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.

A comprehensive picture of the photoinduced non-adiabatic relaxation dynamics of trans-N-1-methyl-2-(tolylazo) imidazole (trans-MTAI) in different electronic excited states has been revealed using on-the-fly surface hopping method at the ab initio CASSCF level. The decay process is mainly driven by the twisting motion around the NN double bond upon photoexcitation to the S1 state. However, after photoexcitation to the S2 state, an ultrafast S2 → S1 non-adiabatic transition occurs in less than 150 fs accompanied by a stretching of the NN bond. Afterwards, an additional NNC bending reaction pathway is activated, competing with the photoisomerization channel. The activation of multiple reaction pathways following excitation to the S2 state is expected to provide a reasonable explanation for the wavelength-dependent isomerization property of trans-MTAI.

The photophysical properties of N-butyl-4-hydroxyl-1, 8-naphthalimide (BOH) and N-(morpholinoethyl)−4-hydroxy-1, 8-naphthalimide (MOH) in various solvents are presented and the density functional theory (DFT)/time-dependent density functional theory (TDDFT) methods at the B3LYP/TZVP theoretical level are adopted to investigate the UV–visible absorption and emission data. An efficient intermolecular excited-state proton transfer (ESPT) reaction occurs for both compounds in DMSO, methanol and water. In aqueous solution, both BOH and MOH can be used as ratiometric pH probes and perform as strong photoacids with pKa*=−2.2, −2.4, respectively. Most interestingly, in the steady-state fluorescence spectra of BOH and MOH in concentrated HCl, an unexpected blue-shifted band is observed and assumed to originate from the contact ion pair (CIP) formed by hydronium ion and the anionic form of the photoacid resulted from ESPT. Theoretical calculations are used to simulate the CIP in the case of BOH, which afford reasonable results compared with the experimental data.

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.

Subpicosecond fluorescence depletion spectroscopy (FDS) was used to measure the solvation dynamics of coumarin 153 (C153) in methanol. The FDS mechanisms were discussed. A quasi-continuous model was used to describe the solvational relaxation of excited states. The perturbations of the probe pulse on the excited sample system, including up-conversion and stimulated emission, were sufficiently discussed. For a probe molecule used in the FDS experiment, ensuring that the up-conversion perturbation can be negligible is important. FDS was found to be a good technique for measuring the solvation dynamics of C153 in methanol.Highlights► Mechanisms of subpicosecond fluorescence depletion spectroscopy. ► Quasi-continuous model was used to describe the solvational relaxation. ► The solvation dynamics of coumarin 153 in methanol has been measured.