Photoelectron spectra of 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil

Abstract

Ground-and excited-state UV photoelectron spectra of thiouracils (2-thiouracil, 4-thiouracil and 2,4-dithiouracil) have been simulated using multireference configuration interaction calculations and Dyson norms as measure for the photoionization intensity. Except for a constant shift, the calculated spectrum of 2-thiouracil agrees very well with experiment, while no experimental spectra are available for the two other compounds. For all three molecules, the photoelectron spectra show distinct bands due to ionization of the sulphur and oxygen lone pairs and the pyrimidine π system. The excited-state photoelectron spectra of 2-thiouracil show bands at much lower energies than in the ground state spectrum, allowing to monitor the excited-state population in time-resolved UV photoelectron spectroscopy (TRPES) experiments. However, the results also reveal that single-photon ionization probe schemes alone will not allow monitoring all photodynamic processes existing in 2-thiouracil. Especially, due to overlapping bands of singlet and triplet states the clear observation of intersystem crossing will be hampered.

Intersystem crossing dynamics in singly substituted thiouracil studied by time-resolved photoelectron spectroscopy: Micro-environmental effects due to sulfur position

“…The orange arrow shows the probe wavelength at 401.5 nm, just below the onset of the UV absorption spectrum to minimize contributions from unwanted probe-pump signals. A two-photon process is required for photoionization of the electronically excited states given a vertical ground-state ionization energy of approximately 8.75 eV [29]. According to the computed S0-photoelectron spectrum in Ref.…”

“…[29], the lowest energy band (IP1) falls within the 8-10 eV region with contribution from sulfur localized n-(D0) and π-holes (D1). The second photoelectron band with ionizations of oxygen-localized orbitals is energetically inaccessible with the total photon energies employed in our TR-PES studies [29]. The analysis of our TR-PES spectra relies on ionization correlations between the electronically excited and ionic states.…”

“…To the best of our knowledge, no previous gas-phase time-resolved photoionization experiments of 4-TU have been reported, although vertical excitation energies and ionization energies are known from quantum chemical calculations, and the static photoelectron spectrum for the ground state ionization has been simulated [28][29][30][31]. Transient absorption spectroscopy experiments of related 4-thionated uracil and thymine derivatives in solution have shown complex photodynamics and strong solvent dependence of intersystem crossing lifetimes [9,14,[32][33][34].…”

Recent Advances and Perspectives on Nonadiabatic Mixed Quantum–Classical Dynamics

“…The nuclear ensemble approach (NEA) can be explored to simulate different steady and time-resolved spectroscopic techniques, including inhomogeneous broadening. [391][392] Usually, working as a post-processing of the dynamics results, the nuclear ensembles have been applied for simulations of a large variety of time-resolved spectra, including twodimensional, differential transmission, 393 photoelectron, 24,217,325,[389][390][394][395][396][397] ultrafast Auger, 24,398 and X-ray photo-scattering 24 spectroscopies. These developments and applications have been based on a broad range of approximations and electronic structure methods, from simple estimates of transition probabilities 24,325,387,395,[399][400][401] to involved modeling of transition moments.…”

A surface-hopping method for semiclassical calculations of cross sections for radiative association with electronic transitions

“…Field-induced couplings were the subject of more studies where the absorption spectra [35][36][37]54 or laser-induced transitions 33,[38][39][40][41] were calculated with the surfacehopping methodology. Spin-orbit and dipole couplings have been recently treated simultaneously with surfacehopping method 31,33,43,44 .…”