Nicolás Ramos-Berdullas

Electronic Properties of p-Xylylene and p-Phenylene Chains Subjected to Finite Bias Voltages: A New Highly Conducting Oligophenyl Structure

Recently, experimental and theoretical determination of electric currents induced by finite bias voltages in p-xylylene chains attached to gold contacts revealed higher conductance of these systems in comparison with p-phenylene homologous chains. To gain more insight into the conducting properties of these oligophenyl structures, ab initio studies were carried out on the electronic properties of two different p-xylylene-like chains (pX1 and pX2) and the p-phenylene (pP) chain attached to gold contacts, with molecular formulas AuCH2 (C6 H4 )n CH2 Au (n=1-5), Au2 C(C6 H4 )n CAu2 (n=1-5), and Au(C6 H4 )n Au (n=1-5), respectively. The molecules were subjected to finite bias voltages ranging from 0 to 5 V. Analysis of the intramolecular electron transfer and electron delocalization revealed a completely opposite response to electric perturbation of pX2 in comparison with pX1 and pP. Thus, in pX2 the applied voltage causes an increase in the electron delocalization within the rings together with a large electron transfer and energetic stabilization. On the contrary, the same voltages partially destroy the electron delocalization in pX1 and pP, produce a large local electron polarization in the benzene rings, and a smaller energetic stabilization. These differences can be rationalized in terms of the role played by polarized valence bond structures in the total wave function. Theoretical estimation of the I/V profiles indicates that pX2 chains are much better electronic conductors than pX1 and pP.

Analyzing the electric response of molecular conductors using “electron deformation” orbitals and occupied‐virtual electron transfer

The concept of “electron deformation orbitals” (EDOs) is used to investigate the electric response of conducting metals and oligophenyl chains. These orbitals and their eigenvalues are obtained by diagonalization of the deformation density matrix (difference between the density matrices of the perturbed and unperturbed systems) and can be constructed as linear combinations of the unperturbed molecular orbitals within “frozen geometry” conditions. This form of the EDOs allows calculating the part of the electron deformation density associated to an effective electron transfer from occupied to virtual orbitals (valence to conduction band electron transfer in the band model of conductivity). It is found that the “electron deformation” orbitals pair off, displaying the same eigenvalue but opposite sign. Each pair represents an amount of accumulation/depletion of electron charge at different molecular regions. In the oligophenyl systems investigated only one pair contributes effectively to the charge flow between molecular ends, resulting from the promotion of electrons from occupied orbitals to close in energy virtual orbitals of appropriate symmetry and overlapping. Analysis of this pair along explains the differences in conductance of olygophenyl chains based on phenyl units.

Aromaticity of closed-shell charged polybenzenoid hydrocarbons.

The aromatic stabilization of closed-shell charged polybenzenoid hydrocarbons (PBHs) has been scrutinized by means of energetic and magnetic aromaticity criteria and by direct measures of electron delocalization. Thus, topological resonance energies and their circuit contributions, ring current maps, and multicenter delocalization indices have been calculated for a series of 18 polybenzenoid cations containing from 3 to 10 benzene rings. All calculations indicate that the closed-shell cations have a similar degree of aromaticity compared to that of the corresponding closed-shell neutral PBHs. All cations investigated display a large degree of electronic delocalization in the ring, accompanied by significant aromatic stabilization and a strong diatropic peripheral electron current. Graph theoretical models describe perfectly the aromatic features of these hydrocarbon fragments, showing how they can be understood as a superposition of specific neutral PBHs. The large aromatic character of these systems suggests they may be relatively stable upon formation at combustion conditions, like those given in the interstellar medium. It has been postulated that closed-shell fragments of PBHs may play an important role in the photoluminescent phenomenon known as extended red emission.

Revisiting the mechanism and the influence of the excitation wavelength on the surface-enhanced Raman scattering of the pyridine–Ag 20 system

This work presents the application of a recent decomposition scheme of the Raman tensor into molecule and surface contributions to the study of the static and resonance Raman spectra of pyridine adsorbed on a Ag20 cluster, a typical probe for the theoretical study of surface-enhanced Raman scattering (SERS) spectra. The results obtained show that both the chemical and electromagnetic enhancements observed are related to changes on the polarizability and polarizability derivatives of the pyridine molecule. No significant contributions from the surface and from vibrational intermolecular coupling are found. Since similar incident lights produce remarkably different SERS spectra, the effect of excitation wavelength on the spectra of the PY–Ag20 complexes is also scrutinized. From the computed Raman excitation profiles and from the analysis of the electron density changes upon electronic transitions, it is established that the differences found can be related to the amount of electron density transferred from the silver cluster to pyridine upon excitation and to the distance between both units. These findings suggest that a proper knowledge of the effect of the excitation wavelength is necessary for obtaining a reliable theoretical interpretation of surface-enhanced Raman spectra.

Study of electron transport in polybenzenoid chains covalently attached to gold atoms through unsaturated methylene linkers

Abstract It is well recognized that chemical properties such as aromaticity influence strongly the transport of electrons within a molecule. Since these properties are calculated from equilibrium wave functions, methods relying on equilibrium electronic distributions to determine the electron transport, such as those stemmed from the energy–time uncertainty relation, may be suitable to connect chemical concepts and electron transport. Here, we explore the relation of molecular conductance with the electric response and aromatic stabilization of molecular junctions formed with chains of polybenzenoid units attached to gold atoms through unsaturated methylene carbons. We have found that voltage dependence of conductance stems from the amount of electrons promoted by the electric potential from occupied to virtual molecular orbitals. We also show that aromaticity assists the electron transport when it arises from resonant polarized structures. Otherwise, aromaticity decreases electron transport as reported in previous theoretical and experimental works.

Can single graphene nanodisks be used as Raman enhancement platforms

Recent works on plasmonic properties of graphene molecules have pointed out the possibility of building optical devices and Raman sensors using individual molecules. Here, the Raman spectra of different biomolecular units adsorbed on a zig-zag graphene nanodisk of ninety six carbon atoms (C96) are investigated using time-dependent perturbation methods. Static and pre-resonance conditions have been simulated to elucidate the Raman enhancement mechanism via ground state and excited state interactions with the nanodisk. Stacking and H-π complexes formed by the pyronine cation, porphine, tetrabenzoporphine and phthalocyanine with C96 have been considered. Static polarizability changes, charge transfer transitions, surface resonance, molecule-surface vibrational couplings and symmetry factors may all influence the Raman spectra of the molecules. We have explored the role played by each factor in the different conformational dispositions. Our results point out the use of small nanographene structures as promising for the development of SERS platforms at the frontier of nanometer and subnanometer scales.