Structural and Spectral Properties of a Nonclassical C₆₆ Isomer with Its Hydrogenated Derivative C₆₆H₄ in Theory

X-ray photoelectron and near-edge X-ray absorption fine structure (NEXAFS) spectra, as well as the ground-state electronic/geometrical structures of a newly discovered nonclassical isomer C2v-C66(NC), and two classical fullerene isomers C2-#4466C66 and Cs-#4169C66 with their hydrogenated derivatives [C2v-C66H4(NC), C2-#4466C66H4, and Cs-#4169C66H4] have been calculated at the density functional theory (DFT) level. Significant differences were observed in the electronic structures and simulated X-ray spectra after hydrogenation. Simultaneously, both X-ray photoelectron and NEXAFS spectra reflected conspicuous isomer dependence, indicating that the “fingerprints” in the X-ray spectra can offer an effective method for identifying the above-mentioned fullerene isomers. The simulated ultraviolet–visible (UV–vis) absorption spectroscopy of C2v-C66H4(NC) has also been generated by means of the time-dependent DFT method, and the calculations are well consistent with the experimental results. Consequently, this work reveals that X-ray and UV–vis spectroscopy techniques can provide valuable information to help researchers explore the fullerene electronic structure and isomer identification on the future experimental and theoretical fullerene domains

Thermally Activated Delayed Fluorescence Mechanism of a Bicyclic “Carbene–Metal–Amide” Copper Compound: DFT/MRCI Studies and Roles of Excited-State Structure Relaxation

Herein we investigated the luminescence mechanism of one “carbene–metal–amide” copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S1 state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S0, S1, and T1) model is sufficient to describe the TADF mechanism, and the T2 state should play a negligible role. The calculated S1–T1 energy gap of 0.10 eV and proper spin–orbit couplings facilitate the reverse intersystem crossing (rISC) from T1 to S1. At 298 K, the rISC rate of T1 → S1 (∼106 s–1) is more than 3 orders of magnitude larger than the T1 phosphorescence rate (∼103 s–1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101 s–1). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck–Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important

Probing the Smallest Molecular Model of MoS₂ Catalyst: S₂ Units in the MoS_<i>n</i>^–/0 (<i>n</i> = 1–5) Clusters

Density functional theory (DFT) and coupled cluster theory (CCSD­(T)) calculations are carried out to investigate the electronic and structural properties of a series of monomolybdenum sulfide clusters, MoSn–/0 (n = 1–5). Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). We found that the additional sulfur atoms have a tendency to successively occupy the terminal sites in the sequential sulfidation until the Mo reaches its maximum oxidation sate of +6. After that, the polysulfide ligands (viz., S2 and S3) emerge in the MoS4 and MoS5–/0 clusters. The MoS4 (C2, 1A) is predicted to be the ground state and may be used as a neutral model for the sulfur-rich edge sites of the fresh MoS2 catalysts. Molecular orbital analyses are performed to analyze the chemical bonding in the monomolybdenum sulfide clusters and to elucidate their electronic and structural evolution

Investigation of Ordered TiMC and TiMCT₂ (M = Cr and Mo; T = O and S) MXenes as High-Performance Anode Materials for Lithium-Ion Batteries

First-principles calculations were used to assess the potential of ordered TiMC and TiMCT2 (M = Cr, Mo; T = O, S) monolayers as high-performance anode materials of lithium-ion batteries (LIBs). The predicted results reveal that Li can easily adsorb on the surfaces of TiMC monolayers, especially on the TiMCT2 monolayers. The density of states analysis shows that TiMC (M = Cr and Mo) and TiCrCO2 monolayers exhibit metallicity with good intrinsic advantages for the application of LIBs. The calculated lowest Li-ion diffusion barriers of pristine TiCrC and TiMoC monolayers on the Ti surface are 0.031 and 0.049 eV, which provide an excellent charge/discharge rate in anode materials. Furthermore, the theoretical capacity of TiCrC is 479 mAh g–1 when the concentration of Li reaches 2, and TiMoC will exhibit a large theoretical capacity when the open circuit voltage drops to 0 V. In addition, TiCrCO2 exhibits relatively high theoretical capacity (373 mAh g–1). Among all studied materials, pristine TiMC (M = Cr and Mo) and functionalized TiCrCO2 monolayers should be promising candidates as anode materials for LIBs

Understanding the Role of Various Dopant Metals (Sb, Sn, Ga, Ge, and V) in the Structural and Electrochemical Performances of LiNi_0.5Co_0.2Mn_0.3O₂

Ni-rich layered oxides have been widely applied commercially due to their high energy density and capacity. However, there are still some drawbacks of capacity fading, O2 release, and Li/Ni exchange. Cation doping has been proven to be one of the most promising strategies to improve the electrochemical performances of Ni-rich layered oxides. Herein, density functional theory (DFT) calculations have been performed to investigate the effects of doping various cations (Sb5+, Sn4+, Ga3+, Ge4+, and V5+) on the structural stability and electrochemical performances of LiNi0.5Co0.2Mn0.3O2 (NCM523). The theoretical results show that Sb, Sn, Ga, Ge, and V doping can reduce the oxidation state of Ni ions. Moreover, doping with these metals can inhibit O2 release and Li/Ni exchange, which improves the safety, capacity retention, and rate capacity. Furthermore, Ga and Ge doping can improve the stability of partially deintercalated states, suppress lattice distortion, and increase the intercalation voltage. In conclusion, Ga and Ge doping is an effective strategy to optimize the electrochemical performances of NCM523

Near-Infrared Dual-Emission of a Thiolate-Protected Au₄₂ Nanocluster: Excited States, Nonradiative Rates, and Mechanism

Both DFT and TD-DFT methods are used to elaborate on the excited-state properties and dual-emission mechanism of a thiolate-protected Au42 nanocluster. A three-state model (S0, S1, and T1) is proposed with respect to the results. The intersystem crossing (ISC) process from S1 to T1 benefits from a small reorganization energy due to the similar geometric structures of S1 and T1. However, the ISC process is suppressed by relatively small spin–orbit coupling resulting from the similarity of the electronic structures of S1 and T1. As a result of the counterbalance, the ISC rate is comparable with the fluorescence emission rate. In the T1 state, the phosphorescence emission prevails the reverse ISC process back to the S1 state. Taken together, fluorescence and phosphorescence are achieved simultaneously. The present work provides deep mechanistic insights to aid the rational design of NIR dual-emissive metal nanoclusters