Computational Investigation into Photoswitching Efficiency of Diarylethene Derivatives: An Insight Based on the Decay Constant of Electron Tunneling

The switching efficiency (SE) of the intramolecular exchange interaction <i>J</i> between the open- and the closed-ring isomers of diarylethenes (DAEs) was investigated using DFT calculations of DAE biradicals with different core structures: DAEs with 3-thienyl, thiophene-<i>S</i>,<i>S</i>-dioxide-3-yl, 2-thienyl, or thiophene-<i>S</i>,<i>S</i>-dioxide-2-yl rings. The SE of DAE with a 3-thienyl ring is calculated to be around 400-fold, which is the largest among the four calculated DAEs. The decay constant β of the exchange interaction <i>J</i> for the DAE molecular wires was evaluated by calculating <i>J</i> for biradicals with different lengths of wires. For the wires of the closed-ring isomers of DAE with 3-thienyl- and thiophene-<i>S</i>,<i>S</i>-dioxide-3-yl rings, which are supposed to take a quinoid structure, allyl nitronyl nitroxide radical was successfully employed. The calculated β values showed a significant difference between the open- and the closed-ring isomers, and this difference of β is considered to be the origin of photoswitching of <i>J</i>. The difference of β upon isomerization, Δβ, is in good agreement with SE, and the largest Δβ was obtained for the DAE with a 3-thienyl ring. We can understand the switching of <i>J</i> as the switching of electron tunneling efficiency β between the open- and the closed-ring isomers

Theoretical Investigation on the Decaying Behavior of Exchange Interaction in Quinoid and Aromatic Molecular Wires

The difference in the decaying behavior of the exchange interaction between quinoid and aromatic molecular wires was investigated by means of density functional theory calculations. The biradical quinoid structure was realized when the molecular wire consists of thiophene-<i>S</i>,<i>S</i>-dioxide and allyl nitronyl nitroxide radical. While the calculated decay constant (β) for oligothiophene was 0.23 Å^–1, the obtained β value for the quinoid structure of oligothiophene-<i>S</i>,<i>S</i>-dioxide was 0.09 Å^–1; this finding suggested that the quinoid molecular wire had a smaller β value than the aromatic wire. It was also found that β decreases upon oxidation of the sulfur atom in the oligothiophene due to an increase in its olefinic nature. The quinoid molecular wire made of thiophene-<i>S</i>,<i>S</i>-dioxide can be thus considered as a suitable system for the charge and spin transport in molecular electronics and spintronics

A Configuration Interaction Picture for a Molecular Environment Using Localized Molecular Orbitals: The Excited States of Retinal Proteins

Electronic excitations of chromophores in proteins and solutions are associated with the electronic response of the molecular environment. The underlying interactions are important origins of solvatochromism. We performed large-scale configuration interaction singles (CIS) calculations (up to 1000 atoms) for retinal chromophores in proteins and methanol solution, in which one-electron processes (polarization and charge-transfer effects of the environment) are included. The present approach also improved the electrostatic potential, as compared to that described by a molecular mechanics (MM) force field. The CIS results were combined with the symmetry adapted cluster (SAC)-CI result using our own N-layer integrated molecular orbital molecular mechanics (ONIOM) method. As compared to the MM description, the CIS reduces the calculated excitation energy by 0.1–0.3 eV and also improves the relative excitation energies among retinal proteins. We applied our localized molecular orbital (LMO) transformation scheme to analyze the CI wave functions. The result clarified the contributions of the amino acids. In bacteriorhodopsin, Tyr185 contributes intermolecular CT excitations. The radial distribution of amino acids’ contributions to the CI wave function was also analyzed. The results of the analysis are useful not only for understanding the molecular interactions and the role of amino acids in color tuning, but also for providing insight into the structure of the excited-state wave function for the molecular environment. An excitation-energy decomposition analysis also supported the results of the excited-state wave functions

Selective Dehydration of Mannitol to Isomannide over Hβ Zeolite

Isomannide is a potential feedstock for the production of super engineering plastics. A prospective route to obtain isomannide is dehydration of mannitol derived from lignocellulosic biomass, but homogeneous acid catalysts reported in the literature produce a large amount of 2,5-sorbitan as a byproduct in the dehydration reaction. In this work, we initially studied the mechanism of proton-induced dehydration of mannitol by density functional theory calculations, which suggested that local steric hindrance around acid sites designed at the angstrom level can tune the selectivity toward isomannide formation. Based on this prediction, we found that the precisely defined microporous confinement offered by Hβ provides improved selectivity and high catalytic activity for the production of isomannide, where 1,4-dehydration is favored by 20 kJ mol^–1 of activation energy. The optimization of the Si/Al ratio of Hβ to balance the acid amount and hydrophobicity improved the catalytic activity and achieved 63% yield of isomannide, far exceeding the best result reported previously (35% yield)

First-Order Interacting Space Approach to Excited-State Molecular Interaction: Solvatochromic Shift of <i>p</i>‑Coumaric Acid and Retinal Schiff Base

