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At least 811 records · Page 45Linked to original sources
PubMed2002

Novel chiral pyromellitdiimide (1,2,4,5-benzenetetracarboxydiimide) dimers and trimers: exploring their structure, electronic transitions, and exciton coupling.

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200-260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.

Benzoates↗
PubMed2001

Theoretical investigation on the electronic structure of pentacyano(L)ferrate(II) complexes with NO(+), NO, and NO(-) ligands. Redox interconversion, protonation, and cyanide-releasing reactions.

Reaction pathways for the one- and two-electron reductions of [Fe(CN)(5)NO](2)(-) have been investigated by means of a density functional theory (DFT) approach combined with the polarized continuum model (PCM) of solvation. In addition, UV-vis spectroscopic data were obtained using ZINDO/S calculations including a point-charge model simulation of solvent effects. DFT methodologies have been used to assess the thermodynamical feasibility of protonation and cyanide-release processes for the reduced species. We conclude that [Fe(CN)(5)NO](3)(-) is a stable species in aqueous solution but may release cyanide yielding [Fe(CN)(4)NO](2)(-), consistent with experimental results. On the other hand, the [Fe(CN)(5)NO](4)(-) complex turns out to be unstable in solution, yielding the product of cyanide release, [Fe(CN)(4)NO](3)(-), and/or the protonated HNO complex. All the structural and spectroscopic (IR, UV-vis) predictions for the [Fe(CN)(5)HNO](3)(-) ion are consistent with the scarce but significant experimental evidence of its presence as an intermediate in nitrogen redox interconversion chemistry. Our computed data support an Fe(II)(LS) + NO(+) assignment for [Fe(CN)(5)NO](2)(-), an Fe(II)(LS) + NO assignment for the one-electron reduction product, but an Fe(I)(LS) + NO(+) for the one-electron product after dissociation of an axial cianide, and an Fe(II) + singlet NO(-) for the two-electron reduction species.

Journal Article↗
PubMed2006

The electronic structure of oxo-Mn(salen): single-reference and multireference approaches.

Using single- and multireference approaches we have examined many of the low-lying electronic states of oxo-Mn(salen), several of which have not been explored previously. Large complete-active-space self-consistent-field (CASSCF) computations have been performed in pursuit of an accurate ordering for the lowest several electronic states. Basis set and relativistic effects have also been considered. For the geometry considered, our best results indicate the ground spin state to be a closed-shell singlet, followed by a pair of low-lying triplet states, with additional singlet states and the lowest quintet state lying significantly higher in energy. Hartree-Fock and density functional theory (DFT) results are obtained and are compared to the more robust CASSCF results. The Hartree-Fock results are qualitatively incorrect for the relative energies of the states considered. Popular density functionals such as BP86 and B3LYP are superior to Hartree-Fock for this problem, but they give inconsistent answers regarding the ordering of the lowest singlet and triplet states and they greatly underestimate the singlet-quintet gap. We obtained multiple Hartree-Fock and DFT solutions within a given spin multiplicity, and these solutions have been subjected to wave function stability analysis.

Journal Article↗
PubMed2006

Spin-resolved electronic structure of nanoscale cobalt islands on Cu(111).

Using spin-polarized scanning tunneling spectroscopy, we reveal how the standing wave patterns of confined surface state electrons on top of nanometer-scale ferromagnetic Co islands on Cu(111) are affected by the spin character of the responsible state, thus experimentally confirming a very recent theoretical result. Furthermore, at the rim of the islands a spin-polarized state is found giving rise to enhanced zero bias conductance. Its polarization is opposite to that of the islands. The experimental findings are in accordance with ab initio spin-density calculations.

Journal Article↗
PubMed2006

Ethyne-bridged (porphinato)zinc(II)-(porphinato)iron(III) complexes: phenomenological dependence of excited-state dynamics upon (porphinato)iron electronic structure.

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5'-[(10',20'- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related supermolecules ([PZn-PFe-(L)1,2]+ species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)1,2]+ ([PFe(L)1,2]+) benchmarks have also been synthesized and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)1,2]+ species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited states of PZn-PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-PFe-(collidine)]+, and [PZn-PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense NIR S1 --> Sn transient absorption manifold. While PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-py)2]+ evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-lutidine)]+ and [PZn-PFe-(collidine)]+ show fast relaxation to a [PZn+-PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while DeltaGCS for [PZn-PFe-(L)1,2]+ species is strongly influenced by the PFe+ ligation state [ligand (DeltaGCS): 4-cyanopyridine (-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent = CH2Cl2], the pump-probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway.

Alkynes↗
PubMed2005

Atomic and electronic structure of mixed and partial dislocations in GaN.

Here we present a detailed study of mixed dislocations in GaN, in which the complexities of the atomic arrangement in the cores have been imaged directly for the first time using an aberration corrected scanning transmission electron microscope. In addition to being present as a full-core structure, the mixed dislocation is observed to dissociate into partial dislocations separated by a stacking fault only a few unit cells in length. The generation of this stacking fault appears to be impurity driven and its presence is consistent with theoretical predictions for dislocation dissociation in materials with hexagonal crystal symmetry.

Journal Article↗