  Organic semiconductor crystals stand out as an efficient, cheap and diverse
platform for realising optoelectronic applications. The optical response of
these crystals is governed by a rich tapestry of exciton physics. So far,
little is known on the phonon-driven singlet exciton dynamics in this class of
materials. In this joint theory-experiment work, we combine the fabrication of
a high-quality oligoacene semiconductor crystal and characterization via
photoluminescence measurements with a sophisticated approach to the microscopic
modeling in these crystals. This allows us to investigate singlet exciton
optics and dynamics. We predict phonon-bottleneck effects in pentacene
crystals, where we find dark excitons acting as crucial phonon-mediated
relaxation scattering channels. While the efficient singlet fission in
pentacene crystals hampers the experimental observation of this bottleneck
effect, we reveal both in theory and experiment a distinct polarisation- and
temperature-dependence in absorption and photoluminescence spectra of tetracene
crystals, including microscopic origin of exciton linewidths, the activation of
the higher Davydov states at large temperatures, and polarisation-dependent
quenching of specific exciton resonances. Our joint theory-experiment study
represents a significant advance in microscopic understanding of singlet
exciton optics and dynamics in oligoacene crystals.
