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The structural, photophysical, and photochemical properties of Ln(depma)(hmpa)2(NO3)3(Ln = La, Ce, Nd, Sm, Eu, Tb, Ho, Er, and Yb) complexes 1-Lnwere investigated with a multidisciplinary approach involving synthesis, photocycloaddition-based crystal engineering, spectroscopic analytical techniques and quantum chemical ab initio calculations. Depending on the Ln3+ion the isostructural 1-Lncomplexes exhibit quite different behavior upon excitation at 350–400 nm. Some 1-Lncomplexes (Ln = La, Ce, Sm, Tb, Yb) emit a broad and strong band near 533 nm arising from paired anthracene moieties, whereas others (Ln = Nd, Eu, Ho, Er) do not. 1-Euis not emissive at all, whereas 1-Nd, 1-Ho, and 1-Erexhibit a Ln3+based luminescence. Upon irradiation with 365 nm ultraviolet (UV) light 1-Ln(Ln = La, Ce, Sm, Tb, Yb) dimerize by means of a photochemically induced [4 + 4] cycloaddition of the anthracene moieties, whereas 1-Ln(Ln = Nd, Eu, Ho, Er) remain monomers. We propose three models, based on the matching of the energy levels between the Ln3+ion and the paired or dimerized anthracene units in the energy-resonance crossing region, as well as on internal conversion-driven and intersystem crossing-driven energy transfer, which explain the Ln3+ion regulated photophysics and photochemistry of the 1-Lncomplexes.

The activation of halogens (X = Cl, Br, I) by N2O5is linked to NOxsources, ozone concentrations, NO3reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N2O5···X–systems at the air–water interface. Two different reaction pathways were identified for the reaction of N2O5with X–at the air–water interface: the formation of XNO2or XONO, along with NO3–. In the case of the Cl–system, the ClNO2generation pathway is more favorable, while for the Br–and I–systems, the formation of BrONO and IONO is barrierless, making them the predominant products. Furthermore, the mechanisms of formation of X2from XNO2and XONO were also investigated. The high energy barriers of reactions and the high free energies of the products compared to those of the reactants indicate that ClNO2is stable at the air–water interface. Contrary to the widely held belief regarding X2producing from the reaction of XNO2with X–, our calculations demonstrate that BrONO and IONO initially form stable BrONO···Br–and IONO···I–complexes, which then subsequently react with Br–and I–to form Br3–and I3–, respectively. Finally, Br3–and I3–decompose to form Br2and I2.These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.

We simulated hot-electron relaxation in black phosphorus using the nonadiabatic molecular dynamics (NA-MD) approach with a non-Condon effect in momentum space beyond the harmonic approximation. By comparing simulations at the Γ point in a large supercell with those using a few k-points in a smaller supercell─while maintaining the same number of electronic states within the same energy range, we demonstrate that both setups yield remarkably consistent energy relaxation times, regardless of the initial state energy. This consistency arises from the complementary effects of supercell size in real space and the number of k-points in the reciprocal space. This finding confirms that simulations at a single k-point in large size supercells are an effective approximation for NA-MD with a non-Condon effect. This approach offers significant advantages for complex photophysics, such as intervalley scattering and indirect bandgap charge recombination, and is particularly suitable for large systems without the need for a harmonic approximation.

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4(BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron–hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron–hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn–I bonds, distorts the [SnI6]4–octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn–I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron–hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Herein, we have used the “on-the-fly” ring-polymer surface-hopping simulation method with the centroid approximation (RPSH-CA), in combination with the multireference OM2/MRCI electronic structure calculations to study the photoinduced dynamics of a green fluorescent protein (GFP) chromophore analogue in the gas phase, i.e., o-HBI, at 50, 100, and 300 K with 1, 5, 10, and 15 beads (3600 1 ps trajectories). The electronic structure calculations identified five new minimum-energy conical intersection (MECI) structures, which, together with the previous one, play crucial roles in the excited-state decay dynamics of o-HBI. It is also found that the excited-state intramolecular proton transfer (ESIPT) occurs in an ultrafast manner and is completed within 20 fs in all the simulation conditions because there is no barrier associated with this ESIPT process in the S1state. However, the other excited-state dynamical results are strongly related to the number of beads. At 50 and 100 K, the nuclear quantum effects (NQEs) are very important; therefore, the excited-state dynamical results change significantly with the bead number. For example, the S1decay time deduced from time-dependent state populations becomes longer as the bead number increases. Nevertheless, an essentially convergent trend is observed when the bead number is close to 10. In contrast, at 300 K, the NQEs become weaker and the above dynamical results converge very quickly even with 1 bead. Most importantly, the NQEs seriously affect the excited-state decay mechanism of o-HBI. At 50 and 100 K, most trajectories decay to the S0state via perpendicular keto MECIs, whereas, at 300 K, only twisted keto MECIs are responsible for the excited-state decay. The present work not only comprehensively explores the temperature-dependent photoinduced dynamics of o-HBI, but also demonstrates the importance and necessity of NQEs in nonadiabatic dynamics simulations, especially at relatively low temperatures.

Herein, we employed linear-response time-dependent functional theory nonadiabatic dynamic simulations to explore the photoinduced exciton dynamics of a chiral single-walled carbon nanotube CNT(6,5) covalently doped with a 4-nitrobenzyl group (CNT65-NO2). The results indicate that the introduction of a sp3defect leads to the splitting of the degenerate VBM/VBM-1 and CBM/CBM+1 states. Both the VBM upshift and the CBM downshift are responsible for the experimentally observed redshifted E11* trapping state. The simulations reveal that the photoinduced exciton relaxation dynamics completes within 500 fs, which is consistent with the experimental work. On the other hand, we also conducted the nonadiabatic carrier (electron and hole) dynamic simulations, which completely ignore the excitonic effects. The comparison demonstrates that excitonic effects are indispensable. Deep analyses show that such effects induce several dark states, which play an important role in regulating the photoinduced dynamics of CNT65-NO2. The present work demonstrates the importance of including excitonic effects in simulating photoinduced processes of carbon nanotubes. In addition, it not only rationalizes previous experiments but also provides valuable insights that will help in the future rational design of novel covalently doped carbon nanotubes with superior photoluminescent properties.

