Ultrafast Excited State Dynamics Of Pyrimidinones And DNA Duplex Strands

DNA/RNA nucleobases have their intrinsic photostability due to the ultrafast internal conversion channel generated from the conical intersection between the exited state(S₁)and the ground state(S₀).In this case,only a small fraction of the singlet excited state population can generate triplet state through intersystem crossing(ISC).Although the triplet quantum yield is low,poential chemcial reactions that lead to DNA/RNA photodamage can take place during the whole triplet lifetime.Therefore,it is important to understanding triplet state generation mechanism in DNA.Early studies have shown that the dark ¹n?*state is a doorway state which mediates the ISC in pyrimidine bases.Recently,it has been reported that that ISC can also take place directly from the initial populated bright ¹??*state to higher energy triplet states.However,this mechanism is still poorly understood due to lack of expeimental data.In this dissertation,we choose pyrimidinones as a model system to clarify the ISC mechanism.Our data reveal that removing the functional group at C4 position can suppress the ultrafast internal conversion in DNA/RNA bases and increase the ISC probility.Meanwhile,the intermolecular hydrogen-bonding between pyrimidinones and protic solvent can accellerate the dissipation of excess vibrational energy in the excited state to the solvent,which inhibits the ISC from singlet state to triplet state.In larger DNA systems,it is impossible that excited state relaxation can still locate on a single bases due to interaction between adjacent bases.However,due to the sturnture flexible,it is diffcult to study the excited state dyanmics in DNA double strands.In this dissertation,we prepared d(GC)₉ double strand with stable Z-DNA structure in deep eutectic solvent and investigate the excited state dynamics of these strands.The radical signal which indicates the existance of the proton coupled electron transfer(PCET)in d(GC)₉ is missing in d(GC)₉ when it is in deep eutectic solvent.Therefore,we proposed that d(GC)₉ in the deep eutectic solvent can dissipate the excited state energy though internal conversion rather than electron transfer.