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. 2017 May 8;4(4):044021.
doi: 10.1063/1.4983157. eCollection 2017 Jul.

Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer

Benjamin E Van Kuiken  1 , Matthew R Ross  1 , Matthew L Strader  2 , Amy A Cordones  2 , Hana Cho  2 , Jae Hyuk Lee  2 , Robert W Schoenlein  2 , Munira Khalil  1
Affiliations

Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer

Benjamin E Van Kuiken et al. Struct Dyn. .

Abstract

Picosecond X-ray absorption (XA) spectroscopy at the S K-edge (∼2.4 keV) is demonstrated and used to monitor excited state dynamics in a small organosulfur molecule (2-Thiopyridone, 2TP) following optical excitation. Multiple studies have reported that the thione (2TP) is converted into the thiol (2-Mercaptopyridine, 2MP) following photoexcitation. However, the timescale and photochemical pathway of this reaction remain uncertain. In this work, time-resolved XA spectroscopy at the S K-edge is used to monitor the formation and decay of two transient species following 400 nm excitation of 2TP dissolved in acetonitrile. The first transient species forms within the instrument response time (70 ps) and decays within 6 ns. The second transient species forms on a timescale of ∼400 ps and decays on a 15 ns timescale. Time-dependent density functional theory is used to identify the first and second transient species as the lowest-lying triplet states of 2TP and 2MP, respectively. This study demonstrates transient S K-edge XA spectroscopy as a sensitive and viable probe of time-evolving charge dynamics near sulfur sites in small molecules with future applications towards studying complex biological and material systems.

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Figures

FIG. 1.
FIG. 1.
The tautomeric equilibrium of 2-thiopyridone (2TP) and 2-mercaptopyridine (2MP).
FIG. 2.
FIG. 2.
(a) UV/Visible spectrum of 2TP in acetonitrile absorptions at 370 nm and 280 nm. (b) Fluorescence yield-detected X-ray absorption spectrum of 2TP in acetonitrile. Two main near-edge features labeled A′ and A are observed at 2471.0 eV and 2473.4 eV.
FIG. 3.
FIG. 3.
(a) Differential XA spectra at several delay times for 2TP pumped by 400 nm pulses. (b) Time-dependence of the XA spectrum at specific energies corresponding to the growth and decay of the transient product species and (c) the recovery of the ground state features of 2TP.
FIG. 4.
FIG. 4.
Comparison of the 500 ps time-resolved XA spectrum (b) with spectra calculated by TDDFT for the S K-edge of (a) singlet 2MP and 2TP and (c) triplet states of 2MP and 2TP.
FIG. 5.
FIG. 5.
The proposed photochemistry of 2TP as viewed by time-resolved S K-edge XA spectroscopy. The 400 nm pulse populates the initial excited state, which undergoes a fast (<70 ps) intersystem crossing (ISC) to a triplet state. 2TP and 2MP triplet states show a 400 ps decay and rise, respectively. The gray arrows depict multiple possible pathways by which the triplet states could decay.
See this image and copyright information in PMC

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