A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initiocalculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310–330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin–orbit coupling of 8.3 cm–1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min(3.36 ± 0.15 ps) rather than 1ππ*min(5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*minconstitutes the main relaxation pathway in me6-TG. Although this ΦTis inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C–S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

The presence of many degenerate d/f orbitals makes polynuclear transition-metal compounds, such as iron–sulfur clusters in nitrogenase, challenging for state-of-the-art quantum chemistry methods. To address this challenge, we present the first distributed multi-graphics processing unit (GPU) ab initiodensity matrix renormalization group (DMRG) algorithm suitable for modern high-performance computing (HPC) infrastructures. The central idea is to parallelize the most computationally intensive part─the multiplication of O(K2) operators with a trial wave function, where Kis the number of spatial orbitals, by combining operator parallelism for distributing the workload with a batched algorithm for performing contractions on GPU. With this new implementation, we are able to reach an unprecedentedly large bond dimension D= 14,000 on 48 GPUs (NVIDIA A100 80 GB SXM) for an active space model (114 electrons in 73 active orbitals) of the P-cluster, which is nearly 3 times larger than the bond dimensions reported in previous DMRG calculations for the same system using only central processing units (CPUs).

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.

Ultrafast charge and spin dynamics have immense effects on the applications of topological insulators (TIs). By performing spin-adiabatic nonadiabatic molecular dynamics simulations in the presence of electron–phonon (e-ph) and spin–phonon couplings, we investigate temperature-dependent intra- and interband charge and spin relaxation dynamics via the bulk and surface paths in the three-dimensional TI Bi2Te3. The e-phcoupling dominates charge relaxation in the bulk path, and the relaxation rate is positively correlated with temperature due to the large energy gaps and weak spin polarization. Conversely, the relaxation dynamics exhibits an opposite temperature dependence in the surface path because of electron re-excitation and spin mismatching induced by spin–phonon coupling, which arises from small energy gaps and strong spin polarization. The two mechanisms rationalize the charge carriers being long-lived in the bulk and surface phases at low and room temperature, respectively. Additionally, strong thermal fluctuations of the topological states’ magnetic moments destroy the spin-momentum locking and trigger backscattering at room temperature.

The contradictory behaviors in light harvesting and non-photochemical quenching make xanthophyll lutein the most attractive functional molecule in photosynthesis. Despite several theoretical simulations on the spectral properties and excited-state dynamics, the atomic-level photophysical mechanisms need to be further studied and established, especially for an accurate description of geometric and electronic structures of conical intersections for the lowest several electronic states of lutein. In the present work, semiempirical OM2/MRCI and multi-configurational restricted active space self-consistent field methods were performed to optimize the minima and conical intersections in and between the 1Ag–, 2Ag–, 1Bu+, and 1Bu–states. Meanwhile, the relative energies were refined by MS-CASPT2(10,8)/6-31G*, which can reproduce correct electronic state properties as those in the spectroscopic experiments. Based on the above calculation results, we proposed a possible excited-state relaxation mechanism for lutein from its initially populated 1Bu+state. Once excited to the optically bright 1Bu+state, the system will propagate along the key reaction coordinate, i.e., the stretching vibration of the conjugated carbon chain. During this period of time, the 1Bu–state will participate in and forms a resonance state between the 1Bu–and 1Bu+states. Later, the system will rapidly hop to the 2Ag–state via the 1Bu+/2Ag–conical intersection. Finally, the lutein molecule will survive in the 2Ag–state for a relatively long time before it internally converts to the ground state directly or via a twisted S1/S0conical intersection. Notably, though the photophysical picture may be very different in solvents and proteins, the current theoretical study proposed a promising calculation protocol and also provided many valuable mechanistic insights for lutein and similar carotenoids.

Hybrid organic–inorganic perovskites (HOIPs) contain light hydrogen atoms that exhibit significant nuclear quantum effects (NQEs). We demonstrate that NQEs have a strong effect on HOIP geometry and electron–vibrational dynamics at both low and ambient temperatures, even though charges in HOIPs reside on heavy elements. By combining ring-polymer molecular dynamics (MD) and ab initio MD with nonadiabatic MD and time-dependent density functional theory and focusing on the most studied tetragonal CH3NH3PbI3, we show that NQEs increase the disorder and thermal fluctuations through coupling of the light inorganic cations to the heavy inorganic lattice. The additional disorder induces charge localization and decreases electron–hole interactions. As a result, the nonradiative carrier lifetimes are extended by a factor of 3 at 160 K and 1/3 at 330 K. The radiative lifetimes are increased by 40% at both temperatures. The fundamental band gap decreases by 0.10 and 0.03 eV at 160 and 330 K, respectively. By enhancing atomic motions and introducing new vibrational modes, NQEs strengthen electron–vibrational interactions. Decoherence, determined by elastic scattering, accelerates almost by a factor of 2 due to NQEs. However, the nonadiabatic coupling, driving nonradiative electron–hole recombination, decreases because it is more sensitive to structural distortions than atomic motions in HOIPs. This study demonstrates, for the first time, that NQEs should be considered to achieve an accurate understanding of geometry evolution and charge carrier dynamics in HOIPs and provides important fundamental insights for the design of HOIPs and related materials for optoelectronic applications.

Direct electrochemical ammonia synthesis from nitric oxide is regarded as an attractive strategy to degrade NO contaminants into valuable NH3, but the scarcity of high-efficiency and durable electrocatalysts to facilitate such a process impedes its applications. The emerging transition metal 1,3,5-triethynylbenzene frameworks (TM-TEB), combining the superiorities of single-atom metal centers and graphynes, offer attractive possibilities in electrochemical catalysis. This work comprehensively studies the TM-TEB systems (for TM from the first three d-block series) and explores their potential applications as electrocatalysts for the NO reduction reaction (NORR) to value-added NH3. By developing rational strategies, the systems with TM = Cr, Pd, and Pt are successfully screened out as the NORR-to-NH3electrocatalysts with superiority catalytic activity. With further consideration of the selectivity, Cr-TEB is identified as a promising candidate with high catalytic activity and outstanding selectivity. Interestingly, both the ΔG*NOand integrated crystal orbital Hamilton population (ICOHP) of TM-NO or N–O could be suitable descriptors to demonstrate the catalytic activity of the NORR to NH3. This work has fundamental scientific implications for understanding the TM-TEB systems and demonstrates their great potential as the electrocatalysts for the NORR to NH3, which will motivate further exploration of the applications of TM-TEB in catalysis.

Organic photocatalysis has been developed flourishingly to rely on bimolecular energy transfer (EnT) or oxidative/reductive electron transfer (ET), promoting a variety of synthetic transformations. However, there are rare examples to merge EnT and ET processes rationally within one chemical system, of which the mechanistic investigation still remains in its infancy. Herein, the first mechanistic illustration and kinetic assessments of the dynamically associated EnT and ET paths were conducted for realizing the C–H functionalization in a cascade photochemical transformation of isomerization and cyclization by using the dual-functional organic photocatalyst of riboflavin. An extended single-electron transfer model of transition-state-coupled dual-nonadiabatic crossings was explored to analyze the dynamic behaviors in the proton transfer-coupled cyclization. This can also be used to clarify the dynamic correlation with the EnT-driven E→ Zphotoisomerization that has been kinetically evaluated by using Fermi’s golden rule with the Dexter model. The present computational results of electron structures and kinetic data contribute to a fundamental basis for understanding the photocatalytic mechanism of the combined operation of EnT and ET strategies, which will guide the design and manipulation for the implementation of multiple activation modes based on a single photosensitizer.

