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Theoretical elucidation of the turn‐off mechanism of the luminescence of a chemosensor based on a metal‐organic framework (MOF) [Zn2(OBA)4(BYP)2] (BYP: 4,4′‐bipyridine; H2OBA: 4,4′‐oxybis[benzoic acid]), selective to nitrobenzene (NB) via quantum chemical computations, is presented. The electronic structure and optical properties of Zn‐MOF were investigated through the combination of density functional theory (DFT) and time‐dependent DFT methods. Our results indicate that the fluorescence emission is governed by a linker (BPY)‐to‐linker (OBA) charge transfer (LLCT) involving orbitals π‐type. Next, the interaction with the analyte was analyzed, where very interesting results were obtained, that is, the lowest unoccupied molecular orbital is now composed of orbitals from NB, which changes the emissive state of the Zn‐MOF. This suggests that the LLCT process is blocked, inducing the fluorescence quenching. Otherwise, the Morokuma‐Ziegler energy decomposition and natural orbitals for chemical valence on the Zn‐MOF‐NB interactions were studied in detail, which illustrate the possible channels of charge transfer between Zn‐MOF and NB. Finally, we believe that this proposed methodology can be applied to different chemosensor‐analyte systems to evidence the molecular and electronic factors that govern the sensing mechanisms. [ABSTRACT FROM AUTHOR]

The possibility of dye charge recombination in DSSCs remains a challenge for the field. This consists of: (a) back‐transfer from the TiO2 to the oxidized dye and (b) intermolecular electron transfer between dyes. The latter is attributed to dye aggregation due to dimeric conformations. This leads to poor electron injection which decreases the photocurrent conversion efficiency. Most organic sensitizers are characterized by an Acceptor‐Bridge‐Donor (A‐Bridge‐D) arrangement that is commonly employed to provide charge separation and, therefore, lowering the unwanted back‐transfer. Here, we address the intermolecular electron transfer by studying the dimerization and photovoltaic performance of a group of A‐Bridge‐D structured dyes. Specifically, eight famous sulfur containing π‐bridges were analyzed (A and D remained fixed). Through quantum mechanical and molecular dynamics approaches, it was found that the formation of weakly stabilized dimers is allowed. The dyes with covalently bonded and fused thiophene rings as Bridges, 6d and 7d as well as 8d with a fluorene, would present high aggregation and, therefore, high probability of recombination processes. Conversely, using TiO2 cluster and surface models, delineated the shortest bridges to improve the adsorption energy and the stability of the system. Finally, the elongation of the bridge up to 2 and 3 units and their photovoltaic parameters were studied. These results showed that all the sensitizers are able to provide similar photocurrent outcomes, regardless of whether the bridge is elongated. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Two related families of molybdenum complexes of the form [MoBr2L(NNR1R2)2], where L=2,2′-bipyridine (bipy) or ortho-phenanthroline (o-phen) and NNR1R2 =N,N substituted hydrazido(2−) ligands where R1 and R2 correspond to methyl and/or phenyl groups. The derivatives have been fully theoretically characterized, carrying on the geometrical characterization of molecular structure, energy decomposition analysis (EDA) and calculation of the absorption spectra using TD-DFT. The study has been performed using Density Functional Theory (DFT), using LDA and GGA(PW91 and BP86) functionals, with the TZP slater type basis set. We found that the complexes are isostructural, have a very similar orbital composition and also show very similar energy values for the frontier molecular orbitals (FMO), except in case of the LUMO+1 which exhibit a special behaviour. The calculated excitations energies are in agreement with the previously reported experimental data. Due to the good agreement between the experimental and theoretical report, a proposal of not yet synthesized complexes was performed. [Copyright &y& Elsevier]

Abstract: A theoretical study of the electronic structure, UV–Vis absorption spectra, reactivity and EPR parameters using density functional theory (DFT) and its extension time dependent-density functional theory (TD-DFT) was performed for a family of paramagnetic Ni(II)-Porphyrazines dimers connected by Ni(III)-dithiolene, with general formula [Ni(II)PzR6Ni(III)S4PzR6Ni(II)]1− where the peripheral substituents R are –NH2 (1), –OH (2), –CH3 (3), –H (4), –C6H5 (5), –Cl (6), –CO2H (7) and –NO2 (8)). The simulated UV–Vis absorption spectra exhibit the usual B or Soret and Q bands. The energies of the electronic transitions, the g-tensors and the simulated EPR spectrum are in good agreement with previously reported experimental data. The results show that the odd electron in complexes with R being electron donor are delocalized over the bridge fragment and the systems with R being electron withdrawing are delocalized over the macrocycles Pz. Thus, along with the frontier MOs analysis and reactivity indexes, it was possible to conclude that the character of the peripheral substituents R affect the reactivity of this kind of systems where the most reactive are those with R being electron donor. In addition, the effect of the R groups on the frontier MOs energies is showed by the correlation of HOMO, LUMO and HOMO–LUMO gap energies with the Hammett constants. On the other hand, charge transfer from the ligands to the Ni atoms and back-donation from the Ni atoms to the ligands is observed in the charge transfer analysis. [Copyright &y& Elsevier]

