4 research outputs found
Charge-Transfer Dynamics in the Lowest Excited State of a PentaceneāFullerene Complex: Implications for Organic Solar Cells
We
characterize the dynamic nature of the lowest excited state
in a pentacene/C<sub>60</sub> complex on the femtosecond time scale,
via a combination of ab initio molecular dynamics and time-dependent
density functional theory. We analyze the correlations between the
molecular vibrations of the complex and the oscillations in the electron-transfer
character of its lowest excited state, which point to vibration-induced
coherences between the (pentacene-based) local-excitation (LE) state
and the complex charge-transfer (CT) state. We discuss the implications
of our results on this model system for the exciton-dissociation process
in organic solar cells
Benchmarking Density Functional Theory Approaches for the Description of Symmetry Breaking in Long Polymethine Dyes
Long polymethines are well-known
experimentally to symmetry-break,
which dramatically modifies their linear and nonlinear optical properties.
Computational modeling could be very useful to provide insight into
the symmetry-breaking process, which is not readily available experimentally;
however, accurately predicting the crossover point from symmetric
to symmetry-broken structures has proven challenging. Here, we benchmark
the accuracy of several density functional theory approaches relative
to CCSDĀ(T) geometries. In particular, we compare analogous hybrid
and long-range-corrected (LRC) functionals to clearly show the influence
of the functional exchange term. Although both hybrid and LRC functionals
can be tuned to reproduce the CCSDĀ(T) geometries, the LRC functionals
are better performing at reproducing the geometry evolution with chain
length and provide a finite upper limit for the gas-phase crossover
point; these methods also provide good agreement with the experimental
crossover points for more complex polymethines in polar solvents.
Using an approach based on LRC functionals, a reduction in the crossover
length is found with increasing medium dielectric constant, which
is related to localization of the excess charge on the end groups.
Symmetry breaking is associated with the appearance of an imaginary
frequency of b<sub>2</sub> symmetry involving a large change in the
degree of bond-length alternation. Examination of the IR spectra shows
that short, isolated streptocyanines have a mode at ā¼1200 cm<sup>ā1</sup> involving a large change in bond-length alternation;
as the polymethine length or the medium dielectric increases, the
frequency of this mode decreases before becoming imaginary at the
crossover point
Copper-Catalyzed Ring-Expansion Cascade of Azirines with Alkynes: Synthesis of Multisubstituted Pyridines at Room Temperature
The
first intermolecular ring-expansion cascade of azirines with
alkynes for the synthesis of pyridines, enabled by a copper/triethylamine
catalytic system via simultaneous generation and utilization of yne-enamine
and skipped-yne-imine intermediates, is reported. Experimental as
well as computational mechanistic studies revealed that the role of
triethylamine is crucial in deciding the reaction pathway toward the
pyridine products. This process offers a novel, one-step, direct,
and practical strategy for the rapid construction of highly substituted
pyridines under exceedingly mild conditions, and an installed alkyne
functionality
Ionization Energies, Electron Affinities, and Polarization Energies of Organic Molecular Crystals: Quantitative Estimations from a Polarizable Continuum Model (PCM)-Tuned Range-Separated Density Functional Approach
We propose a new
methodology for the first-principles description
of the electronic properties relevant for charge transport in organic
molecular crystals. This methodology, which is based on the combination
of a nonempirical, optimally tuned range-separated hybrid functional
with the polarizable continuum model, is applied to a series of eight
representative molecular semiconductor crystals. We show that it provides
ionization energies, electron affinities, and transport gaps in very
good agreement with experimental values, as well as with the results
of many-body perturbation theory within the GW approximation at a
fraction of the computational costs. Hence, this approach represents
an easily applicable and computationally efficient tool to estimate
the gas-to-crystal phase shifts of the frontier-orbital quasiparticle
energies in organic electronic materials