New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum

The deactivation process of salicylidene methylamine in the gas phase has been explored using static calculations (CASSCF, CASPT2, and CC2) and on-the-fly surface hopping dynamics simulations (CASSCF). Five minimum energy conical intersections (MECIs) were located upon the geometry optimization calculations. One corresponds to the excited state intramolecular proton transfer (ESIPT) process, and the remaining four arise from CN bond rotational motion. Our calculation results found that the molecule prefers to decay to the ground state through the four rotational motion related MECIs rather than the ESIPT related one. This mechanistic scenario is verified by the energy profiles connecting the Franck–Condon point and the MECIs at CASSCF, CASPT2, and CC2 levels. Our proposed new decay mechanism can explain the previous experimental findings of femtosecond pump–probe photoionization spectroscopy and can provide additional guidance to the rational design of photochemically switchable molecules

Effect of Methylation on the Photodynamical Behavior of Arylazoimidazoles: New Insight from Theoretical ab Initio Potential Energy Calculations and Molecular Dynamics Simulations

Arylazoimidazoles are a series of azobenzene derivatives possessing the ability to undergo photoinduced trans–cis isomerization. Their isomerization quantum yields are found to be dependent on the excitation wavelength and chemical substituents. The current work investigated the ultrafast nonadiabatic decay behaviors of three arylazoimidazoles (Pai-H, Tai-H, and Tai-Me) after being photoexcited to the S₁ and S₂ states by means of high-level ab initio potential energy calculations and on-the-fly surface hopping dynamical simulations in gas phase to explore the effect of the methylation. The results found that the Pai-H with no methylation substituents only decay along a NNC bending reaction pathway for both the S₁ and S₂ states. The Tai-H with a methylation substituent on the six-membered ring can decay along both the NNC bending and twisting motion pathways for the S₁ and S₂ states. The Tai-Me has methylation substituents on both the six- and five-membered rings prefers to decay by a twisting motion in the S₁ state, while a NNC bending channel is activated following excitation to the S₂ state. The position and numbers of methylation substituents has important influence on the dynamical behaviors of arylazoimidazoles. The current work provides fundamental knowledge of the arylazoimidazoles and will be helpful for advanced and further exploration and application

Mechanism of Fluorescence Quenching by Acylamino Twist in the Excited State for 1‑(Acylamino)anthraquinones

Nitrogen-containing anthraquinone derivatives are widely applied in vegetable fiber dyes. In this paper, the fluorescence quenching mechanism by an acylamino group twist in the excited state for the 1-(acylamino)­anthraquinones (AYAAQs) derivatives in acetonitrile is investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The calculated Stokes shift is in good agreement with the experimental data. The energy profiles show that each AYAAQs derivative reveals a barrierless twist process, indicating that the involvement of acylamino group rotation in addition to proton transfer becomes as another important coordinate in the excited state relaxation pathway. The effects of electron-substituted group promote twist process compared with 1-aminoanthraquinone (AAQ). Then, the cross points are searched by the constructed linearly interpolated internal coordinate (LIIC) pathways for AYAAQs, demonstrating that the potential energy curves of the S₁ and T₂ states intersect each other and are in accord with the El-Sayed rules. So one can conclude that the acylamino group twist and following intersystem crossing (ISC) processes are important nonradiative inactivation channel for the S₁ state of the AYAAQs derivatives, which is more prone to proton transfer process and can explain the low fluorescence efficiency. In addition, we have measured the phosphorescence spectra of AAQ, and on this basis, it can be predicted that the phosphorescence may occur for the AYAAQs derivatives