Organoboron Complexes as Thermally Activated Delayed Fluorescence (TADF) Materials for Organic Light-Emitting Diodes (OLEDs): A Computational Study.

We report on organoboron complexes characterized by very small energy gaps (ΔEST) between their singlet and triplet states, which allow for highly efficient harvesting of triplet excitons into singlet states for working as thermally activated delayed fluorescence (TADF) devices. Energy gaps ranging between 0.01 and 0.06 eV with dihedral angles of ca. 90° were registered. The spin-orbit couplings between the lowest excited S1 and T1 states yielded reversed intersystem crossing rate constants (KRISC) of an average of 105 s-1. This setup accomplished radiative decay rates of ca. 106 s-1, indicating highly potent electroluminescent devices, and hence, being suitable for application as organic light-emitting diodes.

Zn2+-Schiff's Base Complex as an "On-Off-On" Molecular Switch and a Fluorescence Probe for Cu2+ and Ag+ Ions.

The present study presents a thorough theoretical analysis of the electronic structure and conformational preference of Schiff's base ligand N,N-bis(2-hydroxybenzilidene)-2,4,6-trimethyl benzene-1,3-diamine (H2L) and its metal complexes with Zn2+, Cu2+ and Ag+ ions. This study aims to investigate the behavior of H2L and the binuclear Zn2+ complex (1) as fluorescent probes for the detection of metal ions (Zn2+, Cu2+ and Ag+) using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The six conformers of the H2L ligand were optimized using the B3LYP/6-311 + + G** level of theory, while the L-2-metal complexes were optimized by applying the B3LYP functional with the LANL2DZ/6-311 + + G** mixed basis set. The gas-phase and solvated Enol-cis isomer (E-cis) was found to be the most stable species. The absorption spectra of the E-cis isomer and its metal complexes were simulated using B3LYP, CAM-B3LYP, M06-2X and ωB97X functionals with a 6-311 + + G** basis set for C, O, N and H atoms and a LANL2DZ basis set for the metal ions (Zn2+, Cu2+ and Ag+). The computational results of the B3LYP functional were in excellent agreement with the experimental results. Hence, it was adopted for performing the emission calculations. The results indicated that metal complex (1) can act as a fluorescent chemosensor for the detection of Ag+ and Cu2+ ions through the mechanism of intermolecular charge transfer (ICT) and as a molecular switch "On-Off-On" via the replacement of Cu2+ by Ag+ ions, as proved experimentally.

Experimental and theoretical study of donor-π-acceptor compounds based on malononitrile.

A set of different donor-π-acceptor compounds having dicyanovinyl as the acceptor and aryl moieties as donors were synthesized by Knoevenagel condensation. The UV-visible absorption and fluorescence spectra were investigated in different solvents. The optical band gab energy (Eg) was linearly correlated with the Hammett resonance effect of the donor to reveal that the higher the value of Hammett resonance effect of a donor, the lower the Eg of the molecule. The photophysical data revealed that compounds M4-M6 are typical molecular rotors with fluorescence due to twisted intramolecular charge transfer. Compound M5 revealed the largest Stokes shift (11,089 cm-1) making it a useful fluorescent sensor for the changes of the microenvironment. The effect of substituents on the optical properties of donor-π-acceptor compounds having dicyanovinyl as the acceptor are studied using density functional theory and time-dependent density functional theory (DFT/TD-DFT). The optical transitions are thoroughly examined to reveal the impact of subtituents on both absorption and fluorescence, mainly through the modification of the structure in the excited state. The theoretical results have shown that TD-DFT calculations, with a hybrid exchange-correlation and the long-range corrected density functional PBEPBE with a 6-311++G** basis set, was reasonably capable of predicting the excitation energies, the absorption and the emission spectra of these molecules.

Optical Absorption Spectra and Electronic Properties of Symmetric and Asymmetric Squaraine Dyes for Use in DSSC Solar Cells: DFT and TD-DFT Studies.

