Computational Evidence of Inversion of ¹L_a and ¹L_b‑Derived Excited States in Naphthalene Excimer Formation from <i>ab Initio</i> Multireference Theory with Large Active Space: DMRG-CASPT2 Study

The naphthalene molecule has two important lowest-lying singlet excited states, denoted ¹L_a and ¹L_b. Association of the excited and ground state monomers yields a metastable excited dimer (<i>excimer</i>), which emits characteristic fluorescence. Here, we report a first computational result based on <i>ab initio</i> theory to corroborate that the naphthalene excimer fluorescence is ¹L_a parentage, resulting from inversion of ¹L_a and ¹L_b-derived dimer states. This inversion was hypothesized by earlier experimental studies; however, it has not been confirmed rigorously. In this study, the advanced multireference (MR) theory based on the density matrix renormalization group that enables using unprecedented large-size active space for describing significant electron correlation effects is used to provide accurate potential energy curves (PECs) of the excited states. The results evidenced the inversion of the PECs and accurately predicted transition energies for excimer fluorescence and monomer absorption. Traditional MR calculations with smaller active spaces and single-reference theory calculations exhibit serious inconsistencies with experimental observations

Superior Low-Temperature Power and Cycle Performances of Na-Ion Battery over Li-Ion Battery

The most simple and clear advantage of Na-ion batteries (NIBs) over Li-ion batteries (LIBs) is the natural abundance of Na, which allows inexpensive production of NIBs for large-scale applications. However, although strenuous research efforts have been devoted to NIBs particularly since 2010, certain other advantages of NIBs have been largely overlooked, for example, their low-temperature power and cycle performances. Herein, we present a comparative study of spirally wound full-cells consisting of Li_0.1Na_0.7Co_0.5Mn_0.5O₂ (or Li_0.8Co_0.5Mn_0.5O₂) and hard carbon and report that the power of NIB at −30 °C is ∼21% higher than that of LIB. Moreover, the capacity retention in cycle testing at 0 °C is ∼53% for NIB but only ∼29% for LIB. Raman spectroscopy and density functional theory calculations revealed that the superior performance of NIB is due to the relatively weak interaction between Na⁺ ions and aprotic polar solvents

Ab Initio Molecular Orbital Study on the Excited States of [2.2]‑, [3.3]‑, and Siloxane-Bridged Paracyclophanes

Paracyclophanes are simple idealized model molecules for the study of interacting π-stacking systems. In this study, the excited states of [2.2]­paracyclophane ([2.2]­PCP), [3.3]­paracyclophane ([3.3]­PCP), and siloxane-bridged paracyclophane (SiPCP) are systematically investigated using the multiconfiguration quasi-degenerated perturbation theory (MCQDPT) method. The excited states of the alkyl- and silyl-substituted benzene monomers and benzene dimer, which can be regarded as the building blocks of paracyclophanes, are also examined at the same level of theory for more detailed understanding. The accuracy of the time-dependent density functional theory (TD-DFT) method required for excited state geometry optimization of the paracyclophanes is confirmed from calculations of the benzene dimer. The equilibrium distances between the benzene rings of the paracyclophanes in the first excited states are shorter than those in the ground state, and the benzene rings at the excited state optimized geometries are in an almost eclipsed parallel configuration, which indicates excimer formation. The calculated transition energies and oscillator strengths are generally in good agreement with the corresponding experimental results. A clear correlation between the excited state properties and the molecular structures is systematically demonstrated based on the calculation results for the substituted benzene monomers and benzene dimer. The transition energies of SiPCP are close to the corresponding absorption and fluorescence energies of the experimentally studied phenylene–silica hybrids, which indicates that the electronic properties of organic–silica hybrids, which is a new class of material with potential in photofunctional applications, can be approximated by simple siloxane-bridged cyclophane derivatives

Upward Shift in Conduction Band of Ta₂O₅ Due to Surface Dipoles Induced by N‑Doping

Density functional theory calculations were executed to clarify the mechanism of the experimentally observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta₂O₅, which is used as a photosensitizer in CO₂ reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of nondoped Ta₂O₅ and N-doped Ta₂O₅. Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta₂O₅, and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface is not high enough to compensate positive charges induced at the surface defects, the remaining positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta₂O₅, allowing photogenerated electrons to transfer from N-doped Ta₂O₅ to the catalytic active sites for CO₂ reduction as realized with Ru complex on the surface in experiment

