We performed the density functional calculation of bulk β-Ni(OH)2 and monolayer Ni(OH)2 as a surface model to investigate their chemical features. The nickel hydroxide (Ni(OH)2) and nickel oxyhydroxide (NiOOH) redox pair is one of the most attractive materials for electrocatalytic oxygen evolving reaction (OER) with low overpotential. However, its reaction mechanism has not yet been fully understood. Their band structures and visualized pseudo-charge density contributions indicate hydrogen interaction between the layers in the β-Ni(OH)2. In the experimental electrocatalytic OER reaction, the Fe atom is required; the structure is called NiFe-layered double hydroxide (LDH). We report the Fe effect in the monolayer Ni(OH)2 framework as the surface model of NiFe-LDH. The Fe atom is suitable for receiving electrons from substrate waters. This study provides crucial basic information for the OER mechanism on the surface of Ni(OH)2/NiOOH redox pairs.

A comparative study of van der Waals and ionic crystals can provide vital information for the medical and food industries. In this work, we investigated the coenzyme pyrroloquinoline quinone (PQQ), which contains three carboxyl groups coupled to imidazole, pyridine, and quinone. Whole-crystal analysis (crystal-ome) was attempted for NanPQQ (n = 0–4) crystals. All deprotonation sites were found to be dependent on pKa except for the Na sites, which cannot be explained by pKa. The Na1PQQ crystal exhibited an unusual ionic bond, forming COOH–Na+ at one of the carboxyl sites in the structure. The difference in the solubility of the van der Waals and ionic crystals was also investigated, with a focus on the dissolution processes of Na0PQQ and Na2PQQ, by combining molecular dynamics simulations with experiments that define the crystal surfaces. This study is the first step toward developing a general rule to link the different types of crystal structures with different dissolution mechanisms and rates.

The electrochemical reduction of CO2 to CO by an ionic liquid EMIM–BF4 is one of the most promising CO2 reduction processes proposed so far with its high Faradaic efficiency and low overpotential. However, the details of the reaction mechanism are still unknown due to the absence of fundamental understandings. In this study, the most probable and stable geometries of EMIM–BF4 and CO2 were calculated by quantum chemistry in combination with exhaustive search. A possible reaction pathway from CO2 to CO catalyzed by EMIM–BF4, including the most plausible intermediates and the corresponding transition states, was proposed. The role of EMIM–BF4 is explained as forming a complex of [EMIM–COOH]− with CO2 followed by decomposing to CO.

Highlights•An exhaustive search was performed for intermediate state in Zn and H2O reaction.•Zn–H formation effectively reduces the energy.•Zn–H is an intermediate in the reaction of Zn + H2O → ZnO + H2.•H+ in Zn–H indicates a metal hydride character.This paper explores the potentials of a systematic and exhaustive approach for searching intermediate states in the reactions between Zn and H2O. A system consisting of five Zn atoms (Zn5) forming a trigonal dipyramidal structure and fragments decomposed from two H2O to {H2O, OH−, O2− and H+}, were used. All the possible conformations consisting of Zn5 and fragments complex, 859 initial structures in total, were generated and optimized using quantum chemistry calculations. The optimized structure with the lowest energy was used as the initial structure for quantum MD simulation to generate the various conformations, and 600 snapshots including the initial structure were extracted and optimized using quantum chemistry. The sorting of energies revealed that Zn–H formation stabilized the Zn5 clusters and the water fragments. Thus, the intermediate states in the Zn and H2O reactions could be rationally detected. The current approach is not limited to special cases and can be used for a variety of reactions, in particular, for reactions between metal clusters and water molecules.

An unusual intermolecular carbon–carbon short contact, observed previously in the crystal structure of the copper complex of pyridoxal-5-phosphate- pyridoxamine-5-phospate Schiff base, was investigated from a standpoint of quantum chemistry by DFT calculations with plane wave basis sets. The DFT-optimized structure qualitatively reproduced the short contact (2.6–2.8 Å) of the intermolecular carbon–carbon pairs for the dimer of the copper complexes in the unit cell, compared to that (∼2.3 Å) of the X-ray diffraction data. By the occupied and unoccupied orbitals, the dimer showed the in-phase and out-of-phase interactions along the direction of the intermolecular distance. The dimer of the copper complexes was confirmed as the stable intermediate between nonbonding and σ-covalent bonding by the electronic energy curve along the distance of the monomers.

We report here a theoretical study with quantum chemical calculations based on experimental results to understand highly efficient reduction of CO2 to formic acid by using zinc under hydrothermal conditions. Results showed that zinc hydride (Zn–H) is a key intermediate species in the reduction of CO2 to formic acid, which demonstrates that the formation of formic acid is through an SN2-like mechanism.

