W-Shaped π-Extended Double Undecabenzo[7]helicene.

A chiral W-shaped fully π-extended double [7]helicene (ED7H) has been synthesized and fully characterized. It displays fluorescence emission (λem = 636 nm) with a quantum yield (Φf) of 0.10. In comparison to its X-shaped and monomict π-extended [7]helicene analogues, enantiopure W-shaped ED7H exhibited superior chiral optical characteristics, including distinct circular dichroism signals from 400 to 650 nm, a good dissymmetric emission factor |glum| of 4 × 10-3, and a circularly polarized luminescence brightness value BCPL of 42 M-1 cm-1.

Photocatalytic Reduction of CO2 to HCOOH and CO by a Phosphine-Bipyridine-Phosphine Ir(III) Catalyst: Photophysics, Nonadiabatic Effects, Mechanism, and Selectivity.

Photocatalytic CO2 reduction is one of the best solutions to solve the global energy crisis and to realize carbon neutralization. The tetradentate phosphine-bipyridine (bpy)-phosphine (PNNP)-type Ir(III) photocatalyst, Mes-IrPCY2, was reported with a high HCOOH selectivity but the photocatalytic mechanism remains elusive. Herein, we employ electronic structure methods in combination with radiative, nonradiative, and electron transfer rate calculations, to explore the entire photocatalytic cycle to either HCOOH or CO, based on which a new mechanistic scenario is proposed. The catalytic reduction reaction starts from the generation of the precursor metal-to-ligand charge transfer (3 MLCT) state. Subsequently, the divergence happens from the 3 MLCT state, the single electron transfer (SET) and deprotonation process lead to the formation of one-electron-reduced species and Ir(I) species, which initiate the reduction reaction to HCOOH and CO, respectively. Interestingly, the efficient occurrence of proton or electron transfer reduces barriers of critical steps. In addition, nonadiabatic transitions play a nonnegligible role in the cycle. We suggest a lower free-energy barrier in the reaction-limiting step and the very efficient SET in 3 MLCT are cooperatively responsible for a high HCOOH selectivity. The gained mechanistic insights could help chemists to understand, regulate, and design photocatalytic CO2 reduction reaction of similar function-integrated molecular photocatalyst.

Nonadiabatic Dynamics Simulations for Photoinduced Processes in Molecules and Semiconductors: Methodologies and Applications.

Nonadiabatic dynamics (NAMD) simulations have become powerful tools for elucidating complicated photoinduced processes in various systems from molecules to semiconductor materials. In this review, we present an overview of our recent research on photophysics of molecular systems and periodic semiconductor materials with the aid of ab initio NAMD simulation methods implemented in the generalized trajectory surface-hopping (GTSH) package. Both theoretical backgrounds and applications of the developed NAMD methods are presented in detail. For molecular systems, the linear-response time-dependent density functional theory (LR-TDDFT) method is primarily used to model electronic structures in NAMD simulations owing to its balanced efficiency and accuracy. Moreover, the efficient algorithms for calculating nonadiabatic coupling terms (NACTs) and spin-orbit couplings (SOCs) have been coded into the package to increase the simulation efficiency. In combination with various analysis techniques, we can explore the mechanistic details of the photoinduced dynamics of a range of molecular systems, including charge separation and energy transfer processes in organic donor-acceptor structures, ultrafast intersystem crossing (ISC) processes in transition metal complexes (TMCs), and exciton dynamics in molecular aggregates. For semiconductor materials, we developed the NAMD methods for simulating the photoinduced carrier dynamics within the framework of the Kohn-Sham density functional theory (KS-DFT), in which SOC effects are explicitly accounted for using the two-component, noncollinear DFT method. Using this method, we have investigated the photoinduced carrier dynamics at the interface of a variety of van der Waals (vdW) heterojunctions, such as two-dimensional transition metal dichalcogenides (TMDs), carbon nanotubes (CNTs), and perovskites-related systems. Recently, we extended the LR-TDDFT-based NAMD method for semiconductor materials, allowing us to study the excitonic effects in the photoinduced energy transfer process. These results demonstrate that the NAMD simulations are powerful tools for exploring the photodynamics of molecular systems and semiconductor materials. In future studies, the NAMD simulation methods can be employed to elucidate experimental phenomena and reveal microscopic details as well as rationally design novel photofunctional materials with desired properties.

