Photochemical Sciences Ph.D. Dissertations

Ultrafast Excited State Dynamics of Inorganic Molecules Related to Modern Light Harvesting Applications

Date of Award

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky (Committee Chair)

Second Advisor

Kei Nomaguchi (Other)

Third Advisor

Malcolm Forbes (Committee Member)

Fourth Advisor

H Peter Lu (Committee Member)

Abstract

Halide complexes of heavy metals attract great attention due to their applications in photonics and light harvesting (collection of solar energy through photochemical processes with subsequent conversion into chemical energy). Ultrafast spectroscopic studies deliver the molecular level understanding of very initial physicochemical processes, which often define the useful functions these complexes provide, and can be used for rationally designing the materials in which these complexes are the building blocks. We aim to investigate ultrafast excited-state reaction dynamics and photophysical/photochemical relaxation mechanisms of hexahalide complexes of heavy metals. In the first part of this work, the excited-state relaxation pathways in a Bi(III) hexahalide, BiX63–, (X = I, Br) a paradigm complex for understanding the ns2 main group metals, are studied by means of femtosecond transient absorption spectroscopy supported by nanosecond transient absorption spectroscopy, steady-state absorption/emission measurements and DFT computations. Radiationless relaxation out of the initially excited, predominantly metal-centered (MC) triply degenerate 3T1u state, populates two lower-energy states: a ligand-to-metal-charge transfer (LMCT) excited state of the 3π LMCT I(npπ)®Bi(6p) nature and a luminescent ‘trap’ 3A1u(3P0) MC state. On a nanosecond timescale, the 3π LMCT population decays through other intermediates to generate the triplet species (acetonitrile solvent), which is identified as the η2 metal ligated dihalide-bismuth adduct with the intramolecularly formed X–X bond, [(η2-X2)Bi(II)X4]3−, the product of interest for applications in solar energy conversion and storage. In the second part of this work, the ultrafast dynamics of IrCl62-, a prototype low-spin d5 metal complex, excited into intraconfigurational short-wavelength-infrared MC and visible LMCT electronic states is studied by femtosecond transient absorption spectroscopy. Upon any excitation of the complex in water or acetonitrile solutions, the excited-state molecules are found to relax into the ground electronic state via a cascade-like relaxation mechanism. Excited-state Jahn-Teller effect is a driving force for the relaxation dynamics even in this case where substantial spin-orbit interactions in the molecular system are present. Femtosecond pulsed state-selective excitation undertaken in this project is shown to be essential for unravelling the complex excited-state relaxation mechanism of the heavy metal complexes, providing new understanding of their structure and function.

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