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

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

doi:10.1063/1.4983157

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.

Highlights

In recent years, picosecond and femtosecond time-resolved X-ray spectroscopies have become sensitive probes of time-evolving molecular structural dynamics in photoexcited materials and molecular systems in the condensed phase.1,2 X-ray absorption (XA) spectroscopy is unique in its ability to measure dynamics in solution with atomic specificity by probing excitations of highly localized core electrons to unoccupied valence orbitals
We extend time-resolved S K-edge spectroscopy to identify the formation and follow the dynamics of multiple photoproducts during an excited state proton transfer (ESPT) reaction in the solution
This study suggested that 2MP is formed following the initial population of the 2TP triplet state, but the authors were not able to draw conclusions regarding the electronic state of the 2MP product

Summary

Introduction

Picosecond and femtosecond time-resolved X-ray spectroscopies have become sensitive probes of time-evolving molecular structural dynamics in photoexcited materials and molecular systems in the condensed phase. X-ray absorption (XA) spectroscopy is unique in its ability to measure dynamics in solution with atomic specificity by probing excitations of highly localized core electrons to unoccupied valence orbitals. X-ray absorption (XA) spectroscopy is unique in its ability to measure dynamics in solution with atomic specificity by probing excitations of highly localized core electrons to unoccupied valence orbitals. This is unlike more common transient spectroscopies in the optical and IR regimes, which often report on delocalized electronic and vibrational states. Synchrotron and X-ray free electron laser (XFEL) facilities are being used to perform transient metal K and L-edge XA and X-ray emission measurements These experiments have proven to be effective probes of metal-ligand bonding interactions, charge and electron transfer dynamics, and spin crossover phenomena in solution..

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