Studies On The Excited State Dynamics Of Uracils By Femtosecond Transient Absorption Spectroscopy

Interest in excited electronic states dynamics of DNA/RNA has continued unabated for decades because of their role in initiating ultraviolet photodamage. The UV photostability of DNA/RNA is generally determined by its excited state relaxation processes. The nucleic acid bases are the building blocks of DNA and RNA, and the nucleobase photophysics following UV absorption constitutes a fundamental step in UV light induced DNA damage.In this dissertation, we report femtosecond transient absorption measurement for uracil and its C5, C6 substituted derivatives, i.e., thymine,6-methyluracil and 6-azauracil in solvents. Continuous UV probe extended down to 250 nm is utilized for these systems for the first time. The population dynamics of the ground states revealed with the deep UV probe provides a comprehension of the ultrafast dynamics of the excited states of uracil and its various derivatives in water. Vibrational cooling on the ground state of uracil, thymine and 6-methyluracil has been directly observed for the first time, which provides strong evidence of the ultrafast 1ππ*â†'S0 decay. And the initial pumped 1nn* state for those three molecules is found to decay in parallel to ground state So or to an intermediate 1nÏ€* state. More specifically, for the first decay pathway, the direct IC of the excited state leads to highly vibrational excited ground state, followed by vibrational cooling within 2 ps, while for the latter, the 1nÏ€* state further decays to So with a much slower rate. We also find that the contribution of the 1ππ*â†'1nÏ€* channel is much smaller in thymine or 6-methyluracil than in uracil, an indication of the methyl-substitution effect.On the other hand, the excited-state dynamics of 6-azauracil in water have been studied using femtosecond transient absorption spectroscopy. Broad-band white light continuum which covers from 270 to 600 nm is used as the probe. With a global fitting analysis of the measured transient spectra, three decay time constants, i.e.,< 0.3 ps,～4 ps and>1000 ps, are directly obtained. These newly observed lifetime constants are important in clarifying its decay dynamics as well as in providing a criterion for the ultrafast dynamics simulations in 6-azauracil using quantum chemical theories. In combination with previous theoretical works, the main decay channel of 6-azauracil is proposed:the initially populated 1ππ* decays to 1nÏ€* through internal conversion in< 0.3 ps, followed by an intersystem crossing from 1nÏ€* to T1 in 4 ps. The>1000 ps component is due to the decay of the T1 state. Our transient absorption measurement reveals that C6 as well as C5 methyl-substitution on uracil in water inhibits the 1ππ*â†'1nÏ€* channel and promotes the direct IC 1ππ*â†'S0, while aza-substitution at C6 position blocks the direct IC channel and leads the decay of 1ππ* state almost to 1nÏ€* state.A comparison of the excited-state dynamics of uracil and 6-azauracil in different solvents has been provided. For uracil, we present the excited state dynamics of uracil is mainly affected by the protic of solvent, while the viscosity of solvent has no effect. For 6-azauracil, the experimental results reveal that the time constant of decay from 1nÏ€* to T1 shows a clear dependence on the polarity of the solvents. In the solvent with higher polarity, the 6-azauracil 1nÏ€* excited state decays faster. The protic and viscosity of solvent has no obvious effect.