Organic semiconductors exhibit properties of individual molecules and
extended crystals simultaneously. The strongly bound excitons they host are
typically described in the molecular limit, but excitons can delocalize over
many molecules, raising the question of how important the extended crystalline
nature is. Using accurate Green's function based methods for the electronic
structure and non-perturbative finite difference methods for exciton-vibration
coupling, we describe exciton interactions with molecular and crystal degrees
of freedom concurrently. We find that the degree of exciton delocalization
controls these interactions, with thermally activated crystal phonons
predominantly coupling to delocalized states, and molecular quantum
fluctuations predominantly coupling to localized states. Based on this picture,
we quantitatively predict and interpret the temperature and pressure dependence
of excitonic peaks in the acene series of organic semiconductors, which we
confirm experimentally, and we develop a simple experimental protocol for
probing exciton delocalization. Overall, we provide a unified picture of
exciton delocalization and vibrational effects in organic semiconductors,
reconciling the complementary views of finite molecular clusters and periodic
molecular solids