The electronic absorption spectra, ground-state geometries and electronic structures of symmetric and asymmetric squaraine dyes (SQD1-SQD4) were investigated using density functional theory (DFT) and time-dependent (TD-DFT) density functional theory at the B3LYP/6-311++G** level. The calculated ground-state geometries reveal pronounced conjugation in these dyes. Long-range corrected time dependent density functionals Perdew, Burke and Ernzerhof (PBE, PBE1PBE (PBE0)), and the exchange functional of Tao, Perdew, Staroverov, and Scuseria (TPSSh) with 6-311++G** basis set were employed to examine optical absorption properties. In an extensive comparison between the optical data and DFT benchmark calculations, the BEP functional with 6-311++G** basis set was found to be the most appropriate in describing the electronic absorption spectra. The calculated energy values of lowest unoccupied molecular orbitals (LUMO) were 3.41, 3.19, 3.38 and 3.23 eV for SQD1, SQD2, SQD3, and SQD4, respectively. These values lie above the LUMO energy (-4.26 eV) of the conduction band of TiO2 nanoparticles indicating possible electron injection from the excited dyes to the conduction band of the TiO2 in dye-sensitized solar cells (DSSCs). Also, aromaticity computation for these dyes are in good agreement with the data obtained optically and geometrically with SQD4 as the highest aromatic structure. Based on the optimized molecular geometries, relative positions of the frontier orbitals, and the absorption maxima, we propose that these dyes are suitable components of photovoltaic DSSC devices.

Tautomerization, acidity, basicity, and stability of cyanoform: a computational study.

BACKGROUND: Cyanoform is long known as one of the strongest acid. Cyanoform is only stable below -40 °C. The issue of the stability and tautomeric equilibria of cyanoform (CF) are investigated at the DFT and MP2 levels of theory. The present work presents a detailed study of structural tautomer interconversion in three different media, namely, in the gas phase, in a solvent continuum, and in a microhydrated environment where the first solvation layer is described explicitly by one or two water molecule. In all cases, the transition state has been localized and identified. Proton affinities, deprotonation energies and the Raman spectra are reported analyzed and discussed. RESULTS: The 1 tautomer of cyanoform is shown to be more stable than 2 form by only 1.8 and 14.1 kcal/mol in the gas phase using B3LYP/6-311 ++G** and MP2/6-311 ++G** level of theory, respectively. This energy difference is reduced to 0.7 and 13.4 kcal/mol in water as a solvent using CPCM model using B3LYP/6-311 ++G** and MP2/6-311 ++G** level of theory, respectively. The potential energy barrier for this proton transfer process in the gas phase is 77.5 kcal/mol at MP2/6-311 ++G** level of theory. NBO analysis, analysis of the electrostatic potential (ESP) of the charge distribution, donor-acceptor interactions and charge transfer interactions in 1 and 2 are performed and discussed. CONCLUSIONS: Gross solvent continuum effects have but negligible effect on this barrier. Inclusion of one and two water molecules to describe explicitly the first solvation layer, within the supermolecule model, lowers the barrier considerably (29.0 and 7.6 kcal/mol, respectively). Natural bond orbital (NBO) analysis indicated that the stability of the cyanoform arising from charge delocalization. A very good agreement between experimental and theoretical data has been found at MP2/6-311 ++G** for the energies. On other hand, B3LYP/6-311 ++G** level of theory has good agreement with experimental spectra for CF compound.

Regioselectivity of 1,3-dipolar cycloadditions and antimicrobial activity of isoxazoline, pyrrolo[3,4-d]isoxazole-4,6-diones, pyrazolo[3,4-d]pyridazines and pyrazolo[1,5-a]pyrimidines.

