PROBING THE ULTRAFAST INTERMEDIATE STATES OF A DIVALENT CO-MN COMPLEX WITH FEMTOSECOND M-EDGE XANES

In this work we perform ultrafast transient M$_{2,3}$-edge XANES on divalent cobalt-manganese(N,N,N-tri(2-(2-pyridylamino)ethyl)amine)Cl, a heterobimetallic system with directly-interacting dual metal centers held within a non-innocent ligand scaffold. The strong metal-metal interaction facilitated by the ‘double-decker’ type ligand scaffold leads to a highly multiconfigurational electronic structure with relaxation pathways unavailable in monometallic analogues. With the ultrashort broadband probe pulse used in transient high-harmonic generation spectroscopy, we are able to perform M$_{2,3}$-edge XANES on both metal edges simultaneously with high specificity for each metal center and ligand environment. By combining transient XANES with transient UV-visible spectroscopy we have compiled a full picture of the electronic relaxation dynamics of this complex molecule. Photoinduced MM’CT is followed by distinct electronic dynamics at both metal edges and within the ligand scaffold, suggesting complex interplay of the Co, Mn, and ligand redox centers. Increased understanding of the relation of function to metal-specific photodynamics will help lay essential groundwork for the development of multimetallic catalysts with efficiencies comparable to those found in nature

Ultrafast photophysics of transition metal coordination complexes studied with femtosecond extreme ultraviolet transient absorption spectroscopy

Ultrafast femtosecond dynamics of various simple metalloporphryinates (M = Fe3+, Ni2+, Mn3+, Co2+) and a heterobimetallic complex CoMnCl(py3tren) are studied using a homebuilt extreme ultraviolet (XUV) transient absorption apparatus. XUV photons are produced via high-harmonic generation, allowing M-edge XANES measurements to be performed as an in-lab technique. This developing technique is coupled with semi-empirical ligand field multiplet (LFM) simulations to enable the application of tabletop M-edge XANES to resolve the metal-specific ultrafast relaxation mechanism of coordination complexes, complementing information gained from ligand-specific techniques such as optical transient absorption. The experimental apparatus for transient absorption pump-probe XANES of molecular thin films is described in detail. The research described in this thesis establishes M-edge spectroscopy as a powerful tool for studying the steady-state and transient electronic structure of coordination complexes. This technique has been applied to four metalloporphyrinates (FeTPPCl, NiOEP, CoOEP, and MnOEPCl); results show diverse M-edge transient spectra and kinetics that reveal the strong impact of the metal d-orbital occupation on the early time relaxation mechanism and intermediate states of these catalytically-relevant systems. This project is the first instance of using tabletop transient XUV/VUV spectroscopy on coordination complexes and furthermore highlights the importance of directly probing of the metal center in these systems. Ongoing work on the transient XANES of heterobimetallic complex CoMnCl(py3tren) (triply-deprotonated N,N,N-tri(2-(2-pyridylamino)ethyl)amine) is presented, the aim of which is to detangle the complex photophysics present in a system with directly-interacting dual metal centers within a non-innocent ligand scaffold. The relation of function to metal-specific photodynamics will help lay essential groundwork for the development of multimetallic catalysts with efficiencies comparable to those found in nature

High harmonic generation XUV spectroscopy for studying ultrafast photophysics of coordination complexes

\newcommand{\midtilde}{\raisebox{-0.25\baselineskip}{\textasciitilde}} Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M$_{2,3}$-edge excitation of atoms. Absorption of the XUV photon causes a 3p$\rightarrow$3d transition, the energy and shape of which is directly related to the element and ligand environment. This technique is thus element-, oxidation state-, spin state-, and ligand field specific. A process called high-harmonic generation (HHG) enables the production of ultrashort (\midtilde20fs) pulses of collimated XUV photons in a tabletop instrument. This allows transient XUV spectroscopy to be conducted as an in-lab experiment, where it was previously only possible at accelerator-based light sources. Additionally, ultrashort pulses provide the capability for unprecedented time resolution (\midtilde70fs IRF). This technique has the capacity to serve a pivotal role in the study of electron and energy transfer processes in materials and chemical biology. I will present the XUV transient absorption instrument we have built over the past two years, along with preliminary data and simulations of the M$_{2,3}$-edge absorption data of a battery of small inorganic molecules to demonstrate the high specificity of this ultrafast tabletop technique

