  We have synthesized, crystallized and studied the structural and electric
transport properties of organic molecular crystals based on a rubrene
derivative with {\em t}-butyl sidegroups at the 5,11 positions. Two crystalline
modifications are observed: one (A) distinct from that of rubrene with larger
spacings between the naphtacene backbones, the other (B) with a in-plane
structure presumably very similar compared to rubrene. The electric transport
properties reflect the different structures: in the latter phase (B) the
in-plane hole mobility of 12 cm$^2$/Vs measured on single crystal FETs is just
as high as in rubrene crystals, while in the A phase no field-effect could be
measured. The high crystal quality, studied in detail for B, reflects itself in
the density of gap states: The deep-level trap density as low as $10^{15}$
cm$^{-3}$ eV$^{-1}$ has been measured, and an exponential band tail with a
characteristic energy of 22 meV is observed. The bulk mobility perpendicular to
the molecular planes is estimated to be of order of $10^{-3}$ -- $10^{-1}$
cm$^2$/Vs.
