Real-Time CASSCF (Ehrenfest) Modeling of Electron Dynamics in Organic Semiconductors. Dynamics Reaction Paths Driven by Quantum Coherences. Application to a Radical Organic Semiconductor.

We present a strategy for the modeling of charge carrier dynamics in organic semiconductors using conventional quantum chemistry methods, including the analytic gradient for nuclear motion. The theoretical approach uses real-time CASSCF (Ehrenfest) all-electron dynamics coupled to classical nuclear dynamics for the special case of a small number (4-8) of molecular units. The objective is to obtain mechanistic/atomistic insight at the electronic structure level, relating to spin density dynamics, to the effect of crystal structure (e.

Enhancing Intramolecular Ferromagnetic Coupling in Tetrathiafulvalene-Nitronyl Nitroxide-Based Compounds through Spin Polarization Mechanism.

Spin-polarized donor radicals based on tetrathiafulvalene (TTF) derivatives and nitronyl nitroxide (NN) radicals in which one-electron oxidation involves the HOMO instead of the SOMO are well known for exhibiting magnetoresistance. In particular, BTBN consists of one dibromo-TTF and one NN radical, which are linked by a phenyl coupler group. One of the key factors driving magnetoresistance is the presence of intramolecular ferromagnetic (FM) coupling between the oxidized π-donor (TTF⋅, D unit) and NN (R unit).

Satellite ligand effects on magnetic exchange in dimers. A structural, magnetic and theoretical investigation of CuLX (L = methylisothiazolinone and X = Cl, Br).

Halide-bridged polymers have gained significant interest due to their diverse properties and potential applications. Stacked CuLX dimers, where L is an organic ligand and X can be Cl or Br, are of interest because a chloride analogue where L = 2-pyridone, had previously been reported to exhibit bulk ferromagnetism, which augured great potentiality for this class of compounds. The synthesis, structural characterization, magnetic susceptibility measurements, and computational studies of two isostructural CuClMI (MI = methylisothiazolinone) and CuBrMI polymers of Cu(II), along with a related CuClPYR (PYR = 2-pyridone) is reported.

Electronic structure and magnetic coupling in selenium substituted pyridine-bridged bisdithiazolyl multifunctional molecular materials.

Bisdithiazolyl radicals have furnished in recent years multiple examples of molecular materials with promising conductive and magnetic properties. The electronic band structure and magnetic ordering in four different isostructural pyridine-bridged bisdithiazolyl and Selenium substituted compounds have been studied by means of hybrid DFT based methods as implemented in the CRYSTAL code. The full rationalization of the properties of these multifunctional magnetic molecular materials requires a careful description of their complex open-shell electronic structure.

Approaching the isotropic spin-ladder regime: structure and magnetism of all-pyrazine-bridged copper(II)-based antiferromagnetic ladders.

The crystal structure and magnetic properties of two all-pyrazine-bridged antiferromagnetic spin ladders are reported. The complexes, -(bis(3-X-4-pyridone)(μ-pyrazine)copper(II)(-μ-pyrazine)diperchlorate ([Cu(pz)(L)](ClO) where L = 3-X-4-pyridone and X = Br (1) or Cl (2)), contain copper(II)-based ladders in which both the rung and rail bridges are pyrazine molecules bonded through the - orbital of the copper(II) ions. This structural scaffold is proposed to approach the isotropic spin-ladder regime.

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. π-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.

Revisiting the Role of Hydrogen Bonding in the Strong Dimer Superexchange of a 2D Copper(II) Halide Honeycomb-Like Lattice: Structural and Magnetic Study.

The title compound HL(CuClHO)Cl (HL = 1-(4'-pyridinium)pyridin-4-ol-ium), ) was synthesized and investigated structurally and magnetically as well as via a first-principles, bottom-up theoretical analysis of the potential magnetic superexchange pathways. Compound can be described structurally as a well-isolated, distorted 2D-honeycomb lattice with two potential exchange pathways: a dimeric interaction via hydrogen-bonded pairs of (CuClHO) ions and a chain structure via bridging chloride ions. Surprisingly, the experimental magnetic data are best fitted using both a simple dimer model with a Curie-Weiss correction for interdimer exchange ( = -107.

Revising the common understanding of metamagnetism in the molecule-based bisdithiazolyl BDTMe compound.

