Femtosecond time-resolved transient absorption (fs-TA), nanosecond time-resolved transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) methods were used to study the behaviors of the transient intermediates involved in the photophysical and photochemical processes of 3-methylbenzophenone (3-MeBP), 3-(hydroxymethyl)benzophenone (m-BPOH), and 2-(1-hydroxyethyl) 9,10-anthraquinone (2-HEAQ). A particular focus of this work was to study
the unusual meta methyl activation reactions of these compounds in
water-containing solutions. Density functional theory (DFT) calculations were
conducted to help make assignments of the observed experimental transient
species and to better understand the reaction mechanisms. First, the
photophysical and photochemical reactions of m-BPOH were investigated in
selected solvents. In acetonitrile (MeCN) the formation of the triplet state of
m-BPOH, (denoted as (m-BPOH)3 ), was detected via an intersystem crossing
(ISC). In 2-propanol (IPA), (m-BPOH)3 abstracted a hydrogen atom from the
solvent molecule to form an aryl ketyl radical. In an acidic mixed aqueous
solution at pH 2, the photoredox reaction appeared to be the predominant
reaction. In a more acidic aqueous solution with [H+] =1.0 M, the photoredox
reaction faced some competition from the overall photohydration reaction.
Second, the photophysical and photochemical reactions of 2-HEAQ in MeCN,
IPA, and neutral, acid and basic aqueous solutions were studied. The ISC
process of 2-HEAQ took place in MeCN with generation of the triplet excited
state species of 2-HEAQ, (2-HEAQ)3. In IPA solvent, (2-HEAQ)3 underwent a
hydrogen abstraction with the solvent. A photoredox reaction takes place via
an initial protonation process of the AQ group that is followed by deprotonation of the methylene C-H bond in aqueous solutions within a pH
range from 2 to 10. Under a stronger acidic aqueous condition with [H+] =1.0
M, the photohydration reaction becomes the major reaction. In strong basic
solutions (pH=12) only ISC was observed to take place. The unusual
photoredox reaction takes place via protonation of the carbonyl oxygen first
followed by deprotonation of the C-H bond in the side chain for both m-BPOH
and 2-HEAQ. The protonation of the excited carbonyl oxygen group has been
widely studied. On the other hand, the deprotonation of methylene C-H bond
is unusual. Therefore, this photoredox reaction for m-BPOH and 2-HEAQ is
termed as a meta methyl activation reaction. Third, the photophysical and
photochemical reactions of 3-MeBP were explored and compared to those of
4-methylbenzophenone (4-MeBP). This work found that 3-MeBP and 4-MeBP
exhibit similar behaviors with m-BPOH and 2-HEAQ in MeCN and IPA. In
MeCN, both 3-MeBP and 4-MeBP undergo an efficient ISC process producing
triplet excited state species, (3-MeBP)3 and (4-MeBP)3, respectively. In IPA,
the (3-MeBP)3 and (4-MeBP)3 intermediates were quenched by the hydrogen
abstraction reaction with the solvent. In acidic aqueous solutions (pH 2), the
protonated carbonyl oxygen species (3-MeBPH+)3 and (4-(MeBPH+)3 are
directly observed by fs-TA spectra. In the case of 4-MeBP, a photohydration is detected and the m-(4-MeBPH2O)3 and o-(4-MeBPH2O)1 species are
observed. In contrast, an unusual meta methyl activation reaction is observed
for 3-MeBP. In a stronger acid aqueous solution ([H+] =1.0 M) the meta
methyl activation reaction becomes the predominant reaction. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/196012 |
| Date | January 2013 |
| Creators | Ma, Jiani, 马佳妮 |
| Contributors | Phillips, DL |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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