In photosynthesis, calcium is crucial for oxygen evolution. In the absence of Ca2+, the Kok cycle has been proven to stop at S2 with Yz•. To explore the reason, photosystem II (PSII) S2 models (in total 32452 atoms) with different metal ions (Ca2+, Sr2+, and K+) or without Ca2+ involved in the oxygen evolution complex (OEC) have been theoretically studied based on the previous dynamic study of PSII. It is found that the portion of the Mn1 d-orbital decreases in the highest occupied molecular orbitals for Ca2+-depleted PSII. This feature is unfavorable for the electron transfer from the OEC to the Yz•. Furthermore, the proton donor–acceptor distance was found elongated by the alternation of the binding water in the absence of Ca2+. The isolated vibrational modes of the key water molecules along the path and their high frequency of the OH stretching modes also suggested the difficulty of the proton transfer from the OEC toward the proton exit channel. This work provides one mechanistic explanation for the inactivity of Ca2+-depleted PSII.

A formula for an anisotropic dissymmetry factor g evaluating the chiroptical response of orientationally fixed molecules is derived. Incorporating zeroth- and first-order multipole expansion terms, it is applied to bridged triarylamine helicene molecules to examine the experimental results of single-molecule chiroptical spectroscopy. The ground- and excited-state wave functions and a series of transition moments required for the evaluation of the anisotropic g value are calculated using time-dependent density functional theory (TDDFT). The probability histograms obtained for simulated g values, uniformly sampled in regard to the direction of light propagation toward the fixed molecule, show that even for a given diastereomer, the dissymmetry factors have positive and negative values and can deviate from their averages to a considerable extent when the angle between the electric dipole transition moment and the propagation vector of the incident light is near 0 or 180°.

The molecular dynamics simulation is reported. The latest data on photosystem II structure, a thylakoid membrane model with the same lipid class distribution and fatty acid composition as the native thylakoid membrane, are used. The results indicate that the transfer of water, oxygen and protons has different pathways. The root mean square (rms)-fluctuation analysis of trajectory revealed that the residues surrounding the oxygen-evolving center (OEC) show small fluctuations and that most of the water molecules there show large fluctuation and are on proposed pathways for water and oxygen transfer. The water molecules near the OEC having small fluctuation could be involved in proton transfer. We assume that each kind of pathway characterized in this study plays a role in photosynthesis.

•A simple concept for generating hydrogen from water splitting is proposed.•The system is direct-connected an electrochemical cell and a concentrated photovoltaic cell.•The system operates stably and high repeatability.•The energy conversion efficiency from light to hydrogen is over 12%.•The conditions for stable and high efficient operation are discussed.Energy storage is a key technology for establishing a stand-alone renewable energy system. Current energy-storage technologies are, however, not suitable for such an energy system because the technologies are cost ineffective and achieve low energy-conversion efficiency. The most realistic and expected technology is hydrogen generation from water splitting by an electrochemical cell directly connected with photovoltaic cell. In this study, a simple concept is proposed for generating hydrogen from water splitting by using a direct-electrically-connected polymer electrolyte electrochemical cell and a separately-located concentrated photovoltaic cell, named a “concentrated photovoltaic electrochemical cell (CPEC)”. The CPEC operates stably and achieves relatively high-energy conversion efficiency from light to hydrogen of over 12%. The conditions are comparison with those of the electrochemical cell connected with a polycrystalline Si solar cell.

Diarylethene photochromic switches use light to drive structural changes through reversible electrocyclization reactions. High efficiency in dynamic photoswitching is a prerequisite for applications, as is thermal stability and the selective addressability of both isomers ring-opened and -closed diarylethenes. These properties can be optimized readily through rational variation in molecular structure. The efficiency with regard to switching as a function of structural variation is much less understood, with the exception of geometric requirements placed on the reacting atoms. Ultimately, increasing the quantum efficiency of photochemical switching in diarylethenes requires a detailed understanding of the excited-state potential energy surface(s) and the mechanisms involved in switching. Through studies of the temperature dependence, photoswitching and theoretical studies demonstrate the occurrence or absence of thermal activation barriers in three constitutional isomers that bear distinct π-conjugated systems. We found that a decrease in the thermal barriers correlates with an increase in switching efficiency. The origin of the barriers is assigned to the decrease in π-conjugation that is concomitant with the progress of the photoreaction. Furthermore, we show that balanced molecular design can minimize the change in the extent of π-conjugation during switching and lead to optimal bidirectional switching efficiencies. Our findings hold implications for future structural design of diarylethene photochromic switches.