The photolysis of tetrazoles has undergone extensive research. However, there are still some problems to be solved in terms of mechanistic understanding and reactivity analyses, which leaves room for theoretical calculations. Herein, multiconfiguration perturbation theory at the CASPT2//CASSCF level was employed to account for electron correction effects involved in the photolysis of four disubstituted tetrazoles. Based on calculations of vertical excitation properties and evaluations of intersystem crossing (ISC) efficiencies in the Frank–Condon region, the combination of space and electronic effects is found in maximum-absorption excitation. Two types of ISC (1ππ* → 3nπ*, 1ππ* → 3ππ*) are determined in disubstituted tetrazoles, and the obtained rates follow the El-Sayed rule. Through mapping three representative types of minimum energy profiles for the photolysis of 1,5-, and 2,5-disubstituted tetrazoles, a conclusion can be drawn that the photolysis of tetrazoles exhibits reactivity characteristic of bond-breaking selectivity. Kinetic evaluations show that the photogeneration of singlet imidoylnitrene operates predominately over that in the triplet state, which can be confirmed by a double-well model in the triplet potential energy surface of 1,5-disubstituted tetrazole. Similar mechanistic explorations and reactivity analyses were also applied to the photolysis of 2,5-disubstituted tetrazole to unveil fragmentation patterns of nitrile imine generation. All computational efforts allow us to better understand the photoreactions of disubstituted tetrazoles and to provide useful strategies for regulating their unique reactivity.

Identifying photolysis and photothermolysis during a photochemical reaction has remained challenging because of the highly non-equilibrium and ultrafast nature of the processes. Using state-of-the-art ab initio adiabatic and nonadiabatic molecular dynamics, we investigate N2O photodissociation on the reduced rutile TiO2(110) surface and establish its detailed mechanism. The photodecomposition is initiated by electron injection, leading to the formation of a N2O–ion-radical, and activation of the N2O bending and symmetric stretching vibrations. Photothermolysis governs the N2O dissociation when N2O–is short-lived. The dissociation is activated by a combination of the anionic excited state evolution and local heating. A thermal fluctuation drives the molecular acceptor level below the TiO2band edge, stabilizes the N2O–anion radical, and causes dissociation on a 1 ps timescale. As the N2O–resonance lifetime increases, photolysis becomes dominant since evolution in the anionic excited state activates the bending and symmetric stretching of N2O, inducing the dissociation. The photodecomposition occurs more easily when N2O is bonded to TiO2through the O rather than N atom. We demonstrate further that a thermal dissociation of N2O can be realized by a rational choice of metal dopants, which enhance p–dorbital hybridization, facilitate electron transfer, and break N2O spontaneously. By investigating the charge dynamics and lifetime, we provide a fundamental atomistic understanding of the competition and synergy between the photocatalytic and photothermocatalytic dissociation of N2O and demonstrate how N2O reduction can be controlled by light irradiation, adsorption configuration, and dopants, enabling the design of high-performance transition-metal oxide catalysts.

Solar cells and optoelectronic devices are exposed to heat that degrades performance. Therefore, elucidating temperature-dependent charge carrier dynamics is essential for device optimization. Charge carrier lifetimes decrease with temperature in conventional semiconductors. The opposite, anomalous trend is observed in some experiments performed with MAPbI3(MA = CH3NH3+) and other metal halide perovskites. Using ab initio quantum dynamics simulation, we establish the atomic mechanisms responsible for nonradiative electron–hole recombination in orthorhombic-, tetragonal-, and cubic MAPbI3. We demonstrate that structural disorder arising from the phase transitions is as important as the disorder due to heating in the same phase. The carrier lifetimes grow both with increasing temperature in the same phase and upon transition to the higher-temperature phases. The increased lifetime is rationalized by structural disorder that induces partial charge localization, decreases nonadiabatic coupling, and shortens quantum coherence. Inelastic and elastic electron–vibrational interactions exhibit opposite dependence on temperature and phase. The partial disorder and localization arise from thermal motions of both the inorganic lattice and the organic cations and depend significantly on the phase. The structural deformations induced by thermal fluctuations and phase transitions are on the same order as deformations induced by defects, and hence, thermal disorder plays a very important role. Since charge localization increases carrier lifetimes but inhibits transport, an optimal regime maximizing carrier diffusion can be designed, depending on phase, temperature, material morphology, and device architecture. The atomistic mechanisms responsible for the enhanced carrier lifetimes at elevated temperatures provide guidelines for the design of improved solar energy and optoelectronic materials.

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO2(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initioadiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti3+surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate–polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp3C–H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electron-withdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

Nonlinear optical (NLO) crystals are the key materials in modern laser technology and science because of their intrinsic capability to convert the wavelength of the light source. The search for new NLO materials is still very active in both scientific and industrial communities. Machine learning (ML) becomes a powerful tool to explore new candidates of NLO materials and to reveal the underlying relationship between structures and properties. In this work, we have proposed multilevel features that are relevant to the atomic properties, the characters of fundamental structural groups, and the crystal structures to describe inorganic NLO crystals for machine learning. The first-level and second-level descriptors can be obtained based on chemical compositions of crystals without prior knowledge about crystal structures. Several ML classifiers have been optimized using a database that consists of hundreds of NLO crystals to identify the samples with desired birefringence (Δn) and second-order nonlinear coefficients (dij). In particular, almost all of the ML models that only involve the first-level and second-level features, called as the crystal-structure-free model, exhibit good classification performance. It is still far from perfect but suitable to act as a filter in the first step of high-throughput materials discovery. Using the optimized ML models, feature importance analyses and virtual screening processes have been performed to understand the relationship between the features and targeted properties and to extract the statistical pictures on elements and fundamental structural groups. Several unexplored crystals are also picked out as ML-proposed candidates, and three of them are suggested as new potential NLO materials based on further first-principle calculations. The present ML models are expected to accelerate the inverse design for new NLO crystals with desired properties.

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)2PbI4with 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)2PbI4that 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)2PbI4may be generalized to similar organic–inorganic hybrid 2D perovskites having heavy Pb atoms.

