12 research outputs found
Solvent Effects on Electronically Excited States Using the Conductor-Like Screening Model and the Second-Order Correlated Method ADC(2)
The conductor-like screening model (COSMO) is used to
treat solvent
effects on excited states within a correlated method based on the
algebraic-diagrammatic construction through second-order ADC(2). The
origin of solvent effects is revisited, and it is pointed out that
two types of contributions have to be considered. One effect is due
to the change of the soluteās charge distribution after excitation,
which triggers a reorganization of the solvent. Initially, only the
electronic degrees of freedom adapt to the new charge distribution
(nonequilibrium case); for sufficiently long-lived states, the reorientation
of the solvent molecules contributes, as well (equilibrium case).
The second effect is the coupling of the transition densities to the
fast (purely electronic) response of the solvent molecules, which
can be viewed as excitonic coupling between solute and solvent molecules.
This interaction is also responsible for the screening of excitonic
couplings between spatially separated chromophores. While most previous
implementations of comparable continuum solvation models only include
either of both effects, we argue that both contributions should be
taken into account. Both effects can significantly influence the excitation
energy and excited state properties of the solute, as exemplified
for the ĻāĻ* and <i>n</i>āĻ*
excitations of acrolein, and no a priori reason exists to neglect
either. The implementation is also tested for the excitonic coupling
of the ethene dimer where linear response contributions are indispensable
for recovering the screening effects due to the solvent. Example applications
to larger cases are provided, too. We discuss the excitonic coupling
in a linked dyad consisting of two perylene-tetracarboxy-diimide chromophores,
and the solvent effects on an intramolecular charge-transfer state
of 4-(<i>N</i>,<i>N</i>-dimethylamino)Ābenzonitrile
Solvent Effects on Electronically Excited States Using the Conductor-Like Screening Model and the Second-Order Correlated Method ADC(2)
The conductor-like screening model (COSMO) is used to
treat solvent
effects on excited states within a correlated method based on the
algebraic-diagrammatic construction through second-order ADC(2). The
origin of solvent effects is revisited, and it is pointed out that
two types of contributions have to be considered. One effect is due
to the change of the soluteās charge distribution after excitation,
which triggers a reorganization of the solvent. Initially, only the
electronic degrees of freedom adapt to the new charge distribution
(nonequilibrium case); for sufficiently long-lived states, the reorientation
of the solvent molecules contributes, as well (equilibrium case).
The second effect is the coupling of the transition densities to the
fast (purely electronic) response of the solvent molecules, which
can be viewed as excitonic coupling between solute and solvent molecules.
This interaction is also responsible for the screening of excitonic
couplings between spatially separated chromophores. While most previous
implementations of comparable continuum solvation models only include
either of both effects, we argue that both contributions should be
taken into account. Both effects can significantly influence the excitation
energy and excited state properties of the solute, as exemplified
for the ĻāĻ* and <i>n</i>āĻ*
excitations of acrolein, and no a priori reason exists to neglect
either. The implementation is also tested for the excitonic coupling
of the ethene dimer where linear response contributions are indispensable
for recovering the screening effects due to the solvent. Example applications
to larger cases are provided, too. We discuss the excitonic coupling
in a linked dyad consisting of two perylene-tetracarboxy-diimide chromophores,
and the solvent effects on an intramolecular charge-transfer state
of 4-(<i>N</i>,<i>N</i>-dimethylamino)Ābenzonitrile
Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules
Tripletātriplet annihilation (TTA) leads to a
reduced efficiency
of organic light-emitting diodes (OLEDs) at high current densities.
Spacial confinement of the triplet excitons, which is mainly dependent
on triplet energy differences, can reduce the TTA rate. Therefore,
a deliberate choice of the organic semiconductor materials with particular
attention to their triplet energies can help to considerably increase
the device efficiency. Organic solid-state lasers are, on the other
hand, efficiently quenched by singletātriplet annihilation
(STA), which is closely related to the tripletātriplet absorption
of the organic semiconductors. To establish a useful set of parameters
related to the processes in organic semiconducting devices, we provide
theoretical estimates for the triplet energy of 31 organic semiconductor
molecules using state-of-the art ab initio quantum chemical methods.
For a subset of 22 molecules, the tripletātriplet absorption
spectra were calculated as well. We also discuss related features
like localizations of excitations to molecular fragments, driven by
the structural changes of the molecules in the excited triplet state.
The calculated excited-state properties can assist experimentalists
and serve as input parameters in simulations of organic electronics
Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules
Tripletātriplet annihilation (TTA) leads to a
reduced efficiency
of organic light-emitting diodes (OLEDs) at high current densities.
Spacial confinement of the triplet excitons, which is mainly dependent
on triplet energy differences, can reduce the TTA rate. Therefore,
a deliberate choice of the organic semiconductor materials with particular
attention to their triplet energies can help to considerably increase
the device efficiency. Organic solid-state lasers are, on the other
hand, efficiently quenched by singletātriplet annihilation
(STA), which is closely related to the tripletātriplet absorption
of the organic semiconductors. To establish a useful set of parameters
related to the processes in organic semiconducting devices, we provide
theoretical estimates for the triplet energy of 31 organic semiconductor
molecules using state-of-the art ab initio quantum chemical methods.