A triple-layer QM/sQM/MM method was developed for accurately describing the excited-state molecular interactions between chromophore and the molecular environment (Hasegawa, J.; Yanai, K.; Ishimura, K. <i>ChemPhysChem</i> <b>2015</b>, <i>16</i>, 305). A first-order-interaction space (FOIS) was defined for the interactions between QM and secondary QM (sQM) regions. Moreover, configuration interaction singles (CIS) and its second-order perturbation theory (PT2) calculations were performed within this space. In this study, numerical implementation of this FOISPT2 method significantly reduced the computing time, which realized application to solvatochromic systems, <i>p</i>-coumaric acid in neutral (<i>p</i>-CA) and anionic forms in aqueous solution, retinal Schiff base in methanol (MeOH) solution, and bacteriorhodopsin (bR). The results were consistent with the experimentally observed absorption spectra of the applied systems. The QM/sQM/MM result for the opsin shift was in better agreement to the experimental result than that of the ordinary QM/MM. A decomposition analysis was performed for the excited-state molecular interactions. Among the electronic interactions, charge-transfer (CT) effect, excitonic interaction, and dispersion interaction showed significant large contributions, while the electronic polarization effect presented only minor contribution. Furthermore, the result was analyzed to determine the contributions from each environmental molecule and was interpreted based on the distance of the molecules from the π system in the chromophores

Synergy of Vicinal Oxygenated Groups of Catalysts for Hydrolysis of Cellulosic Molecules

Carbon materials bearing carboxylic acids and phenolic groups efficiently catalyze the hydrolysis of cellulose. In this work, we demonstrate that salicylic acid and phthalic acid show higher activity than other substituted benzoic acids as models of catalytic sites on carbons in the hydrolysis of cellobiose and cellulose. Notably, their turnover frequencies are larger than those of <i>o</i>-chlorobenzoic acid and <i>o</i>-trifluoromethylbenzoic acid, despite their lower acid strength. The high catalytic performance of salicylic acid and phthalic acid is not attributed to a reduction of activation energy but to an increase in the frequency factor. Nuclear magnetic resonance and density functional theory studies indicate that one oxygenated group forms a hydrogen bond with a hydroxyl group in cellobiose, which boosts the probability of attack of the neighboring carboxylic acid on the glycosidic bond. The computation also predicts a hydrolysis mechanism including an S_N1 reaction with anomeric inversion, which reasonably accounts for the experimental results in the conversion of cellobiose

Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO₂: Structural Optimization and Mechanistic Study

We prepared bifunctional Mg^II porphyrin catalysts <b>1</b> for the solvent-free synthesis of cyclic carbonates from epoxides and CO₂. The activities of <b>1d</b>, <b>1h</b>, and <b>1i</b>, which have Br^–, Cl^–, and I^– counteranions, respectively, increased in the order <b>1i</b> < <b>1h</b> < <b>1d</b>. Catalysts <b>1d</b> and <b>1j</b>–<b>m</b>, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst <b>1d</b>, we synthesized Mg^II porphyrin <b>1o</b> with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h^–1). The catalytic mechanism was studied by using <b>1d</b>. The yields were nearly constant at initial CO₂ pressures in the 1–6 MPa range, suggesting that CO₂ was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO₂, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO₂. Experiments with ¹⁸O-labeled CO₂ and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br^– on the less hindered side of the epoxide to generate an oxyanion, which underwent CO₂ insertion to afford a CO₂ adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of <b>1d</b> and <b>1o</b> was due to the cooperative action of the Mg^II and Br^– and a conformational change (induced-fit) of the quaternary ammonium cation

Investigation on CD Inversion at Visible Region Caused by a Tilt of the π‑Conjugated Substituent: Theoretical and Experimental Approaches by Using an Asymmetric Framework of Diarylethene Annulated Isomer

A substituent effect of asymmetric diarylethene annulated isomer on their chiroptical properties was investigated by means of theoretical and experimental approaches. The absolute configuration of the annulated isomer was determined by X-ray structural analysis and DFT calculation. The TD–DFT calculation successfully reproduced not only the sign but also the shape and magnitude of experimental CD spectrum by considering the Boltzmann-weighted average of four atrop-isomers. A fragment decomposition (FD) analysis of rotatory strength clearly revealed a noteworthy effect; the tilting motion concomitant with the rotating motion of the substituent affects the sign and magnitude of CD signals. It was found that even when the absolute structure of the chiral core moiety does not change, the slight motion of the substituent can trigger the inversion of the CD signal

Correction to “Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO₂: Structural Optimization and Mechanistic Study”

Correction to “Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO₂: Structural Optimization and Mechanistic Study

Spin-Blocking Effect in CO and H₂ Binding Reactions to Molybdenocene and Tungstenocene: A Theoretical Study on the Reaction Mechanism via the Minimum Energy Intersystem Crossing Point

Potential energy profiles and electronic structural interpretation of the CO and H₂ binding reactions to molybdenocene and tungstenocene complexes [MCp₂] (M = Mo and W, Cp = cycropentadienyl) were studied using density functional theory calculations and ab initio multiconfigurational electronic structure calculations. Experimentally observed slow H₂ binding was reasonably explained in terms of the spin-blocking effect. Electronic structural analysis at the minimum-energy intersystem crossing point (MEISCP) revealed that the singly occupied molecular orbital’s π-bonding/σ-antibonding character in the M-CO/H₂ moiety determines the energy levels of the MEISCP. Analysis of the reaction coordinate showed that the singlet-triplet gap significantly depends on the Cp-M-Cp angle. Therefore, not only the metal–ligand distance but also the Cp-M-Cp angle is an important reaction coordinate to reach the MEISCP, the transition state of H₂ binding. The role of spin–orbit coupling is also discussed