BACKGROUND: Isoxazoles exhibit interesting biological activities, and the 1,3-dipolar cycloaddition(13DC) reactions play an important role in both mechanistic and synthetic organic chemistry. Pyrazoles and annulated pyrazoles exhibit some diverse biological activities. They are used as antipyretic, analgesic drugs, tranquilizing, and herbicidal agents. Pyrazoles are also used extensively as useful synthons in organic synthesis. Pyrazolo[3,4-d]pyridazines showed good antimicrobial, anti-inflammatory and analgesic activities. Several oximes are found to be hyperglycemic, anti-neoplastic, anti-inflammatory, anti-leishmanial and VEGFR-2 kinase inhibitors. RESULTS: The present work describes an efficient synthesis protocol and molecular orbital calculations of isoxazoline and pyrrolo[3,4-d]isoxazole-4,6-dione derivatives from the reaction of hydroximoyl chloride with acrylonitrile, acrylamide, and N-arylmalemides. In addition, pyrazoles and pyrazolo[3,4-d]pyridazines are obtained via the reaction of 3-(dimethylamino)-1-(2,4-dimethyl-1,3-thiazol-5-yl)prop-2-ene-1-one with hydrazonoyl halides. Pyrazolo[1,5-a]pyrimidines were derived from condensation of either Sodium Salt of 3-Hydroxy-1-(2,4-dimethylthiazol-5-yl)prop-2-en-1-one (10) or 3-(dimethylamino)-1-(2,4-dimethyl)(1,3-thiazol-5-yl)prop-2-en-1-one (11) with aiminopyrozoles. A comparative study of the biological activity of the synthesized compounds with ampicillin and tetracycline is compiled in Table 3. Generally, all synthesized compounds showed an adequate inhibitory efficiency of growth of gram-positive and gram-negative bacteria. Structures of the newly synthesized compounds were elucidated by elemental analysis, spectral data and a computational study. CONCLUSIONS: In summary, new and efficient synthetic routes of isoxazoline, pyrrolo[3,4-d]isoxazole-4,6-dione derivatives, pyrazoles, pyrazolo[3,4-d]pyridazines and pyrazolo[1,5-a]pyrimidines have been achieved and the biological activity has been investigated.Graphical abstractNew and efficient synthetic routes of isoxazoline, pyrrolo[3,4-d]isoxazole-4,6-dione derivatives, pyrazolo[3,4-d]pyridazines and pyrazolo [1,5-a] pyrimidines.

Gas-phase acidity and dynamics of the protonation processes of carbidopa and levodopa. A QM/QD study.

The present work details, our efforts to investigate and compare the acid-base properties of levodopa (LD) and carbidopa (CD), a drug combination being used in the treatment of Parkinson's disease. Protonation and deprotonation were examined for all possible sites in the two drugs. Proton affinity and proton detachment enthalpies were computed at the B3LYP/6-311++G** level of theory. Results of the present work reveal that CD is more basic and can abstract protons in solution much more efficiently than LD. Furthermore, at all deportation sites considered, CD is more acidic than LD. DFT-based ADMP, dynamic simulations have been performed to explore the dynamics of the protonation processes in LD and CD. Thus, while the dynamics of the protonation process of LD is very straightforward leading to the formation of the corresponding cation, the protonation process in CD is very complex involving a major geometry change and rearrangement. Results of the present work reveal that the active species in acid medium is not CD in its normal geometry but a carbonyl hydrazine form instead. The presence of the carbonyl group ß to the hydrazine group may very well underlie its enhanced activity which allows it to bind to the active site of the DDC enzyme. The relative stabilities of various water-water-CD complexes have been computed and compared.

Unconventional CH(δ+)···N hydrogen bonding interactions in the stepwise solvation of the naphthalene radical cation by hydrogen cyanide and acetonitrile molecules.

