Confinement on the optical response in h-BNCs: towards highly efficient SERS-active 2D substrates

Several experimental and theoretical studies have shown that 2D hybrid structures formed by boron, nitrogen and carbon atoms (h-BNCs) possess a highly tunable linear and non-linear optical responses. Recent advances towards the controlled synthesis of these unique structures have motivated an important number of experimental and theoretical work. In this work, the confinement on the optical response induced by boron-nitride (BN) strings in h-BNC 2D structures is investigated using time-dependent density functional theory (TDDFT) and electron density response properties. The number of surrounding BN strings (NBN) necessary to “isolate” the optical modes of a carbon nanoisland (nanographene) from the remaining substrate has been characterized in two different nanoisland models: benzene and pyrene. It was found that for NBN ≥ 3 the excitation wavelengths of the optically active modes remain constant and the changes in the transition densities, the ground to excited state density differences and their associated electron deformation orbitals are negligible and strongly confined within the carbon nanoisland. Using a water molecule as model system, Raman enhancement factors of 10^6 for the water vibrational modes are obtained when these electromagnetic “hot spots” are activated by an external electromagnetic field. The high tunability of the optical absorption bands of nanographenes through changes in size and morphology makes h-BNCs be perfect materials to construct platforms for surface enhancement Raman spectroscopy (SERS) for a wide range of laser sources.Xunta de Galicia | Ref. GRC2019/2

A new method to analyze and understand molecular linear and nonlinear optical responses via field-induced functions: a straightforward alternative to sum-over-states (SOS) analysis

The sum-over-states (SOS) method allows the computation of polarizabilities and hyperpolarizabilities additively from the contributions of different electronic excited states in a given molecule or cluster. Subsequent analysis of the main excited configurations contributing to the relevant excited states allows characterizing the orbitals involved in the linear and nonlinear optical response. Unfortunately, the chemically relevant information that can be obtained by SOS is hindered by a series of methodological and computational drawbacks. Among these drawbacks, we can highlight the high computational cost, problems arising from nonconvergent series and errors caused by the inaccurate description of excitation energies and/or higher excited state matrix elements. For this reason, coupled-perturbed schemes are currently widely used to determine the NLO potential of molecules and materials. However, such a choice limits the amount of intuitive chemical information that, on the other hand, can be retrieved by a successful SOS computation. In this work, we present and discuss a novel computational strategy that offers the means to extract the useful chemical insights from a coupled-perturbed calculation at almost negligible extra computational cost providing a transparent picture about orbital contributions to the properties of interest. The proposed method is based on the generation and further analysis of field-induced orbitals, FIOs, from the analytic or numerical derivatives of the dipole moment. Orbital symmetry rules are derived using group theory and the method is tested for a series of small and medium size systems.Xunta de Galicia | Ref. GRC2015/01

Can aromaticity enhance the electron transport in molecular wires?

An interesting debate has been recently raised around the role played by aromaticity in the electron transport ability of molecular wires. Normally, it is associated to destructive interference effects, so that the more aromatic the wire the less conductor. This rule was observed experimentally in a series of homologous wires containing ring units of different aromaticity, but theoretical calculations and other recent experiments demonstrate the rule cannot be generalized and depends, for instance, in the type of molecule-electrode contact. However, neither chemical explanation nor qualitative rules were given yet to allow predicting the specific behavior of different molecular junctions. In this work, using series of polymeric molecular wires of different length and formed by different aromatic units, it is proven how it is possible to change from an expected destructive to a constructive interference effect of the aromaticity in the electron transport. Thus, aromaticity may be also employed to enhance the electron transport in a molecular wire. A chemical explanation to the experimental and theoretical observations is given and a simple way of tuning the response of a molecular wire to an external electric voltage by increasing/decreasing its aromaticity and changing its type of molecule-electrode contact is provided.Xunta de Galicia | Ref. GRC2015/01

Effect of the QM size, basis set, and polarization on QM/MM interaction energy decomposition analysis

