Mechanisms for Solvatochromic Shifts of Free-Base Porphine Studied with Polarizable Continuum Models and Explicit Solute–Solvent Interactions

Solvatochromic shifts of free-base porphine in the Q-band and B-band were studied using the polarizable continuum model (PCM) and explicit solvent molecules employing time-dependent density functional theory (TDDFT) and the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The state-specific (SS) and linear-response (LR) methods were examined in the PCM calculations. These models involve different types of solute–solvent interactions. The LR PCM and explicit solvation models reproduced the experimentally observed trends of the solvatochromic shifts, while the SS PCM failed to reproduce the experimental findings. The origin of the solvatochromic shifts of free-base porphine was dispersive interactions between the solute and solvent. Specific solute–solvent interactions would be important for a decrease of the splitting width between Q-bands. Based on the Casimir–Polder formula and a decomposition analysis, it was found that the dominant part of the solute–solvent interactions can be considered using independent particle approximations

Diels–Alder Cycloaddition of Cyclopentadiene and C₆₀ at the Extreme High Pressure

High-pressure Diels–Alder cycloaddition reaction of fullerenes is an important synthetic method for the thermally stable cycloadducts. The effects of high pressure on the potential energy surfaces of Diels–Alder cycloaddition of cyclopentadiene and C₆₀ were studied with a recently developed approach, the polarizable continuum model for extreme pressure (XP-PCM). It is revealed that the high pressure reduces the activation energies and increases reaction energies drastically, making the DA reaction more favorable. The pressure effects on the reaction energetics can be divided into the cavitation and electronic contributions. For the activation energy, the cavitation contribution is significant in comparison with the electronic contribution. To assist future experiments, the activation volume and reaction volume were computed on the basis of the relationship between activation energy or reaction energy with the pressure as a consequence of the fitting linear correlation between activation energy or reaction energy with the pressure

Structures of Bimetallic Copper–Ruthenium Nanoparticles: Incoherent Interface and Surface Active Sites for Catalytic Nitric Oxide Dissociation

Bimetallic alloy nanoparticles are promising candidates for replacing platinum group metals utilized in the catalytic removal of nitrogen oxides in exhaust gas. In this study, we investigated the electronic, interfacial, and surface structures of copper/ruthenium alloy nanoparticles by quantum chemical computations using 135-atomic cluster models. We employed Ru-core/Cu-shell models in which the Ru-core takes both fcc (face-centered cubic) and hcp (hexagonal closed-packed) structures. The fcc-core model has a coherent Cu/Ru interface, while the hcp-core model involves an incoherent interface. This incoherence results in discontinuity in the lattice structure and the valence electronic structure, and generates step-like structures on the surface of the particle. Such a step-like site enhances the catalytic activities for nitric oxide dissociation. The orbital energies suggest that the alloying can control the oxidation tendency of clusters. Charge-transfer occurs between the Cu shell and Ru core; the surface layer of the clusters has a positive charge, although the surface atoms are not directly binding to the core Ru atoms. The interfacial structure of core–shell interphase is a crucial factor to be considered in designing the properties of alloy nanoparticles

Comparative Study of C^∧N and N^∧C Type Cyclometalated Ruthenium Complexes with a NAD⁺/NADH Function

