Theoretical Study of the Local and Charge-Transfer Excitations in Model Complexes of Pentacene-C60 Using Tuned Range-Separated Hybrid Functionals.

The characteristics of the electronic excited states and the charge-transfer processes at organic-organic interfaces play an important role in organic electronic devices. However, charge-transfer excitations have proven challenging to describe with conventional density functional theory (DFT) methodologies due to the local nature of the exchange-correlation potentials often employed. Here, we examine the excited states of model pentacene-C60 complexes using time-dependent DFT with, on one hand, one of the most popular standard hybrid functionals (B3LYP) and, on the other hand, several long-range corrected hybrid functionals for which we consider both default and nonempirically tuned range-separation parameters.

Accurate description of torsion potentials in conjugated polymers using density functionals with reduced self-interaction error.

We investigate the torsion potentials in two prototypical π-conjugated polymers, polyacetylene and polydiacetylene, as a function of chain length using different flavors of density functional theory. Our study provides a quantitative analysis of the delocalization error in standard semilocal and hybrid density functionals and demonstrates how it can influence structural and thermodynamic properties. The delocalization error is quantified by evaluating the many-electron self-interaction error (MESIE) for fractional electron numbers, which allows us to establish a direct connection between the MESIE and the error in the torsion barriers.

Tuning the electronic and photophysical properties of heteroleptic iridium(III) phosphorescent emitters through ancillary ligand substitution: a theoretical perspective.

The development and application of phosphorescent emitters in organic light-emitting diodes (OLEDs) have played a critical role in the push to commercialization of OLED-based display and lighting technologies. Here, we use density functional theory methods to study how modifying the ancillary ligand influences the electronic and photophysical properties of heteroleptic bis(4,6-difluorophenyl) pyridinato-N,C [dfppy] iridium(III) complexes. We examine three families of bidentate ancillary ligands based on acetylacetonate, picolinate, and pyridylpyrazolate.

Understanding the Density Functional Dependence of DFT-Calculated Electronic Couplings in Organic Semiconductors.

We present an analysis of the magnitude of density functional theory (DFT)-calculated intermolecular electronic couplings (transfer integrals) in organic semiconductors to give insight into the impact that the choice of functional has on the value of this parameter, which is particularly important in the context of charge transport. The major factor determining the magnitude of the calculated transfer integrals is the amount of nonlocal Hartree-Fock (HF) exchange within a given functional, with the transfer integrals increasing by up to a factor of 2 when going from 0 to 100% HF exchange for a series of conventional functionals. We underline that these variations in the transfer integrals are in fact to be expected, with the computed transfer integrals evolving linearly with the amount of HF exchange.

On the relationship between bond-length alternation and many-electron self-interaction error.

Predicting accurate bond-length alternations (BLAs) in long conjugated molecular chains has been a major challenge for electronic-structure theory for many decades. While Hartree-Fock (HF) overestimates BLA significantly, second-order perturbation theory and commonly used density functional theory (DFT) approaches typically underestimate it. Here, we discuss how this failure is related to the many-electron self-interaction error (MSIE), which is inherent to both HF and DFT approaches.

Lowest excited states and optical absorption spectra of donor-acceptor copolymers for organic photovoltaics: a new picture emerging from tuned long-range corrected density functionals.

Polymers with low optical gaps are of importance to the organic photovoltaics community due to their potential for harnessing a large portion of the solar energy spectrum. The combination along their backbones of electron-rich and electron-deficient fragments contributes to the presence of low-lying excited states that are expected to display significant charge-transfer character. While conventional hybrid functionals are known to provide unsatisfactory results for charge-transfer excitations at the time-dependent DFT level, long-range corrected (LRC) functionals have been reported to give improved descriptions in a number of systems.

Long-range corrected hybrid functionals for π-conjugated systems: dependence of the range-separation parameter on conjugation length.

Long-range corrected (range-separated hybrid) functionals represent a relatively new class of functionals for generalized Kohn-Sham theory that have proven to be very successful, for instance, when it comes to predicting ionization potentials and energy gaps for a wide range of molecules and solids. The results obtained from long-range corrected density functional theory approaches can be improved dramatically, if the range-separation parameter (ω) is optimized for each system separately. In this work, we have optimized ω for a series of π-conjugated molecular systems of increasing length by forcing the resulting functionals to obey the ionization potential-theorem, i.

Communication: orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals.

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals.

Evaluating the Performance of DFT Functionals in Assessing the Interaction Energy and Ground-State Charge Transfer of Donor/Acceptor Complexes: Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ) as a Model Case.

