Gas-Phase Cytosine and Cytosine-N1-Derivatives Have 0.1-1 ns Lifetimes Near the S1 State Minimum.

Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S-1 -> S-0 internal conversion of its nucleobases. Using picosecond pumpionization delay measurements, we find that the S-1((1)pi pi*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is tau = 730 ps or similar to 700 times longer than that measured by femtosecond pumpprobe ionization at higher vibrational excess energy, E-exc. N-1-Alkylation increases the S-1 lifetime up to tau = 1030 ps for N-1-ethyl-Cyt but decreases it to 100 ps for N-1-isopropyl-Cyt. Increasing the vibrational energy to E-exc = 300550 cm(1) decreases the lifetimes to 2030 ps. The nonradiative dynamics of S-1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N-1-substituent. Correlated excited-state calculations predict that the gap between the S-2((1)n(O)pi*) and S-1((1)pi pi*) states decreases along the series of N-1-derivatives, thereby influencing the S-1 state lifetime.