1 research outputs found
Gas-Phase Cytosine and CytosineāN<sub>1</sub>āDerivatives Have 0.1ā1 ns Lifetimes Near the S<sub>1</sub> State Minimum
Ultraviolet
radiative damage to DNA is inefficient because of the
ultrafast S<sub>1</sub> ā S<sub>0</sub> internal conversion
of its nucleobases. Using picosecond pumpāionization delay
measurements, we find that the S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine
(Cyt) is Ļ = 730 ps or ā¼700 times longer than that measured
by femtosecond pumpāprobe ionization at higher vibrational
excess energy, <i>E</i><sub>exc</sub>. N<sub>1</sub>-Alkylation
increases the S<sub>1</sub> lifetime up to Ļ = 1030 ps for N<sub>1</sub>-ethyl-Cyt but decreases it to 100 ps for N<sub>1</sub>-isopropyl-Cyt.
Increasing the vibrational energy to <i>E</i><sub>exc</sub> = 300ā550 cm<sup>ā1</sup> decreases the lifetimes
to 20ā30 ps. The nonradiative dynamics of S<sub>1</sub> cytosine
is not solely a property of the amino-pyrimidinone chromophore but
is strongly influenced by the N<sub>1</sub>-substituent. Correlated
excited-state calculations predict that the gap between the S<sub>2</sub>(<sup>1</sup><i>n</i><sub>O</sub>Ļ*) and S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) states decreases
along the series of N<sub>1</sub>-derivatives, thereby influencing
the S<sub>1</sub> state lifetime