  Organic molecules have recently gained attention as novel sources of single
photons. We present a joint experiment--theory analysis of the
temperature-dependent emission spectra, zero-phonon linewidth, and second-order
correlation function of light emitted from a single molecule. We observe
spectra with a zero-phonon-line together with several additional sharp peaks,
broad phonon sidebands, and a strongly temperature dependent homogeneous
broadening. Our model includes both localised vibrational modes of the molecule
and a thermal phonon bath, which we include non-perturbatively, and is able
capture all observed features. For resonant driving we measure Rabi
oscillations that become increasingly damped with temperature, which our model
naturally reproduces. Our results constitute an essential characterisation of
the photon coherence of these promising molecules, paving the way towards their
use in future quantum information applications.
