Impact of methyl rotor in the excited state level mixing of doubly hydrogen-bonded complexes of 2-pyridone

We have presented in this paper the laser-induced fluorescence excitation and resolved fluorescence spectra of five 1:1 hydrogen-bonded complexes of 2-pyridone (2PY) with formic acid (FA), acetic acid (AA), propanoic acid (PA), formamide (FM), and acetamide (AM). The resolved fluorescence spectra, measured following excitation to different single vibronic levels of the dimers indicate that the intermolecular hydrogen bond vibrations undergo mixing with a number of intramolecular modes of the 2PY moiety in the excited state. A comparison of the emission spectral features of these dimers clearly indicates that the methyl groups belonging to the AA and AM moieties spectacularly accelerate the vibrational energy redistribution (IVR) in the 2PY moiety. On the other hand, although the molecular size of PA is bigger than AA, the spectral features of the 2PY−PA dimer bear signatures of a slower IVR rate compared to those of 2PY−AA. We propose that hyperconjugation of the methyl group with the cyclic hydrogen-bonded network involving AA and AM is responsible for the observed phenomenon