, a number of ad hoc applications of intermolecular screens to evaluate the functional group tolerance of a reaction or the stability of functional groups to a given set of reaction conditions have been reported, and will be discussed. More recently, we have developed a formal high-throughput intermolecular screening protocol that can be utilized to rapidly evaluate new chemical reactions. This simple and rapid protocol enables a much broader evaluation of a reaction in terms of functional group tolerance and the stability of chemical motifs to the reaction conditions than is feasible with a typical reaction scope. The development, evaluation, and application of this method within our group will be discussed in detail, with both the potential benefits and limitations highlighted and discussed. In addition, we will discuss more recent applications of intermolecular screens from both industrial and academic groups. Modifications in protocols and applications will be highlighted, including problem based evaluations, assessment of biomolecule compatibility, establishment of relative rate data, and the identification of new reactivity. Such screens have been applied in diverse chemistries including C-H functionalization reactions, frustrated Lewis-pair-catalyzed hydrogenations, heterogeneous catalysis, photoredox catalysis, enantioselective organocatalysis, and polymer science. We feel that the application of intermolecular screens to such a diversity of reactions highlights the practical simplicity of such screens. A summary of the applications and potential utility of intermolecular reaction evaluation is provided.

Determination of screened Coulomb repulsion energies in organic molecular crystals: A real space approach

We present a general method for determining screened Coulomb parameters in molecular assemblies, in particular organic molecular crystals. This allows us to calculate the interaction parameters used in a generalized Hubbard model description of correlated organic materials. In such a model only the electrons in levels close to the Fermi level are included explicitly, while the effect of all other electrons is included as a renormalization of the model parameters. For the Coulomb integrals this renormalization is mainly due to screening. For molecular materials we can split the screening into intra- and inter-molecular screening. Here we demonstrate how the inter-molecular screening can be calculated by modeling the molecules by distributed point-polarizabilities and solving the resulting self-consistent electrostatic screening problem in real space. For the example of the quasi one-dimensional molecular metal TTF–TCNQ we demonstrate that the method gives remarkably accurate results.

Thermodynamics and Kinetic Processes of Polymer Blends and Block Copolymers in the Presence of Pressurized Carbon Dioxide

AbstractEnvironmentally‐responsible materials processing is becoming an important global consideration in a wide variety of technologies, especially those wherein volatile and/or residual organic solvents cannot be tolerated. In this context, polymer processing has benefited tremendously from advances achieved using high‐pressure CO2 as a viscosity modifier, plasticizing agent, foaming agent, and reaction medium. Pressurized CO2 is environmentally benign, inexpensive, sustainable, and versatile owing to its gas‐like viscosity and liquid‐like solubility, which can be tuned through judicious choice of temperature and pressure. The addition of high‐pressure CO2 to homopolymer blends and block copolymers can have a profound impact on polymer thermodynamics and kinetic processes since the number of interacting species increases. As a result, the compressibility as well as plasticization and intermolecular screening effects become non‐negligible. In this work, we review how these factors influence such polymeric systems, and discuss commercial polymer processes and applications that benefit from the use of high‐pressure CO2.

Spectral broadening due to long-range Coulomb interactions in the molecular metal TTF-TCNQ

We employ density-functional theory to calculate realistic parameters for an extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra- and intermolecular screening in the crystal, we find significant longer-range Coulomb interactions along the molecular stacks, as well as inter-stack coupling. We show that the long-range Coulomb term of the extended Hubbard model leads to a broadening of the spectral density, likely resolving the problems with the interpretation of photoemission experiments using a simple Hubbard model only.

Screening and Surface States in Molecular Monolayers Adsorbed on Silicon

We performed density functional theory calculations of the atomic and electronic structure of a dense monolayer of phenyl-terminated alkyl chains chemisorbed onto the (100) Si surface. Different adsorption sites were characterized for both the pristine and (2 x 1) reconstructed surface. A strong effect on the ordering and alignment of the molecular energy levels with respect to the Fermi level of silicon is observed, consequent to intermolecular screening in the monolayer and of the appearance of surface localized states, as a function of the different bonding arrangements. Some possible consequences of these findings are discussed in the framework of the experimental synthesis of such monolayers as molecular current rectifiers in silicon-integrated nanoscale electronics.

Prediction of the Phase Behaviour of Hydrogen-Bonded Polymer Blends

In the early 1990s your authors believed that they had essentially solved the problem of predicting the phase behaviour of hydrogen-bonded polymer blends. A text devoted to this subject, Specific Interactions and the Miscibility of Polymer Blends, was published and we thought that it was now time to look around for something else to do. This was before a colleague, Boris Veytsman, pointed out that there was a flaw in our derivation of the free energy of mixing equation. It has taken us some 15 years to correct the theory and match the predictions of the phase behaviour of hydrogen-bonded blends that we presented in our 1991 book. So we have come full circle. The subject has become far more complicated, but at the same time far more interesting. Along the way we have discovered that the phase behaviour of hydrogen-bonded polymer blends can be successfully predicted using equilibrium constants determined from appropriate low molecular weight analogues, if chain connectivity effects such as intermolecular screening and functional group accessibility are included.

Screening effects in C60 solids

The dielectric function for solid C60 is calculated within local-density-functional theory. The screened Coulomb interaction shows a remarkable orientation and position dependence. Inter-molecular screening is stronger than intra-molecular screening and screening increases with inter-molecular separation. Some of the inequivalent on-site atomic screened Coulomb integrals are comparable to the nearest-neighbour terms and the second-nearest-neighbour terms are smaller than the third-nearest-neighbour terms. The screened on-site molecular Coulomb integral U0 is found to be approximately 2.1 eV and the local multiplet splitting for three electrons in the t1u is approximately 0.12 eV. suggesting that electrons in the derived band should be highly correlated.

Screening trends in C 60 solids

The dielectric function for solid C 60 is calculated within the RPA. The screened coulomb interaction shows a remarkable spatial and orientation dependence. Inter-molecular screening is considerably better than intra-molecular screening and inter-molecular screening increases with separation of molecules.

Dynamic effects in VUV- and XUV-Spectroscopy of Organic Molecular Solids

Dynamic effects, i.e., shake up transitions in photoemission of non-polar, polar and, in particular, dipolar organic compounds are reviewed. Gas phase spectra are contrasted with solid phase spectra. For the non-polar molecules a one-to-one correspondence between the two phases is well documented. For polar and dipolar molecules, however, this one-to-one correspondence is lost, i.e., the dynamics of the hole creation process changes. It is shown that intermolecular screening becomes important. We develop simple physical criteria that allow to rationalize and predict under which conditions such effects are to be expected. In general it is concluded, that the interpretation of solid state spectra on the basis of calculations on the free molecule is not necessarily sound. We show, that even in cases where solid and gas phase spectra appear to be similar this similarity may be accidental and that it may be caused by particularly intermolecular interactions in the solid.