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Herein we report on circularly polarized luminescence (CPL) emission originating from supramolecular chirality of organic microcrystals with a |glum| value up to 0.11. The microcrystals were prepared from highly emissive difluoroboron β‐diketonate (BF2dbk) dyes R‐1 or S‐1 with chiral binaphthol (BINOL) skeletons. R‐1 and S‐1 exhibit undetectable CPL signals in solution but manifest intense CPL emission in their chiral microcrystals. The chiral superstructures induced by BINOL skeletons were confirmed by single‐crystal XRD analysis. Spectral analysis and theoretical calculations indicate that intermolecular electronic coupling, mediated by the asymmetric stacking in the chiral superstructures, effectively alters excited‐state electronic structures and facilitates electron transitions perpendicular to BF2bdk planes. The coupling increases cosθμ,m from 0.05 (monomer) to 0.86 (tetramer) and triggers intense optical activity of BF2bdk. The results demonstrate that optical activity of chromophores within assemblies can be regulated by both orientation and extent of intermolecular electronic couplings. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Phytochrome proteins are light receptors that play a pivotal role in regulating the life cycles of plants and microorganisms. Intriguingly, while cyanobacterial phytochrome Cph1 and cyanobacteriochrome AnPixJ use the same phycocyanobilin (PCB) chromophore to absorb light, their excited‐state behavior is very different. We employ multiscale calculations to rationalize the different early photoisomerization mechanisms of PCB in Cph1 and AnPixJ. We found that their electronic S1, T1, and S0 potential minima exhibit distinct geometric and electronic structures due to different hydrogen bond networks with the protein environment. These specific interactions influence the S1 electronic structures along the photoisomerization paths, ultimately leading to internal conversion in Cph1 but intersystem crossing in AnPixJ. This explains why the excited‐state relaxation in AnPixJ is much slower (ca. 100 ns) than in Cph1 (ca. 30 ps). Further, we predict that efficient internal conversion in AnPixJ can be achieved upon protonating the carboxylic group that interacts with PCB. [ABSTRACT FROM AUTHOR]

The stationary and intersection structures on the S0 and S1 potential energy surfaces of CH3COCH2Cl have been determined by the CAS(10,8)/cc-pVDZ optimizations and their relative energies are refined by the CASPT2//CAS(10,8)/cc-pVDZ single-point calculations. Non-adiabatic molecular dynamics simulations were performed on the basis of the state-averaged CAS(10,8)/cc-pVDZ calculated energies, energy gradients, and Hessian matrix for the S0 and S1 states. It is found that the features of the S1 potential energy surface and non-adiabatic effect control the selectivity of the two α-C-C bond fissions, which provides a reasonable explanation why one α-C-C bond was observed as a primary channel and the other is ruled out even if CH3COCH2Cl is excited at 193 nm. The β-C-Cl fission is determined to be a dominant channel once the CH3COCH2Cl molecule is excited to the S1 state and the β-C-Cl:α-C-C branching ratio is estimated by the RRKM rate theory to be 15:1 at 193 nm, which is overestimated in comparison with the value of ∼11:1 inferred experimentally. The present calculation reveals that the α-C-C fission might take place in the ground electronic state as a result of the S1 → S0 internal conversion upon photolysis at 308 nm. However, the measured kinetic energy distributions of the α-C-C fission products suggest that the fission does not involve internal conversion to the ground state. To solve this issue, we need to perform non-adiabatic quantum dynamics simulation on accurate S0, S1, and S2 potential energy surfaces, which is still a challenging task currently. [ABSTRACT FROM AUTHOR]

In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S0) and 1ππ* (S2) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S2(1ππ*) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S0 state via a S2/S0 conical intersection. The ultrafast S2 → S0 nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S2(1ππ*) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states. [ABSTRACT FROM AUTHOR]

The potential energy surfaces for Cl2CO dissociation into CO+Cl+Cl in the lowest two electronic singlet states (S0 and S1) have been determined by the complete active space self-consistent field, coupled-cluster method with single and double excitations (CCSD), and equation-of-motion CCSD calculations, which are followed by direct ab initio molecular dynamics simulations to explore its photodissociation dynamics at 230 nm. It is found that the C–O stretching mode is initially excited upon irradiation and the excess internal energies are transferred to the C–Cl symmetric stretching mode within 200 fs. On average, the first and the second C–Cl bonds break completely within subsequent 60 and 100 fs, respectively. Electronic structure and dynamics calculations have thus provided a strong evidence that the photoinitiated dissociation of Cl2CO at 230 nm or shorter wavelengths is an ultrafast, adiabatic, and concerted three-body process. Since the two C–Cl bonds begin to break at the same time and the time interval between the two C–Cl bond broken fully is very short (∼40 fs), the photoinitiated dissociation of Cl2CO to CO+2Cl can be considered as the synchronous concerted process. [ABSTRACT FROM AUTHOR]