Cyclometalated ruthenium complexes having C^∧N and N^∧C type coordinating ligands with NAD⁺/NADH function have been synthesized and characterized by spectroscopic methods. The variation of the coordinating position of σ-donating carbon atom leads to a drastic change in their properties. Both the complex Ru­(<b>phbn</b>)­(phen)₂]­PF₆ ([<b>1</b>]­PF₆) and [Ru­(<b>pad</b>)­(phen)₂]­PF₆ ([<b>2</b>]­PF₆) reduced to Ru­(<b>phbnHH</b>)­(phen)₂]­PF₆ ([<b>1HH</b>]­PF₆) and [Ru­(<b>padHH</b>)­(phen)₂]­PF₆ ([<b>2HH</b>]­PF₆) by chemical and electrochemical methods. Complex [<b>1</b>]­PF₆ photochemically reduced to [<b>1HH</b>]­PF₆ in the presence of the sacrificial agent triethylamine (TEA) upon irradiation of visible light (λ ≥ 420 nm), whereas photochemical reduction of [<b>2</b>]­PF₆ was not successful. Both experimental results and theoretical calculations reveal that upon protonation the energy level of the π* orbital of either of the ligands <b>phbn</b> or <b>pad</b> is drastically stabilized compared to the nonprotonated forms. In the protonated complex [Ru­(<b>padH</b>)­(phen)₂]­(PF₆)₂ {[<b>2H</b>]­(PF₆)₂}, the Ru–C bond exists in a tautomeric equilibrium with RuC coordination and behaves as a remote N-heterocyclic carbene (<i>r</i>NHC) compex; on the contrary, this behavior could not be observed in protonated complex [Ru­(<b>phbnH</b>)­(phen)₂]­(PF₆)₂ {[<b>1H</b>]­(PF₆)₂}

Photoisomerization and Proton-Coupled Electron Transfer (PCET) Promoted Water Oxidation by Mononuclear Cyclometalated Ruthenium Catalysts

Photoisomeric transformations in ruthenium polypyridyl complexes have been rarely reported. Herein we report the geometrical transformation of cyclometalated <i>trans</i>-[Ru­(tpy)­(PAD)­(OH₂)]⁺ ([<b>1</b>]⁺) to the <i>cis</i>-[Ru­(tpy)­(PAD)­(OH₂)]⁺ ([<b>1a</b>]⁺) (tpy = 2,2′;6′,2″-terpyridine, PAD = 2-(pyrid-2′-yl)­acridine) isomer upon irradiation of visible light (λ ≥420 nm). Due to a proton-induced tautomeric equilibrium between the Ru–C bond and RuC coordination, the π* energy levels of PADH are lower than those of tpy by 12.61 and 12.24 kcal mol^–1, respectively, in [<b>1</b>]⁺ and [<b>1a</b>]⁺. Isomers [<b>1</b>]⁺ and [<b>1a</b>]⁺ both act as catalytic oxygen-evolving complexes (OECs) chemically as well as electrochemically

Rydberg and π–π* Transitions in Film Surfaces of Various Kinds of Nylons Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations: Peak Shifts in the Spectra and Their Relation to Nylon Structure and Hydrogen Bondings

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra in the 145–260 nm region were measured for surfaces (thickness 50–200 nm) of various kinds of nylons in cast films to explore their electronic transitions in the FUV region. ATR-FUV spectra show two major bands near 150 and 200 nm in the surface condensed phase of nylons. Transmittance (Tr) spectra were also observed in particular for the analysis of valence excitations. Time-dependent density functional theory (TD-DFT/CAM-B3LYP) calculations were carried out using the model systems to provide the definitive assignments of their absorption spectra and to elucidate their peak shifts in several nylons, in particular, focusing on their crystal alignment structures and intermolecular hydrogen bondings. Two major bands of nylon films near 150 and 200 nm are characterized as σ-Rydberg 3p and π–π* transitions of nylons, respectively. These assignments are also coherent with those of liquid <i>n</i>-alkanes (<i>n</i> = 5–14) and liquid amides observed previously. The Rydberg transitions are delocalized over the hydrocarbon chains, while the π–π* transitions are relatively localized at the amide group. Differences in the peak positions and intensity were found in both ATR- and Tr-FUV spectra for different nylons. A red-shift of the π–π* amide band in the FUV spectra of nylon-6 and nylon-6/6 models in α-form is attributed to the crystal structure pattern and the intermolecular hydrogen bondings, which result in the different delocalization character of the π–π* transitions and transition dipole coupling

