Gas-Phase Cytosine and Cytosine-N1-Derivatives Have 0.1-1 ns Lifetimes Near the S1 State Minimum.
J Phys Chem Lett
; 7(5): 752-7, 2016 Mar 03.
Article
em En
| MEDLINE
| ID: mdl-26863095
ABSTRACT
Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S1 â S0 internal conversion of its nucleobases. Using picosecond pump-ionization delay measurements, we find that the S1((1)ππ*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or â¼ 700 times longer than that measured by femtosecond pump-probe ionization at higher vibrational excess energy, Eexc. N1-Alkylation increases the S1 lifetime up to τ = 1030 ps for N1-ethyl-Cyt but decreases it to 100 ps for N1-isopropyl-Cyt. Increasing the vibrational energy to Eexc = 300-550 cm(-1) decreases the lifetimes to 20-30 ps. The nonradiative dynamics of S1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N1-substituent. Correlated excited-state calculations predict that the gap between the S2((1)nOπ*) and S1((1)ππ*) states decreases along the series of N1-derivatives, thereby influencing the S1 state lifetime.
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J Phys Chem Lett
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2016
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Article
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