Abstract: The weak photoluminescence of silver nanoclusters prevents their broad application as luminescent nanomaterials. Recent experiments, however, have shown that gold doping can significantly enhance the photoluminescence intensity of Ag29 nanoclusters but the molecular and physical origins of this effect remain unknown. Therefore, we have computationally explored the geometric and electronic structures of Ag29 and gold‐doped Ag29−xAux (x=1–5) nanoclusters in the S0 and S1 states. We found that 1) relativistic effects that are mainly due to the Au atoms play an important role in enhancing the fluorescence intensity, especially for highly doped Ag26Au3, Ag25Au4, and Ag24Au5, and that 2) heteronuclear Au−Ag bonds can increase the stability and regulate the fluorescence intensity of isomers of these gold‐doped nanoclusters. These novel findings could help design doped silver nanoclusters with excellent luminescence properties. [ABSTRACT FROM AUTHOR]

N-alkenyl maleimides are found to exhibit spin state-specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high-level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N-alkenyl maleimide in the S1 and T1 states as well as relevant photophysical processes. It is found that in the S1 state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T1 state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo-addition reaction. Mechanistically, both S1 [5 + 2] and T1 [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S1 [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S1 intermediate, which then decays to the S0 state as a result of an S1 → S0 internal conversion. In the T1 [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T1 → S0 intersystem crossing process to the S0 state. In the S0 state, the second CC bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z- E photoisomerization mechanism and excited-state decay dynamics of two arylazopyrazole photoswitches. Two chiral S1/S0 conical intersections with associated enantiomeric S1 relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S1 excited-state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S1 relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited-state relaxation. [ABSTRACT FROM AUTHOR]

The article discusses research on gold photochemistry, particularly the photoinduced gold(I)-gold(I) chemical bonding in dicyanoaurate oligomers. It talks about gold chemistry as one of the fields of chemistry. It also offers information of the spectroscopic and excited-state properties of prototypical dicyanoaurate.

Abstract: In the present Letter, density functional theory has been used to optimize geometries of Zn2(η5-Cp*)2 (Cp*=C5Me5). The CCSD(T) calculated dissociation energies of 41.9 and 58.5kcal/mol for the metal–metal and metal–ligand bonds predict that Zn2(η5-Cp*)2 is a stable complex at room temperature. Ionic bond dominates the interaction between metal atom and ligand and a unique unit of was found to exist in the complex. The Zn–Zn bond is a σ single bond, which is further strengthened by a weak d–d interaction. [Copyright &y& Elsevier]

Photo-induced multiple body dissociation is of fundamental interest in chemistry and physics. A description of the mechanism associated with n-body (n >= 3) photodissociation has proven to be an intriguing and yet challenging issue in the field of chemical dynamics. Oxalyl chloride, (ClCO)2, is the sole molecule reported up to date that can undergo four-body dissociation following absorption of a single UV photon, with a rich history of mechanistic debate. In the present work, the combined electronic structure calculations and dynamics simulations have been performed at the advanced level, which provides convincing evidence for resolving the mechanistic debate. More importantly, synchronous and asynchronous concertedness were explored for the first time for the (ClCO)2 photodissociation, which is based on the simulated time constants for the C-C and C-Cl bond fissions. Upon photoexcitation of (ClCO)2 to the S1 state, the adiabatic C-C or C-Cl fission takes place with little possibility. The four-body dissociation to 2Cl(2P) and 2CO(1Σ) was determined to a dominant channel with its branch of ∼0.7, while the three-body dissociation to ClCO(2A′) + CO(1Σ) + Cl(2P) was predicted to play a minor role in the (ClCO)2 photodissociation at 193 nm. Both the four-body and three-body dissociations are non-adiabatic processes, which proceed in a synchronous concerted way as a result of the S1 → S0 internal conversion. There is a little possibility for two-body dissociation to occur in the S0 and S1 states. [ABSTRACT FROM AUTHOR]

In this article, structures and energiesof cyclopropenone in thelow-lying electronic states have been determined by the CASSCF andMS-CASPT2 calculations with different basis sets. Two minimum-energyconical intersections (CI-1 and CI-2) between S0and S1were obtained and their topographic characters were characterizedby the SA4-CAS(10,9) calculated energy gradients and nonadiabaticcoupling vectors. The AIMS method was used to carry out nonadiabaticdynamics simulation with ab initio calculation performed at the SA4-CAS(10,9)level. On the basis of time evolution of wave functions simulatedhere, the S1lifetime is fitted to be 125 fs with a pureexponential decay for the S1electronic population. TheCI-1 intersection is mainly responsible for ultrafast S1→S0nonadiabatic transition and the photoinduceddecarbonylation is a sequential process, where the first CCbond is broken in the S1state and fission of the secondCC bond occurs in the S0state as a result of theS1→S0internal conversion via the CI-1region. As a minor channel through the CI-2 region, the decarbonylationproceeds in an asynchronous concerted way. Effects of the S1excess energies and the S1–S0energygap on the nonadiabatic dynamics were examined, which reveals thatthe S1→S0nonadiabatic transition occurswithin a small energy gap and high-energy conical intersection regionscan play an important role. The present study provides new insightsinto mechanistic photochemistry of cyclopropenones and reveals thatthe AIMS dynamics simulation at a high-accuracy ab initio level isa powerful tool for exploring a mechanism of an ultrafast photochemicalreaction. [ABSTRACT FROM AUTHOR]