Search

In the present work, the direct ab initio quantum trajectory mean‐field and molecular mechanical (QTMF/MM) nonadiabatic dynamics approach has been numerically implemented and used to simulate the photo‐induced ring‐opening process of 2(5H)‐thiophenone in CH3CN solution. Meanwhile, the subsequent processes in the S0 state have been simulated employing a quantum mechanics/molecular mechanics (QM/MM) adiabatic dynamics method that is derived from the direct ab initio QTMF/MM nonadiabatic dynamics approach. Upon irradiation at 267 nm, 2(5H)‐thiophenone is initially populated to its bright 1ππ* state. Subsequently, the ring‐opening reaction is predicted to be an ultrafast process with a time constant of about 228 fs. As a result, the ring‐opening reaction proceeds mainly along the diabatic S2(1ππ*) pathway and the solute‐solvent interaction has little influence on this initial process. Meanwhile, the 1ππ*→1πσ* transition takes place with little probability via the 1πσ*/1ππ* conical intersection and the 1πσ* excited state plays a minor role in the ring‐opening reaction, which is quite different from those reported for many heterocyclic molecules. Reformation of the parent molecule in the S0 state is determined to have a quantum yield of 33.9 % within the simulated period of 3.5 ps, which is considerably underestimated in comparison with a value of ∼60.0 % inferred experimentally. Thioaldehyde‐ketene was experimentally inferred as a product, which is confirmed by the simulations reported herein. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • In Os1 and Os2, the MLCT state character changes in the excited-state relaxation dynamics. • Inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics. • Ligand properties have significant influence on the excited-state relaxation dynamics. Abstract Photoinduced excited-state relaxation dynamics of organometallic compounds, due to their potential applications in solar cells, organic light-emitting diodes, photocatalysis, etc., have been recently explored by a wealth of ultrafast experimental techniques. However, the corresponding nonadiabatic dynamics simulations are rarely reported. In this work, the early-time excited-state relaxation dynamics of two Os(II) compounds i.e. Os(bpy) 3 and Os(bpy) 2 (dpp) (Os1 and Os2) have been investigated by our recently implemented TD-DFT based generalized surface-hopping dynamics simulation method in combination with the quantum mechanics/molecular mechanics (QM/MM) approach. The metal-to-ligand charge transfer (MLCT) excited singlet states from the Os atom to the bpy and dpp ligands are first populated in the Franck-Condon region. These initially populated excited singlet states will be converted through a series of ultrafast internal conversion and intersystem crossing processes to the lowest triplet state as a result of significant nonadiabatic and spin-orbit couplings among singlet and triplet electronic excited states. The intersystem crossing rates are estimated to be 72 and 53 fs for Os1 and Os2, respectively, which are consistent with experimental measured 100 fs and less than 50 fs. The time-dependent transition density analysis reveals that the MLCT excited states are dominant in the entire relaxation dynamics for either Os1 or Os2; however, the MLCT character changes in these processes. In Os1, the initially populated MLCT state is mainly from the Os atom to one bpy ligand, which is changed to the other two MLCT states. In Os2, the MLCT state that is mainly from the Os atom to the two bpy ligands is converted to the MLCT state from the Os to the dpp ligand. Finally, we have found that inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics of Os1 and Os2; while, the hole transfer is minor. Our results also show that the ligand properties have significant influence on the excited-state relaxation dynamics of organometallic compounds. The insights gained in the present study provide atomistic insights for early-time excited-state relaxation dynamics of Osmium(II) compounds. [ABSTRACT FROM AUTHOR]

Abstract: The photoresponsive azobenzene‐tethered DNAs have received growing experimental attention because of their potential applications in biotechnology and nanotechnology; however, little is known about the initial photoisomerization of azobenzene in these systems. Herein we have employed quantum mechanics/molecular mechanics (QM/MM) methods to explore the photoisomerization dynamics of an azobenzene‐tethered DNA duplex. We find that in the S1 state the trans–cis photoisomerization path is much steeper in DNA than in vacuo, which makes the photoisomerization much faster in the DNA environment. This acceleration is primarily caused by complex steric interactions between azobenzene and the nearby unpaired thymine nucleobase, which also change the photoisomerization mechanism of azobenzene in the DNA duplex. [ABSTRACT FROM AUTHOR]

The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S1→T1 intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C−S bond formation in the S0 state. The energy profiles for these transformations are different for the four conformers of the dark-adapted state considered. The photochemistry of the light-adapted state does not involve the triplet state: photoexcitation to the S1 state triggers C−S bond cleavage followed by recombination in the S0 state; both these processes are essentially barrierless and thus ultrafast. The present work offers new mechanistic insights into the photoresponse of flavin-containing blue-light photoreceptors. [ABSTRACT FROM AUTHOR]

Photoswitchable azobenzene cross-linkers can control the folding and unfolding of peptides by photoisomerization and can thus regulate peptide affinities and enzyme activities. Using quantum mechanics/molecular mechanics (QM/MM) methods and classical MM force fields, we report the first molecular dynamics simulations of the photoinduced folding and unfolding processes in the azobenzene cross-linked FK-11 peptide. We find that the interactions between the peptide and the azobenzene cross-linker are crucial for controlling the evolution of the secondary structure of the peptide and responsible for accelerating the folding and unfolding events. They also modify the photoisomerization mechanism of the azobenzene cross-linker compared with the situation in vacuo or in solution. [ABSTRACT FROM AUTHOR]

Bacterial bioluminescence (BL) has been successfully applied in water-quality monitoring and in vivo imaging. The attention of researchers has been attracted for several decades, but the mechanism of bacterial BL is still largely unknown due to the complexity of the multistep reaction process. Debates mainly focus on three key questions: How is the bioluminophore produced? What is the exact chemical form of the bioluminophore? How does the protein environment affect the light emission? Using quantum mechanics (QM), combined QM and molecular mechanics (QM/MM) and molecular dynamic (MD) calculations in gas-phase, solvent and protein environments, the entire process of bacterial BL was investigated, from flavin reduction to light emission. This investigation revealed that: 1) the chemiluminescent decomposition of flavin peroxyhemiacetal does not occur through the intramolecular chemical initiated electron exchange luminescence (CIEEL) or the 'dioxirane' mechanism, as suggested in the literature. Instead, the decomposition occurs according to the charge-transfer initiated luminescence (CTIL) mechanism for the thermolysis of dioxetanone. 2) The first excited state of 4a-hydroxy-4a,5-dihydroFMN (HFOH) was affirmed to be the bioluminophore of bacterial BL. This study provides details regarding the mechanism by which bacterial BL is produced and is helpful in understanding bacterial BL in general. [ABSTRACT FROM AUTHOR]