The BDTMe molecule-based material is the first example of a thiazyl radical to exhibit metamagnetic behavior. Contrary to the common idea that metamagnetism occurs in low-dimensional systems, it is found that BDTMe magnetic topology consists of a complex 3D network of almost isotropic ferromagnetic spin-ladders that are coupled ferromagnetically and further connected by some weaker antiferromagnetic interactions. Calculated magnetic susceptibility χT(T) data is in agreement with experiment.

The magnetic fingerprint of dithiazolyl-based molecule magnets.

Magnetic bistability in organic-radical based materials has attracted significant interest due to its potential application in electronic devices. The first-principles bottom-up study herein presented aims at elucidating the key factors behind the different magnetic response of the low and high temperature phases of four different switchable dithiazolyl (DTA)-based compounds. The drastic change in the magnetic response upon spin transition is always due to the changes in the JAB magnetic interactions between adjacent radicals along the π-stacks of the crystal, which in turn are driven mostly by the changes in the interplanar distance and degree of lateral slippage, according to the interpretation of a series of magneto-structural correlation maps.

A Definition of the Magnetic Transition Temperature Using Valence Bond Theory.

Macroscopic magnetic properties are analyzed using Valence Bond theory. Commonly the critical temperature T for magnetic systems is associated with a maximum in the energy-based heat capacity C(T). Here a more broadly applicable definition of the magnetic transition temperature T is described using the spin moment expectation value (i.

The quest for rationalizing the magnetism in purely organic semiquinone-bridged bisdithiazolyl molecular magnets.

Semiquinone-bridged bisdithiazolyl-based radicals (XBBO) are appealing purely organic magnetic building blocks for the synthesis of new functional materials. Remarkably, for the phenyl-derivative PhBBO, the rationalization of its magnetism becomes a proof of concept that DFT can dramatically fail to evaluate JAB magnetic interactions between purely organic radical pairs. Instead, wavefunction-based methods are required.

Dynamical effects on the magnetic properties of dithiazolyl bistable materials.

The magnetic properties of molecule-based magnets are commonly rationalized by considering only a single nuclear configuration of the system under study (usually an X-ray crystal structure). Here, by means of a computational study, we compare the results obtained using such a approach with those obtained by explicitly accounting for thermal fluctuations, and uncover the serious limitations of the perspective when dealing with magnetic crystals whose radicals undergo wide-amplitude motions. As a proof of concept, these limitations are illustrated for the magnetically bistable 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material.

The origin of the antiferromagnetic behaviour of the charge-transfer compound (HMTTF)[Ni(mnt)2].

The charge-transfer (HMTTF)[Ni(mnt)2] material (1) was the first mixed-stack compound reported to present ferromagnetic (FM) interactions between the donor (D) and acceptor (A) units within each mixed-stack. Despite the presence of a dominant FM interaction, its magnetic susceptibility curve, χ(T), corresponded to that of an antiferromagnetic (AFM) compound at low temperatures, a fact that was tentatively explained in terms of a FM-to-AFM magnetic transition. In this work, the First-Principles Bottom-Up procedure has been applied to rationalize the magnetic properties of 1.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials.

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer.

S=1/2 one-dimensional random-exchange ferromagnetic zigzag ladder, which exhibits competing interactions in a critical regime.

The synthesis, crystal structure, and magnetic properties (from a combined experimental and First-Principles Bottom-Up theoretical study) of the new compound catena-dichloro(2-Cl-3Mpy)copper(II), 1, [2-Cl-3Mpy=2-chloro-3-methylpyridine] are described and rationalized. Crystals of 1 present well isolated magnetic 1D chains (no 3D order was experimentally observed down to 1.8 K) and magnetic frustration stemming from competing ferromagnetic nearest-neighbor (J(NN)) interactions and antiferromagnetic next-nearest neighbor (J(NNN)) interactions, in which α=J(NNN)/J(NN) <-0.

Elucidating the 2D magnetic topology of the 'metal-radical' TTTA⋅Cu(hfac)2 system.

The TTTA⋅Cu(hfac)2 polymer (1; in which TTTA = 1,3,5-trithia-2,4,6-triazapentalenyl, and hfac = (1,1,1,5,5,5)-hexafluoroacetylacetonate) is one of the most prominent examples of the rational use of the 'metal-radical' synthetic approach to achieve ferromagnetic interactions. Experimentally, the magnetic topology of 1 could not be fully deciphered. Herein, the first-principles bottom-up procedure was applied to elucidate the nature and strength of the magnetic JAB exchange interactions present in 1.