Equilibrium thermochemical measurements using the mass-selected ion mobility (MSIM) technique have been utilized to investigate the binding energies and entropy changes of the stepwise association of hydrogen cyanide (HCN) and acetonitrile (CH3CN) molecules with the naphthalene radical cation (C10H8˙(+)) in the gas phase forming the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters with n = 1-3 and 1-5, respectively. The lowest energy structures of the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters for n = 1-2 have been calculated using the M062X and ω97XD methods within the 6-311+G** basis set, and for n = 1-6 using the B3LYP method within the 6-311++G** basis set. In both systems, the initial interaction occurs through unconventional CH(δ+)···N ionic hydrogen bonds between the hydrogen atoms of the naphthalene cation and the lone pair of electrons on the N atom of the HCN or the CH3CN molecule. The binding energy of CH3CN to the naphthalene cation (11 kcal mol(-1)) is larger than that of HCN (7 kcal mol(-1)) due to a stronger ion-dipole interaction resulting from the large dipole moment of CH3CN (3.9 D). On the other hand, HCN can form both unconventional hydrogen bonds with the hydrogen atoms of the naphthalene cation (CH(δ+)···NCH), and conventional linear hydrogen bonding chains involving HCN···HCN interactions among the associated HCN molecules. HCN molecules tend to form "externally solvated" structures with the naphthalene cation where the naphthalene ion is hydrogen bonded to the exterior of an HCN···HCN chain. For the C10H8˙(+)(CH3CN)n clusters, "internally solvated" structures are favored where the acetonitrile molecules are directly interacting with the naphthalene cation through CH(δ+)···N unconventional ionic hydrogen bonds. In both the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters, the sequential binding energy decreases stepwise to about 6-7 kcal mol(-1) by three HCN or CH3CN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal mol(-1)).

Gas-Phase Thermal Tautomerization of Imidazole-Acetic Acid: Theoretical and Computational Investigations.

The gas-phase thermal tautomerization reaction between imidazole-4-acetic (I) and imidazole-5-acetic (II) acids was monitored using the traditional hybrid functional (B3LYP) and the long-range corrected functionals (CAM-B3LYP and ωB97XD) with 6-311++G** and aug-cc-pvdz basis sets. The roles of the long-range and dispersion corrections on their geometrical parameters, thermodynamic functions, kinetics, dipole moments, Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) energy gaps and total hyperpolarizability were investigated. All tested levels of theory predicted the preference of I over II by 0.750-0.877 kcal/mol. The origin of predilection of I is assigned to the H-bonding interaction (nN8→σ*O14-H15). This interaction stabilized I by 15.07 kcal/mol. The gas-phase interconversion between the two tautomers assumed a 1,2-proton shift mechanism, with two transition states (TS), TS1 and TS2, having energy barriers of 47.67-49.92 and 49.55-52.69 kcal/mol, respectively, and an sp³-type intermediate. A water-assisted 1,3-proton shift route brought the barrier height down to less than 20 kcal/mol in gas-phase and less than 12 kcal/mol in solution. The relatively high values of total hyperpolarizability of I compared to II were interpreted and discussed.

Towards understanding the decomposition/isomerism channels of stratospheric bromine species: ab initio and quantum topology study.

The present study aims at a fundamental understanding of bonding characteristics of the C-Br and O-Br bonds. The target molecular systems are the isomeric CH3OBr/BrCH2OH system and their decomposition products. Calculations of geometries and frequencies at different density functional theory (DFT) and Hartree-Fock/Møller-Plesset (HF/MP2) levels have been performed. Results have been assessed and evaluated against those obtained at the coupled cluster single-double (Triplet) (CCSD(T)) level of theory. The characteristics of the C-Br and O-Br bonds have been identified via analysis of the electrostatic potential, natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM). Analysis of the electrostatic potential (ESP) maps enabled the quantitative characterization of the Br σ-holes. Its magnitude seems very sensitive to the environment and the charge accumulated in the adjacent centers. Some quantum topological parameters, namely Ñ2ρ, ellipticity at bond critical points and the Laplacian bond order, were computed and discussed. The potential energy function for internal rotation has been computed and Fourier transformed to characterize the conformational preferences and origin of the barriers. NBO energetic components for rotation about the C-Br and O-Br bonds as a function of torsion angle have been computed and displayed.

Experimental and theoretical study of o-substituent effect on the fluorescence of 8-hydroxyquinoline.

