Dynamic Covalent Properties of a Novel Indolo[3,2-b]carbazole Diradical.

This work describes the synthesis and properties of a dicyanomethylene-substituted indolo[3,2-b]carbazole diradical ICz-CN. This quinoidal system dimerises almost completely to (ICz-CN)2 , which contains two long C(sp3 )-C(sp3 ) σ-bonds between the dicyanomethylene units. The minor open-shell ICz-CN component in the solid-state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV-visible spectroelectrochemical data, as well as comparison with reference monomer ICz-Br reveal that the nature of the one-electron oxidation of (ICz-CN)2 at ambient temperature and ICz-CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co-existence of (ICz-CN)2 and ICz-CN. The involvement of the dicyanomethylene groups stabilizes the close-lying LUMO and LUMO+1 of (ICz-CN)2 and especially ICz-CN compared to ICz-Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz-CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(-NIR) electronic transitions in parent (ICz-CN)2 and ICz-CN and their different redox forms have been assigned convincingly with the aid of TD-DFT calculations. The σ-bond in neutral (ICz-CN)2 is cleaved in solution and in the solid-state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue-green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa).

Exchange Rules for Diradical π-Conjugated Hydrocarbons.

A variety of planar π-conjugated hydrocarbons such as heptauthrene, Clar's goblet and, recently synthesized, triangulene have two electrons occupying two degenerate molecular orbitals. The resulting spin of the interacting ground state is often correctly anticipated as S = 1, extending the application of Hund's rules to these systems, but this is not correct in some instances. Here we provide a set of rules to correctly predict the existence of zero mode states as well as the spin multiplicity of both the ground state and the low-lying excited states, together with their open- or closed-shell nature. This is accomplished using a combination of analytical arguments and configuration interaction calculations with a Hubbard model, both backed by quantum chemistry methods with a larger Gaussian basis set. Our results go beyond the well established Lieb's theorem and Ovchinnikov's rule, as we address the multiplicity and the open-/closed-shell nature of both ground and excited states.

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment.

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π-π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.

DLPNO-CCSD(T) scaled methods for the accurate treatment of large supramolecular complexes.

In this work, we present scaled variants of the DLPNO-CCSD(T) method, dubbed as (LS)DLPNO-CCSD(T) and (NS)DLPNO-CCSD(T), to obtain accurate interaction energies in supramolecular complexes governed by noncovalent interactions. The novel scaled schemes are based on the linear combination of the DLPNO-CCSD(T) correlation energies calculated with the standard (LoosePNO and NormalPNO) and modified (Loose2PNO and Normal2PNO) DLPNO-CCSD(T) accuracy levels. The scaled DLPNO-CCSD(T) variants provide nearly TightPNO accuracy, which is essential for the quantification of weak noncovalent interactions, with a noticeable saving in computational cost. Importantly, the accuracy of the proposed schemes is preserved irrespective of the nature and strength of the supramolecular interaction. The (LS)DLPNO-CCSD(T) and (NS)DLPNO-CCSD(T) protocols have been used to study in depth the role of the CH-π versus π-π interactions in the supramolecular complex formed by the electron-donor truxene-tetrathiafulvalene (truxTTF) and the electron-acceptor hemifullerene (C30 H12 ). (NS)DLPNO-CCSD(T)/CBS calculations clearly reveal the higher stability of staggered (dominated by CH-π interactions) versus bowl-in-bowl (dominated by π-π interactions) arrangements in the truxTTF•C30 H12 heterodimer. Hemifullerene and similar carbon-based buckybowls are therefore expected to self-assemble with donor compounds in a richer way other than the typical concave-convex π-π arrangement found in fullerene-based aggregates. © 2017 Wiley Periodicals, Inc.

Quadratic integrand double-hybrid made spin-component-scaled.

We propose two analytical expressions aiming to rationalize the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) schemes for double-hybrid exchange-correlation density-functionals. Their performances are extensively tested within the framework of the nonempirical quadratic integrand double-hybrid (QIDH) model on energetic properties included into the very large GMTKN30 benchmark database, and on structural properties of semirigid medium-sized organic compounds. The SOS variant is revealed as a less computationally demanding alternative to reach the accuracy of the original QIDH model without losing any theoretical background.

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position.