For a subset of 22 molecules, the tripletātriplet absorption
spectra were calculated as well. We also discuss related features
like localizations of excitations to molecular fragments, driven by
the structural changes of the molecules in the excited triplet state.
The calculated excited-state properties can assist experimentalists
and serve as input parameters in simulations of organic electronics
Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules
Tripletātriplet annihilation (TTA) leads to a
reduced efficiency
of organic light-emitting diodes (OLEDs) at high current densities.
Spacial confinement of the triplet excitons, which is mainly dependent
on triplet energy differences, can reduce the TTA rate. Therefore,
a deliberate choice of the organic semiconductor materials with particular
attention to their triplet energies can help to considerably increase
the device efficiency. Organic solid-state lasers are, on the other
hand, efficiently quenched by singletātriplet annihilation
(STA), which is closely related to the tripletātriplet absorption
of the organic semiconductors. To establish a useful set of parameters
related to the processes in organic semiconducting devices, we provide
theoretical estimates for the triplet energy of 31 organic semiconductor
molecules using state-of-the art ab initio quantum chemical methods.
For a subset of 22 molecules, the tripletātriplet absorption
spectra were calculated as well. We also discuss related features
like localizations of excitations to molecular fragments, driven by
the structural changes of the molecules in the excited triplet state.
The calculated excited-state properties can assist experimentalists
and serve as input parameters in simulations of organic electronics
Embedded Multireference Coupled Cluster Theory
Internally contracted multireference
coupled cluster (icMRCC) theory
is embedded within multireference perturbation theory (MRPT) to calculate
energy differences in large strongly correlated systems. The embedding
scheme is based on partitioning the orbital spaces of a complete active
space self-consistent field (CASSCF) wave function, with a truncated
virtual space constructed by transforming selected projected atomic
orbitals (PAOs). MRPT is applied to the environment using a subtractive
embedding approach that also allows for multilayer embedding. Benchmark
calculations are presented for biradical bond dissociation, spin splitting
in a heterocyclic carbene and hydrated FeĀ(II), and for the super-exchange
coupling constant in solid nickel oxide. The method is further applied
to two large transition metal complexes with a triple-Ī¶ basis
set: an iron complex with 175 atoms and 2939 basis functions, and
a nickel complex with 231 atoms, and 4175 basis functions
Emergence of Coherence through Variation of Intermolecular Distances in a Series of Molecular Dimers
Quantum
coherences between electronically excited molecules are
a signature of entanglement and play an important role for energy
transport in molecular assemblies. Here we monitor and analyze for
a homologous series of molecular dimers embedded in a solid host the
emergence of coherence with decreasing intermolecular distance by
single-molecule spectroscopy and quantum chemistry. Coherent signatures
appear as an enhancement of the purely electronic transitions in the
dimers which is reflected by changes of fluorescence spectra and lifetimes.
Effects that destroy the coherence are the coupling to the surroundings
and to vibrational excitations. Complementary information is provided
by excitation spectra from which the electronic coupling strengths
were extracted and found to be in good agreement with calculated values.
By revealing various signatures of intermolecular coherence, our results
pave the way for the rational design of molecular systems with entangled
states
How To Arrive at Accurate Benchmark Values for Transition Metal Compounds: Computation or Experiment?
With
the objective of analyzing which kind of reference data is
appropriate for benchmarking quantum chemical approaches for transition
metal compounds, we present the following, (a) a collection of 60
transition metal diatomic molecules for which experimentally derived
dissociation energies, equilibrium distances, and harmonic vibrational
frequencies are known and (b) a composite computational approach based
on coupled-cluster theory with basis set extrapolation, inclusion
of coreāvalence correlation, and corrections for relativistic
and multireference effects. The latter correction was obtained from
internally contracted multireference coupled-cluster (icMRCC) theory.
This composite approach has been used to obtain the dissociation energies
and spectroscopic constants for the 60 molecules in our data set.
In accordance with previous studies on a subset of molecules, we find
that multireference corrections are rather small in many cases and
CCSDĀ(T) can provide accurate reference values, if the complete basis
set limit is explored. In addition, the multireference correction
improves the results in cases where CCSDĀ(T) is not a good approximation.
For a few cases, however, strong deviations from experiment persist,
which cannot be explained by the remaining error in the computational
approach. We suggest that these experimentally derived values require
careful revision. This also shows that reliable reference values for
benchmarking approximate computational methods are not always easily
accessible via experiment and accurate computations may provide an
alternative way to access them. In order to assess how the choice
of reference data affects benchmark studies, we tested 10 DFT functionals
for the molecules in the present data set against experimental and
calculated reference values. Despite the differences between these
two sets of reference values, we found that the ranking of the relative
performance of the DFT functionals is nearly independent of the chosen
reference
Intramolecular Charge-Transfer Excited-State Processes in 4ā(<i>N</i>,<i>N</i>āDimethylamino)benzonitrile: The Role of Twisting and the ĻĻ* State
The
structural processes leading to dual fluorescence of 4-(diĀmethylĀamino)ĀbenzoĀnitrile
in the gas phase and in acetonitrile solvent were investigated using
a combination of multireference configuration interaction (MRCI) and
the second-order algebraic diagrammatic construction (ADC(2)) methods.
