  Electron-phonon coupling plays a central role in the transport properties and
photophysics of organic crystals. Successful models describing charge- and
energy-transport in these systems routinely include these effects. Most models
for describing photophysics, on the other hand, only incorporate local
electron-phonon coupling to intramolecular vibrational modes, while nonlocal
electron-phonon coupling is neglected. One might expect nonlocal coupling to
have an important effect on the photophysics of organic crystals, because it
gives rise to large fluctuation in the charge-transfer couplings, and
charge-transfer couplings play an important role in the spectroscopy of many
organic crystals. Here, we study the effects of nonlocal coupling on the
absorption spectrum of crystalline pentacene and 7,8,15,16-tetraazaterrylene.
To this end, we develop a new mixed quantum-classical approach for including
nonlocal coupling into spectroscopic and transport models for organic crystals.
Importantly, our approach does not assume that the nonlocal coupling is linear,
in contrast to most modern charge-transport models. We find that the nonlocal
coupling broadens the absorption spectrum non-uniformly across the absorption
line shape. In pentacene, for example, our model predicts that the lower
Davydov component broadens considerably more than the upper Davydov component,
explaining the origin of this experimental observation for the first time. By
studying a simple dimer model, we are able to attribute this selective
broadening to correlations between the fluctuations of the charge-transfer
couplings. Overall, our method incorporates nonlocal electron-phonon coupling
into spectroscopic and transport models with computational efficiency,
generalizability to a wide range of organic crystals, and without any
assumption of linearity.