Cooperative H₂ Activation at Ag Cluster/θ-Al₂O₃(110) Dual Perimeter Sites: A Density Functional Theory Study

H₂ dissociation by Ag clusters supported on the θ-Al₂O₃(110) surface has been investigated using density functional theory calculations. The crucial role of the dual perimeter site of Ag cluster and the surface oxygen (O) site of the alumina support is demonstrated with three theoretical models: anchored cluster, isolated cluster, and anchored cluster on hydroxylated alumina. The heterolytic cleavage of H₂ at the silver–alumina interface, yielding Ag–H^δ− and O–H^δ+, is thermodynamically and kinetically preferred compared with H₂ cleavage at two Ag atomic sites on top of the Al₂O₃-supported Ag cluster and the homolytic cleavage of H₂ on the isolated Ag cluster. The hydroxylation at the O site of the alumina reduces the H₂ dissociation activity, which indicates that the interfacial bare O site is indispensible. It is concluded that the interfacial cooperative mechanism between the Ag cluster and Lewis acid–base pair site (bare Al–O site) is essentially relevant for the H₂ activation over Ag-loaded Al₂O₃ catalysts

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer Active π‑Conjugated Benzimidazole Compounds: Influence of Structural Rigidification by Ring Fusion

Two excited-state intramolecular proton transfer (ESIPT) active benzimidazole derivatives (<b>1</b> and <b>2</b>) were synthesized by acid-catalyzed intramolecular cyclization. The steady-state fluorescence spectrum in THF revealed that ring-fused derivative <b>1</b> exhibits a dual emission, namely, the major emission was from the K* (keto) form (ESIPT emission) at 515 nm with a large Stokes shift of 11 100 cm^–1 and the minor emission was from the E* (enol) form at below 400 nm. In contrast, the normal emission from the E* form was dominant and the fluorescence quantum yield was very low (Φ ∼ 0.002) for nonfused derivative <b>2</b>. The time-resolved fluorescence spectroscopy of <b>1</b> suggested that ESIPT effectively occurs due to the restricted conformational transition to the S₁–T_ICT state, and the averaged radiative and nonradiative decay rate constants were estimated as ⟨<i>k</i>_f⟩ = 0.15 ns^–1 and ⟨<i>k</i>_nr⟩ = 0.60 ns^–1, respectively. The fluorescence emission of <b>1</b> was influenced by the measurement conditions, such as solvent polarity and basicity, as well as the presence of Lewis base. The ESIPT process and solvatochromic behavior were nicely reproduced by the DFT/TDDFT calculation using the PCM model. In the single-crystal fluorescent spectra, the ESIPT emissions were exclusively observed for both fused and nonfused compounds as a result of hydrogen-bonding interactions

Synthesis and Optical Properties of Imidazole- and Benzimidazole-Based Fused π‑Conjugated Compounds: Influence of Substituent, Counteranion, and π‑Conjugated System

Fused π-conjugated imidazolium chlorides having hydrogen (<b>1-Cl</b>), octyloxy (<b>2-Cl</b>), <i>N</i>,<i>N</i>-dibutylamino (<b>3-Cl</b>), trifluoromethyl (<b>4-Cl</b>), and cyano (<b>5-Cl</b>) groups substituted on the benzene ring at the 2-position of imidazole were prepared. Counteranion exchanges from chloride to bis­(trifluoromethanesulfonyl)­imidate (<b>2-TFSI</b>) and tetrafluoroborate (<b>2-BF4</b>) were performed. The optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism) were influenced by both the substituent and anion character, which was investigated by theoretical calculations using the density functional theory (DFT) and symmetry-adapted cluster–configuration interaction (SAC–CI) methods. Fused π-conjugated benzimidazolium chlorides having <i>N</i>,<i>N</i>-dibutylamino (<b>6-Cl</b>) and cyano (<b>7-Cl</b>) groups were also prepared to observe the different solvatochromic shifts