A Static Picture of the Relaxation and Intersystem Crossing Mechanisms of Photoexcited 2-Thiouracil

Abstract

Accurate excited-state quantum chemical calculations on 2-thiouracil, employing large active spaces and up to quadruple-ζ quality basis sets in multistate complete active space perturbation theory calculations, are reported. The results suggest that the main relaxation path for 2-thiouracil after photoexcitation should be S2 → S1 → T2 → T1, and that this relaxation occurs on a subpicosecond time scale. There are two deactivation pathways from the initially excited bright S2 state to S1, one of which is nearly barrierless and should promote ultrafast internal conversion. After relaxation to the S1 minimum, small singlet–triplet energy gaps and spin–orbit couplings of about 130 cm–1 are expected to facilitate intersystem crossing to T2, from where very fast internal conversion to T1 occurs. An important finding is that 2-thiouracil shows strong pyramidalization at the carbon atom of the thiocarbonyl group in several excited states.

Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution

“…The parent MBT bleach bands at 1318 and 1414 cm -1 recover with respective time constants of 11 ± 1 ps and 12 ± 1 ps, and thereafter (57 ± 1)% and (59 ± 1)% of the initial band depletions remain over our experimental time window. Assuming that internal conversion from the S2 state to S1 state is ultrafast and proceeds with a quantum yield of almost unity, as discussed earlier and suggested for other thione compounds, 17,19 the decay with time constant of 11 -12 ps can be ascribed to internal conversion from the S1 state to the S0 ground state through S0 (v>0) levels, where v>0 represents excited vibrational states of the ground electronic state. The time constant is dominated by vibrational cooling because the S1 state lifetime determined by TEAS is only 0.4 ps.…”

“…14,15 Ultrafast thione T1 state formation dynamics have been investigated by TEAS, as well as by ab initio electronic structure calculations and surface hopping simulations that include spin-orbit coupling. [16][17][18][19][20][21][22][23][24][25][26][27] The emerging picture is of a complicated competition between IC and efficient ISC.…”

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

“…The character of the neutral excited states at the equilibrium geometry is listed in Table I, together with vertical excitations energies and oscillator strengths, calculated at the MRCIS level of theory and compared to our recently published CASPT2 results. 13 Irrespective of the level of theory, the order of the states is the same and the S 1 state is an nπ * excitation while the S 2 is a ππ * state. In both states, an electron is excited from an orbital located at the sulphur atom and promoted to a π * orbital on the pyrimidine ring.…”

“…Instead, it is very likely that the system undergoes intersystem crossing into the energetically close T 2 state and from there internally converts into one of two T 1 minima (named "boat" and "pyramidalized" in Ref. 13). An interesting question is to investigate whether it will be possible to gain insight into this intersystem crossing process of 2TU by TRPES.…”

Visible‐Light‐Mediated Excited State Relaxation in Semi‐Synthetic Genetic Alphabet: d5SICS and dNaM

“…[5][6][7] The high photochemical stability of natural DNA is attributedt oaquick dissipation of excess energy into vibra-tionalm otion.C hemical modification of natural nucleobases has been thoroughly investigated to increaset he genetics torage potential as ad rug delivery agent in chemotherapy. [9,10] For example, many nucleobases (2-thiothymine, [11,12] 6-thioguanine [13] )a re an alternative substitute to canonical DNA and serve as anti-cancer and anti-inflammatory drugs over the last four decades. [9,10] For example, many nucleobases (2-thiothymine, [11,12] 6-thioguanine [13] )a re an alternative substitute to canonical DNA and serve as anti-cancer and anti-inflammatory drugs over the last four decades.…”