RESOLVING ULTRAFAST PHOTOCHEMISTRY OF COORDINATION COMPLEXES USING HIGH HARMONIC GENERATION XANES SPECTROSCOPY

Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M$_{2,3}$ -edge excitation of atoms. Absorption of the XUV photon causes a 3p $\rightarrow$ 3d transition, the energy and multiplet of which is directly related to the element and ligand environment. This in-lab technique is thus element-, oxidation state-, spin state-, and ligand field specific and is a useful tool for the study of electron and energy transfer processes in materials and chemical biology. With the use of this technique and semi-empirical simulations, I have collected ultrafast transient M$_{2,3}$ -edge absorption data of four different metalloporphyrinates (M = Fe, Co, Ni, Mn) in order to resolve the early time relaxation mechanism of these catalytically-relevant coordination complexes with femtosecond time resolution. This is the first instance of using tabletop transient XUV/VUV spectroscopy on coordination complexes and furthermore highlights the importance of directly probing of the metal center in these systems. I will additionally present ongoing work on applying this technique to the study of heterobimetallic systems with directly-interacting dual metal centers within a non-innocent ligand scaffold. The relation of function to metal-specific photodynamics will help lay essential groundwork for the development of multimetallic catalysts with efficiencies comparable to those found in nature

OPTICALLY PUMPED RESTSTRAHLEN BAND TUNING OF WIDE BANDGAP SEMICONDUCTORS

Photoinjection studies of two wide bandgap semiconductors, 4H-SiC and GaN, are conducted with fs infrared reflectivity measurements in order to characterize how the relative penetration depths of UV-pump/IR-probe light affects the modulation of the IR reflectivity spectrum. The infrared spectrum of these materials is dominated by the high reflectivity reststrahlen band region that occurs between the longitudinal optical and transverse optical phonons. The injection of free carriers shifts this metal-like region to higher frequencies via coupling of the longitudinal optical phonon to the free carrier plasma (LOPC effect). The result of this LOPC active tuning is strongly perturbed by the charge carrier spatial distribution, and is thus highly sensitive to the means of carrier generation. We probe the effects of charge carrier spatial distribution on the photomodulated reflectivity of two promising wide bandgap semiconductors, indirect bandgap 4H-SiC and direct bandgap GaN, by comparing the transient reflectivity following photoexcitation with light of short and long penetration depths relative to the IR probe depth. This work shows sensitivity of bulk electronic properties to the charge carrier distribution that is critical to understanding the contributions of these materials to complex nanophotonic devices

Microfluidic liquid sheets as large-area targets for high repetition XFELs

The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined

HIGH HARMONIC GENERATION XUV SPECTROSCOPY FOR STUDYING ULTRAFAST PHOTOPHYSICS OF_x000d_ COORDINATION COMPLEXES

Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M$_{2,3}$-edge excitation of atoms. Absorption of the XUV photon causes a 3p$rightarrow$3d transition, the energy and shape of which is directly related to the element and ligand environment. This technique is thus element-, oxidation state-, spin state-, and ligand field specific. A process called high-harmonic generation (HHG) enables the production of ultrashort ($sim$20fs) pulses of collimated XUV photons in a tabletop instrument. This allows transient XUV spectroscopy to be conducted as an in-lab experiment, where it was previously only possible at accelerator-based light sources. Additionally, ultrashort pulses provide the capability for unprecedented time resolution ($sim$50fs IRF). This technique has the capacity to serve a pivotal role in the study of electron and energy transfer processes in materials and chemical biology. _x000d_ I will present the XUV transient absorption instrument we have built, along with ultrafast transient M$_{2,3}$-edge absorption data of a series of small inorganic molecules in order to demonstrate the high specificity and time resolution of this tabletop technique as well as how our group is applying it to the study of ultrafast electronic dynamics of coordination complexes._x000d