Theoretical studies on thermally activated delayed fluorescence of "carbene-metal-amide" Cu and Au complexes: geometric structures, excitation characters, and mechanisms.

"Carbene-metal(I)-amide" (CMA) complexes have garnered significant attention due to their remarkable properties and potential TADF applications in organic electronics. However, the atomistic working mechanism is still elusive. Herein, we chose two CMA complexes, i.e., cyclic (alkyl)(amino) carbene-copper[gold](I)-carbazole (CAAC-Cu[Au]-Cz), and employed both DFT and TD-DFT methods, in combination with radiative and nonradiative rate calculations, to investigate geometric and electronic structures of these two complexes in the ground and excited states, including orbital compositions, electronic transitions, absorption and emission spectra, and the luminescence mechanism. It is found that the coplanar or perpendicular conformations are coexistent in the ground state (S0), the lowest excited singlet state (S1), and the triplet state (T1). Both the coplanar and perpendicular S1 and T1 states have similar ligand-to-ligand charge transfer (LLCT) character between CAAC and Cz, and some charge-transfer character between metal atoms and ligands, which is beneficial to minimize the singlet-triplet energy gaps (ΔEST) and increase the spin-orbit coupling (SOC). An interesting three-state (S0, S1, T1) model involving two regions (coplanar and perpendicular) is proposed to rationalize the experimental TADF phenomena in the CMA complexes. In addition to the coplanar ones, the perpendicular S1 and T1 states also play a role in promoting the repopulation of the coplanar S1 exciton, which is a primary source for the delayed fluorescence.

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.

Chalcogen Bond Catalysis with Telluronium Cations for Bromination Reaction: Importance of Electrostatic and Polarization Effects.

Recently, chalcogen bond catalysts with telluronium cations have garnered considerable attention in organic reactions. In this work, chalcogen bond catalysis on the bromination reaction of anisole with N-bromosuccinimide (NBS) with the telluronium cationic catalysts has been explored with density functional theory (DFT). The catalytic reaction is divided into two stages: the bromine transfer step and the proton transfer step. Based on the computational results, one can find the rate-determining step is the bromine transfer step. Moreover, the present study elucidates that a stronger chalcogen bond between catalysts and NBS will give better catalytic performance. Additionally, this work also clarified the importance of the electrostatic and polarization effects in the chalcogen bond between the oxygen atom of NBS and the Te atom of the catalyst in this bromination reaction. The electrostatic and polarization effects are significantly influenced by the electron-withdrawing ability of the substitution groups on the catalysts. Moreover, the structure-property relationship between the strength of chalcogen bond, electrostatic effect, polarization effect and catalytic performance are established for the design of more efficient chalcogen bond catalysts.

Nonadiabatic Derivative Couplings Calculated Using Information of Potential Energy Surfaces without Wavefunctions: Ab Initio and Machine Learning Implementations.