Abstract: A theoretical study of the electronic and molecular structure of [Cu(binap)2]+ (1) and [Pd(binap)2] (2) complexes (binap=2,20-bis(diphenylphosphino)-1,10-binaphthyl) was performed. We examined the ground states and the lowest excited states (S1 and T1) in order to get insights about the luminescence processes of these systems. Geometry optimizations were performed at density functional theory DFT (PW91 functional) employing Slater type orbitals (STO) including two polarization functions (TZ2P) and scalar relativistic corrections via the ZORA Hamiltonian. The UV–Vis absorption spectra were simulated employing time-dependent density functional theory (TD-DFT) and solvent effects were estimated using the COSMO model. The calculated excitation energies are in good agreement with experimental data, which is reflected by the calculated UV–Vis spectra. The results suggest that the lowest-energy excited state in each compound corresponds to a ML-LCT (metal–ligand to ligand charge transfer) state. We constructed Jablonsky diagrams for the luminescence phenom. In (1), this process consists of flourescence originated from the S1 state and the phosphorescence from the T1 state. In (2) the emission energy only results in phosphorescence. We found mainly differences in the geometrical parameters that envolve the Pd metal. The structural changes observed in the calculated geometries of the first excited states respect to those of the ground state have significative relevance in the values of the calculated Stokes shifts showed by the complexes in their UV–Vis/emission spectra. [Copyright &y& Elsevier]

Abstract: Relativistic density functional calculations were carried out on several Tantalum cluster of the general formula [Ta6X12Y6]3−, with the aim to characterize and analyze their molecular structure and electronic properties, in order to gain more insights into their stability and reactivity. Herein are reported the geometrical parameters, electronic structures, excitation energies and magnetic properties, of a series of clusters that have been and have not been yet synthesized. The calculated Δg tensor shows that as the halide capping ligand become heavier the Δg tensor values increases due to spin–orbit effects. Through the use of the reactivity indexes it is shown that when the axial ligand is iodine it becomes the most reactive and labile ligand. The TD-DFT calculations on the complete [Ta6X12Y6]3− cluster family show good agreement with the available experimental data. [Copyright &y& Elsevier]

The turn‐on luminescent chemosensor [2‐Hydroxy‐1‐naphthaldehyde‐(2‐pyridyl) hydrazone] (L), selective to Al3+ ions, was studied by means of density functional theory (DFT) and time‐dependent‐DFT quantum mechanics calculations. The UV‐Vis absorption and the radiative channel from the adiabatic S1 excited state were assessed in order to elucidate the selective sensing mechanism of L to Al3+ ions. We found that twisted intramolecular charge transfer (TICT) and photoelectron transfer (PET), which alter the emissive state, are responsible for the luminescence quenching in L. After coordination with Al3+, the TICT is blocked, and PET is no longer possible. So, the emission of the coordination complex is activated, and a fluorescence effect enhanced by chelation is observed. For compounds with Zn2+ and Cd2+, the luminescence quenching is caused by PET, while for Ni2+, ligand to metal charge transfer is the prominent mechanism. To go into more detail, the metal‐ligand interaction was analyzed via the Morokuma‐Ziegler energy decomposition scheme and the natural orbital of chemical valence. [ABSTRACT FROM AUTHOR]

The study of the electronic structure and optical properties of natural pigments using state of the art time-dependent first-principles calculations is presented to highlight their usefulness for photo electrochemical devices. Ground state geometries, UV–vis spectra and photovoltaic properties are reported. In the family of chosen anthocyanidins, it is observed that the frontier molecular orbitals (FMOs) are mainly localized over the whole molecule with exceptions noted for Delphinidin and Petunidin, while in the anthocyanins all the FMOs are localized over the three rings of the molecule, without any contribution of the glycoside motifs. Conversely, the interaction between Cyanidin and Cyanidin 3,5-diglucoside with TiO 2 as the semiconductor in its cluster and surface form was also studied using periodic density functional calculations for suitable supercell models representing the systems of interest. For the Cyanidin 3,5-diglucoside/TiO 2 system the results showed that its highest occupied molecular orbital (HOMO) is located in the TiO 2 bandgap and its lowest unoccupied molecular orbital (LUMO) is close to the TiO 2 conduction band minimum (CB) leading to greatly enhanced visible light absorption. [ABSTRACT FROM AUTHOR]