Internal conversion and intersystem crossing dynamics of uracil upon double thionation: a time-resolved photoelectron spectroscopy study in the gas phase

Abstract

<p>Gas-phase photophysics of 2,4-dithiouracil studied by time-resolved photoelectron spectroscopy.</p>

The Effect of Methylation on the Triplet-State Dynamics of 2-Thiouracil: Time-Resolved Photoelectron Spectroscopy of 2-Thiothymine

“…While the photophysics of the thiobases in the solution phase have been studied extensively, ,,, gas-phase experiments to date have primarily focused on thiouracils. − These studies indicate significant differences between the solution-phase and gas-phase triplet-state dynamics. , In solution, the two T 1 minima are nondegenerate due to a stabilization of T 1 boat with respect to T 1 op‑S , although this was recently questioned by experimental evidence for a T 1 double-well potential consistent with the gas phase . The TRPES study presented here extends the earlier gas-phase work to thiothymine and further interrogates the double-well topography of the triplet state associated with the 2-thionation of uracil and thymine.…”

“…Furthermore, the T 1 electronic character and coordinate promoting ISC to and from T 1 are affected by the substituent position . This was reflected in experimental TRPES studies on a series of thiouracils, − which demonstrated tens to hundreds of picoseconds ISC from T 1 op‑S back to the ground state in the case of 2-thiouracil (2TU); in contrast, 4-thiouracil (4TU) and 2,4-dithiouracil (DTU) are subject to nanosecond trapping on the lowest triplet state, i.e., T 1 boat . Barbatti et al extended the idea to other thiobase derivatives and proposed a more general two-step mechanistic model with rate-determining steps of either a slow barrier crossing between the two minima or fast ISC to the GS.…”

A Deep Understanding on the Effective Generation of Twisted Intramolecular Charge Transfer by Protonation in Thiazolo[5,4-d]thiazole Derivatives

“…To gain insight into the detailed time evolution information on the ESIPT triggered TICT process, the AIMD simulations based on the TD-DFT calculations was conducted to verify the time-scaled proton transfer and the bond rotation of the DP-DA-TzTz molecule (Figure a) . Many experiments showed that the solvent-dependent TICT processes of different complexes, which TICT in the polar solvent is more obvious than in the gas phase or nonpolar solvents, therefore the methanol solvent was selected. − After the energy equilibrating of the system in 30 fs, the proton transfer smoothly starts from 330 to 361 fs (Region I) within the dual potential channels as N1–H2–N3 and N4–H5–N6, respectively. However, only one channel can completely form the ESIPT process with the increase of the Mayer bond order in H2–N3 as well as the reduction and disappearance of the Mayer bond order in N1–H2 (Figure b; AIMD results, Figures S18–S23).…”

Excited States of Thio-2′-deoxyuridine Bearing an Extended π-Conjugated System: 3′,5′-Di-O-acetyl-5-phenylethynyl-4-thio-2′-deoxyuridine

“…[5][6][7][8][9][10][11][12][13][14][15][16] Whereas the relaxation process of native nucleosides in the excited singlet state is known to be ultrafast internal conversion (IC) to the ground state (the lifetime in the excited singlet state was reported to be within 1 ps), [17][18][19] that of thionucleosides is highly efficient intersystem crossing (ISC) to the triplet manifolds, triggered by the thiocarbonyl group. [5][6][7][8][9][10][11][12][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The triplet thionucleosides enable effective triplet energy transfer and/or electron transfer to generate singlet molecular oxygen ( 1 O 2 * ), superoxide and other oxygen radical species through the photosensitization reaction, [5][6][7][8][9][10][11][12] eventually causing cell death. [34]…”

“…Thionucleosides are sulfur-containing analogues of canonical nucleosides. As the replacement of an oxygen atom by a sulfur atom introduces a minimum structural change, these compounds can retain many of their original biological properties and can be readily incorporated into synthetic DNA and RNA for structural studies. − However, the introduction of a sulfur atom on the nucleobases gives thionucleosides certain unique photophysical characters, such as a prominent absorption band in the UVA region (320–400 nm), where native nucleosides are transparent. − Whereas the relaxation process of native nucleosides in the excited singlet state is known to be ultrafast internal conversion (IC) to the ground state (the lifetime in the excited singlet state was reported to be within 1 ps), − that of thionucleosides is highly efficient intersystem crossing (ISC) to the triplet manifolds, triggered by the thiocarbonyl group. − ,− The triplet thionucleosides enable effective triplet energy transfer and/or electron transfer to generate singlet molecular oxygen ( 1 O 2 * ), superoxide, and other oxygen radical species through the photosensitization reaction, − eventually causing cell death. − Therefore, such a thionucleoside can be expected to act as a photosensitizer for photochemotherapy such as photodynamic therapy (PDT) . Currently, porphyrin derivatives such as Photodyn (Hematoporphyrin) and Photofrin (Porfimer Sodium) are widely used in the actual treatment for PDT. , On the other hand, metal-free triplet photosensitizers including a thiocarbonyl group have been, in recent years, investigated as alternative PDT agents because they have the advantages of lower molecular weight and easier dosage control compared with porphyrin derivatives. ,− …”