Dividing the spoils: role of pyrazine ligands and perchlorate counterions in the magnetic properties of bis(pyrazine)diperchloratecopper(II), [Cu(pz)2](ClO4)2.

A complete first-principles bottom-up computational study of the magnetic properties of [Cu(pz)2](ClO4)2 is presented. A remarkable agreement is observed in the whole range of temperatures between simulated and experimental magnetic susceptibility data. Interestingly, the simulated heat capacity values show an anomaly close to the Néel temperature of 4.

Tracing the sources of the different magnetic behavior in the two phases of the bistable (BDTA)2[Co(mnt)2] compound.

A complete computational study of the magnetic properties of the two known phases of the bistable (BDTA)(2)[Co(mnt)(2)] compound is presented. The origin of their different magnetic properties can be traced to a variation in the values of the g tensor, together with a hitherto unknown change in the J(AB) values and their magnetic topology.

Synthesis, structure, magnetic behavior, and theoretical analysis of diazine-bridged magnetic ladders: Cu(quinoxoline)X2 and Cu(2,3-dimethylpyrazine)X2 (X = Cl, Br).

The synthesis, structure, and magnetic behavior of the complexes Cu(qnx)Br(2) (1), Cu(2,3-dmpz)Br(2) (2), Cu(qnx)Cl(2) (3), and Cu(2,3-dmpz)Cl(2) (4) (qnx = quinoxaline, dmpz = dimethylpyrazine) are described. Both X-ray structural data and fitting of the magnetic data suggest that the compounds are well-described as strong-rung, two-leg magnetic ladders with J(rung) ranging from -30 K to -37 K, and J(rail) ranging from -14 K to -24 K. An unexpected decrease in the exchange constant for J(rail) (through the diazine ligand) is observed when the halide ion is changed from bromide to chloride, along with a small decrease in the magnetic exchange through the halide ion.

Origin of the magnetic bistability in molecule-based magnets: a first-principles bottom-up study of the TTTA crystal.

The magnetic bistability present in some molecule-based magnets is investigated theoretically at the microscopic level using the purely organic system TTTA (1,3,5-trithia-2,4,6-triazapentalenyl). The TTTA crystal is selected for being one of the best-studied molecule-based systems presenting magnetic bistability. The magnetic properties of the high- and low-temperature structures (HT and LT phases, respectively) are accurately characterized by performing a First-Principles Bottom-Up study of each phase.

The magnetism of (5MAP)2CuBr4 [5MAP = 5-methyl-2-aminopyridinium]: a quasi-2D or a 3D magnetic system?

In order to determine if its magnetic topology is actually two- or three-dimensional (2D or 3D), the mechanism of the magnetic interaction in (5MAP)(2)CuBr(4), a previously thought quasi-2D antiferromagnet, is re-examined using the first-principles bottom-up methodology. Once the magnitude and sign of all unique magnetic interactions present in the room-temperature (5MAP)(2)CuBr(4) crystal are evaluated, it is found that, even at room temperature, the magnetic topology of the crystal corresponds to a 3D antiferromagnet. Such 3D nature cannot be determined by examination of the magnetic susceptibility curve, chi(T), because it is found that the chi(T) curve computed using this 3D magnetic topology is very similar to that obtained using a 2D model where all interplane interactions have been deleted.

Studying the origin of the antiferromagnetic to spin-canting transition in the beta-p-NCC6F4CNSSN* molecular magnet.

The crystal structure of the spin-canted antiferromagnet beta-p-NCC(6)F(4)CNSSN* at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first-principles bottom-up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=-33.

Synthesis, structure, and magnetic behavior of bis(2-amino-5-fluoropyridinium) tetrachlorocuprate(II).

Reaction of CuCl2 with 2-amino-5-fluoropyridine and HCl in aqueous solution yields bis(2-amino-5-fluoropyridinium) tetrachlorocuprate(II), (5FAP)2CuCl4, (1). The complex crystallizes in the monoclinic space group P21/c with cell dimensions a = 6.926(7) A, b = 21.

A first-principles analysis of the magnetism of CuII polynuclear coordination complexes: the case of [Cu4(bpy)4(aspartate)2(H2O)3](ClO4)4.2.5H2O.

The magnetic structure of the [Cu4(bpy)4(aspartate)2(H2O)3](ClO4)4.2.5 H2O crystal -- using fractional coordinates determined at room-temperature -- has been analysed in detail.