Metal halide perovskites exhibit enhanced photoluminescence and long-lived carriers in experiments under strain. Using ab initio nonadiabatic molecular dynamics, we demonstrate that compressive and tensile strain can eliminate charge recombination centers created by defect states, by shifting traps from bandgap into bands. A compressive strain enhances coupling of Pb-s and I-p orbitals, pushes the valence band (VB) up in energy, and moves the trap state due to iodine interstitial (Ii) into the VB. The strain distorts the system and breaks the I-dimer responsible for the Iitrap. A tensile strain increases Pb–Pb distance, weakens overlap of Pb-p orbitals, and pushes the conduction band (CB) down in energy. The trap state created by replacement of iodine with methylammonium (MAI) is moved into the CB. Application of strain to the defective systems not only eliminates midgap traps but also creates moderate disorder that reduces overlap of electron and hole wave functions, activates phonon modes accelerating coherence loss within the electronic subsystem, and extends carrier lifetimes even beyond those in pristine MAPbI3. Our investigation rationalizes the high performance of perovskites solar cells under strain and reveals how strain passivates Iiand MAIdefects in MAPbI3, providing a new nonchemical strategy for defect control and engineering.

Anatase TiO2is an intensely investigated photocatalytic material due to its abundance and chemical stability. However, it suffers from weak light harvesting and low photocatalytic efficiency. Experiments show that light absorption and photocatalytic properties can be enhanced simultaneously by TiO2doping with well-dispersed Cu atoms, forming a single-atom catalyst (Cu/TiO2) that can be used for solar water splitting and other applications. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrate that Cu/TiO2is inactive before light irradiation due to rapid electron–hole recombination via both shallow and deep traps. Surprisingly, the shallow trap is more detrimental to the Cu/TiO2performance than the deep trap because it couples better to free carriers. After light irradiation, leading to electron transfer and Cu/TiO2protonation, the shallow trap is eliminated, and a local distortion around the Cu atom stabilizes the deep trap state on the Cu d-orbital, decoupling it from free charges and giving rise to high photocatalytic hydrogen generation activity. We further demonstrate that the photocatalytic performance of Cu/TiO2can be enhanced by spin selection, achievable experimentally via optical intersite spin transfer or chiral semiconductor coating. Both H adsorption and spin selection enhance charge carrier lifetimes by an order of magnitude. The spin selection mechanism does not require formation of the H species, which necessitates concurrent sources of electrons and protons and which is intrinsically unstable because water splitting involves frequent proton shuffling. Our results rationalize the experimental observations at the atomistic level, provide mechanistic insights into operation of single atom photocatalysis, and demonstrate that spin selection can be used to develop advanced and efficient systems for solar energy conversion.

Singlet fission (SF), the conversion of one high-energy singlet to two low-energy triplets, provides the potential to increase the efficiency of photovoltaic devices. In the SF chromophores with C2hsymmetry, exemplified by polyenes, singlet-to-triplet conversion generally involves a low-lying 21Agdark state, which serves as either a multiexciton (ME) intermediate to promote the SF process or a parasitic trap state to shunt excited-state populations via internal conversion. This controversial behavior calls for a deep understanding of dark-state-related photophysics involving the higher-lying singlet state. However, the optical “dark” and “transient” nature of these dark states and strong correlation feature of double exciton species make their characterization and interpretation challenging from both experimental and computational perspectives. In the present work combining transient spectroscopy and multireference electronic structure calculations (XDW-CASPT2), we addressed a new photophysical model, i.e., a high-lying 31Agdark-state-mediated ultrafast SF process in the benzodipyrrolidone (BDPP) skeleton. Such a 31Agdark state with distinctive double excitation character, described as the ME state, could be populated from the initial 11Bubright state on an ultrafast time scale given the quasi-degeneracy and intersection of the two electronic states. Furthermore, the suitable optical band gap and triplet energy, high triplet yield, and excellent photostability render BDPP a promising SF candidate for photovoltaic devices. These results not only enrich the arsenal of SF materials but also shed new insights into the understanding of dark-state-related photophysics, which could promote the development of new SF-active materials.

Understanding of photoinduced interfacial carrier dynamics in organic-transition metal dichalcogenides heterostructures is very important for the enhancement of their potential photoelectronic conversion efficiencies. In this work we have used density functional theory (DFT) calculations and DFT-based fewest-switches surface-hopping dynamics simulations to explore the photoinduced hole transfer and subsequent nonadiabatic electron–hole recombination dynamics taking place at the interface of pentacene and MoS2in pentacene@MoS2. Upon photoexcitation the electronic transition mainly occurs on the MoS2monolayer, which corresponds to moving an electron to the MoS2conduction band. As a result, a hole is left in the valence band. This hole state is energetically lower than certain occupied states of the pentacene molecule; thus, the interfacial hole transfer from MoS2to pentacene is favorable in energy. In terms of nonadiabatic dynamics simulations, the hole transfer time to the HOMO–1 state of the pentacene is estimated to be about 600 fs; however, the following hole relaxation process from HOMO–1 to HOMO takes much longer time of ca. 15 ps due to the large energy gap between HOMO–1 and HOMO. Moreover, our results also show that the subsequent radiationless recombination process between the hole transferred to the pentacene molecule and the remaining electron on the MoS2CBM needs about 10.2 ns. The computational results shed important mechanistic insights on the interfacial carrier dynamics of mixed-dimensional pentacene@MoS2. These insights could help to design excellent interfaces for organic-TMDs heterostructures.

Chlorine oxides play crucial roles in ozone depletion, and the final oxidation steps of chlorine oxide potentially result in the formation of chloric acid (HClO3) or perchloric acid (HClO4). Herein, the solvation and reactive uptake of three stable isomers of chlorine trioxide (Cl2O3), namely, ClOCl(O)O, ClClO3, and ClOOOCl, at the air–water interface were investigated using classical and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) coupled with advanced free energy methods. Two distinct mechanisms were revealed for the hydrolysis of ClOCl(O)O and ClClO3: molecular and ionic mechanisms. A comparison of the computed free-energy profiles for the gaseous and air–water interfacial systems indicated that the air–water interface could markedly lower the free-energy barrier for ClO3−or HClO3formation while stabilizing the product state. In particular, the hydrolysis of ClClO3at the air–water interface was barrierless. In contrast, our calculations showed that the hydrolysis of ClOOOCl was very slow, indicating that ClOOOCl was inert to water at the air–water interface. This study provides theoretical evidence for the hypothesis that HClO3is a sink for chlorine oxides and for the widespread distributions of HClO3recently observed in the Arctic region.