Effects of Ta₂O₅ Surface Modification by NH₃ on the Electronic Structure of a Ru-Complex/N–Ta₂O₅ Hybrid Photocatalyst for Selective CO₂ Reduction

This work examined a Ru-complex/N–Ta₂O₅ (N–Ta₂O₅: nitrogen-doped Ta₂O₅) hybrid photocatalyst for CO₂ reduction. In this material, electrons are transferred from the N–Ta₂O₅ to the Ru-complex in response to visible light irradiation, after which CO₂ reduction occurs on the complex. N-doping is believed to produce an upward shift in the conduction band minimum (CBM) of the Ta₂O₅, thus allowing more efficient electron transfer, although the associated mechanism has not yet been fully understood. In the present study, the effects of NH₃ adsorption (the most likely surface modification following nitrification) were examined using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy data suggest that NH₃ molecules are adsorbed on the N–Ta₂O₅ surface, and it is also evident that the photocatalytic activity of the Ru-complex/N–Ta₂O₅ is decreased by the removal of this adsorbed NH₃. Calculations show that both the occupied and unoccupied orbital levels of Ta₁₆O₄₀(NH₃)_<i>x</i> clusters (<i>x</i> = 4, 8, 12, or 16) are shifted upward as <i>x</i> is increased. Theoretical analyses of Ru-complex/cluster hybrids demonstrate that the gap between the lowest unoccupied molecular orbital of the Ta₁₆O₄₀ moiety and the unoccupied orbitals of the Ru-complex in Ru-complex/Ta₁₆O₄₀(NH₃)₁₂ is much smaller than that in Ru-complex/Ta₁₆O₄₀. The highest occupied molecular orbital of [Ru-complex/Ta₁₆O₄₀]⁻ is evidently localized on the Ta₁₆O₄₀ moiety, whereas that of [Ru-complex/Ta₁₆O₄₀(NH₃)₁₂]⁻ is spread over both the Ta₁₆O₄₀ and Ru-complex. These results indicate that the NH₃ adsorption associated with N-doping can result in an upward shift of the CBM of Ta₂O₅. Additional calculations for Ta₁₆O_{40–<i>y</i>}(NH)_<i>y</i> (<i>y</i> = 2, 4, 6, 8, or 10) suggest that the substitution of NH groups for oxygen atoms on the Ta₂O₅ surface may be responsible for the red shift in the adsorption band edge of the oxide but makes only a minor contribution to the upward shift of the CBM

A Solid Chelating Ligand: Periodic Mesoporous Organosilica Containing 2,2′-Bipyridine within the Pore Walls

Synthesis of a solid chelating ligand for the formation of efficient heterogeneous catalysts is highly desired in the fields of organic transformation and solar energy conversion. Here, we report the surfactant-directed self-assembly of a novel periodic mesoporous organosilica (PMO) containing 2,2′-bipyridine (bpy) ligands within the framework (BPy-PMO) from a newly synthesized organosilane precursor [(<i>i</i>-PrO)₃Si–C₁₀H₆N₂–Si­(O<i>i</i>-Pr)₃] without addition of any other silane precursors. BPy-PMO had a unique pore-wall structure in which bipyridine groups were densely and regularly packed and exposed on the surface. The high coordination ability to metals was also preserved. Various bipyridine-based metal complexes were prepared using BPy-PMO as a solid chelating ligand such as Ru­(bpy)₂(BPy-PMO), Ir­(ppy)₂(BPy-PMO) (ppy = 2-phenylpyridine), Ir­(cod)­(OMe)­(BPy-PMO) (cod = 1,5-cyclooctadiene), Re­(CO)₃Cl­(BPy-PMO), and Pd­(OAc)₂(BPy-PMO). BPy-PMO showed excellent ligand properties for heterogeneous Ir-catalyzed direct C–H borylation of arenes, resulting in superior activity, durability, and recyclability to the homogeneous analogous Ir catalyst. An efficient photocatalytic hydrogen evolution system was also constructed by integration of a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay molecules. These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform for construction of efficient molecular-based heterogeneous catalysis systems