The synthesis and characterization of different ether and ester derivatives of 8-hydroxyquinoline have been made. UV-visible and fluorescence spectra of these compounds have revealed spectral dependence on both solvent and O-substituent. The fluorescence intensity of ether derivatives revealed higher intensity for 8-octyloxyquinoline compared with 8-methoxyquinoline, whereas those of ester derivatives had less fluorescence than 8-hydroxyquinoline. Theoretical calculations based on Time-dependent density functional theory (TD-DFT) were carried out for the quinolin-8-yl benzoate (8-OateQ) compound to understand the effect of O-substituent on the electronic absorption of 8-hydroxyquinaline (8-HQ). The calculations revealed comparable results with those obtained from the experimental data. Optimized geometrical structure was calculated with DFT at B3LYP/6-311++G** level of theory. The results indicated that 8-OateQ is not a coplanar structure. The absorption spectra of the compound were computed in gas-phase and solvent using B3LYP and CAM-B3LYP methods with 6-311++G ** basis set. The agreement between calculated and experimental wavelengths was very good at CAM-B3LYP/6-311++G** level of theory.

Electronic structure of alloxan and its dimers: QM/QD simulations and quantum chemical topology analysis.

This study aims to identify the origin of the extra stability of alloxan, a biologically active pyrimidine. To achieve this goal, detailed DFT computations and quantum dynamics simulations have been performed to establish the most stable conformation and the global minimum structure on the alloxan potential energy surface. The effects of the solvent, basis set, and DFT method have been examined to validate the theoretical model adopted throughout the work. Two non-covalent intermolecular dimers of alloxan, the H-bonded and dipolar dimers, have been investigated at the ωB97X-D and M06-2X levels of theory using the triple zeta 6-311++G** to establish their relative stability. Quantum chemical topology features and natural bond orbital analysis (NBO) have been performed to identify and characterize the forces that govern the structures and underlie the extra stability of alloxan.

Interaction of polar and nonpolar organic pollutants with soil organic matter: sorption experiments and molecular dynamics simulation.

The fate of organic pollutants in the environment is influenced by several factors including the type and strength of their interactions with soil components especially SOM. However, a molecular level answer to the question "How organic pollutants interact with SOM?" is still lacking. In order to explore mechanisms of this interaction, we have developed a new SOM model and carried out molecular dynamics (MD) simulations in parallel with sorption experiments. The new SOM model comprises free SOM functional groups (carboxylic acid and naphthalene) as well as SOM cavities (with two different sizes), simulating the soil voids, containing the same SOM functional groups. To examine the effect of the hydrophobicity on the interaction, the organic pollutants hexachlorobenzene (HCB, non-polar) and sulfanilamide (SAA, polar) were considered. The experimental and theoretical investigations explored four major points regarding sorption of SAA and HCB on soil, yielding the following results. 1--The interaction depends on the SOM chemical composition more than the SOM content. 2--The interaction causes a site-specific adsorption on the soil surfaces. 3--Sorption hysteresis occurs, which can be explained by inclusion of these pollutants inside soil voids. 4--The hydrophobic HCB is adsorbed on soil stronger than the hydrophilic SAA. Moreover, the theoretical results showed that HCB forms stable complexes with all SOM models in the aqueous solution, while most of SAA-SOM complexes are accompanied by dissociation into SAA and the free SOM models. The SOM-cavity modeling had a significant effect on binding of organic pollutants to SOM. Both HCB and SAA bind to the SOM models in the order of models with a small cavity>a large cavity>no cavity. Although HCB binds to all SOM models stronger than SAA, the latter is more affected by the presence of the cavity. Finally, HCB and SAA bind to the hydrophobic functional group (naphthalene) stronger than to the hydrophilic one (carboxylic acid) for all SOM models containing a cavity. For models without a cavity, SAA binds to carboxylic acid stronger than to naphthalene.

Protonation and deprotonation enthalpies of alloxan and implications for the structure and energy of its complexes with water: a computational study.