Oxetane has been extensively studied for its applications in medicinal chemistry and as a reactive intermediate in synthesis. Experiments report a Cu-catalyzed [2 + 2] photocycloaddition of acetone and norbornene to oxetane, which is proposed to deviate from the conventional Paternò-Büchi reaction. However, its mechanism at the atomic level is not clear. In this study, we used a combination of multistate complete active space second-order perturbation theory (MS-CASPT2) and density functional theory to systematically investigate the reaction mechanism and elucidate the factors contributing to the diastereomeric selectivity. Initially, the formation of the TpCu(Norb) complex is achieved by strong interaction between tris(pyrazolyl)borate Cu(I) (TpCu) and norbornene in the ground state (S0). Upon photoexcitation, TpCu(Norb) eventually decays to the T1state, in which TpCu(Norb) attacks acetone to initiate subsequent reactions and produces final endo- or exo-oxetane products. All these reactions initially involve the C–C bond formation in the T1state thereto leading to a ring-opening intermediate. This intermediate then undergoes a nonradiative transition to the S0state, producing a five-membered ring intermediate, from which the C–O bond is formed, leading to the experimentally dominant exo-product. In contrast, the endo-oxetane formation requires a rearrangement process after the C–C bond is formed because of the large steric effects. As a consequence, the different reaction pathways generating exo- and endo-products exhibit large differences in the free-energy barriers, which results in a diastereomeric selectivity observed experimentally. Additionally, the nonradiative transition is found to play an important role in facilitating these reaction steps. The present computational study provides valuable mechanistic insights into Cu-catalyzed photocycloaddition reactions.

Variational wave function ansätze are at the heart of solving quantum many-body problems in physics and chemistry. Previous designs of hardware-efficient ansatz (HEA) on quantum computers are largely based on heuristics and lack rigorous theoretical foundations. In this work, we introduce a physics-constrained approach for designing HEA with rigorous theoretical guarantees by imposing a few fundamental constraints. Specifically, we require that the target HEA to be universal, systematically improvable, and size-consistent, which is an important concept in quantum many-body theories for scalability but has been overlooked in previous designs of HEA. We extend the notion of size-consistency to HEA and present a concrete realization of HEA that satisfies all these fundamental constraints while only requiring linear qubit connectivity. The developed physics-constrained HEA is superior to other heuristically designed HEA in terms of both accuracy and scalability, as demonstrated numerically for the Heisenberg model and some typical molecules. In particular, we find that restoring size-consistency can significantly reduce the number of layers needed to reach a certain accuracy. In contrast, the failure of other HEA to satisfy these constraints severely limits their scalability to larger systems with more than 10 qubits. Our work highlights the importance of incorporating physical constraints into the design of HEA for efficiently solving many-body problems on quantum computers.

The arylazopyrazole 3pzH as a novel photoswitch exhibits quantitative switching and high thermal stability. In this work, combined electronic structure calculations and ab initio multiple spawning (AIMS) dynamic simulations were performed to systemically investigate the cis↔ transphotoisomerization mechanism and the chiral preference after photoexcitation of 3pzH to the first excited singlet state (S1). Unlike most of the azoheteroarene photoswitches reported previously, many twisted and T-shaped cisisomers were found to be stable for 3pzH in the S0state, owing to the moderate interaction between the hydrogen atom and π electrons of the aromatic ring. Two twisted cisisomers with different chirality ((M)-Z1and (P)-Z1), the most stable T-shaped cisisomer ((T)-Z2), and the most stable planar transisomer (E2) were selected as the initial structures to carry out the AIMS nonadiabatic dynamic simulations. Following excitation to the S1state, all of the cisisomers decayed to conical intersection (CI) regions via the same bicycle pedal mechanism, while the evolution of the transisomers to their CI regions was achieved via rotation around the N═N bond. More importantly, chiral preferences were found for the twisted cisisomers in the S1state through the AIMS dynamic simulations due to the steric effect and static electronic repulsion. Notably, chirality was also observed in S1isomerization starting from the planar E2isomer because of the dynamic effect. After the nonadiabatic transition to the S0state, the bicycle pedal mechanism was found to play a crucial role in cis↔ transphotoisomerization. The simulated photoisomerization productivities were generally consistent with past experimental observations. Our calculations not only uncover the underlying reason for the excellent photoswitching properties of 3pzH but also enrich the knowledge of photoisomerization for azoheteroarene photoswitches, which will surely benefit their rational design.

Machine learning is capable of effectively predicting the potential energies of molecules in the presence of high-quality data sets. Its application in the construction of ground- and excited-state potential energy surfaces is attractive to accelerate nonadiabatic molecular dynamics simulations of photochemical reactions. Because of the huge computational cost of excited-state electronic structure calculations, the construction of a high-quality data set becomes a bottleneck. In the present work, we first built two data sets. One was obtained from surface hopping dynamics simulations at the semiempirical OM2/MRCI level. Another was extracted from the dynamics trajectories at the CASSCF level, which was reported previously. The ground- and excited-state potential energy surfaces of ethylene-bridged azobenzene at the CASSCF computational level were constructed based on the former low-level data set. Although non-neural network machine learning methods can achieve good or modest performance during the training process, only neural network models provide reliable predictions on the latter external test data set. The BPNN and SchNet combined with the Δ-ML scheme and the force term in the loss functions are recommended for dynamics simulations. Then, we performed excited-state dynamics simulations of the photoisomerization of ethylene-bridged azobenzene on machine learning potential energy surfaces. Compared with the lifetimes of the first excited state (S1) estimated at different computational levels, our results on the Eisomer are in good agreement with the high-level estimation. However, the overestimation of the Zisomer is unimproved. It suggests that smaller errors during the training process do not necessarily translate to more accurate predictions on high-level potential energies or better performance on nonadiabatic dynamics simulations, at least in the present case.

The resonance Raman spectroscopic study of the excited state structural dynamics of 1,3-dimethyluracil (DMU), 5-bromo-1,3-dimethyluracil (5BrDMU), uracil, and thymine in water and acetonitrile were reported. Density functional theory calculations were carried out to help elucidate the ultraviolet electronic transitions associated with the A-, and B-band absorptions and the vibrational assignments of the resonance Raman spectra. The effect of the methylation at N1, N3 and C5 sites of pyrimidine ring on the structural dynamics of uracils in different solvents were explored on the basis of the resonance Raman intensity patterns. The relative resonance Raman intensities of DMU and 5BrDMU are computed at the B3LYP-TD level. Huge discrepancies between the experimental resonance Raman intensities and the B3LYP-TD predicted ones were observed. The underlying mechanism was briefly discussed. The decay channel through the S1(1nπ*)/S2(1ππ*) conical intersection and the S1(1nπ*)/T1(3ππ*) intersystem crossing were revealed by using the CASSCF(8,7)/6-31G(d) level of theory calculations.