In this study, a set of 10 positional indolocarbazole (ICz) isomers substituted with dicyanomethylene groups connected via para or meta positions are computationally investigated with the aim of exploring the efficiency of structural isomerism and substitution position in controlling their optical and electronic properties. Unrestricted density functional theory (DFT), a spin-flip time-dependent DFT approach, and the multireference CASSCF/NEVPT2 method have been applied to correlate the diradical character with the energetic trends (i.e., singlet-triplet energy gaps). In addition, the nucleus-independent chemical shift together with ACID plots and Raman intensity calculations were used to strengthen the relationship between the diradical character and (anti)aromaticity. Our study reveals that the substitution pattern and structural isomerism represent a very effective way to tune the diradical properties in ICz-based systems with meta-substituted systems with a V-shaped structure displaying the largest diradical character. Thus, this work contributes to the elucidation of the challenging chemical reactivity and physical properties of diradicaloid systems, guiding experimental chemists to produce new molecules with desirable properties.

Conductance enhancement in nanographene-gold junctions by molecular pi-stacking.

First-principles calculations on gold-pentacene-gold and several gold-circumacene-gold nanojunctions indicate that their low-bias conductance is due to the onset of a HOMO-derived resonance, thus being quite sensitive to the detailed interaction between the molecule and the gold leads. It is also found that such interaction is dominated by the electrophilic binding of Au to the (circum)acene, in agreement with previous theoretical and experimental results on pentacene. Therefore, the alignment of the HOMO resonance with the Fermi level, and thus the conductance, increases as the ionization potential and the HOMO-LUMO gap of the molecular arrangement diminish. It is shown here that both quantities are inversely proportional to the molecular length and the number of molecules present on a pi stack. It is also found that the conductance depends dramatically on the amount of pi overlap between the molecules in the stack, as well as on the particular disposition of the metallic tips with respect to the molecule. The conclusions reached point toward pi-stacked arrangements of large circumacenes as potential candidates to build useful nanodevices for molecular electronics made out of nanographene-based materials.

Using circumacenes to improve organic electronics and molecular electronics: design clues.

Theoretical modeling is used here to ascertain the potential use of circumacenes to improve the transport parameters of pi-conjugated materials acting as: (i) the layered molecular constituent for organic electronic devices; and (ii) the molecular component of gold-molecule-gold nanobridges for molecular electronic device use. It is concluded that, to a first approximation, the molecular length or, alternatively, the HOMO-LUMO gap (HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital) can be used to relate the two transport regimes usually found in these two fields, thus serving as a key design parameter for guaranteeing good performance of circumanthracene for both regimes. It is also clearly established that going beyond this simple relationship requires knowledge of the detailed molecule-contact geometry of the molecular nanobridge, and how its tremendous impact on the binding strength and the conductance prevents blind extrapolation of results obtained for molecular nanobridges built by means of different experimental set-ups.

Accurate Treatment of Large Supramolecular Complexes by Double-Hybrid Density Functionals Coupled with Nonlocal van der Waals Corrections.

In this work, we present a thorough assessment of the performance of some representative double-hybrid density functionals (revPBE0-DH-NL and B2PLYP-NL) as well as their parent hybrid and GGA counterparts, in combination with the most modern version of the nonlocal (NL) van der Waals correction to describe very large weakly interacting molecular systems dominated by noncovalent interactions. Prior to the assessment, an accurate and homogeneous set of reference interaction energies was computed for the supramolecular complexes constituting the L7 and S12L data sets by using the novel, precise, and efficient DLPNO-CCSD(T) method at the complete basis set limit (CBS). The correction of the basis set superposition error and the inclusion of the deformation energies (for the S12L set) have been crucial for obtaining precise DLPNO-CCSD(T)/CBS interaction energies. Among the density functionals evaluated, the double-hybrid revPBE0-DH-NL and B2PLYP-NL with the three-body dispersion correction provide remarkably accurate association energies very close to the chemical accuracy. Overall, the NL van der Waals approach combined with proper density functionals can be seen as an accurate and affordable computational tool for the modeling of large weakly bonded supramolecular systems.

Nonlocal van der Waals Approach Merged with Double-Hybrid Density Functionals: Toward the Accurate Treatment of Noncovalent Interactions.

