Pyran-Squaraine as Photosensitizers for Dye-Sensitized Solar Cells: DFT/TDDFT Study of the Electronic Structures and Absorption Properties

In an effort to provide, assess, and evaluate a theoretical approach which enables designing efficient donor-acceptor dye systems, the electronic structure and optical properties of pyran-squaraine as donor-acceptor dyes used in dye-sensitized solar cells were investigated. Ground state properties have been computed at the B3LYP/6-31+G** level of theory. The long-range corrected density functionals CAM-B3LYP, PBEPBE, PBE1PBE (PBE0), and TPSSH with 6-311++G** were employed to examine absorption properties of the studied dyes. In an extensive comparison between experimental results and ab initio benchmark calculations, the TPSSH functional with 6-311++G** basis set was found to be the most appropriate in describing the electronic properties for the studied pyran and squaraine dyes. Natural transition orbitals (NTO), frontier molecular orbitals (FMO), LUMO, HOMO, and energy gaps, of these dyes, have been analyzed to show their effect on the process of electron injection and dye regeneration. Interaction between HOMO and LUMO of pyran and squaraine dyes was investigated to understand the recombination process and charge-transfer process involving these dyes. Additionally, we performed natural bond orbital (NBO) analysis to investigate the role of charge delocalization and hyperconjugative interactions in the stability of the molecule

Ultrafast radiationless decay mechanisms through conical intersections in cytosine: Computational insight and topological analysis of the charge density distributions

27-34<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family: Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US">The quantum theory of atoms-in-molecules (QTAIM) in conjunction with the DFT/B3LYP/6-311++G(2d,2p) wave function are used to compute the atomic, bonded and non-bonded interactions, distributions of the charge density, (r)<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family: Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA;mso-bidi-font-weight: bold" lang="EN-US">, and its Laplacian, 2(r), for the ground equilibrium structure of cytosine. The study has been further extended to include two conical intersection (CI) structures that underlie the radiationless decay of cytosine. Complete Active Space Multi-configuration SCF level of theory with the 6-311++G** basis set are used to identify, characterize and to optimize the geometrical structures of the conical intersections between So and S1. In the case of cytosine, all ring bond critical points show 2(r)<span style="font-size:9.0pt; mso-fareast-font-family:Calibri;mso-bidi-font-family:Calibri;mso-ansi-language: EN-US;mso-bidi-language:AR-SA" lang="EN-US"> <span style="font-size: 9.0pt;font-family:Symbol;mso-ascii-font-family:" times="" new="" roman";mso-fareast-font-family:="" calibri;mso-hansi-font-family:"times="" roman";mso-bidi-font-family:calibri;="" mso-ansi-language:en-us;mso-bidi-language:ar-sa;mso-char-type:symbol;="" mso-symbol-font-family:symbol;mso-bidi-font-weight:bold"="" lang="EN-US"><<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family: Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA;mso-bidi-font-weight: bold" lang="EN-US"> 0<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family: Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US"> <span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family: Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US">indicating covalent binding and accumulation of the electron density in the bonding regions. On the other hand, the C1-O11 bond shows, depletion of charge density, indicating the increased ionic character of this bond. This point might very well underlie the reactivity and the low keto-enol barrier in cytosine. Contour plots and relief maps have been analyzed for regions of valence shell charge concentrations and depletions in the ground state and the three CI structures of cytosine. NBO analysis reveals that the conformational and overall stability of the studied cytosine conformations is facilitated by the competitive conjugative and the lone-pair interactions. In cytosine, there is a delicate balance between these two forces, whereas, in each of the two CI configurations there is one dominant force that underlies the stability of the cytosine structure. The present work indicates that the QTAIM provide not only a graphical presentation of very important critical points on the PES but also unique quantitative descriptors of CI's that characterize it. </span

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

Electronic structure and acid–base properties of Kojic acid and its dimers. A DFT and quantum topology study

<p>Kojic acid is a polyfunctional heterocyclic compound, with several important reaction centres; it has a wide range of applications in the cosmetic, medicine, food, agriculture and chemical industries. The present study aims at better insight into its electronic structure and bonding characteristics. Thus, density functional theory at the M06-2x /6-311++G^** level of theory is used to investigate its ground state electronic and acid–base properties. Protonation and deprotonation enthalpies are computed and analysed. The ability of Kojic acid to form both water complexes and dimers is explored. Several different complexes and dimer structures were examined. Natural bond order and quantum topology features of the charge density were analysed. The origin of the stability of the studied complexes and dimer structures can be traced to hydrogen bonding, π-conjugative and non-covalent dispersive interactions.</p

Substituent effect on the intramolecular hydrogen bond and the proton transfer process in pyrimidine azo dye : A computional study

This study provides a complete analysis of the electronic and photophysical properties of, the derivative of uracil, IsoOrotic (IOA) azo dyes. The ability of the dye to work as an excited state intramolecular proton transfer (ESIPT) was investigated by using Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT) methods. The effect of electron-donating substituents (CH3 and NMe2) and an electron-withdrawing one (NO2) was examined. In addition, the effect of the solvent polarity on the ESIPT process is studied. All the geometrical structures in the singlet ground (S0) and excited (S1) states, were optimized using B3LYP/6–311 + G** level of theory. The intramolecular hydrogen bond parameters (IHBs), and the Infra-Red vibrational analysis of the O-H bond show that the IHBs are enhanced in the S1 state. Furthermore, the absorption and emission spectra were simulated and the values of stokes shifts were observed. The PAIOA derivative with an electron withdrawing group shows large stokes shift compared with those having electron-donating ones. Therefore, we can safely conclude that the substituent groups and the different solvents are extremely impactful on the ESIPT process

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

<div><p>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.</p></div

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