In this work, we implemented an approximate algorithm for calculating nonadiabatic coupling matrix elements (NACMEs) of a polyatomic system with ab initio methods and machine learning (ML) models. Utilizing this algorithm, one can calculate NACMEs using only the information of potential energy surfaces (PESs), i.e., energies, and gradients as well as Hessian matrix elements. We used a realistic system, namely CH2NH, to compare NACMEs calculated by this approximate PES-based algorithm and the accurate wavefunction-based algorithm. Our results show that this approximate PES-based algorithm can give very accurate results comparable to the wavefunction-based algorithm except at energetically degenerate points, i.e., conical intersections. We also tested a machine learning (ML)-trained model with this approximate PES-based algorithm, which also supplied similarly accurate NACMEs but more efficiently. The advantage of this PES-based algorithm is its significant potential to combine with electronic structure methods that do not implement wavefunction-based algorithms, low-scaling energy-based fragment methods, etc., and in particular efficient ML models, to compute NACMEs. The present work could encourage further research on nonadiabatic processes of large systems simulated by ab initio nonadiabatic dynamics simulation methods in which NACMEs are always required.

Extending multi-layer energy-based fragment method for excited-state calculations of large covalently bonded fragment systems.

Recently, we developed a low-scaling Multi-Layer Energy-Based Fragment (MLEBF) method for accurate excited-state calculations and nonadiabatic dynamics simulations of nonbonded fragment systems. In this work, we extend the MLEBF method to treat covalently bonded fragment ones. The main idea is cutting a target system into many fragments according to chemical properties. Fragments with dangling bonds are first saturated by chemical groups; then, saturated fragments, together with the original fragments without dangling bonds, are grouped into different layers. The accurate total energy expression is formulated with the many-body energy expansion theory, in combination with the inclusion-exclusion principle that is used to delete the contribution of chemical groups introduced to saturate dangling bonds. Specifically, in a two-layer MLEBF model, the photochemically active and inert layers are calculated with high-level and efficient electronic structure methods, respectively. Intralayer and interlayer energies can be truncated at the two- or three-body interaction level. Subsequently, through several systems, including neutral and charged covalently bonded fragment systems, we demonstrate that MLEBF can provide accurate ground- and excited-state energies and gradients. Finally, we realize the structure, conical intersection, and path optimizations by combining our MLEBF program with commercial and free packages, e.g., ASE and SciPy. These developments make MLEBF a practical and reliable tool for studying complex photochemical and photophysical processes of large nonbonded and bonded fragment systems.

Thermally activated delayed fluorescence of a Ir(III) complex: absorption and emission properties, nonradiative rates, and mechanism.

One recent experimental study reported a Ir(III) complex with thermally activated delayed fluorescence (TADF) phenomenon in solution, but its luminescent mechanism is elusive. In this work, we combined density functional theory (DFT), time-dependent DFT (TDDFT) and multi-state complete active space second-order perturbation theory (MS-CASPT2) methods to investigate excited-state properties, photophysics, and emission mechanism of this Ir(III) complex. Two main absorption bands observed in experiments can be attributed to the electronic transition from the S0 state to the S1 and S2 states; while, the fluorescence and phosphorescence are generated from the S1 and T1 states, respectively. Both the S1 and T1 states have clear metal-to-ligand charge transfer (MLCT) character. The present computational results reveal a three-state model including the S0, S1 and T1 states to rationalize the TADF behavior. The small energy gap between the S1 and T1 states benefits the forward and reverse intersystem crossing (ISC and rISC) processes. At 300 K, the rISC rate is five orders of magnitude larger than the phosphorescence rate therefore enabling TADF. At 77 K, the rISC rate is sharply decreased but remains close to the phosphorescence rate; therefore, in addition to the phosphorescence, the delayed fluorescence could also contribute to the experimental emission. The estimated TADF lifetime agrees well with experiments, 9.80 vs. 6.67 µs, which further verifies this three-state model.

Thermally Activated Delayed Fluorescence of a Dinuclear Platinum(II) Compound: Mechanism and Roles of an Upper Triplet State.

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited-state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out in this work. In terms of the results, the S1 and T2 states show mixed intraligand charge transfer (ILCT)/metal-to-ligand CT (MLCT) characters while the T1 state exhibits mixed ILCT/ligand-to-metal CT (LMCT) characters. Mechanistically, a four-state (S0 , S1 , T1 , and T2 ) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T1 to final S1 states involves two up-conversion channels (direct T1 →S1 and T2 -mediated T1 →T2 →S1 pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T1 phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances.

