Out-of-Plane Low-Frequency Vibrations and Nonradiative Decay in the 1ππ* State of Jet-Cooled 5-Methylcytosine

Abstract

We investigate the UV vibronic spectrum and excited-state nonradiative processes of jet-cooled 5-methylcytosine (5MCyt) using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. In contrast to cytosine, which shows only five bands above its electronic origin, the lowest electronic transition of 5MCyt exhibits about 25 low-frequency vibronic bands that extend to 0(0)(0) + 450 cm(–1), allowing to extract detailed information on the excited-state electronic and nuclear structure. Most bands are overtones and combinations of the out-of-plane vibrations ν'(1), ν'(2), and ν'(3). Their large intensities reflect butterfly-, boat-, and twist-deformations of the 5MCyt framework upon electronic excitation. From the rotational contours of the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands, the transition is found to be polarized along the in-plane a/b axes, characteristic of a (1)ππ* transition. Approximate second-order coupled-cluster (CC2) and time-dependent B3LYP calculations both predict that 5MCyt undergoes an out-of-plane deformation in its (1)ππ* (S(2)) state but both methods overestimate the out-of-plane ν'(1), ν'(2), and ν'(3) vibrational frequencies by a factor of 3–5. The TD-B3LYP (1)ππ* transition dipole moment direction is 10%:90% a:b, in good agreement with experiment. From the Lorentzian line shape contributions needed to fit the rotational contours, a lower limit to the 5MCyt (1)ππ* state lifetime at the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands is determined as τ ≥ 30 ps. These values are in stark contrast to the ultrafast (picosecond) lifetimes measured for jet-cooled cytosine by femtosecond pump–probe techniques. They also confirm the observation from the R2PI spectrum that 5-methylation of cytosine increases its excited-state lifetime. The higher out-of-plane overtone and combination bands disappear from the spectrum by ~460 cm(–1), signaling the onset of lifetimes τ < 0.5 ps, induced by excitation of the ν'(1), ν'(2), and ν'(3) modes. Above 460 cm(–1) the in-plane fundamentals 9(0)(1), 10(0)(1), and 17(0)(1) are weakly observed, implying that the in-plane vibrations couple weakly to the out-of-plane modes and have lifetimes τ > 3 ps up to 720 cm(–1) excess vibrational energy.