Photochemical Sciences Ph.D. Dissertations

Ultrafast photophysical and photochemical dynamics of polyhalogenated alkanes, cycloalkanes, and transition metal complexes

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky (Advisor)

Second Advisor

John Cable (Committee Member)

Third Advisor

R. Marshall Wilson (Committee Member)

Fourth Advisor

Kristen Rudisill (Other)

Abstract

This work aims at deeper understanding of chemical reaction dynamics, ultrafast photochemical and photophysical relaxation pathways, and energy redistribution and dissipation in the liquid phase on a femto-to-picosecond timescale. Breaking and making of chemical bonds, structural reorganization, and energy flow are all the essence of chemistry, and all occur on the time scale of molecular vibrations (tens of femtoseconds). Important insights about the structure of evolving molecules, relaxation phenomena, and ultrafast photochemical and photophysical population dynamics can be obtained using sub-50-fs time-resolved transient absorption spectroscopy. As a result, highly viable and/or unique information about the mechanism of relaxation dynamics and the nature of short-lived intermediates can be gained. Furthermore, ultrashort laser pulses with duration of 100 fs or shorter can help set up and detect coherent motion in vibronic excited and ground states. Coherence observed in product states provides the knowledge about electronic and nuclear motion along the relaxation path. This yields the information about the potential energy surfaces involved in the photochemical or photophysical process that occurs. The knowledge acquired from such experiments can be used as an entry point for control of femtochemical processes in solution.

The technical aspect of this work focuses on developing and optimizing the ultrafast transient absorption setup for studying ultra-rapid photochemical relaxation dynamics in the molecules of interest, namely, geminal dibromocycloalkanes and halogen-containing transition metal complexes. Excitation pulses from deep-UV to near-IR were temporally compressed using prisms compressors to generate sub-40 fs laser pulses. Also, stable white-light continuum pulses used for probing were generated in the wide spectral range of 280 – 760 nm.

This thesis is divided into two main parts. The first part provides information about ultrafast dynamics (10-fs time resolution after deconvolution) of geminal dibromocycloalkanes. The second part covers ultrafast radiationless relaxation dynamics of halogen-containing transition metal complexes (IrBr62-, IrCl62-, etc.). A particular focus in this part is on excitation into several vibronic states of the different nature (ligand-to-metal charge-transfer (LMCT) and intraconfigurational), where the lowest-excited intraconfigurational state is accessed via direct femtosecond near-IR (2000 nm) excitation.

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