Picosecond X-ray Absorption Spectroscopy of Photochemical Transient Species in Solution

A photoinduced Fe(II) spin crossover reaction in solution is studied with ultrafast x-ray absorption spectroscopy. The iron-nitrogen bond lengthens by 0.21+-0.03 Angstrom in the high-spin transient excited state relative to the ground state

Theoretical investigation of the electronic structure of Fe(II) complexes at spin-state transitions

The electronic structure relevant to low spin (LS)high spin (HS) transitions in Fe(II) coordination compounds with a FeN6 core are studied. The selected [Fe(tz)6]2+(1) (tz=1H-tetrazole), [Fe(bipy)3]2+(2) (bipy=2,2’-bipyridine) and [Fe(terpy)2]2+ (3) (terpy=2,2’:6’,2’’-terpyridine) complexes have been actively studied experimentally, and with their respective mono-, bi-, and tridentate ligands, they constitute a comprehensive set for theoretical case studies. The methods in this work include density functional theory (DFT), time-dependent DFT (TD-DFT) and multiconfigurational second order perturbation theory (CASPT2). We determine the structural parameters as well as the energy splitting of the LS-HS states (ΔEHL) applying the above methods, and comparing their performance. We also determine the potential energy curves representing the ground and low-energy excited singlet, triplet, and quintet d6 states along the mode(s) that connect the LS and HS states. The results indicate that while DFT is well suited for the prediction of structural parameters, an accurate multiconfigurational approach is essential for the quantitative determination of ΔEHL. In addition, a good qualitative agreement is found between the TD-DFT and CASPT2 potential energy curves. Although the TD-DFT results might differ in some respect (in our case, we found a discrepancy at the triplet states), our results suggest that this approach, with due care, is very promising as an alternative for the very expensive CASPT2 method. Finally, the two dimensional (2D) potential energy surfaces above the plane spanned by the two relevant configuration coordinates in [Fe(terpy)2]2+ were computed both at the DFT and CASPT2 levels. These 2D surfaces indicate that the singlet-triplet and triplet-quintet states are separated along different coordinates, i.e. different vibration modes. Our results confirm that in contrast to the case of complexes with mono- and bidentate ligands, the singlet-quintet transitions in [Fe(terpy)2]2+ cannot be described using a single configuration coordinate

Recent advances and future directions to optimize the performances of p-type dye-sensitized solar cells

This review provides a summary of the most important developments in the field of solar cells based on the sensitization of p-type semiconductors, such as NiO, and identifies the future challenges and opportunities to enhance their overall performance. In particular, the main factors responsible for the low open-circuit voltage, short circuit photocurrent and fill factor are discussed in detail. (C) 2012 Elsevier BM. All rights reserved

Cobalt Polypyridyl-Based Electrolytes for p-Type Dye-Sensitized Solar Cells

Polypyridyl Co complexes with different substituents were applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimide-naphthalenediimide (PMI-NDI) dyad. The photocurrent and photovoltages of the devices were found to depend on the steric bulk of the redox species rather than their electrochem. potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (VOC) of each of the devices was superior to the equiv. PMI-NDI-sensitized p-DSCs contg. the triiodide/iodide redox couple

Cobalt Polypyridyl-Based Electrolytes for p-Type Dye-Sensitized Solar Cells

A series of polypyridyl cobalt complexes with different substituents was applied as redox mediators in p-type dye-sensitized solar cells (p-DSCs), consisting of mesoporous NiO sensitized with a perylenemonoimide naphthalenediimide (PMI-NDI) dyad. The photocurrent and photovoltages of the devices were found to depend on the steric bulk of the redox species rather than their electrochemical potential. Bulky substituents were found to slow the detrimental charge recombination reactions between holes in the NiO semiconductor and the reduced form of the redox couple. The open-circuit potential (V(OC)) of each of the devices was superior to the equivalent PMI-NDIsensitized p-DSCs containing the triiodide/iodide redox couple

A p-type NiO-based dye-sensitized solar cell with an open-circuit voltage of 0.35 V

Employing a mol. dyad and a cobalt-based electrolyte gives a threefold-increase in open-circuit voltage (VOC) for a p-type NiO device (VOC=0.35 V), and a fourfold better energy conversion efficiency. Incorporating these improvements in a TiO2/NiO tandem dye-sensitized solar cell (TDSC), results in a TDSC with a VOC=0.91 V

Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X-ray Absorption Spectroscopy

For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/lowspin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specifi c deformation in the photoinduced high-spin state of solvated [Fe(terpy)2 ]2+ , a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal− ligand bonds. The correlated changes in Fe−NAxial , Fe− NDistal , and bite angle NDistal− Fe− NAxial extracted from the measurements are in very good agreement with those predicted by DFT calculations in D2d symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order