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Time‐dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+), which has an intense UV–visible absorption near 600 nm. The redox properties of the MV2+/MV•+ couple and light‐sensitivity of MV•+ have made the system appealing for photo‐electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo‐induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+. Using a conventional hybrid exchange functional (B3‐LYP) and a long‐range corrected hybrid exchange functional (ωB97X‐D3), including with a conductor‐like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions. [ABSTRACT FROM AUTHOR]

Benchmark ab initio calculations have been performed for poly(p-phenylenevinylene) (PPV) dimers, a paradigmatic material for studying excitation energy transfer mechanisms. Second-order Møller–Plesset perturbation theory was utilized with the scaled opposite spin approach (SOS-MP2) and correlation consistent basis sets to determine the geometric properties and interaction energies in the ground state. Vertical excitations and optimized structures for the S1 state were computed using the SOS second-order algebraic diagrammatic construction method. For the ground state properties, extrapolation to the complete basis set (CBS) limit and correction for the basis set superposition error (BSSE) were performed. While all results computed with different basis sets and considering BSSE correction or not agreed at the CBS limit, a strong bias was observed either using augmented basis sets or BSSE corrections, proving that these approaches are not advisable for calculating intermolecular distances and interaction energies with smaller basis sets. The lower states for vertical excitations were largely local excitons where the hole/electron pair was confined to single chains. For higher excited states, interchain charge transfer (CT) states were also observed. Geometry optimization of the S1 state led to significant reductions in the intermolecular distances and energetic stabilization, with Stokes shifts between 1.4 eV and 0.9 eV (with increasing chain length), and significant CT values between 0.5e and 0.4e. [ABSTRACT FROM AUTHOR]

The chemical stability and the low‐lying singlet and triplet excited states of BN‐n‐acenes (n = 1–7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet‐triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon‐only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet‐triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN‐n‐acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low‐lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron. [ABSTRACT FROM AUTHOR]

Progress in the hydrogen fuel field requires a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen nuclei, any theoretical studies of these systems must be treated quantum dynamically. One class of material that has been examined in this context are dihydrogen complexes. Since their discovery by Kubas in 1984, many such complexes have been studied both experimentally and theoretically. This particular study examines the rotational dynamics of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 complex, allowing for full motion in both the rotational degrees of freedom and treating the quantum dynamics (QD) explicitly. A "gas-phase" global potential energy surface is first constructed using density functional theory with the Becke, 3-parameter, Lee-Yang-Parr functional; this is followed by an exact QD calculation of the corresponding rotation/libration states. The results provide insight into the dynamical correlation of the two rotation angles as well as a comprehensive analysis of both ground- and excited-state librational tunneling splittings. The latter was computed to be 6.914 cm-1--in excellent agreement with the experimental value of 6.4 cm-1. This work represents the first full-dimensional ab initio exact QD calculation ever performed for dihydrogen ligand rotation in a coordination complex. [ABSTRACT FROM AUTHOR]

The color of mature red wines is due in large part to the chemical transformation of grape anthocyanins into pyranoanthocyanins. Given the difficulties of isolation and purification of pyranoanthocyanins from wines, experimental investigations have focused on pyranoflavylium cations, synthetic analogs that contain the basic chromophoric moiety of pyranoanthocyanins. Quantum chemical methodologies have been extensively employed to predict the physical, spectroscopic and photophysical properties of anthocyanins and, more recently, pyranoflavylium cations. In the present work, we employ TD-DFT with the B3-LYP functional and the def2-TZVP basis set, combined with estimation of solvation free energies via COSMO, to estimate a priori the pKa values of the ground state and the lowest excited singlet and triplet states of a series of seven substituted hydroxypyranoflavylium cations. For the ground state and the first excited singlet state, the quantum chemical results compare favorably to experimental values for most of these compounds. Although there are currently no experimental data for the lowest excited triplet state, the acidity is predicted to be more similar to that of the ground state than to the excited singlet state, as is generally found experimentally for photoacids. [ABSTRACT FROM AUTHOR]

Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst. [ABSTRACT FROM AUTHOR]

Theoretical descriptions of excited state proton transfer (ESPT) have had various degrees of success. This work presents a theoretical description of the photodissociation of the 7‐hydroxyflavylium cation (7‐HF), the fundamental chromophoric moiety of anthocyanin natural plant pigments. ESPT of 7‐HF is promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. Several factors contribute to the consistency of the results of the present study: (1) the theoretical approach (TD‐DFT with the B3‐LYP functional and def2‐TZVP basis set utilizing Grimme's D3 dispersion correction); (2) the modeling of the solvent effect combining hydrogen bonding of the photoacid to a cluster of discrete water molecules in a water‐like continuum solvent (COSMO); (3) the large S1‐S2 energy gap of flavylium cations; and (4) the electrostatics of the ESPT in which a proton is transferred from a cationic photoacid to water without Coulombic interaction between the proton and the conjugate base. [ABSTRACT FROM AUTHOR]

The behavior of monomers and dimers of methylated xanthine derivatives in their excited states is investigated by means of the ADC(2), CASSCF, and CASPT2 methods. The results of the calculations of stationary points in the ground and excited states, minima on the S0/S1 crossing seams and the relaxation pathways are used to provide the interpretation of experimental observations of the monomer xanthine derivatives. The effect of dimerization on the excited state properties is studied for various relative orientations of the monomers in the dimer complexes in comparison with the relevant monomer species. A significant stabilization in the excited state minima of dimers is observed. These can act as trapping sites. Various types of conical intersections, with both localized and delocalized characters of wavefunctions, have been found, mainly energetically above the lowest bright excited state in the FC region. In addition, structures with the bonds formed between the two monomers were also found on the crossing seams. The possibility of ultrafast relaxation via these conical intersections is discussed. [ABSTRACT FROM AUTHOR]