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Time-Resolved Spectroscopic Studies of the Photophysics and Photochemistry of Selected Benzoin and Benzophenone Compounds




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Time-Resolved Spectroscopic Studies of the Photophysics and Photochemistry of Selected Benzoin and Benzophenone Compounds by Yong Du
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This dissertation, "Time-resolved Spectroscopic Studies of the Photophysics and Photochemistry of Selected Benzoin and Benzophenone Compounds" by Yong, Du, 杜勇, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled TIME-RESOLVED SPECTROSCOPIC STUDIES OF THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF SELECTED BENZOIN AND BENZOPHENONE COMPOUNDS Submitted by Yong DU for the degree of Doctor of Philosophy at The University of Hong Kong in Mar 2008 Nanosecond time-resolved Resonance Raman (ns-TR ) and picosecond transient absorption (ps-TA) were performed to characterize the structure and dynamics of selected benzoin and benzophenone compounds. The dynamics and mechanisms of the excited state deactivation and deprotection processes for benzoin diethyl phosphate (BDP) and its methoxy-benzylic-ring substituted benzoin derivative (e.g., 3΄,5΄- dimethoxy benzoin diethyl phosphate which is denoted as dmBDP) in neat acetonitrile (MeCN) and 75%H O/25%MeCN solvents were studied using ns-TR and ps-TA spectroscopy experiments. These spectroscopic results combined with results from density functional theory (DFT) calculations were used to characterize the structure and vibrational frequencies so as to help identify the important intermediates involved in the excited state processes and to elucidate the relevant reaction mechanism(s). This study provided direct time-resolved evidence that for the deprotection reaction of BDP, the phenacyl localized nπ* triplet state of BDP was the common and immediate precursor for the photo-deprotection reaction. This work also observed that the triplet deprotection reaction follows different pathways in neat MeCN and in the largely water containing solvent systems. For the dmBDP molecule, the deprotection reaction occurs predominantly from the lowest excited singlet state and the primary photoproduct was identified as the lowest nπ* singlet state S of dmBDP that subsequently decays by two different pathways. One pathway for the decay of the S state of dmBDP is the singlet- triplet intersystem crossing (ISC) to form the triplet state of dmBDP. This triplet state was found to be non-reactive and subsequently returns to the ground state of dmBDP. The other pathway observed was a cyclization reaction to produce a preoxetane biradical intermediate B. This biradical intermediate B species was found to be (i) the reactive precursor to the deprotection-cyclization reaction in neat MeCN and (ii) the common precursor to the deprotection-cyclization and deprotection leading to photosolvolysis in the largely water containing solvent. A ns-TR spectroscopic investigation of the T state benzophenone (BP) reaction with 2-propanol solvent was performed. The TR spectra observed that the BP T state (nπ*) hydrogen-abstraction reaction with 2-propanol occurred on the tens of nanosecond (ns) time scale to produce a diphenyl ketyl (DPK) radical and an associated 3 dimethyl ketyl (DMK) radical. The TR spectra also observed the recombination of these two radical species via a cross coupling reaction to produce a light absorbing transient (LAT) intermediate. Comparison of the TR spectra to results from DFT calculations were used to explicitly identify the LAT intermediate as the para-DPK- DMK species. The intermediates formed from para-hydroxybenzophenone (p-hBP) in neat acetonitrile and aqueous solution after 266 nm photolysis were examined by ns-TR spectroscopy. The deprotonation reaction occurred on the p-hBP
Release date NZ
January 27th, 2017
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Country of Publication
United States
colour illustrations
Open Dissertation Press
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