X-ray transient absorption reveals the 1Au (n\u03c0*) state of pyrazine in electronic relaxation

Valeriu Scutelnic,Anna I Krylov,Shota Tsuru,Stephen R Leone,Sonia Coriani,Michael Epshtein,Yuki Kobayashi,Eric Haugen,Mátyás Pápai,Klaus B Møller,Zheyue Yang,Tian Xue

doi:10.1038/s41467-021-25045-0

Abstract

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1B2u (ππ*) (S2) and 1B3u (nπ*) (S1) states, the participation of the optically dark 1Au (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1Au (nπ*) and 1B3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1Au (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

Highlights

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation
Mignolet et al.[23], who later simulated the time-resolved photoelectron imaging (TR-PEI) spectra, supported the conclusions of Kanno et al and Horio et al The controversy regarding the involvement of the 1Au state persists because, on the one hand, it is difficult to experimentally disentangle individual contributions of various electronic states involved in the photoinduced dynamics and, on the other hand, the energy ordering of the states depends on the level of theory used in the electronic structure calculations
We investigate the implication of the nuclear motion on the X-ray resonances by calculating the spectra of the excited states averaged over the 59 trajectories from the fewestswitches surface hopping (FSSH) simulation at 220 fs—at this time point, only two trajectories remain in the 1B2u state and the other 57 trajectories are in the lowest excited state having a mixed 1B3u/1Au character

Summary

Introduction

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. A theoretical study by Werner and coworkers[19] set forth the hypothesis that an additional dipole-forbidden 1Au(nπ*) [(6ag)−1(1au)+1] state is involved in the photoinduced dynamics of pyrazine on the 10 fs to sub-picosecond timescale. Alternative time-resolved experimental techniques capable of tracking the photoinduced dynamics of pyrazine and separating the contributions of the individual excited states are highly desirable, along with additional theoretical simulations. According to the theoretical study by Sun et al.[27] it should be possible to identify spectroscopic signatures of the 1Au state with three techniques: time- and frequency-resolved fluorescence spectroscopy, electronic two-dimensional spectroscopy, and transient absorption pump-probe spectroscopy

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Results

Conclusion