Thomas Cauchy

Magnetic structure of the large-spin Mn10 and Mn19 complexes: a theoretical complement to an experimental milestone.

High-spin molecules have been proposed as candidates for the storage of information at the molecular level. The electronic structure of two complex magnetic molecular systems, Mn 10 and Mn 19, is characterized by means of a computational study based on density functional theory. All the exchange interactions in the recently reported Mn 19 complex with the highest known spin value of 83/2, and in its highly symmetric Mn 10 parent compound, are ferromagnetic. In these complexes, there are two kinds of ferromagnetic coupling: the first one corresponds to Mn (II)-Mn (III) interactions through a double mu 2-alkoxo-mu 4-oxo bridge where the high coordination number of the Mn (II) cations results in long Mn (II)-O bond distances, while the second one involves Mn (III)-Mn (III) interactions through mu 2-alkoxo-mu 3-eta (1):eta (1):eta (1) azido bridging ligands with long Mn (III)-N distances due to a Jahn-Teller effect.

Exchange coupling in transition-metal complexes via density-functional theory: Comparison and reliability of different basis set approaches

Theoretical methods based on density-functional theory with Gaussian, plane waves, and numerical basis sets were employed to evaluate the exchange coupling constants in transition-metal complexes. In the case of the numerical basis set, the effect of different computational parameters was tested. We analyzed whether and how the use of pseudopotentials affects the calculation of the exchange coupling constants. For the three different basis sets, a comparison of the exchange coupling constants and spin distributions shows that both the plane-wave and the numerical basis set approaches are accurate and reliable alternatives to the more established Gaussian basis functions.

Tetrathiafulvalene-amido-2-pyridine-N-oxide as Efficient Charge-Transfer Antenna Ligand for the Sensitization of YbIII Luminescence in a Series of Lanthanide Paramagnetic Coordination Complexes

The tetrathiafulvalene-amido-2-pyridine-N-oxide (L) ligand has been employed to coordinate 4f elements. The architecture of the complexes mainly depends on the ionic radii of the lanthanides. Thus, the reaction of L in the same experimental protocol leads to three different molecular structure series. Binuclear [Ln(2)(hfac)(5)(O(2)CPhCl)(L)(3)]·2 H(2)O (hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate anion, O(2)CPhCl(-)=3-chlorobenzoate anion) and mononuclear [Ln(hfac)(3)(L)(2)] complexes were obtained by using rare-earth ions with either large (Ln(III)=Pr, Gd) or small (Ln(III)=Y, Yb) ionic radius, respectively, whereas the use of Tb(III) that possesses an intermediate ionic radius led to the formation of a binuclear complex of formula [Tb(2)(hfac)(4)(O(2)CPhCl)(2)(L)(2)]. Antiferromagnetic interactions have been observed in the three dinuclear compounds by using an extended empirical method. Photophysical properties of the coordination complexes have been studied by solid-state absorption spectroscopy, whereas time-dependent density functional theory (TD-DFT) calculations have been carried out on the diamagnetic Y(III) derivative to build a molecular orbital diagram and to reproduce the absorption spectrum. For the [Yb(hfac)(3)(L)(2)] complex, the excitation at 19,600 cm(-1) of the HOMO→LUMO+1/LUMO+2 charge-transfer transition induces both line-shape emissions in the near-IR spectral range assigned to the (2)F(5/2)→(2)F(7/2) (9860 cm(-1)) ytterbium-centered transition and a residual charge-transfer emission around 13,150 cm(-1). An efficient antenna effect that proceeds through energy transfer from the singlet excited state of the tetrathiafulvalene-amido-2-pyridine-N-oxide chromophore is evidence of the Yb(III) sensitization.

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis((4-(4-pyridyl)ethynyl)bicyclo[2.2.2]oct-1-yl)buta-1,3-diyne, 1, contains two 1,4-bis(ethynyl)bicyclo[2.2.2]octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic C2/c centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70-300 K) proton spin-lattice relaxation, (1)H T1(-1), at two different (1)H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol(-1), to the two (1)H spin-lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of 1 should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis(3-((trimethylsilyl)ethynyl)bicyclo[1.1.1]pent-1-yl)buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response.

In solution sensitization of Er(III) luminescence by the 4-tetrathiafulvalene-2,6-pyridinedicarboxylic acid dimethyl antenna ligand.

In the [Er(hfac)(3)(L)](2) complex (1) (L = 4-tetrathiafulvalene-2,6-pyridinecarboxylic acid dimethyl ester), the Er(III) ion is bonded to the tridentate coordination site. Electrochemical and photophysical measurements in solution reveal that the tetrathiafulvalene moiety is a versatile antenna for erbium luminescence sensitization at 6540 cm(-1) upon excitation in the low-energy charge transfer transition (donor to acceptor charge transfer) at 16600 cm(-1) assigned via time-dependent density functional theory calculations.

Ethylenedithio-tetrathiafulvalene-helicenes: electroactive helical precursors with switchable chiroptical properties.

Electroactive fused ethylenedithio-tetrathiafulvalene-[4]helicene and -[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3-dibromo-helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single-crystal X-ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid-state architectures show interplay between S⋅⋅⋅S and π⋅⋅⋅π intermolecular interactions. The chiroptical properties of the enantiopure EDT-TTF-[6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical-cation species has been achieved.

