  Starting from a microscopic model, we investigate the optical spectra of
molecules in strongly-coupled organic microcavities examining how they might
self-consistently adapt their coupling to light. We consider both rotational
and vibrational degrees of freedom, focusing on features which can be seen in
the peak in the center of the spectrum at the bare excitonic frequency. In both
cases we find that the matter-light coupling can lead to a self-consistent
change of the molecular states, with consequent temperature-dependent
signatures in the absorption spectrum. However, for typical parameters, these
effects are much too weak to explain recent measurements. We show that another
mechanism which naturally arises from our model of vibrationally dressed
polaritons has the right magnitude and temperature dependence to be at the
origin of the observed data.