Noncovalent interactions drive the self-assembly of weakly interacting molecular systems to form supramolecular aggregates, which play a major role in nanotechnology and biochemistry. In this work, we present a thorough assessment of the performance of different double-hybrid density functionals (PBE0-DH-NL, revPBE0-DH-NL, B2PLYP-NL, and TPSS0-DH-NL), as well as their parent hybrid and (meta)GGA functionals, in combination with the most modern version of the nonlocal (NL) van der Waals correction. It is shown that this nonlocal correction can be successfully coupled with double-hybrid density functionals thanks to the short-range attenuation parameter b, which has been optimized against reference interaction energies of benchmarking molecular complexes (S22 and S66 databases). Among all the double-hybrid functionals evaluated, revPBE0-DH-NL and B2PLYP-NL behave remarkably accurate with mean unsigned errors (MUE) as small as 0.20 kcal/mol for the training sets and in the 0.25-0.42 kcal/mol range for an independent database (NCCE31). They can be thus seen as appropriate functionals to use in a broad number of applications where noncovalent interactions play an important role. Overall, the nonlocal van der Waals approach combined with last-generation density functionals is confirmed as an accurate and affordable computational tool for the modeling of weakly bonded molecular systems.

Is There Still Room for Parameter Free Double Hybrids? Performances of PBE0-DH and B2PLYP over Extended Benchmark Sets.

The performances of two double hybrids, namely B2PLYP and PBE0-DH, are tested over the large GMNTK30 benchmark and compared with the results obtained with the related global hybrids, B3LYP and PBE0 with the aim of defining if there is still room for the development on nonparametrized functionals at DH level. Beyond the intrinsic interest in figures, these functionals' pairs are chosen as representative of the parametrized (B2PLYP/B3LYP) and parameter-free (PBE0-DH/PBE0) approaches to density functional theory. The obtained results show that the behavior of the double hybrids in general parallel the performances of the corresponding global hybrids, thus showing that either using a parametrized or using a nonparameterized approach to design new double hybrids, the performances are generally ameliorated with respect to the corresponding global hybrids. Nevertheless, the accuracy of B2PLYP is still higher than that of PBE0-DH, especially for thermochemistry. Albeit a link between performances and functional physics is difficult to extricate, it could be argued that this last result is not surprising since both B3LYP and B2PLYP are tuned on this last property.

Ab Initio Modeling of Donor-Acceptor Interactions and Charge-Transfer Excitations in Molecular Complexes: The Case of Terthiophene-Tetracyanoquinodimethane.

This work presents a thorough quantum chemical study of the terthiophene-tetracyanoquinodimethane complex as a model for π-π donor-acceptor systems. Dispersion-corrected hybrid (B3LYP-D) and double hybrid (B2PLYP-D), hybrid meta (M06-2X and M06-HF), and recently proposed long-range corrected (LC-wPBE, CAM-B3LYP, and wB97X-D) functionals have been chosen to deal with π-π intermolecular interactions and charge-transfer excitations in a balanced way. These properties are exhaustively compared to those computed with high-level ab initio SCS-MP2 and CASPT2 methods. The wB97X-D functional exhibits the best performance. It provides reliable intermolecular distances and interaction energies and predicts a small charge transfer from the donor to the acceptor in the ground state. In addition, wB97X-D is also able to yield an accurate description of the charge-transfer excitations in comparison to benchmark CASPT2 calculations.

Combining two-body density correlation functionals with multiconfigurational wave functions using natural orbitals and occupation numbers.

We propose a procedure that combines multiconfigurational (MC) wave functions with two-body density correlation functionals by transforming the latter into functionals of the MC natural orbitals and occupation numbers. The method is tested with the spectroscopic constants of a set of 11 diatomics, the diradical-involved automerization barrier of cyclobutadiene, the energy difference between triplet and open-shell singlet states in He and the methylene molecule, and the magnetic coupling constants of several systems, such as NiO, KNiF(3), K(2)NiF(4), La(2)CuO(4), alpha-4-dehydrotoluene, 1,1('),5,5(')-tetramethyl-6,6(')-dioxo-3,3(')-biverdazyl, [Cu(2)Cl(6)](-2), copper(II) acetate monohidrate and H-He-H. The procedure is applied to the Colle-Salvetti [Theor. Chim. Acta 37, 329 (1975); 53, 55 (1979)], functional and to a size-consistent functional depending on the on-top pair density (F1-5-N(eff)). On average, the best results are provided by the transformed F1-5-N(eff) [J. Chem. Phys. 114, 2022 (2001)] functional.

Torsional potential of 4,4'-bipyridine: ab initio analysis of dispersion and vibrational effects.

Ab initio calculations using restricted Hartree-Fock, second-order Møller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.