Corneliu I. Oprea

Density Functional Theory (DFT) Study of Coumarin-based Dyes Adsorbed on TiO2 Nanoclusters—Applications to Dye-Sensitized Solar Cells

Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.

On exchange coupling and bonding in the Gd2@C80 and Gd2@C79N endohedral dimetallo-fullerenes

A series of computational experiments performed with various methods belonging to wave-function and density functional theories approaches the issue of bonding regime and exchange coupling in the title compounds. Gd2@C80 is computed with a very weak exchange coupling, the sign depending on the method, while Gd2@C79N has resulted with a strong coupling and ferromagnetic ground state, irrespective of the computational approach. The multi-configuration calculation and broken symmetry estimation are yielding closely coincident coupling constants, of about J ∼ 400 cm−1. No experimental estimation exists, but the ferromagnetic ground state of Gd2@C79N is confirmed from paramagnetic resonance data. The different behaviour is due to particularities of electron accommodation in the orbital scheme. The exchange effects localised on atom lead to preference for parallel alignment of the electrons placed in the 4f and 5d lanthanide shells, determining also a ferromagnetic inter-centre coupling. The structural insight is completed with a ligand field analysis of the density functional theory results in the context of frozen density embedding. The energy decomposition analysis of bonding effects is also discussed. Finally, with the help of home-made codes (named Xatom+Xsphere), a model for the atom encapsulated in a cage is designed, the exemplified numeric experiments showing relevance for the considered endohedral metallo-fullerene issues.

Broken symmetry DFT calculations of exchange coupling constants for manganese–porphyrin quasi-one-dimensional molecular magnets

We report the results of broken symmetry density functional theory (BS-DFT) calculations providing the exchange coupling constants for two quasi-one-dimensional manganese–porphyrin compounds, [MnOEP][HCBD] and [MnTtBuPP][HCBD] (OEPxa0=xa0octaethylporphyrinato, TtBuPPxa0=xa0meso-tetrakis-(4′-tert-butylphenyl)porphinato, HCBDxa0=xa0hexa-cyanobutadiene). The different magnetic behaviour of these materials is due mainly to the distinct binding modes of the cyanocarbon unit. We compare and contrast the results of BS-DFT calculations for magnetic dinuclears, which properly model the actual molecular magnets, to determine the geometry dependence of the exchange interaction. The exchange coupling constants resulting from BS-DFT calculations vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the constants determined experimentally. Structure-properties correlations reveal the determinant role of the Mn–(N≡C)TCNE bond angle on the ferrimagnetic coupling between the S1xa0=xa02 spin located on the MnIII–porphyrin donor and the S2xa0=xa01/2 spin positioned on the cyanocarbon acceptor. Based on a phenomenological model providing the geometry dependence of the exchange coupling constants, we fitted the results of the DFT calculations and obtained parameters describing the ferromagnetic and the antiferromagnetic parts of the superexchange interaction. The large differences between the magnetic properties of the two Mn–HCBD systems are explained based on the correlation between the exchange coupling constant and the overlap between the Mn(III) d and the HCBD π* orbitals and on the torsion angle of the butadiene backbone, which affects dramatically the nature of the π* orbital in the case of the nonplanar HCBD.

DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

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 bpyu2009=u20092,2′-bipyridyl, dcbpyu2009=u20094,4′-dicarboxy-2,2′-bipyridyl, and nu2009=u20090,u20091,u20092,u20093, 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 Rh3+ 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 Ru2+ 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 nu2009=u20091 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 TiO2 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.

Ab initio study of exchange coupling for the consistent understanding of the magnetic ordering at room temperature in V[TCNE]x

AbstractnWe 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 (TCNExa0=xa0tetracyanoethylene, xxa0~xa02). 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.

DFT study of binding and electron transfer from colorless aromatic pollutants to a TiO2 nanocluster: Application to photocatalytic degradation under visible light irradiation

Summary We report results of density functional theory (DFT) calculations on some colorless aromatic systems adsorbed on a TiO2 nanocluster, in order to explain experimental results regarding the photocatalytic degradation of these pollutants under visible light irradiation. Based on our modeling, we are able to clarify why transparent pollutants can degrade under visible light in the presence of a catalyst that absorbs only in the UV, to explain experimental data regarding differences in the efficiency of the degradation process, and to state the key requirements for effective water-cleaning. For that purpose, we analyze the absorption spectrum of the free and adsorbed molecules, the binding configurations, the matching of the energy levels with the oxide catalyst and the likelihood of the charge-transfer to the substrate. The comparison between several colorless aniline and phenolic systems allows a correlation between the chemical structure and the degradation rate of these pollutants.

Comparative computational IR, Raman and phosphorescence study of Ru- and Rh-based complexes

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.

Noble gas endohedral fullerenes, Ng@C60 (Ng=Ar, Kr): a particular benchmark for assessing the account of non-covalent interactions by density functional theory calculations

AbstractnThis work is dedicated to a special test, checking the capability of density functional theory computations in the account of long-range effects. The particular case of endohedral noble gas atoms in C60 fullerene puts several methodological challenges, such as the numeric problem in the balance of covalent versus non-covalent interactions. We designed a procedure based on the shifts of vibrational frequencies in C60 versus Ng@C60 couples. The energy scale of the investigated vibrations is comparable to that of the extracted van der Waals interaction parameters, achieving then a well-tempered description. A phenomenological model, based on transparent analytical formulas of the totally symmetric modes, is outlined and used to assess the computational results on series of functionals belonging to different classes (genuine forms, long-range corrected and with empiric dispersion ingredients). While the vast majority of the tested functionals undergo failures, good results are obtained for some long-range-corrected functionals (LC-BLYP and LC-wPBE), which follow the Tsuneda and Hirao’s scheme, provided that richer basis sets (with diffuse components) are used. Successes are obtained also for the Grimme B97D functional, when coupled with the D2 and D3 dispersion scheme, the results being rather independent from the basis set, as expected from the empirical nature of this type of amendment.

DFT Study of Optical Properties of Pt‐based Complexes

We report Density Functional Theory (DFT) calculations providing the geometrical and electronic structures, as well as the vibrational and optical properties of the homologous series of Pt‐pyramidalized olefin complexes (CH2)n‐(C8H10)Pt(PH3)2, where nu2009=u20090, 1, and 2, in their neutral and oxidized states. All complexes were geometry optimized for the singlet ground state in vacuum using DFT methods with B3LYP exchange‐correlation functional and the Effective Core Potential LANL2DZ basis set, within the frame of Gaussian03 quantum chemistry package. We find the coordination geometry of Pt to be distorted square planar, with dihedral angles ranging from 0°, for nu2009=u20090 and 1, which have C2V symmetry to 3.4°, for nu2009=u20092 with C2 symmetry. The Mulliken charge analysis allows a discussion of the oxidation state of the Pt ion. Electronic transitions were calculated at the same level of theory by means of Time Dependant‐DFT. For nu2009=u20092 the electronic absorption bands are located in the UV region of the spectrum, the tr...

DFT study of binding and electron transfer from penicillin to a TiO2 nanocluster: Applications to photocatalytic degradation

We report results of density functional theory calculations on penicillin G and penicillin V adsorbed on a TiO2 nanoclusters, in order to study its photocatalytic degradation under ultraviolet and visible light irradiation. We analyze the absorption spectrum of the free and adsorbed penicillin, the binding configurations, and the likelihood of the charge-transfer to the substrate.