  We present a multiscale modeling of the infrared optical properties of the
rubrene crystal. The results are in very good agreement with the experimental
data that point to nonmonotonic features in the optical conductivity spectrum
and small optical effective masses. We find that, in the static-disorder
approximation, the nonlocal electron-phonon interactions stemming from
low-frequency lattice vibrations can decrease the optical effective masses and
lead to lighter quasiparticles. On the other hand, the charge-transport and
infrared optical properties of the rubrene crystal at room temperature are
demonstrated to be governed by localized carriers driven by inherent thermal
disorders. Our findings underline that the presence of apparently light
carriers in high-mobility organic semiconductors does not necessarily imply
band-like transport.
