A computational study of two promising tweezers

A DFT study was carried out to explore the properties of two nonplanar π-conjugated systems that share dibenzo[a,e]-cyclooctatetraene (DBCOT) as a fundamental element. These systems were presented as molecules with potential use as tweezers and in optoelectronic applications due to their expected nonlinear optical effects. Structure optimizations, TD-DFT calculations, molecular orbitals and topological analysis were performed for C 36 N 2 H 18 O 4 and C 26 H 18 structures, formed by one and two 1,3,5,7- cyclooctatetraene rings, respectively, with and without the Na cation. This alkali cation was used as a model to investigate the potential of these molecules as tweezers. The results show a V-shaped structure as a minimum in the ground state for both molecules. This angular structure can trap the Na cation very effectively, especially for one of the cases. In addition, the present analysis opens new studies to explore the use of these systems as tweezers of other cations, anions or aromatic molecules that may exhibit π-stacking. The analysis of the molecular orbitals involved in the main electronic transitions allows us to propose that these systems will have interesting optical properties.Xunta de Galicia | Ref. GRC2019/24Universidade de Vigo/CISU

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

Chiroptical symmetry analysis: exciton chirality-based formulae to understand the chiroptical responses of Cn and Dn symmetric systems

The high sensitivity of chiroptical responses to conformational changes and supramolecular interactions has prompted an increasing interest in the development of chiroptical applications. However, prediction of and understanding the chiroptical responses of the necessary large systems may not be affordable for calculations at high levels of theory. In order to facilitate the development of chiroptical applications, methodologies capable of evaluating the chiroptical responses of large systems are necessary. The exciton chirality method has been extensively used for the interaction between two independent chromophores through the Davydov model. For systems presenting C2 or D2 symmetry, one can get the same results by applying the selection rules. In the present article, the analysis of the selection rules for systems with symmetries Cn and Dn with n = 3 and 4 is used to uncover the origin of their chiroptical responses. We foresee that the use of the Chiroptical Symmetry Analysis (CSA) for systems presenting the symmetries explored herein, as well as for systems presenting higher symmetries will serve as a useful tool for the development of chiroptical applications.Ministerio de Economía y Competitividad | Ref. CTQ2014-58629-RMinisterio de Economía y Competitividad | Ref. CTQ2013-50575-EXPMinisterio de Economía y Competitividad | Ref. RYC-2012-10364Xunta de Galicia | Ref. EM2013/017Xunta de Galicia | Ref. GRC2015/17Xunta de Galicia | Ref. ED431F 2016/00

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

Development of robust chiroptical systems through spirobifluorenes

Chiroptical responses are valuable for the structural determination of dissymmetric molecules. However, the development of everyday applications based on chiroptical systems is yet to come. We have been earlier using axially chiral allenes for the construction of linear, cyclic, and cage‐shaped molecules that present remarkable chiroptical responses. Additionally, we have developed chiral surfaces through upstanding chiral architectures. Since the goal is to obtain robust chiroptical materials, more recently we have been studying spirobifluorenes (SBFs), a well‐established building block in optoelectronic applications. After theoretical and experimental demonstration, the suitability of chiral SBFs for the development of robust chiroptical systems was certified by the construction all‐carbon double helices, flexible shape‐persistent macrocycles, chiral frameworks for surface functionalization, and structures featuring helical or spiroconjugated molecular orbitals. Here, we give an overview of our contribution to these matters.Ministerio de Ciencia e Innovacion | Ref. TED2021-131760B-100Ministerio de UniversidadesXunta de Galicia | Ref. ED431C 2017/51Universidade de Vigo/CISU

Optical and electrochemical properties of spirobifluorene iridanaphthalene complexes

Three new spirobifluorene iridaaromatic compounds bearing electron-withdrawing or electron-donor substituents or another iridanaphthalene moiety have been synthesized and structurally characterized. Thorough experimental and theoretical evaluation revealed that these novel systems present a high thermal, air and electrochemical stability as well as low optical and electronic energy gap values with a significant redshift of the absorption maximum in the UV-Vis spectra and predicted remarkably higher first hyperpolarizabilities compared to their organic counterparts. Therefore, the combination of a metallaaromatic system with a spirobifluorene moiety leads to the design and development of new spirobifluorene derivatives. These new systems have shown interesting optical and electronic properties making them of interest for future applications in optoelectronics.Xunta de Galicia | Ref. GRC2019/2

Spatiotemporal Characteristics of the Largest HIV-1 CRF02_AG Outbreak in Spain: Evidence for Onward Transmissions

