Ground state quantum chemistry, excited state dynamics, and time-resolved x-ray absorption spectroscopy of substituted thiophenols

Time-resolved x-ray absorption spectroscopy is at present rapidly gaining popularity as a powerful analytic technique for studying electronically excited molecules. The method effectively combines the site- and element-specific resolution of x rays with the capability of probing both optically bright and dark states on the femtosecond time scale. However, the high information content in corresponding experimental data usually requires support from theoretical simulations for interpretation. In the present study, we combine trajectory surface hopping (TSH) dynamics, including spin-orbit coupling, with transition potential/Δ-Kohn-Sham x-ray absorption simulations based on density functional theory to simulate time-resolved x-ray absorption spectra of 4-nitrothiophenol (4-NTP), a system well known in the field of plasmonic chemistry and catalysis. We find that 4-NTP in the gas phase undergoes rapid internal conversion via low-lying, dark nπ* states of the NO2-group after excitation to the first bright ππ* state, which can be directly tracked in the time-resolved C1s and O1s x-ray absorption spectra. Furthermore, picosecond intersystem crossing in photoexcited 4-NTP is identified, which agrees with supporting TSH simulations based on second-order algebraic-diagrammatic construction [ADC(2)] theory. We also discuss a variety of ground state quantum chemical and static UV/Vis spectroscopic properties of 4-NTP in comparison to the structurally similar 4-halogenated thiophenols (4-HAT or 4-XTP), which we aim to investigate in greater detail in a future study.