Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. -conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials. Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies but the external dynamical control of these states still awaits experimental realization. Here, via quantum chemical calculations, the authors demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units induces controlled pairing of pi -conjugated electrons in a reversible way

First-principles periodic calculation of four-body spin terms in high-Tc cuprate superconductors

A general mapping between the energy of pertinent magnetic solutions and the diagonal terms of the spin Hamiltonian in a local representation provides the first general framework to extract accurate values for the many body terms of extended spin Hamiltonians from periodic first-principle calculations. Estimates of these terms for La2CuO4, the paradigm of high-Tc superconductor parent compounds, and for the SrCu2O3 ladder compound are reported. For La2CuO4, present results support experimental evidence by Toader et al. [Phys. Rev. Lett. 94, 197202 (2005)]. For SrCu2O3 even larger four-body spin amplitudes are found together with Jl/Jr=1 and non-negligible ferromagnetic interladder exchange

Electronic structure and properties of multifunctional systems: bisdithiazolyl-based materials

Els materials orgànics moleculars cada vegada tenen més aplicacions en la fabricació de dispositius electrònics per les seves propietats òptiques i de conducció. Quan els elements moleculars són radicals, cal tenir en compte alhora la càrrega i l’espín de l’electró desaparellat. La racionalització de l’estructura i de les propietats d’aquests materials multifuncionals requereix una descripció acurada de la seva estructura electrònica. En aquest treball s’analitza l’aplicabilitat dels models actuals en la modelització de la conducció elèctrica d’aquests materials, emprant tota una família de compostos derivats del bisditiazolil com a sistemes model.Paraules clau: Materials moleculars orgànics, radicals, materials multifuncionals, conductivitat elèctrica, estructura electrònica.Molecular organic materials are finding increasing application in the manufacture of electronic devices thanks to their optical and conduction properties. When the molecular moieties are radicals, both charge and spin of the unpaired electron should be taken into account. The full rationalization of the structure and properties of these multifunctional materials requires a careful description of their electronic structure. In this paper, the applicability of the current models in the modeling of the electrical conduction of these materials is analyzed, using the family of bisdithiazolyl-based compounds as model systems.Keywords: Organic molecular materials, radicals, multifunctional materials, electrical conduction, electronic structure

Controlling the Diradical Character of Thiele Like Compounds

Organic diradicals play an important role in many fields of chemistry, biochemistry, and materials science. In this work, by means of high-level theoretical calculations, we have investigated the effect of representative chemical substituents in p-quinodimethane (pQDM) and Thiele's hydrocarbons with respect to the singlet-triplet energy gap, a feature characterizing their diradical character. We show how the nature of the substituents has a very important effect in controlling the singlet-triplet energy gap so that several compounds show diradical features in their ground electronic state. Importantly, steric effects appear to play the most determinant role for pQDM analogues, with minor effects of the substituents in the central ring. For Thiele like compounds, we found that electron-withdrawing groups in the central ring favor the quinoidal form with a low or almost null diradical character, whereas electron-donating group substituents favor the aromatic-diradical form if the electron donation does not exceed 6-π electrons. In this case, if there is an excess of electron donation, the diradical character is reduced. The electronic spectrum of these compounds is also calculated, and we predict that the most intense bands occur in the visible region, although in some cases characteristic electronic transition in the near-IR region may appear.The authors thank the financial support from the Spanish Ministerio de Economía y Competitividad (project nos. PID2019-109518GB-I00 and PID2019-106830GB-I00 and Spanish Structures of Excellence María de Maeztu program, through grant nos. MDM-2017-0767 and CEX2021-001202-M); the Catalan Government (project nos. 2021SGR354 and 2021SGR442); and Consorci de Serveis Universitaris de Catalunya (CSUC) for providing computational resources.Peer reviewe

Emergent Spin Frustration in Neutral Mixed-Valence 2D Conjugated Polymers: A Potential Quantum Materials Platform

