  Vibrationally resolved spectra provide a stringent test of the accuracy of
theoretical calculations. We combine the thawed Gaussian approximation (TGA)
with an on-the-fly ab initio (OTF-AI) scheme to calculate the vibrationally
resolved emission spectra of oligothiophenes with up to five rings. The
efficiency of the OTF-AI-TGA permits treating all vibrational degrees of
freedom on an equal footing even in pentathiophene with 105 vibrational degrees
of freedom, thus obviating the need for the global harmonic approximation,
popular for large systems. Besides reproducing almost perfectly the
experimental emission spectra, in order to provide a deeper insight into the
associated physical and chemical processes, we also develop a novel systematic
approach to assess the importance and coupling between individual vibrational
degrees of freedom during the dynamics. This allows us to explain how the
vibrational line shapes of the oligothiophenes change with increasing number of
rings. Furthermore, we observe the dynamical interplay between the quinoid and
aromatic characters of individual rings in the oligothiophene chain during the
dynamics and confirm that the quinoid character prevails in the center of the
chain.