Sub-100 Fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

Nickel porphyrins have been extenstively studied as photosensitizers due to their long-lived metal-centered excited states. The multiplicity of the (d,d) state, and/or the rate of intersystem crossing between singlet and triplet metal-centered states, has remained uncertain due to the spin-insensitivity of many spectral probes. In this work, we directly probe the metal 3d shell occupation of nickel(II) octaethylporphyrin (NiOEP) using femtosecond M2,3-edge X-ray absorption near-edge structure (XANES). A tabletop high-harmonic source is used to perform 400 nm pump, extreme-ultraviolet probe transient absorption spectroscopy with ~100 fs time resolution. Photoexcitation produces a (π,π*) state that evolves with a time constant of 48 fs to a vibrationally hot metal-centered triplet 3(d,d) excited state with a lifetime of 595 ps. The spin sensitivity of M-edge XANES allows the 3(d,d) state to be distinguished from a potential 1(d,d) state, as shown by charge transfer multiplet simulations and comparison to triplet nickel(II) oxide. Vibrational cooling of the hot triplet state occurs over tens of ps, with minimal change in the electronic structure of the nickel(II) center. No evidence of an LMCT or MLCT intermediate state is seen within the time resolution of the instrument, suggesting that if such a state exists in NiOEP it depopulates in <25 fs. Finally, this study demonstrates the ability of table high-harmonic XUV sources to measure excited-state spin transitions in molecular transition metal complexes

The Morphoregulatory Role of Thidiazuron: Metabolomics-Guided Hypothesis Generation for Mechanisms of Activity

Thidiazuron (TDZ) is a diphenylurea synthetic herbicide and plant growth regulator used to defoliate cotton crops and to induce regeneration of recalcitrant species in plant tissue culture. In vitro cultures of African violet thin petiole sections are an ideal model system for studies of TDZ-induced morphogenesis. TDZ induces de novo shoot organogenesis at low concentrations and somatic embryogenesis at higher concentrations of exposure. We used an untargeted metabolomics approach to identify metabolites in control and TDZ-treated tissues. Statistical analysis including metabolite clustering, pattern and pathway tools, logical algorithms, synthetic biotransformations and hormonomics identified TDZ-induced changes in metabolism. A total of 18,602 putative metabolites with extracted masses and predicted formulae were identified with 1412 features that were found only in TDZ-treated tissues and 312 that increased in response to TDZ. The monomer of TDZ was not detected intact in the tissues but putative oligomers were found in the database and we hypothesize that these may form by a Diels–Alder reaction. Accumulation oligomers in the tissue may act as a reservoir, slowly releasing the active TDZ monomer over time. Cleavage of the amide bridge released TDZ-metabolites into the tissues including organic nitrogen and sulfur containing compounds. Metabolomics data analysis generated six novel hypotheses that can be summarized as an overall increase in uptake of sugars from the culture media, increase in primary metabolism, redirection of terpene metabolism and mediation of stress metabolism via indoleamine and phenylpropanoid metabolism. Further research into the specific mechanisms hypothesized is likely to unravel the mode of action of TDZ and to provide new insights into the control of plant morphogenesis.Science, Faculty ofNon UBCChemistry, Department ofReviewedFacult

Shrinking the Synchrotron : Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes

We show that the electronic structure of molecular first-row transition-metal complexes can be reliably measured using tabletop high-harmonic XANES at the metal M2,3 edge. Extreme ultraviolet photons in the 50-70 eV energy range probe 3p → 3d transitions, with the same selection rules as soft X-ray L2,3-edge absorption (2p → 3d excitation). Absorption spectra of model complexes are sensitive to the electronic structure of the metal center, and ligand field multiplet simulations match the shapes and peak-to-peak spacings of the experimental spectra. This work establishes high-harmonic spectroscopy as a powerful tool for studying the electronic structure of molecular inorganic, bioinorganic, and organometallic compounds