Multireference Ab Initio Calculations on Excited Electronic States of Carbazole-Based Organic Compounds for Thermally Activated Delayed Fluorescence.

The emerging technology of organic light-emitting diodes takes advantage of the thermally activated delayed fluorescence (TADF) mechanism for improved efficiency. Carbazole-based organic molecules are suitable for TADF emission because of charge transfer excitations between the electron-donor carbazole and an electron-acceptor unit. Computational design of new TADF molecules with the desired properties is challenging because charge-transfer excitations cannot be predicted accurately by time-dependent density functional theory. Four groups of carbazole-based donor-acceptor molecules have been studied using multireference ab initio methods to understand the nature of excited electronic states. The state-averaged complete active space self-consistent field (SA-CASSCF) and the N-electron valence state perturbation theory (NEVPT2) were used to calculate energies and oscillator strengths for multiple excited electronic states. The number of active electrons and orbitals and the number of excited states included in state-averaged CASSCF were selected such that the accuracy of ab initio predictions could be improved systematically. The procedure introduced here for the calculation of multiple excited electronic states of TADF candidates can be used to accelerate the computational search for efficient TADF materials.

How does counter-cation substitution influence inter- and intramolecular hydrogen bonding and electrospinnability of alginates.

Despite numerous applications of nanofibrous alginate (Alg) mat, its facile fabrication via electrospinning is still challenging. The low alginate content compared to the carrier polymer and existence of impurities are the main drawbacks of existing approaches. The purpose of this research is both to study and improve alginate electrospinnability by focusing on the effect of inter- and intramolecular hydrogen bonding. Based on hard and soft acids and bases (HSAB) theory, the Na+ cations (carboxylate counter-cation) were substituted with a harder acid, Li+ cation, to increase the strength of ionic interaction and decrease the density of hydrogen bonding. Viscosity and electrical conductivity measurements as well as FTIR and 1H NMR revealed a lower intramolecular hydrogen bonding density in Li-Alg. SEM images showed improvement of alginate electrospinnability for Li-Alg compared to the salts of Na-Alg and K-Alg. This study sheds more light on underlying reasons hindering alginate electrospinning and introduces a simple method for fabrication of nanofibers with high alginate content.

Does single-electron chalcogen bond exist? Some theoretical insights.

Ab initio calculations have been carried out to investigate the σ-hole interaction in XHY···CH3 and XHY···CH2CH3 complexes, where X = F, Cl, Br and Y = S, Se. This interaction, termed "single-electron chalcogen bond interaction" was analyzed in terms of geometric, interaction energies and electronic features of the complexes. This interaction is a weak one, with an interaction energy that varies from a minimum of -1.7 kcal mol(-1) for BrHS···CH3 to -6.0 kcal mol(-1) for FHSe···CH2CH3 at the CCSD(T)/aug-cc-pVTZ level of theory. Energy decomposition analysis indicated that the dominant attraction energy originates in the electrostatic term which is larger for the Se complexes than for the S counterparts. However, the attractive polarization and dispersion components also make an important contribution to the interaction energy for the single-electron chalcogen bond interactions.

Mutual influence between anion-π and pnicogen bond interactions: the enhancement of P⋯N and P⋯O interactions by an anion-π bond.

In this work, the interplay between anion-π and pnicogen bond interactions is investigated by ab initio calculations. Cooperative effects are observed in the studied complexes in which anion-π and pnicogen bond interactions coexist. These effects are analyzed in detail in terms of the energetic, geometric, charge-transfer and electron density properties of the complexes. The cooperative energy ranges from -1.8 to -4.1kcalmol(-1). The effect of an anion-π bond on a pnicogen bond is more pronounced than that of a pnicogen bond on an anion-π bond. The enhancing mechanism is analyzed in views with the charge-transfer, electrostatic potential and electron density analysis.

Exploring σ-hole bonding in XH3Si···HMY (X=H, F, CN; M=Be, Mg; Y=H, F, CH3) complexes: a "tetrel-hydride" interaction.

