Investigation Of Photophysical And Photochemical Reactions Of Nitroindoles Excited States

Nitroindoles are universal bases inserted into the DNA duplex in a x-stacking mode to pair with four natural bases indiscriminately since they only destabilize the duplex slightly.Therefore,they have been widely used in polymerase chain reaction,DNA probes,primer design,etc.In this thesis,4-nitroindole（HN-4NO₂）,5-nitroindole（HN-5NO₂）,and 6-nitroindole（HN-6NO₂）were selected for the first time to study the properties and reactivities of their lowest excited triplet states（T₁）.Furthermore,the reaction mechanism between the T₁ state of 4-Nitroindole and guanosine（G）was investigated primarily.All these results aim to provide the experimental base for using nitroindoles as an in-situ photoinitiator to study the one-electron oxidation of DNA and the kinetics of electron transfer in DNA duplex.The nanosecond transient absorption spectroscopy was used to acquire characteristic spectra and kinetic information on reaction intermediates.Under the help of theoretical calculation,the structures of intermediates were identified,and then the reaction mechanisms were deduced.The main findings are as follows:（1）In the T₁ state,the nitro group of ³HN-4NO₂ can be protonated due to the increased electron density.p K_a^*=2.60 for ³HN-4NO₂H⁺was obtained according to the Ware model.The hydrogen bond configurations of HN-4NO₂in alcoholic solvents were analyzed with the Kamlet-Taft model.The result shows that in the S₀ state,the hydrogen bond is mainly N-H…O-H,while in the T₁ state,the N-O…H-O hydrogen bond also exists.The quantum yield of ³HN-4NO₂ is 0.41 gained by the energy transfer method.³HN-4NO₂ can undergo proton-coupled electron transfer reaction with phenols and electron transfer reaction with electron donors.Based on the relationship between rate constants of electron transfer and oxidation potentials of electron donors fitted with the Rehm-Weller equation,a reduction potential of 1.23 V for ³HN-4NO₂ was obtained.Importantly,³HN-4NO₂ and guanine（G）can undergo an electron transfer reaction through the double-hydrogen-bonded ³HN-4NO₂…G complex to generate radical ions G^+·and HN-4NO₂^-·,that followed by proton transfer to generate radicals HN-4NO₂H^·and G（N₁-H）^·/G（N₂-H）^·.We used the nanosecond transient spectroscopy to capture and characterize the hydrogen-bonded complex and radicals in this reaction process for the first time,which provided an experimental basis for further inserting HN-4NO₂ into the DNA strand as an in-situ photoinitiator.（2）In the T₁ state,the quantum yield of ³HN-6NO₂ is 0.46,p K_a^*（³HN-4NO₂H⁺）=3.37.³HN-6NO₂ can undergo proton-coupled electron transfer reaction with phenols producing HN-6NO₂H^·at 360 nm and electron transfer reaction with various electron donors.The reduction potential of³HN-6NO₂ is 1.27 V obtained by the Rehm-Weller formula.³HN-6NO₂ can also occur proton transfer reaction with isopropyl alcohol（IPA）and tert-butylamine（TBA）generating^-N-6NO₂.The difference between the reactions is that no triplet anion was observed with IPA as the proton acceptor.So it may involve an intersystem crossing accelerated by alcohols and/or a nonadiabatic proton transfer process.（3）The quantum yield of ³HN-5NO₂ is 0.46.³HN-5NO₂ can also undergo proton-coupled electron transfer reaction with various phenols,proton transfer reaction with TBA,and electron transfer reaction with various electron donors.E_red（³HN-5NO₂）=1.33 V,E_0-0（T₁）=2.98 e V.HN-5NO₂has two low-lying excited singlet states with different dipole moments in polar aprotic solvents.While in protic solvents（such as alcohols）,the energy level is reversed due to the hydrogen bond.Transient absorption experiment results show that ³HN-5NO₂ has two low-energy triplet states with degenerate energy in polar aprotic solvents.In contrast,only the more stable T₁（π,π^*）state can be observed in the strong hydrogen bond donating solvents.