Organization of Acenes with a Cruciform Assembly Motif

This study explores the assembly in the crystalline state of a class of pentacenes that are
substituted along their long edges with aromatic rings forming rigid, cruciform molecules. The crystals were
grown from the gas phase, and their structures were compared with DFT-optimized geometries. Both
crystallographic and computed structures show that a planar acene core is the exception rather than the
rule. In the assembly of these molecules, the phenyl groups block the herringbone motif and further guide
the arrangement of the acene core into higher order structures. The packing for the phenyl-substituted
derivatives is dictated by close contacts between the C−H's of the pendant aromatic rings and the carbons
at the fusions in the acene backbone. Using thiophene substituents instead of phenyls creates cofacially
stacked acenes. In thin films, the thiophene-substituted derivative forms devices with good electrical
properties:  relatively high mobility, high ON/OFF ratios, and low threshold voltage for device activation.
An unusual result is obtained for the decaphenyl pentacene when devices are fabricated on its crystalline
surface. Although its acene cores are well isolated from each other, this material still exhibits good electrical
properties.