Interface orientation between zinc phthalocyanine (ZnPc) and fullerene (C60) affects their interfacial charge separation dynamics; however, the underlying physical origin is still elusive. In this work, we have employed the time-dependent density functional theory (TDDFT) method to explore excited-state properties of ZnPc and C60heterojunctions with both face-on and edge-on configurations. Spectroscopically bright absorption is from locally excited (LE) singlet excitons within ZnPc. In the face-on configuration, LE excitons are much higher in energy than charge-transfer (CT) excitons, thereby making charge separation process favorable. However, in the edge-on configuration, LE excitons are the lowest ones and CT ones are higher in energy; thus, charge separation is not efficient. Subsequently, we have carried out TDDFT-based nonadiabatic dynamics method to simulate photoinduced exciton and charge separation dynamics of ZnPc and C60heterojunctions with both edge-on and face-on configurations. In the former, there are no exciton transfer and charge separation processes observed within 300 fs simulation time; while, in the latter, fragment-based electronic transition density matrix analysis reveals that only LE excitons |C60ZnPC*⟩ and CT excitons |C60–ZnPC+⟩ are involved. The exciton transfer from |C60ZnPC*⟩ to |C60–ZnPC+⟩ is completed within about 100 fs in which charge separation takes place with electron–hole distances increasing from 1.0 to 4.5 Å. This exciton transfer process is essentially in company with electron transfer from ZnPc to C60but almost not involving hole transfer. These gained insights not only rationalize experiments but also enrich our knowledge to design donor–acceptor orientations to optimize organic photovoltaic performance.

Exposure to oxygen and light undermines chemical stability of metal halide perovskites, while it surprisingly improves their optical properties. Focusing on CH3NH3PbI3, we demonstrate that material degradation and charge carrier lifetimes depend strongly on the oxidation state of the oxygen species. Nonadiabatic molecular dynamics simulations combined with time-domain density functional theory show that a neutral oxygen molecule has little influence on the perovskite stability, while the superoxide and the peroxide accelerate degradation by breaking Pb–I chemical bonds and enhancing atomic fluctuations. Creating electron and/or hole traps, the neutral oxygen and the superoxide decrease charge carrier lifetimes by over 1 and 2 orders of magnitude, respectively. Importantly, photoinduced reduction of oxygen to the peroxide eliminates trap states and extends carrier lifetimes by more than a factor of 2 because it decreases the nonadiabatic coupling and shortens quantum coherence. The simulations indicate that the superoxide should be strongly avoided, for example, by full reduction to the peroxide because it causes simultaneous degradation of perovskite stability and optical properties. The detailed simulations rationalize the complex interplay between the influence of atmosphere and light on perovskite performance, apply to other solar cell materials exposed to natural elements, and provide valuable insights into design of high-performance solar cells.

Recently, we have developed a multilayer energy-based fragment (MLEBF) method to describe excited states of large systems in which photochemically active and inert regions are separately treated with multiconfigurational and single-reference electronic structure method and their mutual polarization effects are naturally described within the many-body expansion framework. This MLEBF method has been demonstrated to provide highly accurate energies and gradients. In this work, we have further derived the MLEBF method with which highly accurate excited-state Hessian matrices of large systems are efficiently constructed. Moreover, in combination with recently proposed embedded atom neural network (EANN) model we have developed a machine learning (ML) accelerated MLEBF method (i.e., ML-MLEBF) in which photochemically inert region is entirely replaced with trained ML models. ML-MLEBF is found to improve computational efficiency of Hessian matrices in particular for large systems. Furthermore, both MLEBF and ML-MLEBF methods are highly parallel and exhibit low-scaling computational cost with multiple CPUs. The present developments could motivate combining various ML techniques with fragment-based electronic structure methods to explore Hessian-matrix-based excited-state properties of large systems.

All-inorganic perovskites have great potential in photovoltaic applications and their performance is subject to phonon-assisted charge recombination dynamics. Local microstructures, such as ferroelastic domains, are considered to have a significant influence on the charge carrier lifetime in the CsPbBr3perovskite. Employing a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics simulations, we demonstrate that the formation of ferroelastic domains weakens the NA coupling and suppresses the non-radiative electron–hole recombination. This effect originates from the ferroelastic domains separating electron and hole wave functions spatially and decreasing the NA coupling by a factor of 2.4 compared to pristine CsPbBr3, delaying electron–hole recombination by a factor of 4.2. We also show that symmetry breaking enhances electronic–vibrational interactions, activating more phonon modes and accelerating quantum decoherence by 1 fs or so, which further slows recombination. Both factors compete successfully with the slightly reduced bandgap of about 0.2 eV and prolong the charge carrier lifetime to several nanoseconds. Our study advances the understanding of the atomistic mechanism for inhibited recombination in the CsPbBr3perovskite in the presence of ferroelastic domains, providing an effective route to design high-performance all-inorganic halide perovskites.

Two-dimensional (2D) heterostructures of black phosphorus (BP)/bismuth vanadate (BVO) have attracted much attention due to their potential uses in photocatalytic water splitting. However, the interfacial photoinduced electron- and hole-transfer dynamics are not explored computationally. Herein, we have used density functional theory (DFT) calculations and DFT-based fewest-switches surface-hopping dynamics simulations to investigate the light-driven electron and hole dynamics taking place at the interface of BP and the BVO(010) surface. Our results show that the BP monolayer is adsorbed on BVO(010) via van der Waals interaction. Upon irradiation, the electron transfer takes place from BP to BVO(010) within 500 fs but with two distinct processes. In the first phase, the electron transfer proceeds adiabatically and is mainly driven by atomic motions. In the second phase, the electron transfer decays very slowly. The hole-transfer dynamics from BVO(010) to BP exhibits a similar ultrafast decay in the first stage followed by a slow decay; however, there is a comparable amount of hole trapped in a BP state due to a large energy gap from its higher state. These insights may be useful for the design of novel photocatalytic water-splitting materials.

Two-dimensional (2D) Ruddlesden–Popper perovskites form a new class of solar energy materials with high performance, low cost and good stability. Nonradiative electron–hole recombination is the main source of charge and energy losses, limiting material efficiency. Experiments show that edge states in 2D halide perovskites accelerate exciton dissociation into long-lived charge carriers, improving performance. Using a combination of nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that unsaturated chemical bonds of iodine atoms at perovskite edges is the main driving force for hole localization. Chemically unsaturated Pb atoms confine electrons to a much lesser extent, because they more easily support different oxidation states and heal chemical defects. This difference between defects associated with metals and nonmetals is general to many nanoscale systems. Thermal atomic fluctuations play important roles in charge localization, even in the bulk region of 2D perovskite films, a phenomenon that is different from polaron formation. Charge localization at edges is robust to thermal excitation at ambient conditions. The separated charges live a long time, because the nonadiabatic coupling between the excited and ground states is small, under 1 meV, and quantum coherence is short, less than 10 fs. The calculations agree very well with the time-resolved optical measurements on both luminescence lifetime and line width. The detailed understanding of the excited state dynamics in the 2D halide perovskites generated by the simulations highlights the unique chemical properties of these materials, and provides guidelines for design of efficient and inexpensive solar energy materials.