Solvent effects were included on the basis of the conductor-like screening
model. The MRCI method was used for computing the nonadiabatic interaction
between the two lowest excited ĻĻ* states (S<sub>2</sub>(L<sub>a</sub>, CT) and S<sub>1</sub>(L<sub>b</sub>, LE)) and the
corresponding minimum on the crossing seam (MXS) whereas the ADC(2)
calculations were dedicated to assessing the role of the ĻĻ*
state. The MXS structure was found to have a twisting angle of ā¼50Ā°.
The branching space does not contain the twisting motion of the diĀmethylĀamino
group and thus is not directly involved in the deactivation process
from S<sub>2</sub> to S<sub>1</sub>. Polar solvent effects are not
found to have a significant influence on this situation. Applying <i>C<sub>s</sub></i> symmetry restrictions, the ADC(2) calculations
show that CCN bending leads to a strong stabilization and to significant
charge transfer (CT). Nevertheless, this structure is not a minimum
but converts to the local excitation (LE) structure on releasing the
symmetry constraint. These findings suggest that the main role in
the dynamics is played by the nonadiabatic interaction of the LE and
CT states and that the main source for the dual fluorescence is the
twisted internal charge-transfer state in addition to the LE state
A Series of M<sup>II</sup>Cu<sup>II</sup><sub>3</sub> Stars (M = Mn, Ni, Cu, Zn) Exhibiting Unusual Magnetic Properties
The
work in this report describes the syntheses, electrospray ionization
mass spectromtery, structures, and experimental and density functional
theoretical (DFT) magnetic properties of four tetrametallic stars
of composition [M<sup>II</sup>(Cu<sup>II</sup>L)<sub>3</sub>]Ā(ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>, M = Mn; <b>2</b>, M =
Ni; <b>3</b>, M = Cu; <b>4</b>, M = Zn) derived from a
single-compartment Schiff base ligand, <i>N</i>,<i>N</i>ā²-bisĀ(salicylidene)-1,4-butanediamine (H<sub>2</sub>L), which is the [2 + 1] condensation product of salicylaldehyde
and 1,4-diaminobutane. The central metal ion (Mn<sup>II</sup>, Ni<sup>II</sup>, Cu<sup>II</sup>, or Zn<sup>II</sup>) is linked with two
Ī¼<sub>2</sub>-phenoxo bridges of each of the three [Cu<sup>II</sup>L] moieties, and thus the central metal ion is encapsulated in between
three [Cu<sup>II</sup>L] units. The title compounds are rare or sole
examples of stars having these metal-ion combinations. In the cases
of <b>1</b>, <b>3</b>, and <b>4</b>, the four metal
ions form a centered isosceles triangle, while the four metal ions
in <b>2</b> form a centered equilateral triangle. Both the variable-temperature
magnetic susceptibility and variable-field magnetization (at 2ā10
K) of <b>1</b>ā<b>3</b> have been measured and
simulated contemporaneously. While the Mn<sup>II</sup>Cu<sup>II</sup><sub>3</sub> compound <b>1</b> exhibits ferromagnetic interaction
with <i>J</i> = 1.02 cm<sup>ā1</sup>, the Ni<sup>II</sup>Cu<sup>II</sup><sub>3</sub> compound <b>2</b> and Cu<sup>II</sup>Cu<sup>II</sup><sub>3</sub> compound <b>3</b> exhibit
antiferromagnetic interaction with <i>J</i> = ā3.53
and ā35.5 cm<sup>ā1</sup>, respectively. Variable-temperature
magnetic susceptibility data of the Zn<sup>II</sup>Cu<sup>II</sup><sub>3</sub> compound <b>4</b> indicate very weak antiferromagnetic
interaction of ā1.4 cm<sup>ā1</sup>, as expected. On
the basis of known correlations, the magnetic properties of <b>1</b>ā<b>3</b> are unusual; it seems that ferromagnetic
interaction in <b>1</b> and weak/moderate antiferromagnetic
interaction in <b>2</b> and <b>3</b> are possibly related
to the distorted coordination environment of the peripheral copperĀ(II)
centers (intermediate between square-planar and tetrahedral). DFT
calculations have been done to elucidate the magnetic properties.
The DFT-computed <i>J</i> values are quantitatively (for <b>1</b>) or qualitatively (for <b>2</b> and <b>3</b>) matched well with the experimental values. Spin densities and magnetic
orbitals (natural bond orbitals) correspond well with the trend of
observed/computed magnetic exchange interactions