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Title:Ultrafast extreme ultraviolet spectroscopy of transition metal dithiolate coordination complexes
Author(s):Benke, Kristopher
Director of Research:Vura-Weis, Josh
Doctoral Committee Chair(s):Vura-Weis, Josh
Doctoral Committee Member(s):Dlott, Dana D; Lu, Yi; van der Veen, Renske
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):M-edge XANES spectroscopy
ultrafast spectroscopy
transition metal photophysics
ligand noninnocence
Abstract:Transient tabletop M-edge x-ray absorption near edge structure (XANES) spectroscopy using extreme ultraviolet (XUV) light is used as a tool to interrogate the evolution of the metal center in a series of transition metal dithiolate complexes. The behavior of these molecules after absorption of light has implications for the development of catalysts and photosensitizers using earth-abundant transition metals. The cobalt dithiolene complex, [Co(bdt)2]- (bdt = 1,2-benzendithiolate), is primarily known for its ligand-noninnocence and participation in the catalytic production of hydrogen. After excitation of [Co(bdt)2]- with visible light, its relaxation dynamics are tracked with a combination of optical and transient M-edge XANES spectroscopic techniques that allow for the identification of a ligand-to-metal charge transfer (LMCT) excited state whose spin can be determined by comparison to semi-empirical ligand field multiplet calculations. The combination of optical and x-ray techniques is crucial to identifying relaxation processes that affect predominantly either the metal or the ligand. NEVPT2 calculations are used to understand its optical absorption spectrum and rationalize the timescale by which the molecule relaxes to the ground state. A set of three of cobalt tris(dithiolate) complexes with varying ligand field strength have been studied using optical transient absorption spectroscopy which shows a difference of a factor of at least ten in their excited state lifetimes. Understanding the origin of this change is important to further extending the lifetimes of photosensitizers. The sensitivity of M-edge XANES spectroscopy to the oxidation and spin state of a metal enabled the identification of the excited states involved in the relaxation of all three complexes. The contribution of a long-lived charge transfer state was ruled out and the final excited state was determined to be a 5T state. Finally, a nickel dithiocarbamate complex, Ni(dedtc)2 (dedtc = diethyldithiocarbamate, S2CNEt2) was studied as a simple analogue of a square planar nickel-centered hydrogen catalyst. Examination of this complex and the equivalent copper and zinc complexes reveal new phenomena in XUV spectroscopy, including dependence of the ligand absorbance on the metal center and observation of a ligand-based change in absorbance beneath the nickel M-edge after LMCT excitation.
Issue Date:2021-04-14
Rights Information:Copyright 2021 Kristopher Benke
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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