| 1. |
- Cimpoesu, Fanica, et al.
(author)
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Disorder, exchange and magnetic anisotropy in the room-temperature molecular magnet V[TCNE](x) - A theoretical study
- 2014
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 91, s. 320-328
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Journal article (peer-reviewed)abstract
- We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE](x) (TCNE = tetracyanoethylene, x similar to 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra- or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V[TCNE](x). The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D-c = -0.03 K) of the bulk as well as a sizeable random anisotropy (D-r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
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| 2. |
- Frecus, Bogdan, et al.
(author)
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Ab initio study of exchange coupling for the consistent understanding of the magnetic ordering at room temperature in V[TCNE] (x)
- 2014
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In: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 133:5, s. 1470-
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Journal article (peer-reviewed)abstract
- We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE](2) model system, describing the amorphous room temperature molecular magnet V[TCNE] (x) (TCNE = tetracyanoethylene, x similar to 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE](-) anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
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| 3. |
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| 4. |
- Oprea, Corneliu I., et al.
(author)
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Comparative computational IR, Raman and phosphorescence study of Ru- and Rh-based complexes
- 2013
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In: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 111:9-11, s. 1526-1538
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Journal article (peer-reviewed)abstract
- We report density functional theory (DFT) calculations providing the infrared and Raman spectra of [Ru(II)(bpy)(3-n)(dcbpy)(n)](2+) and [Rh(III)(bpy)(3-n)(dcbpy)(n)](3+) complexes, where bpy = 2,2-bipyridyl, dcbpy = 4,4-dicarboxy-2,2-bipyridyl, and n = 0, 1, 2, 3, studied in the context of dye-sensitised solar cells. We compare and contrast the role of the metallic ion and of the COOH groups on the vibration and phosphorescence properties of these complexes. The vibrational spectra are not very sensitive to the replacement of the metal ion, but the presence of carboxyl groups leads to a richer spectrum due to the additional bands caused by the COOH groups. Comparison with the limited experimental data available allowed the assignment of the Raman bands. The calculated phosphorescence lifetimes suffer only modest changes when the COOH groups are introduced but vary significantly when changing the metal ion, being two orders of magnitude larger for Rh(III) than for the Ru(II) complexes.
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| 5. |
- Oprea, Corneliu I., et al.
(author)
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DFT calculations of a Metal-TCNE complex
- 2007
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In: Six International Conference of the Balkan Physical Union. - : AIP. - 9780735404045 ; , s. 716-716
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Conference paper (peer-reviewed)abstract
- For V[TCNE](x) y(solvent) (TCNE = tetracyanoethylene), the first room-temperature molecular magnet, the mechanism for the strong exchange coupling in the family M[TCNE](x) y(solvent) has remained an open question. hi an attempt to understand the magnetic ordering in these systems we performed density functional theory calculations on a six coordinated vanadium complex with four TCNE molecules. For this model system we find that the ferrimagnetic spin configuration is energetically more favorable and that the highest single-occupied orbital has pi* character.
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| 6. |
- Oprea, Corneliu I., et al.
(author)
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DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells
- 2011
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In: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 109:21, s. 2511-2523
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Journal article (peer-reviewed)abstract
- The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)(3-n)(dcbpy)(n)](2+) and [Rh(III)(bpy)(3-n)(dcbpy)(n)](3+) complexes, where bpy = 2,2'-bipyridyl, dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl, and n = 0, 1, 2, 3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic-inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh(3+) centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru(2+) ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)(3-n)(dcbpy)(n)](2+) complexes with n = 1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO(2) substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.
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| 7. |
- Oprea, Corneliu I., et al.
(author)
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DFT study of structure-properties correlations in [MnTPP][TCNE] quasi-one-dimensional molecular magnets
- 2011
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In: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 129:6, s. 847-857
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Journal article (peer-reviewed)abstract
- We report the first band structure calculations of the quasi-one-dimensional [MnTPP][TCNE] compounds (TPP = meso-tetraphenylporphyrinato, TCNE = tetracyanoethylene), based on Density Functional Theory (DFT) methods, in order to interpret the magnetic ordering in these prototypic systems. We compare and contrast the results of broken-symmetry DFT calculations for extended systems, with periodic boundary conditions, and for finite systems, magnetic dimers modeling the actual molecular magnets. By varying systematically the main angles, we are able to determine the geometry dependence of the exchange interaction. Structure-properties correlations in these charge-transfer salts reveal the determinant role of the Mn-(N C) TCNE bond angle on the strength of the ferrimagnetic coupling between the S(1) = 2 spin located on the Mn(III)-porphyrin donor and the S(2) = 1/2 spin positioned on the cyanocarbon acceptor. When the Mn-(N C) TCNE angle is decreased, the intrachain magnetic coupling strengthens, correlated with the increase in the d(z)(2) - pi* orbital overlap. The exchange coupling constants resulting from DFT calculations of extended systems, with periodic boundary conditions, were found to be consistent with those obtained for the dimers, but systematically smaller. The exchange constants vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the experimental mean-field critical temperatures. The coupling constant varies significantly with the type of broken-symmetry approach, depending on the overlap between magnetic orbitals, but weakly on the basis set once polarization effects are included. The electronic structure calculations for the extended systems provide a density of states consistent with the energy spectrum of the corresponding dimer, allowing for an intuitive explanation of the intrachain ferrimagnetic ordering.
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| 8. |
- Oprea, Corneliu I., et al.
(author)
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Theoretical study of neutral and reduced hexacyanobutadiene
- 2007
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In: Journal of Molecular Structure. - : Elsevier BV. - 0166-1280. ; 804:1-3, s. 111-116
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Journal article (peer-reviewed)abstract
- The molecular structure and the electronic densities of neutral and anionic hexacyanobutadiene (HCBD), trans-C-4(CN)(6), as well as the electron affinity (EA) of HCBD have been studied using density functional theory (DFT) and perturbation theory (MP2) procedures. The optimized geometries showed that the HCBD molecule is not planar, with a 140 degrees torsion angle about the central C-C bond. While the optimized geometries are not very sensitive to the choice of either the method or the basis set, the adiabatic electron affinity varies significantly with both. All the DFT computed electron affinities overestimate the experimental value, the best results being obtained with the BLYP functional, whereas the MP2 calculations heavily underestimate it. At the NIP2 level, the EA value obtained after projecting out the major spin contaminating component (PMP2) is a good estimate of the EA, within 0.11 eV of the experiment. Although other studies suggested that single-point PMP2 calculations over DFT-optimized geometries could be a valuable alternative for the study of larger systems at low computational effort, our results indicate that this approach can increase the spin contamination and should be used with caution.
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