73 research outputs found
Theoretical Simulationof the Zeke Spectra ofNaphthalene From SingleVibronic Levels of S1
We present the simulations and analysis of the two-color ZEKE spectra of naphthalene,
performed with the help of quantum chemical calculations of molecular parameters
followed by the modelling of vibronic intensities. Ab initio and semi-empirical
calculations were carried out to obtain molecular structures of neutral and ionic
naphthalene, and vibronic perturbations that couple the electronic states. It is shown
that the intensities, simulated with a model based on the perturbative expansion of
vibronic states, nicely reproduce the observed spectra and contribute to reassign some of
the ground state frequencies of naphthalene cation
Molecular electronics exploiting sharp structure in the electrode density-of-states. Negative differential resistance and Resonant Tunneling in a poled molecular layer on Al/LiF electrodes
Density-functional calculations are used to clarify the role of an ultrathin
LiF layer on Al electrodes used in molecular electronics. The LiF layer creates
a sharp density of states (DOS), as in a scanning-tunneling microscope (STM)
tip. The sharp DOS, coupled with the DOS of the molecule leads to negative
differential resistance (NDR). Electron transfer between oriented molecules
occurs via resonant tunneling. The I-V characteristic for a thin-film of tris
(8-hydroxyquinoline)- aluminum (AlQ) molecules, oriented using electric-field
poling, and sandwiched between two Al/LiF electrodes is in excellent agreement
with theory. This molecular device presents a new paradigm for a convenient,
robust, inexpensive alternative to STM or mechanical break-junction structures.Comment: 5 pages, 3 figure
Doming Modes and Dynamics of Model Heme Compounds
Synchrotron far-IR spectroscopy and density-functional calculations are used to characterize the low-frequency dynamics of model heme FeCO compounds. The “doming” vibrational mode in which the iron atom moves out of the porphyrin plane while the periphery of this ring moves in the opposite direction determines the reactivity of oxygen with this type of molecule in biological systems. Calculations of frequencies and absorption intensities and the measured pressure dependence of vibrational modes in the model compounds are used to identify the doming and related normal modes
Metallic behaviour of carrier-polarized C molecular layers: Experiment and Theory
Although C is a molecular crystal with a bandgap E of ~2.5 eV, we
show that E is strongly affected by injected charge. In sharp contrast to
the Coulomb blockade typical of quantum dots, E is {\it reduced} by the
Coulomb effects. The conductance of a thin C layer sandwiched between
metal (Al, Ag, Au, Mg and Pt) contacts is investigated. Excellent Ohmic
conductance is observed for Al electrodes protected with ultra-thin LiF layers.
First-principles calculations, Hubbard models etc., show that the energy gap of
C is dramatically reduced when electrons hop from C to
C.Comment: 4 PRL style pages, 2 figures. email: [email protected]
Designing molecules to bypass the singlet-triplet bottleneck in the electroluminescence of organic light-emitting-diode materials
Electroluminescence in organic light emitting diode (OLED) materials occurs
via the recombination of excitonic electrons-hole pairs Only the singlet
excitons of commonly used OLED materials, e.g., Aluminum trihydroxyquinoline
(AlQ), decay radiatively, limiting the external quantum efficiency to a
maximum 25%. Thus 75% of the energy is lost due to the triplet bottleneck for
radiative recombination. We consider molecules derived from AlQ which
bypass the triplet bottleneck by designing structures which contain strong
spin-orbit coupling. As a first stage of this work, groundstate energies and
vertical excitation energies of Al-arsenoquinolines and Al-boroarsenoquinolines
are calculated. It is found that the substitution of N by As leads to very
favourable results, while the boron substitution leads to no advantage.Comment: 4 pages, 4 figue
Magnetism and structure at a vacancy in graphene
The electronic structure, bonding and magnetism in graphene containing
vacancies are studied using density-functional methods. The single-vacancy
graphene ground state is spin polarized and structurally flat. The unpolarized
state is non planar only for finite segments. Systems containing periodic
arrays of vacancies displays magnetic transitions and metal-insulator
transitions.Comment: 4 pages, four figure
Photochrome that was not: 2-hydroxynaphtylidene-(8-aminoquinoline)
We report the results of quantum-chemical calculations, which show that the keto form of 2-hydroxynaphtylidene-(8-aminoquinoline) (HNAQ) is slightly more stable than the enol form both in the ground and first excited \u3c0\u3c0* electronic states. The barrier for proton transfer between the enol and the ketone in the ground state is ca. 3300 cm\u20131 (HF), and 770 cm\u20131 (B3LYP), indicating a very fast (ps scale) exchange of protons between the two tautomeric forms. This barrier decreases slightly in the first excited \u3c0\u3c0* electronic state (2500 cm\u20131 - CIS), making proton exchange even faster. We show that the \u3c0\u3c0* state of the ketone tautomer is prone to radiationless transition to a state with nearly perpendicular orientation of the two ring systems, similarly to other Schiff bases that are photochromes (for instance salicydeneaniline). This state arises when an electron from the highest occupied molecular orbital (HOMO) of the ketone ring system is transferred to a LUMO localized on the CHNH group of the bridge connecting the two ring systems of the molecule. The energy minimum of this \u201cperpendicular\u201d state lies only ca. 0.09 eV from the ground state potential-energy surface, thus it is prone to extremely rapid radiationless decay. Further relaxation on this surface leads to a metastable conformation that lies ca. 4440 cm\u20131 above the planar, hydrogen-bonded, ketone conformation. Unfortunately, photochromism of this metastable conformation does not occur, since its absorption spectrum overlaps the spectrum of the stable species (with the predicted absorption around 438 nm vs calculated 440.6 nm in the stable ketone).Peer reviewed: YesNRC publication: Ye
Model, First-Principle Calculation of Ammonia Dissociation on Si(100) Surface. Importance of Proton Tunneling
Abstract: The dissociation of an ammonia molecule on a cluster of Si atoms simulating the 100 silicon crystal structure with two Si dimers has been investigated by means of the DFT and an approximate instanton methods. The model corresponds to the low coverage limit of the surface. Absolute rate constants of two different dissociation paths are evaluated together with deuterium isotope effects. It is demonstrated that, even at room temperatures, the process is dominated by tunneling and that dissociation to a silicon atom of the adjacent dimer, rather than a silicon within the same dimer, is the prevailing mechanism. This leads to creation of a metastable structure which will slowly decay through a two-step hydrogen atom migration towards the absolute minimum on the potential energy surface corresponding to the NH2 group and the hydrogen atom residing in the same dimer
Model, First-Principle Calculation of Ammonia Dissociation on Si(100) Surface. Importance of Proton Tunneling
Abstract: The dissociation of an ammonia molecule on a cluster of Si atoms simulating the 100 silicon crystal structure with two Si dimers has been investigated by means of the DFT and an approximate instanton methods. The model corresponds to the low coverage limit of the surface. Absolute rate constants of two different dissociation paths are evaluated together with deuterium isotope effects. It is demonstrated that, even at room temperatures, the process is dominated by tunneling and that dissociation to a silicon atom of the adjacent dimer, rather than a silicon within the same dimer, is the prevailing mechanism. This leads to creation of a metastable structure which will slowly decay through a two-step hydrogen atom migration towards the absolute minimum on the potential energy surface corresponding to the NH2 group and the hydrogen atom residing in the same dimer
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