Herein, an Energy Decomposition Analysis (EDA) scheme extended to the framework of QM/MM calculations in the context of electrostatic embeddings (QM/MM-EDA) including atomic charges and dipoles is applied to assess the effect of the QM region size on the convergence of the different interaction energy components, namely, electrostatic, Pauli, and polarization, for cationic, anionic, and neutral systems interacting with a strong polar environment (water). Significant improvements are found when the bulk solvent environment is described by a MM potential in the EDA scheme as compared to pure QM calculations that neglect bulk solvation. The predominant electrostatic interaction requires sizable QM regions. The results reported here show that it is necessary to include a surprisingly large number of water molecules in the QM region to obtain converged values for this energy term, contrary to most cluster models often employed in the literature. Both the improvement of the QM wave function by means of a larger basis set and the introduction of polarization into the MM region through a polarizable force field do not translate to a faster convergence with the QM region size, but they lead to better results for the different interaction energy components. The results obtained in this work provide insight into the effect of each energy component on the convergence of the solute−solvent interaction energy with the QM region size. This information can be used to improve the MM FFs and embedding schemes employed in QM/MM calculations of solvated systems.Agencia Estatal de Investigación | Ref. PID2020-117806GA-I00Comunidad de Madrid | Ref. 2018-T1/BMD-10261Xunta de Galicia | Ref. GRC2019/24Universidade de Vigo/CISU

Simulating the detection of dioxin-like pollutants with 2D surface-enhanced Raman spectroscopy using h-BNC substrates

The ability of 2D hybrid structures formed by boron, nitrogen and carbon atoms (h-BNCs) to act as potential substrates for the surface-enhanced Raman spectroscopy (SERS) detection of dioxin-like pollutants is theoretically analyzed. The strong confinement and high tunability of the electromagnetic response of the carbon nanostructures embedded within the h-BNC sheets point out that these hybrid structures could be promising for applications in optical spectroscopies, such as SERS. In this work, two model dioxin-like pollutants, TCDD and TCDF, and a model h-BNC surface composed of a carbon nanodisk of ninety-six atoms surrounded by a string of borazine rings, BNC96, are used to simulate the adsorption complexes and the static and pre-resonance Raman spectra of the adsorbed molecules. A high affinity of BNC96 for these pollutants is reflected by the large interaction energies obtained for the most stable stacking complexes, with dispersion being the most important contribution to their stability. The strong vibrational coupling of some active modes of TCDF and, specially, of TCDD causes the static Raman spectra to show a ”pure” chemical enhancement of one order of magnitude. On the other hand, due to the strong electromagnetic response of BNC96, confined within the carbon nanodisk, the pre-resonance Raman spectra obtained for TCDD and TCDF display large enhancement factors of 108 and 107, respectively. Promisingly, laser excitation wavelengths commonly used in SERS experiments also induce significant Raman enhancements of around 104 for the TCDD and TCDF signals. Both the strong confinement of the electromagnetic response within the carbon domains and the high modulation of the resonance wavelengths in the visible and/or UV region in h-BNCs should lead to a higher sensitivity than that of graphene and white graphene parent structures, thus overcoming one of the main disadvantages of using 2D substrates for SERS applications.Xunta de Galicia | Ref. GRC2019/2

The effect of spin polarization on the electron transport of molecular wires with diradical character

Some of the most promising materials for application in molecular electronics and spintronics are based on diradical chains. Herein, the proposed relation between increasing conductance with length and diradical character is revisited using ab initio methods that account for the static electron correlation effects. Electron transmission was previously obtained from restricted single determinant wavefuntions or tight-binding approximations, which are unable to account for static correlation. Broken Symmetry Unrestricted Kohn-Sham Density Functional Theory (BS-UKS-DFT) in combination with electron transport analysis based on electron deformation orbitals (EDOs) reflects an exponential decay of the electrical conductance with length. Also, other important effects such as quantum interference are correctly accounted for, leading to a decrease of the conductance as the diradical character increases. As a proof-of-concept, the electrical conductance obtained from BS-UKS-DFT and CASSCF(2,2) wavefunctions were compared in diradical graphene strips in the frame of the pseudo-π approach, obtaining very similar resultsXunta de Galicia | Ref. GRC2019/2

Assessing the reversed exponential decay of the electrical conductance in molecular wires: the undeniable effect of static electron correlation

An extraordinary new family of molecular junctions, inaccurately referred to as "anti-Ohmic" wires in the recent literature, has been proposed based on theoretical predictions. The unusual electron transport observed for these systems, characterized by a reversed exponential decay of their electrical conductance, might revolutionize the design of molecular electronic devices. This behavior, which has been associated with intrinsic diradical nature, is reexamined in this work. Since the diradical character arises from a near-degeneracy of the frontier orbitals, the employment of a multireference approach is mandatory. CASSCF calculations on a set of nanowires based on polycyclic aromatic hydrocarbons (PAHs) demonstrate that, in the frame of an appropriate multireference treatment, the ground state of these systems shows the expected exponential decay of the conductance. Interestingly, these calculations do evidence a reversed exponential decay of the conductance, although now in several excited states. Similar results have been obtained for other recently proposed candidates to "anti-Ohmic" wires. These findings open new horizons for possible applications in molecular electronics of these promising systems.Xunta de Galicia | Ref. GRC2019/24Agencia Estatal de Investigación | Ref. PGC2018-095953-B-I0

