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 solute’s 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 π–π* and <i>n</i>–π* 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)­benzonitrile

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 solute’s 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 π–π* and <i>n</i>–π* 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)­benzonitrile

Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet 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 singlet–triplet annihilation (STA), which is closely related to the triplet–triplet 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 triplet–triplet 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

Triplet–triplet 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 singlet–triplet annihilation (STA), which is closely related to the triplet–triplet 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 triplet–triplet 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

Triplet–triplet 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 singlet–triplet annihilation (STA), which is closely related to the triplet–triplet 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 triplet–triplet 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 Fe­(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-ζ 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 core–valence 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 CCSD­(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 CCSD­(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 4‑(<i>N</i>,<i>N</i>‑Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State

The structural processes leading to dual fluorescence of 4-(di­methyl­amino)­benzo­nitrile 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 ππ* states (S₂(L_a, CT) and S₁(L_b, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the di­methyl­amino group and thus is not directly involved in the deactivation process from S₂ to S₁. Polar solvent effects are not found to have a significant influence on this situation. Applying <i>C_s</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^IICu^II₃ 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^II(Cu^IIL)₃]­(ClO₄)₂ (<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>′-bis­(salicylidene)-1,4-butanediamine (H₂L), which is the [2 + 1] condensation product of salicylaldehyde and 1,4-diaminobutane. The central metal ion (Mn^II, Ni^II, Cu^II, or Zn^II) is linked with two μ₂-phenoxo bridges of each of the three [Cu^IIL] moieties, and thus the central metal ion is encapsulated in between three [Cu^IIL] 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 2–10 K) of <b>1</b>–<b>3</b> have been measured and simulated contemporaneously. While the Mn^IICu^II₃ compound <b>1</b> exhibits ferromagnetic interaction with <i>J</i> = 1.02 cm^–1, the Ni^IICu^II₃ compound <b>2</b> and Cu^IICu^II₃ compound <b>3</b> exhibit antiferromagnetic interaction with <i>J</i> = −3.53 and −35.5 cm^–1, respectively. Variable-temperature magnetic susceptibility data of the Zn^IICu^II₃ compound <b>4</b> indicate very weak antiferromagnetic interaction of −1.4 cm^–1, as expected. On the basis of known correlations, the magnetic properties of <b>1</b>–<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 copper­(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