Synthesis of Methoxy-Substituted Picenes: Substitution Position Effect on Their Electronic and Single-Crystal Structures

A series of picenes having methoxy
groups was synthesized through
Pd-catalyzed Suzuki–Miyaura couplings or Wittig reaction/intramolecular
cyclization sequences, and their physicochemical properties and single-crystal
structures were evaluated. The substitution position effects between
the outer 1,12-, 2,11-, and 4,9-position and the inner 3,10-position
are quite different; the former showed the same electronic structure
as that of picene, but the latter results in a HOMO geometry different
from those of picene and other methoxy picenes. In addition, crystal
structures of four types of methoxy-substituted picenes 4a–c,e strongly depend on their substitution
position and number of methoxy groups, which dramatically changes
the structures from the fully anisotropic 1D π-stacked structure
to a unique 3D herringbone structure due to steric hindrance of methoxy
groups. The calculations of transfer integrals based on their single-crystal
structures reveal that the methoxy picenes have intermolecular overlaps
less effective than that of the parent nonsubstituted picene. These
results are attributed not only to the packing structure but also
to electronic structures such as the HOMO distribution. The preliminary
OFET of the representative 4c,e showed hole
mobilities significantly lower than that of picene due to their less
effective intermolecular overlaps, as predicted by the calculated
transfer integrals.