Two-dimensional conjugated polymers (2DCPs)─organic 2D materials composed of arrays of carbon sp2 centers connected by π-conjugated linkers─are attracting increasing attention due to their potential applications in device technologies. This interest stems from the ability of 2DCPs to host a range of correlated electronic and magnetic states (e.g., Mott insulators). Substitution of all carbon sp2 centers in 2DCPs by nitrogen or boron results in diamagnetic insulating states. Partial substitution of C sp2 centers by B or N atoms has not yet been considered for extended 2DCPs but has been extensively studied in the analogous neutral mixed-valence molecular systems. Here, we employ accurate first-principles calculations to predict the electronic and magnetic properties of a new class of hexagonally connected neutral mixed-valence 2DCPs in which every other C sp2 nodal center is substituted by either a N or B atom. We show that these neutral mixed-valence 2DCPs significantly energetically favor a state with emergent superexchange-mediated antiferromagnetic (AFM) interactions between C-based spin-1/2 centers on a triangular sublattice. These AFM interactions are surprisingly strong and comparable to those in the parent compounds of cuprate superconductors. The rigid and covalently linked symmetric triangular AFM lattice in these materials thus provides a highly promising and robust basis for 2D spin frustration. As such, extended mixed-valence 2DCPs are a highly attractive platform for the future bottom-up realization of a new class of all-organic quantum materials, which could host exotic correlated electronic states (e.g., unusual magnetic ordering, quantum spin liquids).I.A. is grateful for a Juan de la Cierva postdoctoral grant (FJC2019-038971-I) from the Ministerio de Ciencia e Innovación. ICN2 is funded by the CERCA Programme from Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). S.T.B acknowledges support from the MICINN-funded project grants: PID2021-127957NB-I00 and TED2021-132550B-C21. I.P.R.M acknowledges support from the MICINN-funded project grant: PID2019-109518GB-I00. J.R.A. acknowledges support from the MICINN-funded project grant: PID2020-117803GB-I00. S.T.B., I.P.R.M., and J.R.A. also acknowledge support from project grant 2021SGR00354 funded by the Generalitat de Catalunya. The IQTC-UB is funded by the María de Maeztu program for Spanish Structures of Excellence (CEX2021-001202-M).With funding from the Spanish government through the ‘María de Maeztu Unit of Excelence’ accreditation (CEX2021-001202-M)Peer reviewe

Magnetic coupling in the weac ferromagnet CuF2

CuF2 is known to be an antiferromagnetic compound with a weak ferromagnetism due to the anisotropy of its monoclinic unit cell (Dzialoshinsky-Moriya mechanism). We investigate the magnetic ordering of this compound by means of ab initio periodic unrestricted Hartree-Fock calculations and by cluster calculations which employ state-of-the-art configuration interaction expansions and modern density functional theory techniques. The combined use of periodic and cluster models permits us to firmly establish that the antiferromagnetic order arises from the coupling of one-dimensional subunits which themselves exhibit a very small ferromagnetic coupling between Cu neighbor cations. This magnetic order could be anticipated from the close correspondence between CuF2 and rutile crystal structures

Differential Behavior of Amino–Imino Constitutional Isomers in Nonlinear Optical Processes

8 páginas, 6 figuras, 2 esquemas.A detailed study of the “blocked” amino–imino tautomers derived from N-acridine-substituted 2-aminobenzothiazole—and their effect on the nonlinear optical response—is presented. The synthesis, characterization, and nonlinear optical properties of these frozen tautomers, namely, N-methyl-N-(2-nitroacridin-6-yl)-2-aminobenzothia-zole and 3-methyl-N-(7-nitroacridin-3-yl)-2-iminobenzothiazole, are reported. A theoretical model based on valence–bond theory is also proposed and used to analyze the effects of the nuclear configuration corresponding to each frozen tautomer structure. In the present case, the aromatic form and the allylic-anion-like system of the [BOND]N[BOND]C[BOND]N[BOND] group inherent to each isomer are crucial for understanding and analyzing the different responses of each “blocked” tautomer.Financial support from the Spanish Ministerio de Ciencia y Tecnología, DGI projects FIS2008–02238/FIS, CTQ-2006–15611-C02–02, CTQ2008–02856/BQU, MAT2008–06522-C02–02 and, in part, from the Generalitat de Catalunya projects 2005SGR-000697 and 2005SGR-00111 are fully acknowledged. We also acknowledge to the staff of NMR Spectroscopy Service of the Servei Cientifico-tècnics of the Universitat de Barcelona, in special to Ma Teresa González, for the assistance in the 2D experiments.Peer reviewe