The recently developed narrow-band blue-emitting organoboron chromophores
based on the multiple-resonance (MR) effect have now become one of
the most important components for constructing efficient organic light
emitting diodes (OLEDs). While they basically emit through fluorescence,
they are also known for showing substantial thermally activated delayed
fluorescence (TADF) even with a relatively large singlet–triplet
gap (Δ
E
ST
). Indeed, understanding
the reverse intersystem crossing (RISC) dynamics behind this peculiar
TADF will allow judicious molecular designs toward achieving better
performing OLEDs. Explaining the underlying nonadiabatic spin-flip
mechanism, however, has often been equivocal, and how the sufficiently
fast RISC takes place even with the sizable Δ
E
ST
and vanishingly small spin–orbit coupling is
not well understood. Here, we show that a vibronic resonance, namely
the frequency matching condition between the vibration and the electronic
energy gap, orchestrates three electronic states together and this
effect plays a major role in enhancing RISC in a typical organoboron
emitter. Interestingly, the mediating upper electronic state is quite
high in energy to an extent that its thermal population is vanishingly
small. Through semiclassical quantum dynamics simulations, we further
show that the geometry dependent non-Condon coupling to the upper
triplet state that oscillates with the frequency Δ
E
ST
/
ℏ
is the main driving force
behind the peculiar resonance enhancement. The existence of an array
of vibrational modes with strong vibronic rate enhancements provides
the ability to sustain efficient RISC over a range of Δ
E
ST
in defiance of the energy gap law, which
can render the MR-emitters peculiar in comparison with more conventional
donor–acceptor type emitters. Our investigation may provide
a new guide for future blue emitting molecule developments.
The simulation of electron beam induced welding of crossed carbon nanotubes is considered with classical molecular dynamics simulations.
Covalent junctions are predicted to form between various types of carbon nanotubes that contain many defects and are likely to be representative
of experimentally welded nanotubes under highly nonequilibrium synthesis conditions. The effect of the junction structure and hydrogen
termination of dangling bonds on the mechanical responses of the junctions is also considered.
The chemical modification of polystyrene through the deposition of a beam of polyatomic fluorocarbon ions (C 3 F 5 + and CF 3 + ) at experimental fluences is studied using classical molecular dynamics simulations with many-body empirical potentials. To facilitate these simulations, a new C-H-F potential is developed on the basis of the second-generation reactive empirical bond-order potential for hydrocarbons developed by Brenner. Lennard-Jones potentials are used to model long-range van der Waals interactions. The incident energy of the ion beam is 50 eV/ion, and it is deposited normal to the surface. The simulations illustrate the important differences in the chemical interactions of these polyatomic ions with the polystyrene. The CF 3 + ions are predicted to be more effective at fluorinating the polystyrene than C 3 F 5 + ions, and the dissociation of the C 3 F 5 + ions produce long-lived precursors to fluorocarbon thin film nucleation.
The role of sliding orientation on the tribological properties of polyethylene (PE) is investigated by using classical molecular dynamics simulations. Cross-linked PE surfaces slide against one another in two different directions: one that is perpendicular to and one that is parallel to the aligned direction of the polymer chains. The results indicate that sliding in the parallel direction occurs with a lower friction coefficient than sliding in the perpendicular direction. In both cases, gross level stick-slip motion is observed to be associated with the sliding of a restrained, corrugated molecular interface. In addition, the simulations demonstrate the way in which the system stores more shear strain energy during sliding in the perpendicular direction. The tribological behavior of these PE surfaces is compared to the behavior of similarly modeled polytetrafluoroethylene surfaces; the differences and similarities between the two systems are discussed.
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