We have evaluated the performance of several density functional theory (DFT) functionals for the description of the ground-state electronic structure and charge transfer in donor/acceptor complexes. The tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) complex has been considered as a model test case. Hybrid functionals have been chosen together with recently proposed long-range corrected functionals (ωB97X, ωB97X-D, LRC-ωPBEh, and LC-ωPBE) in order to assess the sensitivity of the results to the treatment and magnitude of exact exchange.

Effects of electronegative substitution on the optical and electronic properties of acenes and diazaacenes.

Large acenes, particularly pentacenes, are important in organic electronics applications such as thin-film transistors. Derivatives where CH units are substituted by sp(2) nitrogen atoms are rare but of potential interest as charge-transport materials. In this article, we show that pyrazine units embedded in tetracenes and pentacenes allow for additional electronegative substituents to induce unexpected redshifts in the optical transitions of diazaacenes.

Assessing the performance of density functional theory for the electronic structure of metal-salens: the M06 suite of functionals and the d⁴-metals.

We have systematically investigated the electronic structure of the d⁴ metal-salen complexes including the Cr(II)-, Mn(III)-, Fe(IV)-, Mo(II)-, Tc(III)-, and Ru(IV)-salen complexes. Density functional theory (DFT) has been employed, using the BP86 and B3LYP functionals, and the entire M05 and M06 suites of meta-generalized gradient functionals. These results are compared to robust complete active-space self-consistent field (CASSCF) optimized geometries and complete active-space third-order perturbation theory (CASPT3) energies for the lowest singlet, triplet, and quintet states.

Torsion potential in polydiacetylene: accurate computations on oligomers extrapolated to the polymer limit.

The optoelectronic properties of polydiacetylenes can be strongly modulated by torsions along the polymer chains. These as well as other distortions of the nominally coplanar polydiacetylene backbones result in the major color changes observed for these materials in response to a variety of external, low-energy stimuli; such color changes form the basis for the many applications of polydiacetylenes as sensor materials. There has been little theoretical work related to backbone distortions in polydiacetylenes; actually, previous estimates of the torsional barriers in these systems differ by an order of magnitude.

Assessing the performance of density functional theory for the electronic structure of metal-salens: the d6-metals.

The energies and optimized geometries of the lowest lying singlet, triplet, and quintet states for the Fe(II)-, Co(III)-, Ni(IV)-, Ru(II)-, Rh(III)-, and Pd(IV)-salens have been computed with the B3LYP and BP86 density functional theory (DFT) methods, and the results are compared to more robust complete active-space self-consistent field (CASSCF) values. Density functional optimizations are performed using two different models of the salen ligand, and CASSCF relative energies at these DFT geometries show no appreciable difference whether the smaller or the larger model salen is considered. Unlike in our previous studies on the d0 and d2 metal-salens, DFT methods rarely predict the correct ordering of states compared to high-level complete active-space third-order perturbation theory (CASPT3) computations.

Assessing the performance of density functional theory for the electronic structure of metal-salens: the d2-metals.

The performance of three common combinations of density functional theory has been evaluated for the geometries and relative energies of a commonly-employed model complex of the salen ligand [salen = bis(salicylaldehydo)ethylenediamine] with the d2-metals Ti(II), V(III), Cr(IV), Zr(II), Nb(III), and Mo(IV). High-level ab initio methods including complete active-space third-order perturbation theory have been employed both as benchmarks for the density functional theory results and to examine the multireference character of the low-lying electronic states in these systems. The strong multireference character of the systems has been clearly demonstrated.

Assessing the performance of density functional theory for the electronic structure of metal-salens: the 3d(0)-metals.

A series of metal-salen complexes of the 3d(0) metals Sc(III), Ti(IV), V(V), Cr(VI), and Mn(VII) have been explored using high-level electronic structure methods including coupled-cluster theory with singles, doubles, and perturbative triples as well as complete active-space third-order perturbation theory. The performance of three common density functional theory approaches has been assessed for both the geometries and the relative energies of the low-lying electronic states. The nondynamical correlation effects are demonstrated to be extremely large in all of the systems examined.

The electronic structure of oxo-Mn(salen): single-reference and multireference approaches.

Using single- and multireference approaches we have examined many of the low-lying electronic states of oxo-Mn(salen), several of which have not been explored previously. Large complete-active-space self-consistent-field (CASSCF) computations have been performed in pursuit of an accurate ordering for the lowest several electronic states. Basis set and relativistic effects have also been considered.