Tracking the transition from pericyclic to pseudopericyclic reaction mechanisms using multicenter electron delocalization analysis: the [1,3] sigmatropic rearrangement

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGHerein, the power of multicenter electron delocalization analysis to elucidate the intricacies of concerted reaction mechanisms is brought to light by tracking the transition of [1,3] sigmatropic rearrangements from the high-barrier pericyclic mechanism in 1-butene to the barrierless pseudopericyclic mechanism in 1,2-diamino-1-nitrosooxyethane. This transition has been progressively achieved by substituting the migrating group, changing the donor and acceptor atoms, and functionalizing the alkene unit with weak and strong electron-donating and electron-withdrawing groups. Fourteen [1,3] sigmatropic reactions with electronic energy barriers ranging from 1 to 89 kcal/mol have been investigated. A very good correlation has been found between the barrier and the four-center electron delocalization at the transition state, the latter calculated for the atoms involved in the four-centered ring adduct formed along the reaction path. Surprisingly, the barrier has been found to be independent of the bond strength between the migrating group and the donor atom so that only the changes induced in the multicenter bonding control the kinetics of the reaction. Additional insights into the effect of atom substitution and group functionalization have also been extracted from the analysis of the multicenter electron delocalization profiles along the reaction path and qualitatively supported by the topological analysis of the electron density.Xunta de Galicia | Ref. GRC2019/2

A highly efficient neutral anion receptor in polar environments by synergy of anion−π interactions and hydrogen bonding

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGHerein, it is shown how anion recognition in highly polar solvents by neutral metal-free receptors is feasible when multiple hydrogen bonding and anion−π interactions are suitably combined. A neutral aromatic molecular tweezer functionalized with azo groups is shown to merge these two kinds of interactions in a unique system and its efficiency as an anion catcher in water is evaluated using first-principles quantum methods. Theoretical calculations unequivocally prove the high thermodynamic stability in water of a model anion, bromide, captured within the tweezer’s cavity. Thus, static calculations indicate anion–tweezer interaction energies within the range of covalent or ionic bonds and stability constants in water of more than 10 orders of magnitude. First-principles molecular dynamics calculations also corroborate the stability through the time of the anion–tweezer complex in water. It shows that the anion is always found within the tweezer’s cavity due to the combination of the tweezer–anion interactions plus a hydrogen bond between the anion and a water molecule that is inside the tweezer’s cavity.Xunta de Galicia | Ref. GRC2019/2

Potential Application of h-BNC Structures in SERS and SEHRS Spectroscopies: A Theoretical Perspective

In this work, the electronic and optical properties of hybrid boron-nitrogen-carbon structures (h-BNCs) with embedded graphene nanodisks are investigated. Their molecular affinity is explored using pyridine as model system and comparing the results with the corresponding isolated graphene nanodisks. Time-dependent density functional theory (TDDFT) analysis of the electronic excited states was performed in the complexes in order to characterize possible surface and charge transfer resonances in the UV region. Static and dynamic (hyper)polarizabilities were calculated with coupled-perturbed Kohn-Sham theory (CPKS) and the linear and nonlinear optical responses of the complexes were analyzed in detail using laser excitation wavelengths available for (Hyper)Raman experiments and near-to-resonance excitation wavelengths. Enhancement factors around 103 and 108 were found for the polarizability and first order hyperpolarizability, respectively. The quantum chemical simulations performed in this work point out that nanographenes embedded within hybrid h-BNC structures may serve as good platforms for enhancing the (Hyper)Raman activity of organic molecules immobilized on their surfaces and for being employed as substrates in surface enhanced (Hyper)Raman scattering (SERS and SEHRS). Besides the better selectivity and improved signal-to-noise ratio of pristine graphene with respect to metallic surfaces, the confinement of the optical response in these hybrid h-BNC systems leads to strong localized surface resonances in the UV region. Matching these resonances with laser excitation wavelengths would solve the problem of the small enhancement factors reported in Raman experiments using pristine graphene. This may be achieved by tuning the size/shape of the embedded nanographene structure