Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5‑Fluorocytosine

The <i>S</i>₀ → <i>S</i>₁ vibronic spectrum and <i>S</i>₁ state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm^–1 resolution. The 0₀⁰ rotational band contour is polarized in-plane, implying that the electronic transition is ¹<i>ππ</i>*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the ¹<i>ππ</i>* (<i>S</i>₁) transition of 5FCyt. The spectral region from 0 to 300 cm^–1 is dominated by overtone and combination bands of the out-of-plane ν₁^′ (boat), ν₂^′ (butterfly), and ν₃^′ (HN–C₆H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the ¹<i>ππ</i>* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm^–1; this is attributed to the ¹<i>ππ</i>* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν₁^′, ν₂^′, and ν₃^′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm^–1, rapid nonradiative relaxation (<i>k</i>_nr ≥ 10¹² s^–1) sets in, which we interpret as the height of the ¹<i>ππ</i>* state barrier in front of the lowest <i>S</i>₁/<i>S</i>₀ conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν₁^′, 2ν₂^′, 2ν₁^′ + 2ν₂^′, 4ν₂^′, and 2ν₁^′ + 4ν₂^′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν₁^′ + 4ν₂^′ level at +234 cm^–1. These gas-phase lifetimes are twice those of <i>S</i>₁ state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its <i>S</i>₁ (¹<i>ππ</i>*) state is essentially controlled by the same ∼1200 cm^–1 barrier and conical intersection both in the gas phase and in solution

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

Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy

We describe a three-wave mixing experiment using time-separated microwave pulses to detect the enantiomer-specific emission signal of the chiral molecule using Fourier transform microwave (FTMW) spectroscopy. A chirped-pulse FTMW spectrometer operating in the 2–8 GHz frequency range is used to determine the heavy-atom substitution structure of solketal (2,2-dimethyl-1,3-dioxolan-4-yl-methanol) through analysis of the singly substituted ¹³C and ¹⁸O isotopologue rotational spectra in natural abundance. A second set of microwave horn antennas is added to the instrument design to permit three-wave mixing experiments where an enantiomer-specific phase of the signal is observed. Using samples of <i>R</i>-, <i>S</i>-, and racemic solketal, the properties of the three-wave mixing experiment are presented, including the measurement of the corresponding nutation curves to demonstrate the optimal pulse sequence

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