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ABSTRACT TALIAFERRO, CHELSEA MARIE. Photophysical

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ABSTRACT TALIAFERRO, CHELSEA MARIE. Photophysical ABSTRACT TALIAFERRO, CHELSEA MARIE. Photophysical Characterization and Ultrafast Dynamics of Diimine-Containing Metal Hydrides and Carbonyls (Under the direction of Prof. Felix N. Castellano). Typically the center of photochemical reactivity, the metal hydride bond is instrumental in photocatalytic reactions involving transition metal hydrides. The ability to vary between photoacidic and photohydridic character is an extraordinary trait of some transition metal hydrides, such as [Ir(Cp*)(N^N)H]+, and allows for great tuning of photocatalytic behavior. However, this complicates the mechanistic study of these reactions. Time resolved spectroscopies allow for direct observation of photoproduct generation, therefore better elucidating the process(es) by which these reactions occur in different environments. And while transient absorption spectroscopy provides invaluable information on the underlying photophysical and photochemical processes through visualization of electronic transitions of ground and excited states, specific bonds can be probed using transient infrared spectroscopy. While the Ir-H bond typically is in a clear window, far from diimine-ligand breathing modes, the low absorptivity of these modes makes probing them through time-resolved techniques difficult. To aid in the effort of observing metal hydride stretching modes via transient infrared spectroscopy, and to better understand the underlying photophysics of Ir(III) diimine-containing hydrides, a robust dihydride with a short-lived metal-to-ligand charge transfer excited was selected for photophysical characterization. The stability of this complex allowed for direct interrogation of the Ir-H vibrational stretching modes using ultrafast transient infrared spectroscopy despite the low extinction coefficients of such vibrations. The analogous deutero complex was also synthesized and photophysically characterized, producing similar conclusions. The results presented in this document will hopefully lay the groundwork for future time- resolved infrared studies of more reactive metal hydrides. Transient infrared spectroscopy has typically been reserved for complexes containing better IR tags than M-H bonds, such as CO or CN bonds. For example, a wealth of research has been performed on Re(I) tricarbonyl complexes in which the metal-carbonyl IR absorbances are examined after excitation. The backbonding nature of these ligands allows for easy determination of the nature of the excited states as changes in electron density around the metal center lead to large changes in C≡O frequency. The research presented here focuses on the transient infrared spectra and dynamics of a class of Re(I) tricarbonyls containing a highly- conjugated diimine ligand and different ancillary ligands. This structural motif allows for transient infrared spectroscopy to be performed on two separate windows, each visualizing the changes in electron density at the metal C≡O and ligand C=O regions. Majorly ligand localized excited states lead to large changes in the ultrafast spectra of the lower-energy ligand-based carbonyls while changes to the metal C≡O window are not as drastic, further supporting the largely ligand-centered assignment with small MLCT character. The ultrafast dynamics of several Re(I) dicarbonyl complexes containing simple diimine ligands are also presented, revealing minor changes in photophysical characteristics of these complexes upon changing the electronics of the diimine ligands. As these complexes are not complicated by low-lying LC- based excited states, the results are straightforward when compared to the investigated Re(I) tricarbonyls, with excitation into the low energy MLCT absorption band leading to marked increase in C≡O antisymmetric and symmetric stretching frequencies observed using transient infrared spectroscopy. © Copyright 2019 by Chelsea Taliaferro All Rights Reserved Photophysical Characterization and Ultrafast Dynamics of Diimine-Containing Metal Hydrides and Carbonyls. by Chelsea Marie Taliaferro A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Chemistry Raleigh, North Carolina 2019 APPROVED BY: _______________________________ _______________________________ Felix N. Castellano Elena Jakubikova Committee Chair _______________________________ _______________________________ David A. Shultz Thomas Theis DEDICATION To my loving husband ii BIOGRAPHY Chelsea Taliaferro was born in Erie, Pennsylvania and grew up in Stafford, VA, where she attended Colonial Forge High School. She is the proud older sister to Kaitlin, Haley, Nic, Adele, and Ben. Grateful daughter to mother, Melissa, father, Joseph, and step-mother, Tara, she credits most of her personality and values to her large family. She received her Bachelor of Science in Chemistry from Virginia Tech in Blacksburg, VA where she met her husband, Ryland Taliaferro, and gained two more parents through Ellie and Jeff. It was at Virginia Tech where she fostered an interest in research after joining the lab of the late Prof. Karen Brewer, where she realized the beauty of synthetic and spectroscopic research of organometallic compounds. She then joined the Chemistry graduate program at North Carolina State University where she began her research under the direction of Prof. Felix Castellano, in which she was allowed the opportunity to explore the intricacies of time-resolved spectroscopies and photophysical research. iii ACKNOWLEDGMENTS I would like to thank firstly my research advisor, Phil, for his guidance and support throughout my graduate career and for giving me the freedom to explore my scientific interests. I would also like to thank Evgeny for his infinite patience and guidance through the years as I learned ultrafast techniques. To Sofia, who not only was a great mentor, but also a good friend who gave me the outlets to vent on the bad days and celebrate the good days; I still have a collection of sticky notes you gave me to remind me what is important in life. To James, who was a great source of knowledge and provided me with confidence as a researcher, especially in my earlier years, and who was a formidable opponent on board game nights. To Joe Deaton, thank you for passing along your wealth of knowledge, especially in relation to air-free chemistry and glovebox maintenance. To the rest of the Castellano group, past and present, who were an immense source of support and knowledge (also thank you for humoring my Halloween obsession). I would also like to thank the lab of Prof. Alex Miller for the insightful discussions and collaboration. To my family, by both birth and marriage. You are the net that saved me from the rocks many times over the years. Thank you for the unending support and laughter. I am so grateful to each of you for the love you give and your advice to make sure I’m living my best life. To my friends, especially Kayley, Brittany, and Haley, all beautiful scientists who are going to do great things in the world. Occasional trips to DC were necessary for my sanity and I’m so happy to have such kind and intelligent friends who supported me over the past few years. Also thank you to Thomas for being my horror-movie bud and for siding with me every time I suggest burgers and milkshakes for dinner. iv Finally, I would like to thank my husband, Ryland. You loved me the same on the days my experiments failed and I didn’t want to talk and the days I came home excitedly with new results and wanted to celebrate (usually with milkshakes). You listened to me rant about the things I’m passionate about and helped me foster all my new hobbies, no matter how silly. You listened to the same jokes over and over again because they worked the first time and have continued to pretend they land after the hundredth time. You are the most important thing to me, and I am so excited to start a new journey with you across the country! v TABLE OF CONTENTS LIST OF TABLES ......................................................................................................................... ix LIST OF FIGURES AND SCHEMES ............................................................................................x CHAPTER 1: Fundamentals of Photophysical Processes and Spectroscopy .................................1 1.1. Photophysics vs. Photochemistry ..........................................................................1 1.2. Photophysical Processes ........................................................................................1 1.2.1. Light Absorption ..........................................................................................1 1.2.2. Non-radiative Decay .....................................................................................4 1.2.3. Radiative Decay ...........................................................................................5 1.3. Spectroscopic Instrumentation ..............................................................................6 1.3.1. UV-vis Absorption (Electronic) Spectroscopy .............................................6 1.3.2. Transient Absorption (TA) Spectroscopy ..................................................10 1.3.3. FT-IR Spectroscopy ...................................................................................15 1.3.4. Ultrafast Transient Infrared (TRIR) Spectroscopy .....................................18 1.4. References ...........................................................................................................22
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