168 research outputs found
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
Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy
The use of time-resolved photoelectron spectroscopy for analyzing excited state intramolecular proton transfer (ESIPT) and internal conversion dynamics in a model system was investigated. The photoelectron spectra of both the excited state enol and keto tautomers were presented as a function of pump laser wavelength and pump-probe time delay. It was found that the internal conversion dynamics in o-hydroxybenzaldehyde (OHBA) was influenced by interactions with a close-lying n??* state.open958
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
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]
Quantum effective potential, electron transport and conformons in biopolymers
In the Kirchhoff model of a biopolymer, conformation dynamics can be
described in terms of solitary waves, for certain special cross-section
asymmetries. Applying this to the problem of electron transport, we show that
the quantum effective potential arising due to the bends and twists of the
polymer enables us to formalize and quantify the concept of a {\it conformon}
that has been hypothesized in biology. Its connection to the soliton solution
of the cubic nonlinear Schr\"{o}dinger equation emerges in a natural fashion.Comment: to appear in J. Phys.
Mechanoâelectrical interactions and heterogeneities in wildâtype and drugâinduced long QT syndrome rabbits
Mechanoâelectrical interactions and heterogeneities in wildâtype and drugâinduced long QT syndrome rabbits
Electromechanical reciprocity â comprising electro-mechanical (EMC) and mechano-electric coupling (MEC) â provides cardiac adaptation to changing physiological demands. Understanding electromechanical reciprocity and its impact on function and heterogeneity in pathological conditions â such as (drug-induced) acquired long QT syndrome (aLQTS) â might lead to novel insights in arrhythmogenesis. Our aim is to investigate how electrical changes impact on mechanical function (EMC) and vice versa (MEC) under physiological conditions and in aLQTS. To measure regional differences in EMC and MEC in vivo, we used tissue phase mapping cardiac MRI and a 24-lead ECG vest in healthy (control) and IKr-blocker E-4031-induced aLQTS rabbit hearts. MEC was studied in vivo by acutely increasing cardiac preload, and ex vivo by using voltage optical mapping (OM) in beating hearts at different preloads. In aLQTS, electrical repolarization (heart rate corrected RT-interval, RTn370) was prolonged compared to control (P < 0.0001) with increased spatial and temporal RT heterogeneity (P < 0.01). Changing electrical function (in aLQTS) resulted in significantly reduced diastolic mechanical function and prolonged contraction duration (EMC), causing increased apico-basal mechanical heterogeneity. Increased preload acutely prolonged RTn370 in both control and aLQTS hearts (MEC). This effect was more pronounced in aLQTS (P < 0.0001). Additionally, regional RT-dispersion increased in aLQTS. Motion-correction allowed us to determine APD-prolongation in beating aLQTS hearts, but limited motion correction accuracy upon preload-changes prevented a clear analysis of MEC ex vivo. Mechano-induced RT-prolongation and increased heterogeneity were more pronounced in aLQTS than in healthy hearts. Acute MEC effects may play an additional role in LQT-related arrhythmogenesis, warranting further mechanistic investigations
Cardiac Electrophysiological Effects of Light-Activated Chloride Channels
During the last decade, optogenetics has emerged as a paradigm-shifting technique to monitor and steer the behavior of specific cell types in excitable tissues, including the heart. Activation of cation-conducting channelrhodopsins (ChR) leads to membrane depolarization, allowing one to effectively trigger action potentials (AP) in cardiomyocytes. In contrast, the quest for optogenetic tools for hyperpolarization-induced inhibition of AP generation has remained challenging. The green-light activated ChR from Guillardia theta (GtACR1) mediates Clâ-driven photocurrents that have been shown to silence AP generation in different types of neurons. It has been suggested, therefore, to be a suitable tool for inhibition of cardiomyocyte activity. Using single-cell electrophysiological recordings and contraction tracking, as well as intracellular microelectrode recordings and in vivo optical recordings of whole hearts, we find that GtACR1 activation by prolonged illumination arrests cardiac cells in a depolarized state, thus inhibiting re-excitation. In line with this, GtACR1 activation by transient light pulses elicits AP in rabbit isolated cardiomyocytes and in spontaneously beating intact hearts of zebrafish. Our results show that GtACR1 inhibition of AP generation is caused by cell depolarization. While this does not address the need for optogenetic silencing through physiological means (i.e., hyperpolarization), GtACR1 is a potentially attractive tool for activating cardiomyocytes by transient light-induced depolarization
Fragmentation processes of ionized 5-fluorouracil in the gas phase and within clusters
We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF+. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH3 loss from protonated 5-fluorouracil
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