Observation of the Sub-100 Femtosecond Population of a Dark State in a Thiobase Mediating Intersystem Crossing

Abstract

We combined sub-30 fs broadband transient absorption spectroscopy in the ultraviolet with state-of-the-art quantum mechanics/molecular mechanics simulations to study the ultrafast excited-state dynamics of the sulfur-substituted nucleobase 4-thiouracil. We observed a clear mismatch between the time scales for the decay of the stimulated emission from the bright ππ* state (76 ± 16 fs, experimentally elusive until now) and the buildup of the photoinduced absorption of the triplet state (225 ± 30 fs). These data provide evidence that the intersystem crossing occurs via a dark state, which is intermediately populated on the sub-100 fs time scale. Nonlinear spectroscopy simulations, extrapolated from a detailed CASPT2/MM decay path topology of the solvated system together with an excited state mixed quantum-classical nonadiabatic dynamics, reproduce the experimental results and explain the experimentally observed vibrational coherences. The theoretical analysis rationalizes the observed different triplet buildup times of 4-and 2-thiouracil.

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

“…We correlate this continuous population exchange to an increase of the ππ* lifetime in 2TU, as compared to 4TU (results in Ref. ), for which we have recently reported a sub‐100 fs lifetime, associated with the ballistic depopulation of the ππ* state . For the product EADS, the PA bands around 350 nm and between 550–700 nm (highlighted as hollow blue boxes) are in very good agreement with the calculated PAs of the lowest 3 ππ* triplet states [Min 3 ππ*(S2) and Min 3 ππ*′(S2)].…”

“…Thus, rather than a relaxation process in the bright state, the two time constants 109 and 435 fs describe the parallel decay of the two ππ* states ππ*(S4) and ππ*(S2), respectively. We note the obvious similarities with the decay mechanism of the ππ*(S4) state in 4TU, for which we have recently reported an even shorter lifetime of 76 fs associated with a fast ππ*→nπ* decay in planar ring geometry . Pumping at 270 nm, two other bright ππ* states, ππ*′(S2) and ππ*′(S4), denoted by light‐blue and red circles—are populated, the decay paths of which are shown on the right‐ and left‐hand side of Figure b.…”

“…The joint analysis of the thio‐substituted uracil derivatives 2TU, 24TU and 4TU evidences that, although the decay paths activated through excitation of the bright ππ*/ππ*′ states are characterized by different energetics, lifetimes, and molecular distortions, they exhibit a common pattern. The ππ*/ππ*′ states relax either to their corresponding singlet nπ*/nπ*′ states or directly to the triplet manifold ( 3 nπ*/ 3 nπ*′) through an efficient ISC process.…”

“…Eventually, the population is trapped in the lowest triplet state in each channel ( 3 ππ*/ 3 ππ*′). Our calculations strongly suggest the involvement of the dark nπ*/nπ*′ states in the non‐adiabatic decay, in agreement with a previous dynamical calculation and time‐resolved photoelectron spectroscopy experiments in the gas phase . Nonetheless, the fingerprint PA signals of the nπ*/nπ*′ states are either weak in intensity (e.g., 24TU; for more details see Figures S6 and S7 in the Supporting Information) or coincide with more intense PA signals coming from the ππ*/ππ*′ states and the triplet manifold (e.g., 2TU; Figure S4, Supporting Information).…”

“…The ππ*(S4) decay path for 4TU is reported in Ref. . Finally, the ππ*′(S2) transitions on 2TU and 24TU show an evident out‐of‐plane distortion of the C 2 =S 2 group along the decay path, attributed to the nodal structure of the π*′ orbital, showing a nodal plane along the C 2 =S 2 bond (Figure c), which facilitates out‐of‐plane deformations.…”