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 S₁ ⇝ S₀ internal conversion of its nucleobases. Using picosecond pump–ionization delay measurements, we find that the S₁(¹<i>ππ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼700 times longer than that measured by femtosecond pump–probe ionization at higher vibrational excess energy, <i>E</i>_exc. 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 <i>E</i>_exc = 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<i>n</i>_Oπ*) and S₁(¹<i>ππ</i>*) states decreases along the series of N₁-derivatives, thereby influencing the S₁ state lifetime