Background and Aim: The circulating recombinant form 02_AG (CRF02_AG) is the predominant clade among the human immunodeficiency virus type-1 (HIV-1) non-Bs with a prevalence of 5.97% (95% Confidence Interval-CI: 5.41–6.57%) across Spain. Our aim was to estimate the levels of regional clustering for CRF02_AG and the spatiotemporal characteristics of the largest CRF02_AG subepidemic in Spain.Methods: We studied 396 CRF02_AG sequences obtained from HIV-1 diagnosed patients during 2000–2014 from 10 autonomous communities of Spain. Phylogenetic analysis was performed on the 391 CRF02_AG sequences along with all globally sampled CRF02_AG sequences (N = 3,302) as references. Phylodynamic and phylogeographic analysis was performed to the largest CRF02_AG monophyletic cluster by a Bayesian method in BEAST v1.8.0 and by reconstructing ancestral states using the criterion of parsimony in Mesquite v3.4, respectively.Results: The HIV-1 CRF02_AG prevalence differed across Spanish autonomous communities we sampled from (p < 0.001). Phylogenetic analysis revealed that 52.7% of the CRF02_AG sequences formed 56 monophyletic clusters, with a range of 2–79 sequences. The CRF02_AG regional dispersal differed across Spain (p = 0.003), as suggested by monophyletic clustering. For the largest monophyletic cluster (subepidemic) (N = 79), 49.4% of the clustered sequences originated from Madrid, while most sequences (51.9%) had been obtained from men having sex with men (MSM). Molecular clock analysis suggested that the origin (tMRCA) of the CRF02_AG subepidemic was in 2002 (median estimate; 95% Highest Posterior Density-HPD interval: 1999–2004). Additionally, we found significant clustering within the CRF02_AG subepidemic according to the ethnic origin.Conclusion: CRF02_AG has been introduced as a result of multiple introductions in Spain, following regional dispersal in several cases. We showed that CRF02_AG transmissions were mostly due to regional dispersal in Spain. The hot-spot for the largest CRF02_AG regional subepidemic in Spain was in Madrid associated with MSM transmission risk group. The existence of subepidemics suggest that several spillovers occurred from Madrid to other areas. CRF02_AG sequences from Hispanics were clustered in a separate subclade suggesting no linkage between the local and Hispanic subepidemics

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 tetracyanobutadiene spirobifluorene: synthesis, enantiomeric resolution and chiroptical properties

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGPush-pull organic compounds with π-conjugated backbones are attracting considerable interest in terms of their highly efficient (NLO) non-linear optical effects. In this respect, cyano groups constitute one of the strongest electron acceptors suitable for NLO materials. We herein report the first 1,1,4,4-tetracyanobutadiene (TCBD) spirobifluorene (SBF) through a double [2+2] cycloaddition-retroelectrocyclization. Enantiomeric resolution by HPLC using a chiral stationary phase (CSP) and theoretical calculations unveiled not only the nature of the potential energy surface, but also the absolute configuration of each enantiomer. Since both the presence of strong donor and acceptor, as well as the intrinsic non-centrosymmetric character of chiral compounds are a prerequisite for the development of NLO properties, the first hyperpolarizability of the developed system was theoretically calculated. The results suggested a strong dependence on the conformation and an intensity enhancement for certain conformers of 1 as compared to its allenic TCBD analogue. Therefore, the present study opens a new class of SBF compounds suitable for NLO applications.Xunta de Galicia | Ref. ED431F2016/005Xunta de Galicia | Ref. GRC2019/24IBEROS | Ref. 0245_IBEROS_1_EInstituto de Investigación Sanitaria Galicia Su

Chiroptical symmetry analysis of Trianglimines: a case study

It is well established that chiroptical responses, based on the unique reaction to circularly polarized light by chiral non-racemic systems, are sensitive to the stereochemistry of the featuring systems. This behavior has promoted the use of chiroptical spectroscopies as a mandatory tool in the structure determination of molecules for decades. Recently, the higher sensitivity of chiroptical techniques compared to the conventional UV/Vis absorption and fluorescence spectroscopies or electrochemistry has awakened much interest in the development of chiroptical everyday applications. While chiroptical responses could be predicted by ab initio calculations, large systems calculated at a high level of theory may have an important computational cost; therefore, more intuitive methods are desired to design systems with tailored chiroptical responses. In this regard, the exciton chirality method has been often used in conformationally stable systems incorporating at least two independent chromophores. Taking this method into consideration, in our previous work, we described the chiroptical symmetry analysis (CSA) based on symmetry selection rules. To explore the scope of the CSA, herein we perform the chiroptical symmetry analysis of diverse trianglimines and draw general conclusions to assist on the design of chiroptical systems with high symmetry.Xunta de Galicia | Ref. ED431F 2016/005Xunta de Galicia | Ref. GRC2019/24European Commission | Ref. 0245_IBEROS_1_