Molecule of the Month
June 2013
Nonradiative Relaxation of UV Excited Cytosine: From Fast to Ultrafast

Cytosine exhibits a short S1 excited-state lifetime upon UV photoexcitation. We have measured the S0 → S1 vibronic spectrum [1] of supersonic jet-cooled cytosine. All observed vibronic excitations are overtone and combination bands of the low-frequency out-of-plane ν1 (butterfly), ν2 (boat) and ν3 (H-N-C6-H twist) vibrations. The lifetimes of the individual vibronic states are determined from the Lorentzian broadening of the band contours: At the vibrationless level, the lifetime is 44 picoseconds.
Increasing the vibrational energy in the S1 state rapidly decreases the lifetime to below <1 ps above 600 cm-1 vibrational energy [2]. This implies that the out-of-plane vibrations ν1, ν2 and ν3 are the promoting modes for the S1 → S0 internal conversion in cytosine [2].

This work was carried out by Simon Lobsiger, Maria Trachsel and Hans-Martin Frey in the group of Prof. Samuel Leutwyler.

References:

  1. S. Lobsiger, S. Leutwyler;
    "The Adiabatic Ionization Energy and Triplet T1 Energy of Jet-Cooled Keto-Amino Cytosine"
    J. Phys. Chem. Lett., 3, 3576-3580, (2012); doi:10.1021/jz301719h.
  2. S. Lobsiger, M. A. Trachsel, H.-M. Frey, S. Leutwyler;
    "Excited-State Structure and Dynamics of KetoľAmino Cytosine: The 1ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast"
    J. Phys. Chem. B, 117, 6106ľ6115, (2013); doi:10.1021/jp401881b.