The mixed quantum-classical dynamical approaches have been widely used to study nonadiabatic phenomena in photochemistry and photobiology, in which the time evolutions of the electronic and nuclear subsystems are treated based on quantum and classical mechanics, respectively. The key issue is how to deal with coherence and decoherence during the propagation of the two subsystems, which has been the subject of numerous investigations for a few decades. A brief description on Ehrenfest mean-field and surface-hopping (SH) methods is first provided, and then different algorithms for treatment of quantum decoherence are reviewed in the present paper. More attentions were paid to quantum trajectory mean-field (QTMF) method under the picture of quantum measurements, which is able to overcome the overcoherence problem. Furthermore, the combined QTMF and SH algorithm is proposed in the present work, which takes advantages of the QTMF and SH methods. The potential to extend the applicability of the QTMF method was briefly discussed, such as the generalization to other type of nonadiabatic transitions, the combination with multiscale computational models, and possible improvements on its accuracy and efficiency by using machine-learning techniques.

Herein, we have employed OM2/MRCI-based full-dimensional nonadiabatic dynamics simulations to explore the photoisomerization and subsequent excited-state decay of a macrocyclic cyclobiazobenzene molecule. Two S1/S0conical intersection structures are found to be responsible for the excited-state decay. Related to these two conical intersections, we found two stereoselective photoisomerization and excited-state decay pathways, which correspond to the clockwise and counterclockwise rotation motions with respect to the N═N bond of the azo group. In both pathways, the excited-state isomerization is ultrafast and finishes within ca. 69 fs, but the clockwise isomerization channel is much more favorable than the counterclockwise one with a ratio of 74% versus 26%. Importantly, the present work demonstrates that stereoselective pathways exist not only in the photoisomerization of isolated azobenzene (AB)-like systems but also in macrocyclic systems with multiple ABs. This finding could provide useful insights for understanding and controlling the photodynamics of macrocyclic nanostructures with AB units as the main building units.

Hybrid organic–inorganic perovskites have emerged as very successful optically active materials due to their unique electronic and chemical properties. Experiments have shown that oxygen undermines perovskite chemical stability, but enhances charge carrier lifetimes. Focusing on CH3NH3PbI3, which has become the classic material, we demonstrate how and why charge carrier lifetimes change in the presence of oxygen, by carrying out nonadiabatic molecular dynamics simulations combined with time-domain ab initiodensity functional theory. Calculations have shown that superoxide and peroxide are the common forms of oxygen interacting with CH3NH3PbI3and that oxygen most readily interacts with iodine vacancies on the perovskite surface. We establish that the iodine vacancy decreases charge carrier lifetimes, because it localizes both electrons and holes, increasing their overlap. By passivating the vacancy, the oxygen species separate electrons and holes and increase the lifetimes by more than an order of magnitude. Passivating the vacancy by water and Lewis bases, such as pyridine and thiophene, also leads to electron–hole separation. The energy gap changes only by a few percent; however, the nonadiabatic coupling becomes much weaker, and the quantum coherence time decreases significantly. The detailed time-domain atomistic analysis of the excited state dynamics rationalizes why the photogenerated charge carriers in perovskites are robust to defects and interactions with chemical species present in air, such as water and oxygen, even though they undermine perovskite chemical stability. The results can apply to other solar energy materials, which are exposed to atmospheric gases and the performance of which often depends on such exposure.

A comprehensive understanding of interfacial charge transfer dynamics is critical for improving the optoelectronic efficiency of organic–transition metal dichalcogenide heterostructures. In this work we have employed density functional theory (DFT) and developed nonadiabatic dynamics simulation approaches to study the photoinduced electron transfer dynamics at the interface of zinc phthalocyanine (ZnPc) and molybdenum disulfide (MoS2). Our present results show that ZnPc is adsorbed in a parallel orientation on MoS2through a weak van der Waals interaction. Photoirradiation excites an electron of ZnPc into its lowest unoccupied molecular orbital (LUMO), which is primarily located on ZnPc but has a tail on MoS2. This enhances the vibronic coupling between the LUMO of ZnPc and adiabatic states of MoS2, thereby benefiting the interfacial electron transfer. The LUMO of ZnPc is also calculated to be 0.27 eV higher than the conduction band minimum (CBM) of MoS2so that the electron transfer from ZnPc to MoS2is thermodynamically favorable. Further nonadiabatic dynamics simulations verify such ultrafast electron transfer and estimate its time scale of ca. 10 fs. In this process, the low-frequency out-of-plane vibration of MoS2, and low- and high-frequency in-plane and out-of-plane vibrations of ZnPc are found to play an important role in regulating this interfacial electron transfer. In-depth analysis also reveals that atomic motion induced changes of adiabatic states is a dominant factor leading to such ultrafast interfacial electron transfer. These insights could be useful for understanding charge transfer processes at interfaces of heterostructures.

Thermally activated delayed fluorescence (TADF) phenomena have been found in many organometallic complexes, but Au(III) complexes with TADF are rarely reported, possibly due to the existence of efficient nonradiative channels for luminescence states. Recent experiments identified two cyclometalated Au(III) aryl molecules with TADF; however, the underlying photophysical and luminescence mechanisms are elusive. Here, we have employed M06 and TD-M06 methods combined with polarizable continuum model and quantum mechanics/molecular mechanics approaches to comprehensively study the excited-state structures and properties of these two Au(III) complexes in toluene solution and crystal phases, respectively. We have found that both S1and T1states are of ligand-to-ligand charge transfer character. The significant twisting between C∧N∧C and aryl groups leads to good separation and negligible overlap of the highest occupied molecular orbital and lowest unoccupied molecular orbital. This results in a pretty small S1–T1energy gap, which, in conjunction with strong spin–orbit coupling, facilitates the reverse intersystem crossing (rISC) process. In terms of the results of electronic structure calculations, we have calculated the related radiative and nonradiative rates. The forward intersystem crossing (ISC) and rISC processes are estimated to occur on the timescale of 1010s–1, which is significantly faster than the fluorescence and phosphorescence emission rates (106and 103s–1). The faster rISC process relative to the phosphorescence one enables the TADF process. The low-frequency vibrational modes are found to have important contribution to the Huang–Rhys factors and to enhance the ISC and rISC rates. Moreover, environmental effects are found to be important and cannot be completely ignored in realistic simulations. Finally, the substituted −F and −OEt groups have a small influence on geometric structures but visible effects on electronic structures and related radiative and nonradiative rates, which implies that the TADF performance of the Au(III) complexes could be further enhanced through chemical tailoring or tuning these substituting groups.

