Mgr Justyna Krupa
Title of PhD thesis „Conformations and photochemistry of plant phenols and their
derivatives. Spectroscopic and experimental studies.”
Supervisor: Dr hab. Maria Wierzejewska, prof. UWr
Abstract
The main aim of the presented work was to study photoisomerization and photolysis
processes of a group of plant phenols and their derivatives. Thanks to the essential role played
by phenolic compounds in plants as very efficient antioxidant and radical-scavenging agents
as well as UV absorbers they are gaining increasing interest as multi-purpose functional
agents. Thus, it seemed important to find out conformational preferences and photolysis
pathways and to find and explain relation between structure of these species and their
photochemical activity. There were the following objects of the studies: guaiacol, isoeugenol,
eugenol, anisol, anethole and safrole.
The studies were performed by means of infrared spectroscopy coupled with low
temperature matrix isolation technique and quantum chemical calculations.
Conformational analysis performed using DFT method allowed to locate all minima
on the potential energy surfaces, to determine their geometry, stability, spectral characteristics
and abundance for all studied molecules and permit to detect experimentally the most stable
forms present in low temperature matrices.
Thermally induced conformational interconversions were detected for isoeugenol,
anethole and safrole. These transformations appeared due to rotation of allyl or propenyl
groups about C-C bond. The observed phenomena were explained on the basis of the low
values of the predicted by DFT calculations energy barriers for these processes.
The narrowband UV-induced trans-cis photoisomerization observed for isoeugenol
and anethole has been shown to be an efficient and selective way to interconvert two isomers
into each other, with conformational discrimination.
In turn, conformational transformations of safrole taking place in solid xenon allowed
for detection of the allyl group rotation upon UV, not reported so far in low temperature
matrices.
The narrowband UV irradiation technique was also applied to follow the photolysis
processes of all studies derivatives. They led to long chain ketenes, as main products, that
were observed to be produced in several different conformations. In spite of the fact that these
products were of similar type for all precursors, the ways to produce them were diverse. For
the plant phenols (guaiacol, isoeugenol, eugenol), the ketenes were a consequence of the
cleavage of the O-H bond and the breakage of the six membered rings. In anethole and anisol
molecules the ketenes were formed as the result of the methyl radical detachment from the
OCH3 group and the breakage of the six membered rings as well. In the case of safrole the
primary reaction was the cleavage of the 1,3-dioxolane moiety followed by the
decarbonylation process and next by cleavage of the cyclohexadienone derivative.