1,721,029 research outputs found
Spectroscopic properties of cyclometallated iridium complexes by TDDFT
No full textWe provide a unified view of our recent work on TDDFT calculations on the class of Iridium(III) cyclometallated complexes, of relevance for their extensive use in OLED and LEEC devices. Our results, obtained for vastly different systems, allow us to trace some general conclusions concerning the modeling of these transition metal complexes. The effect of relativistic effects on both geometrical structures and emission properties is analyzed considering scalar relativistic corrections and quadratic response theory. Absorption spectra for these compounds can be satisfactorily reproduced by TDDFT calculations neglecting spin-orbit coupling, because of the strong intensity of singlet-singlet transitions which hinders the weaker singlet-triplet transitions. We also underline the importance of being able to simulate the emission spectra line-shapes, thus allowing for a direct comparison of calculated and experimental quantities and to estimate the emission color perceived by the human eye. Approximate treatments of spin-orbit coupling based on excited state analysis are in some case useful to rationalize experimental trends, but more rigorous and quantitative approaches are required for future applications. Finally, we stress the absolute relevance of solvation effects in the description of the excited states of this class of systems, with very large differences in the excited state properties calculated in vacuo and as a function of different solvents. These differences have profound consequences for the comprehension of the photophysical properties of this important class of systems and should always be considered in their effective modeling. On overall, our results indicate that electronic structure-driven tuning of the excited state properties of Iridium(Ill) cyclometallated complexes is possible, thus opening the way to a theoretical and computational strategy for the design of new phosphorescent compounds with specific target characteristics. (C) 2009 Elsevier B.V. All rights reserved
Alignment of the dye's molecular levels with the TiO(2) band edges in dye-sensitized solar cells: a DFT-TDDFT study
No full textWe present a theoretical study of the lineup of the LUMO of Ru(II)-polypyridyl (N3 and N719) molecular dyes with the conduction band edge of a TiO(2) anatase nanoparticle. We use density functional theory (DFT) and the Car-Parrinello scheme for efficient optimization of the dye-nanoparticle systems, followed by hybrid B3LYP functional calculations of the electronic structure and time-dependent DFT (TDDFT) determination of the lowest vertical excitation energies. The electronic structure and TDDFT calculations are performed in water solution, using a continuum model. Various approximate procedures to compute the excited state oxidation potential of dye sensitizers are discussed. Our calculations show that the level alignment for the interacting nanoparticle-sensitizer system is very similar, within about 0.1 eV, to that for the separated TiO(2) and dye. The excellent agreement of our results with available experimental data indicates that the approach of this work could be used as an efficient predictive tool to help the optimization of dye-sensitized solar cells
Time-dependent density functional theory study of the absorption spectrum of [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] in water solution: Influence of the pH
No full textWe present a combined density functional/time-dependent density functional study of the electronic structure and optical absorption spectrum of the charge-transfer sensitizer cis-[Ru(4,4'-COOH-2,2'-bpY)(2)(NCS)(2)] in water solution. To simulate the effect of different pH values, calculations have been performed for both the title complex and the corresponding tetra-deprotonated species, cis-[Ru(4,4'-COO-2,2'-bPY)(2)(NCS)(2)](4-). The experimentally observed blue-shift of the spectrum at high pH is well reproduced by our theoretical model and appears to be related to destabilization of the bipyridines pi(*) LUMOs as a result of the increased electron density on the deprotonated carboxylic groups. (C) 2004 Elsevier B.V. All rights reserved
CASPT2//CASSCF and TDDFT//CASSCF Mapping of the Excited State Isomerization Path of a Minimal Model of the Retinal Chromophore
No full textIn this work we explore the validity of the application of TDDFT methods to the study of excited state reactivity problems. Accordingly, TDDFT//CASSCF calculations have been used to evaluate the excited state isomerization path of a retinal chromophore model and have been compared with the path obtained at the more expensive CASPT2//CASSCF level. We show that the TDDFT and CASPT2 excited state energy profiles are qualitatively similar. Indeed, remarkably, the TDDFT//CASSCF strategy achieves a qualitatively correct description of the intersection region, which is a basic mechanistic feature of photochemical processes. Quantitative differences are found in the region of the energy profile characterized by a coupled stretching-twisting deformation. This discrepancy reflects the difference in the equilibrium values of the bond lengths of the planar excited state structures when evaluated at the TDDFT and CASPT2 levels. We stress that our results support the use of TDDFT for the evaluation of energy profiles along CASSCF reaction coordinates. Thus, in no way shall such results be considered as indicative of the validity of TDDFT for the calculation of excited state equilibrium structures or reaction coordinates