In this work, a σ-hole interaction is predicted theoretically in XH3Si···HMY complexes, where X=H, F, CN; M=Be, Mg and Y=H, F, CH3. The properties of this interaction, termed "tetrel-hydride" interaction, are investigated in terms of geometric, interaction energies, and electronic features of the complexes. The geometry of these complexes is obtained using the second-order Møller-Plesset perturbation theory (MP2) with aug-cc-pVTZ basis set. For each XH3Si···HMY complex, a tetrel-hydride bond is formed between the negatively charged H atom of HMY molecule and the positively charged Si atom of XH3Si molecule. The CCSD(T)/aug-cc-pVTZ interaction energies of this type of σ-hole bonding range from -0.6 to -3.8 kcal mol(-1). The stability of XH3Si···HMY complexes is attributed mainly to electrostatic and correlation effects. The nature of tetrel-hydride interaction is analyzed with atoms in molecules (AIM) and natural bond orbital (NBO) theories.

An ab initio study on the concerted interaction between chalcogen and pnicogen bonds.

We analyzed cooperation between chalcogen-bonding and pnicogen-bonding interactions in XHS···NCH2P···NCY (X = F, Cl; Y = H, OH, NH2, CN and NC) complexes at the MP2/6-311++G** level. These effects were studied in terms of geometric and energetic properties, harmonic frequencies, and nuclear magnetic resonance (NMR). A cooperativity factor was adopted to measure the cooperativity between the two types of interaction in triads based on S-X and P-CN stretching frequencies. The size of the cooperative effect in each complex depends on the strength of S···N and P···N interactions. It is largest for FHSN⋯CH2P⋯NCNH2 and smallest for ClHS⋯NCH2P⋯NCCN and ClHS⋯NCH2P⋯NCNC complexes. The total spin-spin coupling constants across the chalcogen and pnicogen bonds in the ternary complexes are always larger than those in the binary systems. This trend can be also interpreted as a cooperative effect between chalcogen and pnicogen bond interactions. The enhancing mechanism was analyzed in terms of electron redistribution effects in XHS···NCH2P···NCY complexes.

Substituent effects on cooperativity of pnicogen bonds.

Substituent effects on cooperativity of P···N pnicogen bonds are studied in XH2P···NCH(2)P···NCY (X=F, Cl; Y=H, F, CN, OH, NH(2)) complexes using high-level ab initio calculations. An increased attraction or a positive cooperativity is observed on introduction of a third molecule to the XH(2)P···NCH(2)P and NCH(2)P···NCY dyads. The shortening of the each pnicogen bond distance in the triads is dependent on the strength of the P···N bond and is increased in the order Y = NH(2) > OH > H > F > CN. The energy decomposition analysis indicates that the polarization energy is the important element in the interaction energy of P···N bond and may be regarded as being responsible for the stabilization in these systems. Natural bond orbital theory is used to characterize the interactions and analyze their enhancement with varying orbital interactions.

Cooperativity between fluorine-centered halogen bonds: investigation of substituent effects.

This article analyzes the substitution effects on cooperativity between fluorin-centered halogen bonds in NCF · · · NCF · · · NCX and CNF · · · CNF · · · CNX complexes, where X = H, F, Cl, CN, OH, and NH2. These effects are investigated theoretically in terms of geometric and energetic features of the complexes, which are computed by ab initio methods. The topological analysis, based on the quantum theory of atoms in molecules (QTAIM), is used to characterize the interactions and analyze their enhancement with varying electron density at bond critical points. It is found that the complexes with electron-donating groups exhibit a strong cooperativity, while a much weaker cooperativity occurs in the NCF · · · NCF · · · NCCN and CNF · · · CNF · · · CNCN trimers. An excellent correlation is found between the cooperative energy in the ternary complexes and the calculated three-body interaction energies. The energy decomposition analysis (EDA) indicates that the electrostatic and dispersion effects play a main role in the cooperativity of fluorine-centered halogen bonding.