Ultrafast dynamics of vibrational N–H stretching excitations in the 7-azaindole dimer

The ultrafast vibrational response of hydrogen bonds in the 7-azaindole dimer, a model compound for DNA base pairing, is studied in infrared pump–probe experiments. We observe a ∼100 fs relaxation of the v = 1 state of the N–H stretching oscillators via anharmonically coupled over- and combination tones of fingerprint modes. In the monomer with a larger energy mismatch between the v = 1 state and over-/combination tones, a much longer lifetime of 10 ps is found. Femtosecond N–H stretching excitation induces coherent underdamped motions of the dimer, dominated by the anharmonically coupled 110 cm−1 stretching mode of the hydrogen bonds.