Long-Range Effects in Topologically Defective Arm-Chair Graphene Nanoribbons.

The electronic structure of 7/9-AGNR superlattices with up to eight unit cells has been studied by means of state-of-the-art Density Functional Theory (DFT) and also by two model Hamiltonians, the first one including only local interactions (Hubbard model, Hu) while the second one is extended to allow long-range Coulomb interactions (Pariser, Parr and Pople model, PPP). Both are solved within mean field approximation. At this approximation level, our calculations show that 7/9 interfaces are better described by spin non-polarized solutions than by spin-polarized wavefunctions. Consequently, both Hu and PPP Hamiltonians lead to electronic structures characterized by a gap at the Fermi level that diminishes as the size of the system increases. DFT results show similar trends although a detailed analysis of the density of states around the Fermi level shows quantitative differences with both Hu and PPP models. Before improving model Hamiltonians, we interpret the electronic structure obtained by DFT in terms of bands of topological states: topological states localized at the system edges and extended bulk topological states that interact between them due to the long-range Coulomb terms of Hamiltonian. After careful analysis of the interaction among topological states, we find that the discrepancy between ab initio and model Hamiltonians can be resolved considering a screened long-range interaction that is implemented by adding an exponential cutoff to the interaction term of the PPP model. In this way, an adjusted cutoff distance λ=2 allows a good recovery of DFT results. In view of this, we conclude that the correct description of the density of states around the Fermi level (Dirac point) needs the inclusion of long-range interactions well beyond the Hubbard model but not completely unscreened as is the case for the PPP model.

Investigating the (Poly)Radicaloid Nature of Real-World Organic Compounds with DFT-Based Methods.

Recent advances in the synthesis of stable organic (open-shell) polyradicaloids have opened their application as active compounds for emerging technologies. These systems typically exhibit small energy differences between states with different spin multiplicities, which are intrinsically difficult to calculate by theoretical methods. We thus apply here some DFT-based variants (FT-DFT, SF-DFT, and SF-TDDFT) on a test set of large and real-world molecules, as test systems for which such energy differences are experimentally available, also comparing systematically with RAS-SF results to infer if shortcomings of previous DFT applications are corrected. Additionally, we explore the spin-spin contribution to the ZFS tensor, of high interest for EPR spectroscopy, and derive the spatial extent of the corresponding (photoexcited) triplet state.

Transport and Optical Gaps in Amorphous Organic Molecular Materials.

The standard procedure to identify the hole- or electron-acceptor character of amorphous organic materials used in OLEDs is to look at the values of a pair of basic parameters, namely, the ionization potential (IP) and the electron affinity (EA). Recently, using published experimental data, the present authors showed that only IP matters, i.e., materials with IP > 5.7 (<5.7) showing electron (hole) acceptor character. Only three materials fail to obey this rule. This work reports ab initio calculations of IP and EA of those materials plus two materials that behave according to that rule, following a route which describes the organic material by means of a single molecule embedded in a polarizable continuum medium (PCM) characterized by a dielectric constant ε . PCM allows to approximately describe the extended character of the system. This "compound" system was treated within density functional theory (DFT) using several combinations of the functional/basis set. In the preset work ε was derived by assuming Koopmans' theorem to hold. Optimal ε values are in the range 4.4⁻5.0, close to what is expected for this material family. It was assumed that the optical gap corresponds to the excited state with a large oscillator strength among those with the lowest energies, calculated with time-dependent DFT. Calculated exciton energies were in the range 0.76⁻1.06 eV, and optical gaps varied from 3.37 up to 4.50 eV. The results are compared with experimental data.

Herringbone Pattern and CH-π Bonding in the Crystal Architecture of Linear Polycyclic Aromatic Hydrocarbons.

The herringbone pattern is a pervasive structural motive found in most molecular crystals involving aromatic compounds. A plot of the experimental sublimation enthalpies of members of increasing size of the acene, phenacene and p-phenyl families versus the number of carbons uncovers a linear relationship between the two magnitudes, suggesting a major role of CH-π bonding. In this work we undertake the task of evaluating the relevance of the edge-to-face interaction (or CH-π bond) in the overall reticular energy of the crystal, to quantitatively assess the importance of this structural element. Following a heuristic approach, we considered the series of acenes, phenacenes and p-phenyls and analyzed the edge-to-face interaction between the molecules as they occur in the experimental crystal network. Isolation of the relevant molecular dimers allows to incorporate some of the most sophisticated tools of quantum chemistry and get a reliable picture of the isolated bond. When compared to the experimental sublimation energy, our results are conclusive: this sole interaction is the largest contribution to the lattice energy, and definitively dictates the crystal architecture in all the studied cases. Elusive enough, the edge-to-face interaction is mainly dominated by correlation interactions, specifically in the form of dispersion and, to a less extent, of charge-transfer terms. A suggestive picture of the bond has been obtained by displaying the differences in local electron densities calculated by either correlated or non-correlated methods.

Intra- and intermolecular dispersion interactions in [N]cycloparaphenylenes: do they influence their structural and electronic properties?

Cycloparaphenylenes (CPPs) are nanosized structures with unique isolated and bulk properties, and are synthetic targets for the template-driven bottom-up synthesis of carbon nanotubes. Thus, a systematic understanding of the supramolecular order at the nanoscale is of utmost relevance for molecular engineering. In this study, it is found that intramolecular noncovalent (dispersion) interactions must be taken into account for obtaining accurate estimates of the structural and optoelectronic properties of [n]CPP compounds, and their influence as the number of repeat units increases from n=4 to n=12 is also analyzed, both in the gas phase and in solution. The supramolecular self-assembly, for which both intra- and intermolecular noncovalent interactions are relevant, of [6]CPP is also investigated by calculating the binding energies of dimers taken along several crystal directions. These are also used to estimate the cohesive energy of the crystal, which is compared to the value obtained by means of dispersion-corrected DFT calculations using periodic boundary conditions. The reasonable agreement between both computational strategies points towards a first estimate of the [6]CPP cohesive energy of around 50 kcal mol(-1) .

Cyclobutadiene automerization and rotation of ethylene: energetics of the barriers by using spin-polarized wave functions.

Spin-projected spin polarized Møller-Plesset and spin polarized coupled clusters calculations have been made to estimate the cyclobutadiene automerization, the ethylene torsion barriers in their ground state, and the gap between the singlet and triplet states of ethylene. The results have been obtained optimizing the geometries at MP4 and/or CCSD levels, by an extensive Gaussian basis set. A comparative analysis with more complex calculations, up to MP5 and CCSDTQP, together with others from the literature, have also been made, showing the efficacy of using spin-polarized wave functions as a reference wave function for Møller-Plesset and coupled clusters calculations, in such problems.