Generalized Ab Initio Nonadiabatic Dynamics Simulation Methods from Molecular to Extended Systems.

Nonadiabatic dynamics simulation has become a powerful tool to describe nonadiabatic effects involved in photophysical processes and photochemical reactions. In the past decade, our group has developed generalized trajectory-based ab initio surface-hopping (GTSH) dynamics simulation methods, which can be used to describe a series of nonadiabatic processes, such as internal conversion, intersystem crossing, excitation energy transfer and charge transfer of molecular systems, and photoinduced nonadiabatic carrier dynamics of extended systems with and without spin-orbit couplings. In this contribution, we will first give a brief introduction to our recently developed methods and related numerical implementations at different computational levels. Later, we will present some of our latest applications in realistic systems, which cover organic molecules, biological proteins, organometallic compounds, periodic organic and inorganic materials, etc. Final discussion is given to challenges and outlooks of ab initio nonadiabatic dynamics simulations.

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.

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 C 2v -C66(NC), and two classical fullerene isomers C 2-#4466C66 and C s -#4169C66 with their hydrogenated derivatives [C 2v -C66H4(NC), C 2-#4466C66H4, and C s -#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 C 2v -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.

Theoretical Insights into Synergistic Effects at Cu/TiC Interfaces for Promoting CO2 Activation.

The adsorption behaviors of CO2 at the Cu n /TiC(001) interfaces (n = 1-8) have been investigated using the density functional theory method. Our results reveal that the introduction of copper clusters on a TiC surface can significantly improve the thermodynamic stability of CO2 chemisorption. However, the most stable adsorption site is sensitive to the size and morphology of Cu n particles. The interfacial configuration is the most stable structure for copper clusters with small (n ≤ 2) and large (n ≥ 8) sizes, in which both Cu particles and TiC support are involved in CO2 activation. In such a case, the synergistic behavior is associated with the ligand effect introduced by directly forming adsorption bonds with CO2. For those Cu n clusters with a medium size (n = 3-7), the configuration where CO2 adsorbs solely on the exposed hollow site constructed by Cu atoms at the interface shows the best stability, and the charger transfer becomes the primary origin of the synergistic effect in promoting CO2 activation. Since the most obvious deformation of CO2 is observed for the TiC(001)-surface-supported Cu4 and Cu7 particles, copper clusters with specific sizes of n = 4 and 7 exhibit the best ability for CO2 activation. Furthermore, the kinetic barriers for CO2 dissociation on Cu4- and Cu7-supported TiC surfaces are determined. The findings obtained in this work provide useful insights into optimizing the Cu/TiC interface with high catalytic activation of CO2 by precisely controlling the size and dispersion of copper particles.

Spin-Orbit Coupling Is the Key to Promote Asynchronous Photoinduced Charge Transfer of Two-Dimensional Perovskites.

Two-dimensional (2D) perovskites are emerging as promising candidates for diverse optoelectronic applications because of low cost and excellent stability. In this work, we explore the electronic structures and interfacial properties of (4Tm)2PbI4 with both the collinear and noncollinear DFT (PBE and HSE06) methods. The results evidently manifest that explicitly considering the spin-orbit coupling (SOC) effects is necessary to attain correct band alignment of (4Tm)2PbI4 that agrees with recent experiments (Nat. Chem.2019, 11, 1151; Nature2020, 580, 614). The subsequent time-domain noncollinear DFT-based nonadiabatic carrier dynamics simulations with the SOC effects reveal that the photoinduced electron and hole transfer processes are asymmetric and associated with different rates. The differences are mainly ascribed to considerably different nonadiabatic couplings in charge of the electron and hole transfer processes. Shortly, our current work sheds important light on the mechanism of the interfacial charge carrier transfer processes of (4Tm)2PbI4. The importance of the SOC effects on correctly aligning the band states of (4Tm)2PbI4 may be generalized to similar organic-inorganic hybrid 2D perovskites having heavy Pb atoms.

BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen.

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of α,ß-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O2-. ) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S1 ) to triplet (T1 ) states via an intermediate triplet (T2 ) state. The low reduction potential and ultralong lifetime of the T1 state facilitate the efficient generation of O2-. by inter-molecular charge transfer to molecular oxygen. The energy gap of T1 -S0 is smaller than that between 3 O2 and 1 O2 thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.

Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence in the Pd Complex: Mechanism and Dual Upconversion Channels.

The Pd complex PdN3N exhibits an unusual dual emission of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S0, S1, T1, and T2 states are involved in the luminescence. Both the S1 → T1 and S1 → T2 intersystem crossing (ISC) processes are more efficient than the S1 fluorescence and insensitive to temperature. However, the direct T1 → S1 and T2-mediated T1 → T2 → S1 reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T1 phosphorescence and therefore enable TADF. Importantly, the T1 → S1 rISC and T1 phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T1 phosphorescence is recorded experimentally.

Effects of doping high-valence transition metal (V, Nb and Zr) ions on the structure and electrochemical performance of LIB cathode material LiNi0.8Co0.1Mn0.1O2.

Ni-rich layered oxides, like LiNi0.8Co0.1Mn0.1O2 (NCM811), have been widely investigated as cathodes due to their high energy density. However, gradual structural transformation during cycling can lead to capacity degradation and potential decay of cathode materials. Herein, we doped high-valence transition metal (TM) ions (V5+, Nb5+, and Zr4+) at the Ni site of NCM811 by first principles simulations and explored the mechanism of doping TMs in NCMs for enhancing the electrochemical performance. Analysis of the calculations shows that doping V, Nb and Zr has an efficient influence on alleviating the Ni oxidation, reducing the loss of oxygen, and facilitating Li+ migration. Moreover, V doping can further suppress the lattice distortion due to the radius of V5+ being close to the radius of Mn4+. In particular, compared with the barrier of the pristine NCM in Li divacancy, the barrier of V-doped NCM reaches the lowest. In conclusion, V is the most favorable dopant for NCM811 to improve the electrochemical properties and achieve both high capacity and cycling stability.

MicroRNA-29c Acting on FOS Plays a Significant Role in Nonalcoholic Steatohepatitis Through the Interleukin-17 Signaling Pathway.

Nonalcoholic fatty liver disease is the most common hepatic disease in western countries and is even more ubiquitous in Asian countries. Our study determined that TH17/Treg cells were imbalanced in animal models. Based on our interest in the mechanism underlying TH17/Treg cell imbalance in nonalcoholic fatty liver mice, we conducted a joint bioinformatics analysis to further investigate this process. Common gene sequencing analysis was based on one trial from one sequencing platform, where gene expression analysis and enrichment analysis were the only analyses performed. We compared different sequencing results from different trials performed using different sequencing platforms, and we utilized the intersection of these analytical results to perform joint analysis. We used a bioinformatics analysis method to perform enrichment analysis and map interaction network analysis and predict potential microRNA sites. Animal experiments were also designed to validate the results of the data analysis based on quantitative polymerase chain reaction (qPCR) and western blotting. Our results revealed 8 coexisting differentially expressed genes (DEGs) and 7 hinge genes. The identified DEGs may influence nonalcoholic steatosis hepatitis through the interleukin-17 pathway. We found that microRNA-29c interacts with FOS and IGFBP1. Polymerase chain reaction analyses revealed both FOS and microRNA-29c expression in NASH mice, and western blot analyses indicated the same trend with regard to FOS protein levels. Based on these results, we suggest that microRNA-29c acts on FOS via the interleukin-17 signaling pathway to regulate TH17/Treg cells in NASH patients.