Excited State Dynamics Of Aza-nucleobases And Its Derivatives

Excited state dynamics of canonical nucleobases are sensitive to molecular structure.Substitution of a carbon atom with a nitrogen atom at certain position in the nucleobases leads to the family of azabases.Azabases are an important part of DNA/RNA nucleobase derivatives.And azabases generally have unique photophysical and photochemical properties.Moreover,some kind of azabases exhibit characteristics such as anticancer and antimicrobial drugs,which makes them have a great potential application prospects in the field of biomedicine.The photosensitivity of azabases has also been reported,azabases may cause photodamage to cells medicinally,and little related research is reported.The investigation is expected to explain the effect of nitrogen atom substitution on the photochemical properties of canonical bases and provide important information for pharmaceutical research to evaluate medicine prospects of azabases.Although a few studies have been carried out,contradictions still remain and the ultrafast decay mechanism of excited azabases is far from well understood.In particular,experimental studies of azabases only have nanosecond time resolution reported so far.In this work,femto to microsecond transient absorption?TA?spectroscopy,electron paramagnetic resonance spectroscopy?EPR?and time-dependent density functional theory?TDDFT?calculations were used to study the excited state dynamics of 6-azathymine?6-AT?and its more biological relevant derivative 6-azathymidine?6-ATD?.The decay mechanism of azabases 6-AT and 6-ATD at ultra-fast time scale have been revealed.The main researches of this thesis are as follows:1.We found that there are two decay pathways in 6-AT after 266 nm excitation to the bright S₂???*?state.The first one is ultrafast internal conversion?IC?to the dark S₁?n?*?state,and then intersystem crossing?ISC?to T₁???*?state.The other is direct ISC from S₂???*?state to T₂?n?*?state,following ultrafast IC to T₁???*?state.TA spectra of the T₂?n?*?state were resolved for the first time.On the other hand,direct ISC from S₂???*?state to T₂?n?*?is hindered in 6-ATD.2.The nitrogen substitution at position 6 of thymine will greatly improve its ISC efficiency.The triplet quantum yields of 6-AT and 6-ATD are 46%and 43%,respectively,which is about 30 times of thymine.Single oxygen generation is detected using these two molecules as triple sensitizer.This means that when azabases are used as medicines,they may induce unnecessary photodamage after ultraviolet light irradiation,and cause some side effects.Therefore,the effect of 6-AT and 6-ATD as clinical medicines need to be evaluated with cautions.