Absorption spectrum and solvatochromism of the [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] molecular dye by time dependent density functional theory
No full textWe present a combined Density Functional/Time Dependent Density Functional study of the molecular structure, electronic states, and optical absorption spectrum of [Ru(4,4'-COOH-2,2'-bpy)(2)(NCS)(2)], a widely used charge-transfer sensitizer in nanocrystalline TiO2 solar cells. Calculations have been performed both for the complex in vacuo and in ethanol and water solvents, using a continuum model to account for solute-solvent interactions. Inclusion of the solvent leads to important changes of the energies and composition of the molecular orbitals of the complex; as a consequence, whereas the computed spectrum for the Ru-complex in vacuo deviates from the experimental one in both energy and shape, the spectra calculated in the presence of the solvent are in good agreement with the experiment. The first two absorption bands are found to originate from mixed ruthenium-NCS to bipyridine-pi* transitions rather than to pure metal-to-ligand-charge-transfer (MLCT) transitions, whereas the third band arises from intraligand pi -->pi* transitions. The experimentally observed blue-shift of the spectrum in water with respect to ethanol is well reproduced by our calculations and appears to be related to a decreased dipole moment in the excited state
A computational approach to the electronic and optical properties of Ru(II) and Ir(III) polypyridyl complexes: Applications to DSC, OLED and NLO
No full textRuthenium(II) and Iridium(III) polypyridyl complexes have been intensively investigated due to their use in energy conversion and light-emitting devices and materials for non-linear optics. Quantum mechanical computer simulations of molecules and materials have become increasingly popular in the scientific community. Along with experimental investigations, such computational analyses can provide complementary information on the electronic and optical properties of transition metal compounds of interest for optoelectronic applications. Here, we provide a unified review of recent work carried out on computational investigations of a large series of Ruthenium(II) and Iridium(III) polypyridyl complexes, discussing the relations between their electronic structure and optical properties and their device functioning. Our results, obtained for vastly different systems, allow us to trace some general conclusions concerning the modeling of these transition metal complexes, casting the bases for the computational design and screening, even before their synthesis, of new and more efficient transition metal complexes for photonics applications. (C) 2011 Elsevier B.V. All rights reserved
Ab Initio Modeling of Solar Cell Dye Sensitizers: The Hunt for Red Photons Continues
No full textDespite the swift surge of lead halide perovskites, research in dye-sensitized solar cells (DSCs) has continued over the last few years, with a steady increase in record device efficiencies. A major requisite of an efficient solar cell sensitizer is that of showing an extended UV/Vis absorption spectrum closely matching that of solar radiation. This has given rise to what we call here the hunt for red photons, and ab initio computational modeling plays a major role in designing and screening new dyes with tailored characteristics. In this microreview, we highlight recent developments in modeling transition metal polypyridyl dyes by means of advanced ab initio simulations, including solvation and relativistic effects. We illustrate the molecular design rules that have led to the best performing ruthenium and osmium dyes to date, showing the information which can be extracted from ab initio simulations and how to exploit such information for engineering novel dye candidates
A density functional study of Ethylene rearrangements assisted by tungsten calix-4-arene
No full textA Density functional study of tetraphenolate and calix-4-arene complexes of early transition metals
No full textA dynamic density functional study of the stepwise migratory insertion of isocyanides into zirconium-carbon bonds anchored to a calix[4]arene moiety
No full textThe stepwise migratory insertion of methyl isocyanide into the zirconium-carbon bonds in [calix[4](OMe)(2)(O)(2)-ZrMe2] has been investigated by means of both static and dynamic density functional calculations. Dynamics simulations have shown that methyl isocyanide insertion takes place via the initial formation of an eta(1)-iminoacyl species that is suddenly converted into the more stable eta(2)-isomer. The energy profiles for the two pathways branching from the initially formed eta(2)-iminoacyl, i.e., (i) the insertion of a second isocyanide molecule into the residual alkyl group leading to a bis-eta(2)-iminoacyl and (ii) the insertion of the residual alkyl group into the iminoacyl moiety leading to an eta(2)-bound imine, have been characterized. Formation of the bis-eta(2)-iminoacyl species was found to be thermodynamically favored at low temperature (DeltaG(double dagger) =7.2 vs 6.4 kcal mol(-1), DeltaE = -38.5 vs -12.2 kcal mol(-1)). However, the large entropic contribution to the barrier for this intermolecular process kinetically favors the intramolecular imine formation at room temperature (DeltaG(double dagger) = 11.5 vs 6.4 kcal mol(-1)), providing a rationale for the experimentally characterized temperature selectivity of the overall reaction
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