Unraveling The Mechanisms Of Photochemical Reaction Of Thiochromone And Photoinduced Electron Transfer Based Photolabile Protecting Groups

Photolabile protecting group（PPG）released the protected group via light irradiation.With some advantages of photodeprotection,such as simple operation,mild condition,and non-invasive spatiotemporal control,PPG were applied in the field of organic synthesis,biochemistry,medicine,etc.There was great interest in the development and optimization of diverse PPG for particular applications.While,due to the lack of information describing intermediates and transient species produced in photodeprotection reactions,the detailed reaction mechanism of numerous PPG were still ambiguous.In this thesis,（1,1-dioxido-4-oxo-3-phenyl-4H-thiochromen-2-yl）methoxycarbonyl-caged alcohol（TS-PPG）and photoinduced electron transfer-based PPG-protected phenylacetic acid（BP-PPG）were selected as the model compounds to investigate the photodeprotection reaction mechanisms of thiochromone derivatives based-PPG and photoinduced electron transfer-based PPG by time-resolved transient absorption spectroscopy combined with density functional theory calculation.It was shown that in the polar aprotic solvent（acetonitrile）,the excited singlet state of TS-PPG（TS-PPG（S₁））occurred intersystem crossing to produce its excited triplet state（TS-PPG（T））.TS-PPG（T）underwent a Paternò–Büchi reaction to give a biradical intermediate,which took place a C-O bond cleavage and followed by deprotonation reaction to complete the photodeprotection;in the polar protic solvent,TS-PPG（S₁）could either occur the deprotection reaction similar with the route in acetonitrile or directly undergo C-O bond heterolysis to produce a new cation intermediate,which followed by cyclization and deprotonation reactions to yield the photodeprotection reaction products.As for the BP-PPG,upon the irradiation,the generated excited singlet state occurred intersystem crossing to its excited triplet state（BP-PPG（T））,which underwent electron transfer reaction with N,N-dimethylaniline（DMA）to produce radical ion pair.The radical anion took place deprotection and gave the protected phenylacetic acid.Besides,a ketyl radical intermediate,influencing the photodeprotection reaction efficiency possibly,was detected by the time-resolved transient absorption spectroscopy,and it was either be produced through hydrogen atom transfer reaction between BP-PPG（T）and hydrogen-donor solvents or generated via proton transfer reaction between radical ion pair,which was yielded by electron transfer reaction from DMA to BP-PPG（T）.The investigation of photodeprotection reaction mechanism of TS-PPG and BP-PPG not only provides the reference for the optimization of the two PPG platforms but also guides the efficient application of thiochromone derivatives based-PPG and photoinduced electron transfer-based PPG.