In this work, we prepared a new 1,2-bis(3-cyanothiophen-2-yl)perfluorocyclopentene with electro-withdrawing cyano groups at both reactive carbon atoms. Furthermore, we studied the substituent effects of the reactive carbon atoms on the photochromic properties of 1,2-bis(3-R-substituted thiophen-2-yl)perfluorocyclopentene derivatives by comparing the absorption wavelengths and quantum yields of the derivatives having R = cyano, methyl, and methoxy groups. The absorption bands of the closed-ring isomers generated by UV irradiation shifted to longer wavelengths with an increase in the electron-donating characteristic of the substituents. The closed-ring isomer having cyano groups at both reactive carbon atoms has an absorption band at 427 nm (λmax), whereas those of methyl and methoxy derivatives have bands at 432 and 481 nm, respectively. The derivative with cyano groups shows the largest cycloreversion quantum yield (0.45), and this yield decreased with an increase in the substituents’ donating characteristic. Theoretical calculation explains that the excited state of the closed-ring isomer with cyano groups has the highest energy, because there is no barrier to ring-opening on the excited potential surface.

A new mechanism of the oxygen evolving reaction catalyzed by [H2O(terpy)Mn(μ-O)2Mn(terpy)OH2]3+ is proposed by using density functional theory. This proton coupled electron transfer (PCET) model shows reasonable barriers. Because in experiments excess oxidants (OCl– or HSO5–) are required to evolve oxygen from water, we considered the Mn2 complex neutralized by three counterions. Structure optimization made the coordinated OCl– withdraw a H+ from the water ligand and produces the reaction space for H2O2 formation with the deprotonated OH– ligand. The reaction barrier for the H2O2 formation from OH– and protonated OCl– depends significantly on the system charge and is 14.0 kcal/mol when the system is neutralized. The H2O2 decomposes to O2 during two PCET processes to the Mn2 complex, both with barriers lower than 12.0 kcal/mol. In both PCET processes the spin moment of transferred electrons prefers to be parallel to that of Mn 3d electrons because of the exchange interaction. This model thus explains how the triplet O2 molecule is produced.

Diarylethene derivatives incorporating an azulene ring at the ethene moiety were synthesized. One derivative having thiazole rings showed the expected coloration reaction by excitation at 313 nm (to a higher singlet state) but not when excited at 635 nm (S0 to S1 excitation). The system demonstrates that the cyclization reaction can be controlled by selective excitation at different wavelengths of the absorption spectrum. On the other hand, another derivative having thiophene rings did not show any photochromism. The results clearly show the importance of the coplanarity of the system for the photoisomerization.

A diarylethene (DAE) study using thermodynamical physical chemistry, elemental fractal analysis, and quantum chemistry is presented. Attention is focused on the ways the polymer environment affects DAE photochromism and on the ways that DAE photochromism affects surfaces. Non-constant quantum yields in single-molecule measurements, selective metal deposition, and a super-water-repellent fractal surface are discussed after a short summary of the latest experimental results concerning photochromism in DAE molecules.Highlights► Photochromic diarylethene shows non-constant quantum yields, as a result of polymer environmental effect. ► It generates an interesting surface. We call it as fractal surface. This is an active environmental effect by diarylethene. ► It generates another interesting surface. This is an active environmental effect which enables selective metal deposition.

Unusual blue shift of the absorption maxima of two nitronyl nitroxide attached diarylethene through phenyl units (DAE-phe-NN) with increasing number of phenyl units is examined by time dependent density functional theory (TDDFT). The extended π-conjugation between nitronyl nitroxide and diarylethene is rather suppressed by the bridge phenyl units. In comparison, the red shift found in two nitronyl nitroxide attached diarylethenes through thiophene units (DAE-thio-NN) with increasing number of thiophene units is due to the longer π-conjugation induced by smaller dihedral angles between diarylethene and bridge and between bridges.

The electrochemical reduction of CO2 to CO by an ionic liquid EMIM–BF4 is one of the most promising CO2 reduction processes proposed so far with its high Faradaic efficiency and low overpotential. However, the details of the reaction mechanism are still unknown due to the absence of fundamental understandings. In this study, the most probable and stable geometries of EMIM–BF4 and CO2 were calculated by quantum chemistry in combination with exhaustive search. A possible reaction pathway from CO2 to CO catalyzed by EMIM–BF4, including the most plausible intermediates and the corresponding transition states, was proposed. The role of EMIM–BF4 is explained as forming a complex of [EMIM–COOH]− with CO2 followed by decomposing to CO.

We report here a theoretical study with quantum chemical calculations based on experimental results to understand highly efficient reduction of CO2 to formic acid by using zinc under hydrothermal conditions. Results showed that zinc hydride (Zn–H) is a key intermediate species in the reduction of CO2 to formic acid, which demonstrates that the formation of formic acid is through an SN2-like mechanism.