Experimental and theoretical studies on photophysical properties: tuning redox-active amido-tetrathiafulvalene derivatives in paramagnetic coordination complexes.

Amido-5-pyrimidine (1), -4-pyridine (2), -2-pyrazine (3), -2-pyridine (4), and -2-pyridine-N-oxide (5) derivatives of TTF (TTF = tetrathiafulvalene) have been synthesized and characterized. The crystal structure of 1 has been resolved. Their capacities to coordinate paramagnetic transition metal have been explored. The following new molecular compounds have been synthesized and obtained as single crystals: {[Cu(hfac)(2)(1)](H(2)O)}(2) (6), cis-[Mn(hfac)(2)(2)(2)](THF)(2) (7), trans-[Cu(hfac)(2)(3)(2)] (8), trans-[Cu(hfac)(2)(4)(2)] (9), and trans-[M(hfac)(2)(5)(2)] (M = Cu (10), Mn (11), Zn (12)). The crystal structures reveal that the nature of the coordinating substituent plays a fundamental role on the crystalline organization. Cyclic voltammetry measurements have been performed for all the species and they have permitted us to observe the redox activity of the free and linked donors. EPR measurements are in agreement with the solid-state structures. All the ligands and corresponding coordination complexes have been studied by UV-visible absorption spectroscopy. Gaussian deconvolutions have been performed to fit the experimental solid-state absorption curves. Molecular orbital diagram for ligands 4 and 5; and their coordination complexes have been determined. The nature of the thirty to fifty low-lying monoelectronic transitions occurring in the TTF derivatives have been identified by TD-DFT calculations and their corresponding UV-visible absorption spectra have been simulated. Concerning the open-shell complexes, the excitations in the low energy region of their spectra have been calculated to determine the coordination effect on the TTF to acceptor transitions of the ligand fragments.

Tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) revisited: crystal structures, chiroptical properties, theoretical calculations, and a complete series of conducting radical cation salts

The (S,S,S,S) and (R,R,R,R) enantiomers of tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) show equatorial conformation for the four methyl groups in the solid state, according to the single-crystal X-ray analyses. Theoretical calculations at the Density Functional Theory (DFT) and time-dependent (TD) DFT levels indicate higher gas phase stability for the axial conformer than the equatorial one by 1.25 kcal ∙ mole⁻¹ and allow the assignment of the UV-vis and circular dichroism transitions. A complete series of radical cation salts of 1:1 stoichiometry with the triiodide anion I₃⁻ was obtained by electrocrystallization of both enantiopure and racemic forms of the donor. In the packing the donors are organized in dimers that further interact through S ∙ ∙ ∙ S intermolecular contacts and the triiodide anions lie parallel to pairs of oxidized donors. The conductivity of the racemate, which adopts the same, but disordered, structural type, is considerably lower, with much higher activation energy.

Tetrathiafulvalene-1,3,5-triazines as (Multi)Donor–Acceptor Systems with Tunable Charge Transfer: Structural, Photophysical, and Theoretical Investigations

Palladium-catalyzed cross-coupling reactions between chlorinated 1,3,5-triazines (TZ) and tetrathiafulvalene (TTF) trimethyltin derivatives afford mono- and C3 symmetric tris(TTF)-triazines as donor-acceptor compounds in which the intramolecular charge transfer (ICT) is modulated by the substitution scheme on TTF and TZ and by chemical or electrochemical oxidation. The TTF-TZ-Cl2 and (SMe)2TTF-TZ-Cl2 derivatives show fully planar structures in the solid state as a consequence of the conjugation between the two units. Electrochemical and photophysical investigations, supported by theoretical calculations, clearly demonstrate that the lowest excited state can be ascribed to the intramolecular charge transfer (ICT) π(TTF)→π*(TZ) transition. The tris(TTF) compound [(SMe)2TTF]3-TZ shows fluorescence when excited in the ICT band, and the emission is quenched upon oxidation. The radical cations TTF(+•) are easily observed in all of the cases through chemical and electrochemical oxidation by steady-state absorption experiments. In the case of [(SMe)2TTF]3-TZ, a low energy band at 5000 cm(-1), corresponding to a coupling between TTF(+•) and TTF units, is observed. A crystalline radical cation salt with the TTF-TZ-Cl2 donor and PF6(-) anion, prepared by electrocrystallization, is described.

Can theoretical methods go beyond the experimental data? The case of molecular magnetism

The complexity of polynuclear transition metal complexes with the most appealing magnetic properties makes it impossible to extract the values of exchange interactions between the paramagnetic centers using experimental techniques. Hence, theoretical methods based on density functional theory are used because they allow the accurate estimation of such values. Three Mn(6) complexes were studied and the calculated exchange coupling constants used to plot a magnetic susceptibility curve that can be compared with the experimental ones. We propose a new tool to facilitate the understanding of the magnetic properties in systems of this kind. We employed magnetostructural maps to correlate the calculated exchange coupling constants with structural parameters for the dinuclear or polynuclear manganese complexes that we have studied.