A recent experimental study reported a visible-light-mediated aerobic oxidative coupling reaction of phenol with alkynes that produces hydroxyl-functionalized aryl ketones using inexpensive CuCl as catalyst under mild conditions. Here we apply the complete active space self-consistent field (CASSCF) method and multistate second-order perturbation (MS-CASPT2) theory in combination with density functional theory (DFT) to systematically explore the entire photocatalytic reaction between phenol and phenylacetylene in acetonitrile solution in the presence of molecular oxygen and CuCl. Our main findings are as follows: (1) The visible-light-driven conversion of phenylacetylene to PhCCCu(I) occurs thermally because of efficient excited-state deactivation to the S0state. (2) The single electron transfer from PhCCCu(I) to molecular oxygen that leads to the PhCCCu(II) cation takes place in the T1state after an efficient S1→ T1intersystem crossing. (3) During the initial oxidation of phenol, molecular oxygen prefers to attack the para position of the phenol radical intermediate to produce 1,4-benzoquinone, which further reacts with PhCCCu(II) to generate para-hydroxyl-substituted aryl ketones; this is the origin of the experimentally observed regioselectivity. (4) The C≡C bond of the phenylacetylene moiety is not activated by the triplet-state single electron transfer from PhCCCu(I) to molecular oxygen but is cleaved at a later stage, in the [2+2] cycloaddition between PhCCCu(II) and 1,4-benzoquinone. (5) The substrate phenol plays an active role in several hydrogen transfer and decarboxylation reactions; the barriers to these phenol-assisted reactions are lower than those for the corresponding direct or water-assisted reactions, which explains the experimental finding that adding water does not enhance the photocatalytic reaction yield. In summary, while supporting the general features of the experimentally proposed mechanism, our computational study provides detailed mechanistic insights that should be useful for understanding and further improving visible-light-induced copper-catalyzed coupling reactions.

Two-dimensional transition metal dichalcogenides (TMDs), such as WS2, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron–hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS2and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS2/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor–acceptor coupling. The photoexcitation of the WS2leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.15 eV of energy within 1 ps. The strong charge–phonon coupling and a broad range of phonon modes involved in electron dynamics are responsible for the faster electron transfer than the hole transfer. The electron–hole recombination across the WS2/QD interface occurs in several 100 ps, ensuing a long-lived charge-separated state. Particularly, the hole transfer is threefold magnitude faster than the electron–hole recombination inside QD, ensuing that QD can be an excellent light-harvester. The detailed atomistic insights into the photoinduced charge and energy dynamics at the WS2/QD interface provide valuable guidelines for the optimization of solar light-harvesting and photovoltaic efficiency in modern nanoscale materials.

Nitric oxide (NO) is an important air pollutant. Its chemical conversion to ammonia (NH3) and water (H2O) molecules has recently attracted a lot of experimental attention. In this work, we have employed a periodic density functional theory method combined with a slab model to study the catalytic reaction of NO adsorbed on a hydroxylated rutile TiO2(110) surface. We have obtained two favorable NO adsorption structures: in the first one, the terminal N atom is bonded with a Ti5csurface atom (NadO); in the second one, both the N and O atoms are bonded with two nearby Ti5csurface atoms (NadOad). Interestingly, NadOadbecomes more stable than NadO with the increasing coverage of hydroxyl groups, i.e., more than three hydroxyl groups in our slab model, which demonstrates that hydroxyls can seriously influence surface electronic structures and, thus, surface catalysis. Mechanistically, we have found that the N–O bond should be weakened prior to its dissociation. In the NadO adsorption structure, this weakening is achieved through a hydrogen atom transfer to the N atom of the NO molecule; in the NadOadadsorption structure, this N–O bond is already activated upon adsorption on the surface. After the N–O bond is broken, a series of hydrogen atom transfers to either the N or O atom take place, which eventually produces the final products. Our present computational results provide important mechanistic insights into NO removal from TiO2surfaces.

In the present work, the quantum trajectory mean-field (QTMF) approach is numerically implemented by ab initio calculation at the level of the complete active space self-consistent field, which is used to simulate photoisomerization of acetylacetone at ∼265 nm. The simulated results shed light on the possible nonadiabatic pathways from the S2state and mechanism of the photoisomerization. The in-plane proton transfer and the subsequent S2→ S1transition through the E-S2/S1-1intersection region is the predominant route to the S1state. Meanwhile, rotational isomerization occurs in the S2state, which is followed by internal conversion to the S1state in the vicinity of the E-S2/S1-2conical intersection. As a minor pathway, the direct S2→ S1→ S0transition can take place via the E-S2/S1/S0three-state intersection region. The rotamerization in the S1state was determined to be the key step for formation of nonchelated enolic isomers. The final formation yield is predicted to be 0.57 within the simulated period. The time constant for the S2proton transfer was experimentally inferred to be ∼70.0 fs in the gas phase and ∼50.0 fs in dioxane, acetonitrile, and n-hexane, which is well-reproduced by the present QTMF simulation. The S1lifetime of 2.11 ps simulated here is in excellent agreement with the experimentally inferred values of 2.12, 2.13, and 2.25 ps in n-hexane, acetonitrile, and dioxane, respectively. The present study provides clear evidence that a direct ab initio QTMF approach is a reliable tool for simulating multiple-state nonadiabatic dynamics processes.

Interfacial electron transfer (ET) plays a key role in the operation of solar cells based on TiO2sensitized with organohalide perovskites, since it leads to separation of the photogenerated electrons and holes into different materials. The reported experimental ET times range by over 3 orders of magnitude, from sub-200 fs to over 300 ps. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we demonstrate that ET at a CH3NH3PbI3/TiO2interface can be complete within 100 fs, indicating that the longer time scales reflect other processes, such as charge and exciton diffusion in perovskite bulk. The electron injection is fast because the interaction between the donor and acceptor species is strong at ambient conditions. Photoexcitation directly at the interface can create a charge-separated state. Electrons generated farther away inject by a combination of adiabatic and nonadiabatic mechanisms. Thermally activated low frequency vibrational motions at the interface modulate the CH3NH3PbI3/TiO2separation, creating opportunities for chemical bonding and generating channels for adiabatic ET. Higher-frequency modes create large nonadiabatic coupling. The interaction between perovskite and TiO2is purely van der Waals at 0 K, whereas at ambient temperatures I–Ti covalent bonds can form transiently at the interface. The covalent bonding is particularly important for photoexcitation of charge-separated states and adiabatic ET. The ET occurs prior to nonradiative electronic energy losses that can lead to charge trapping and recombination. The ultrafast interfacial charge separation contributes to the high efficiencies of perovskite-sensitized TiO2solar cells. The reported simulations provide a detailed time-domain atomistic description of the interfacial ET and advance our understanding of carrier dynamics in perovskite solar cells.