Book Description

This dissertation, "Resonance Raman, Time-resolved Resonance Raman and Density Functional Theory Study of Benzoin Diethyl Phosphate, Selected P-Hydroxy and P-methoxy Substituted Phenacyl Ester Phototrigger and Model Compounds" by Wing-sum, Chan, 陳穎心, 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: RESONANCE RAMAN, TIME-RESOLVED RESONANCE RAMAN AND DENSITY FUNCTIONAL THEORY STUDY OF BENZOIN DIETHYL PHOSPHATE, SELECTED P-HYDROXY AND P-METHOXY SUBSTITUTED PHENACYL ESTER PHOTOTRIGGER AND MODEL COMPOUNDS Submitted by Wing Sum CHAN for the degree of Doctor of Philosophy at The University of Hong Kong in August 2005 Resonance Raman (RR) spectroscopy and density functional theory (DFT) calculations were done to characterize the structure and dynamics of selected phototrigger and model compounds. Nanosecond time-resolved resonance Raman (ns-TR ) spectra of p-hydroxyphenacyl diethyl phosphate (HPDP), 4-methxoyphenacyl diethyl phosphate (MPDP) and its model compound 4''-methoxyacetophenone (MPA) in acetonitrile and in the water/acetonitrile mixed solvents are reported. The results from the DFT calculations and the ns-TR spectra indicate hydrogen-bonding leads to substantial modification of both the electronic properties and structural conformation of the triplet state MAP molecule. The free triplet MAP molecule was found to have a slightly twisted geometry with a delocalized ππ* character whereas the H-bonded MAP complex has a planar geometry with a significant ring localized biradical ππ nature. A strong solvent effect on the triplet quenching dynamics was observed for the HPDP molecule. This in conjunction with the leaving group dependence of the triplet decay observed in HPDP and related molecules are consistent with a solvent assisted triplet heterolytic cleavage pathway for the HPDP photodeprotection reaction. Intermolecular hydrogen-bonding of the HPDP molecule with the solvent water molecules appears to be essential to drive its cleavage or deprotection reaction. A spiroketone intermediate was proposed to be an intermediate after the deprotection reaction of HPDP and before the rearrangement process that results in the formation of the p-hydroxyphenylacetic acid (HPAA) final product. The results reported here provide important kinetics and structural data for the photodeprotection reactions of p-hydroxy and p-methoxy phenacyl ester phototriggers. This new data provides an improved understanding of the reaction mechanism occurring after photolysis of HPDP and related phototrigger compounds in various solvents. The RR spectra of benzoin diethyl phosphate (BDP) were obtained in acetonitrile solvent with excitation to its nπ* and ππ* excited states. DFT calculations were done to determine the structure and vibrational frequencies for the ground state and the excitation energies and molecular orbitals for the lower excited states of the BDP molecule. The DFT results were used to assign the observed RR bands and for comparison and interpretation of the resonance enhancement of the experimental RR spectra. It was found that Raman modes exhibiting strong enhancement are mainly related to the benzoinyl moiety for both excited states. The results for BDP indicate there is only a relatively small displacement between the nπ* excited state and the ground state but a larger displacement between the ππ* state and the ground state. The absence of RR bands in the RR spectra related to the cleavage motion of the diethyl phosphate group rules out any direct photodissociation pathway for the BDP photodeprotection