Excited-State Structure and Dynamics of Keto–Amino Cytosine: The ¹ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S₀ → S₁ vibronic spectra and S₁ state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm^–1 resolution. The rotational contours of the 0₀⁰ band and nine vibronic bands up to +437 cm^–1 are polarized in the pyrimidinone plane, proving that the electronic excitation is to a ¹ππ* state. All vibronic excitations up to +437 cm^–1 are overtone and combination bands of the low-frequency out-of-plane ν₁^′ (butterfly), ν₂^′ (boat), and ν₃^′ (H–N–C⁶–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and ¹ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν₁^′ and ν₂^′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν₁^′ and ν₂^′ overtone vibronic excitations, this implies that the ¹ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The ¹ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0₀⁰ band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm^–1, and decreases to 25–30 ps up to +437 cm^–1. These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm^–1). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the ¹ππ* state minimum is <i>k</i>_nr ∼ 2.5 × 10¹⁰ s^–1, much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm^–1 excess energy. Since high overtone bands of ν₁^′ and ν₂^′ are observed in the R2PI spectrum but only a single weak 2ν₃^′ band, we propose that ν₃^′ is a promoting mode for nonradiative decay, consistent with the observation that the ν₃^′ normal-mode eigenvector points toward the “C⁶-puckered” conical intersection geometry

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