José Antonio Vergés

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 which interact between them due to the long-range Coulomb terms of Hamiltonian. After careful analysis of the interaction among topological sates, we find that the discrepancy between ab initio and model Hamiltonians can be resolved considering a screened long-range interaction that is implemented adding an exponential cutoff to the interaction term of 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 Hubbard model but not completely unscreened as is the case for the PPP model.

The introduction of thiophene rings to the helical structure of carbohelicenes has electronic effects that may be used advantageously in organic electronics. The performance of these devices is highly dependent on the sulfur atom topology, so a precise knowledge of the synthetic routes that may afford isomeric structures is necessary. We have studied the photocyclization pathway of both 2- and 3-styrylthiophenes on their way to thiahelicenes by experiment and theory. To begin with, the synthesis of stereochemically well-defined 2- and 3-styrylthiophenes allowed us to register first, and simulate later, the UV–vis electronic spectra of these precursors. This information gave us access through time-dependent density functional theory calculations to the very nature of the excited states involved in the photocyclization step and from there to the regio- and stereochemical outcome of the reaction. For the widely known case of a 2-styrylthiophene derivative, the expected naphtho[2,1-b]thiophene type of ring fusion was predicted and experimentally observed by synthesis. On the contrary, 3-styrylthiophene derivatives have been seldom used in synthetic photocyclizations. Among the two possible structural outcomes, only the naphtho[1,2-b]thiophene type of ring fusion was found to be mechanistically sound, and this was actually the only compound observed by synthesis.

The crystalline structure of potassium intercalated para-terphenyl has been theoretically investigated using a van der Waals Density Functional (vdW--DF) scheme. The incorporation of K atoms into the pristine terphenyl crystal continuously improves the compound stability up to a 3:1 stoichiometry. In addition to confirming previously studied structures, several new crystalline phases showing inedited patterns -small clusters formed by four to six potassium atoms- have been found, in particular, for the more stable K3Terphenyl crystals. Initially, the band structure of these compounds shows metallic features. Nevertheless, a better description of the electronic structure using hybrid functionals that go beyond generalized-gradient-approximation (GGA) reveals that potassium electrons completely filled three sub--bands that develop at the conduction band bottom. Consequently, these stable phases should be discarded as metallic phases able to show superconductivity at low temperatures. Moreover, our study evidences that due to the important electronic correlations shown by these compounds only computational schemes that go beyond GGA are able to provide robust conclusions.

New possible structures for the compounds formed by intercalation of an alkali metal into a molecular crystal formed by polycyclic aromatic hydrocarbon (PAH) molecules have been theoretically searched for. Among them, a crystalline framework in which PAH molecules show a slipped parallel geometry provides an interesting alternative to the usual herringbone arrangement shown by pristine crystal structures of PAHs. While these different crystalline structures are energetically unfavorable for potassium alloying, they are energetically preferred at the highest cesium load (3:1 stoichiometry). The most remarkable feature of the proposed structure is the avoidance of the magnetic instability that leads to insulating phases of herringbone crystals when a 3:1 proportion of metal-Picene is reached. After the evaluation of the corresponding energies of formation, a robust metallic scenario is found for Cs3Pentacene, making it possible the observation of superconductivity.

Electronic polarizability of finite chains is accurately calculated from the total energy variation of the system produced by small but finite static electric fields applied along the chain direction. Normalized polarizability, that is, polarizability divided by chain length, diverges as the second power of length for metallic systems but approaches a constant value for insulating systems. This behaviour provides a very convenient way to characterize the wave-function malleability of finite systems as it avoids the need of attaching infinite contacts to the chain ends. Hubbard model calculations at half filling show that the method works for a small U = 1 interaction value that corresponds to a really small spectral gap of 0.005 (hopping t = −1 is assumed). Once successfully checked, the method has been applied to the long-range hopping model of Gebhard and Ruckenstein showing 1/ r hopping decay (Gebhard and Ruckenstein 1992 Phys. Rev. Lett . 68 244; Gebhard et al 1994 Phys. Rev . B 49 10926). Metallicity for U values below the reported metal-insulator transition is obtained but the surprise comes for U values larger than the critical one (when a gap appears in the spectral density of states) because a steady increase of the normalized polarizability with size is obtained. This critical size-scaling behaviour can be understood as corresponding to a molecule which polarizability is unbounded. We have checked that a real transfer of charge from one chain end to the opposite occurs as a response to very small electric fields in spite of the existence of a large gap of the order of U for one-particle excitations. Finally, ab initio quantum chemistry calculations of realistic poly-acetylene chains prove that the occurrence of such critical behaviour in real systems is unlikely.

Potassium is introduced into the crystalline herringbone structure of pentacene searching for a compound showing metallic electronic transport properties and, hopefully, superconductivity at small enough temperatures. Several possible structures for stoichiometric KPentacene (1:1), K2Pentacene (2:1) and K3Pentacene (3:1) compounds are theoretically investigated. Detailed densities of states for all of them are presented. As a more prominent result, a new monoclinic structure has been stabilized for the potassium richer material that could correspond to the recently synthesized superconducting phase of K3Pentacene. Although energetically unfavorable, it is the only metallic candidate found to date.