  We examine the interdependence of structural and electronic properties of two
substituted pyrene crystals by means of combined spectroscopic probes and
density-functional theory calculations. One derivative features n-hexyl side
groups, while the other one contains branched silanyl groups. Both derivatives
form triclinic crystal structures when grown from solution, but the electronic
dispersion behavior is significantly different due to differences in
$\pi$-$\pi$ overlap along the $a$ crystal axis. Both systems display dispersion
of 0.40-0.45 eV in the valence band, suggesting a high intrinsic hole mobility.
However, the dispersion is primarily along the a-axis in the
silanyl-substituted derivative, but less aligned with this crystal axis in the
hexyl-substituted material. This is a direct consequence of the diferences in
co-facial $\pi$ electron overlap revealed by the crystallographic studies. We
find that photophysical defects, ascribed to excimer-like states, point to the
importance of localized trap states. Substituted pyrenes are useful model
systems to unravel the interplay of crystal structure and electronic properties
in organic semiconductors.