The optimized geometries, harmonic vibrational frequencies, and energies of the structures of monohydrated alloxan were computed at the DFT/ωB97X-D and B3LYP/6-311++G** level of theory. Results confirm that the monohydrate exists as a dipolar alloxan-water complex which represents a global minimum on the potential energy surface (PES). Trajectory dynamics simulations show that attempt to reorient this monohydrate, to a more favorable orientation for H-bonding, is opposed by an energy barrier of 25.07 kJ/mol. Alloxan seems to prefer acting as proton donor than proton acceptor. A marked stabilization due to the formation of N-H-OH2 bond is observed. The concerted proton donor-acceptor interaction of alloxan with one H2O molecule does not increase the stability of the alloxan-water complex. The proton affinity of the O and N atoms and the deprotonation enthalpy of the NH bond of alloxan are computed at the same level of theory. Results are compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the four pyrimidines were discussed. Results of the present study reveal that alloxan is capable of forming stronger H-bonds and more stable cyclic complex with water; yet it is of much lower basicity than other pyrimidines.

Does prop-2-ynylideneamine, HC≡CCH=NH, exist in space? A theoretical and computational investigation.

MP2, DFT and CCSD methods with 6-311++G** and aug-cc-pvdz basis sets have been used to probe the structural changes and relative energies of E-prop-2-ynylideneamine (I), Z-prop-2-ynylideneamine (II), prop-1,2-diene-1-imine (III) and vinyl cyanide (IV). The energy near-equivalence and provenance of preference of isomers and tautomers were investigated by NBO calculations using HF and B3LYP methods with 6-311++G** and aug-cc-pvdz basis sets. All substrates have Cs symmetry. The optimized geometries were found to be mainly theoretical method dependent. All elected levels of theory have computed I/II total energy of isomerization (ΔE) of 1.707 to 3.707 kJ/mol in favour of II at 298.15 K. MP2 and CCSD methods have indicated clearly the preference of II over III; while the B3LYP functional predicted nearly similar total energies. All tested levels of theory yielded a global II/IV tautomerization total energy (ΔE) of 137.3-148.4 kJ/mol in support of IV at 298.15 K. The negative values of ΔS indicated that IV is favoured at low temperature. At high temperature, a reverse tautomerization becomes spontaneous and II is preferred. The existence of II in space was debated through the interpretation and analysis of the thermodynamic and kinetic studies of this tautomerization reaction and the presence of similar compounds in the Interstellar Medium (ISM).

Molecular design of donor-acceptor dyes for efficient dye-sensitized solar cells I: a DFT study.

Dye-sensitized solar cells (DSSCs) have drawn great attention as low cost and high performance alternatives to conventional photovoltaic devices. The molecular design presented in this work is based on the use of pyran type dyes as donor based on frontier molecular orbitals (FMO) and theoretical UV-visible spectra in combination with squaraine type dyes as an acceptor. Density functional theory has been used to investigate several derivatives of pyran type dyes for a better dye design based on optimization of absorption, regeneration, and recombination processes in gas phase. The frontier molecular orbital (FMO) of the HOMO and LUMO energy levels plays an important role in the efficiency of DSSCs. These energies contribute to the generation of exciton, charge transfer, dissociation and exciton recombination. The computations of the geometries and electronic structures for the predicted dyes were performed using the B3LYP/6-31+G** level of theory. The FMO energies (EHOMO, ELUMO) of the studied dyes are calculated and analyzed in the terms of the UV-visible absorption spectra, which have been examined using time-dependent density functional theory (TD-DFT) techniques. This study examined absorption properties of pyran based on theoretical UV-visible absorption spectra, with comparisons between TD-DFT using B3LYP, PBE, and TPSSH functionals with 6-31+G (d) and 6-311++G** basis sets. The results provide a valuable guide for the design of donor-acceptor (D-A) dyes with high molar absorptivity and current conversion in DSSCs. The theoretical results indicated 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (D2-Me) can be effectively used as a donor dye for DSSCs. This dye has a low energy gap by itself and a high energy gap with squaraine acceptor type dye, the design that reduces the recombination and improves the photocurrent generation in solar cell.