Packing Polymorphism Affecting the Optoelectronic Properties of a π-Conjugated Organic Compound

International audience[1]Benzothieno[3,2-b]benzothiophene (BTBT) derivatives are widely employed as hole transport materials in organic field-effect transistors. The electronic properties of these materials depend critically on the crystal packing, which in turn depends on the choice of the attached functional group. With symmetrically attached alcohol chains, the structure may display different packing modes depending on the number of CH2 groups in the alkyl chain. The dipentanol BTBT derivative has two polymorphs I and II, crystallizing from different solvents, with distinct packing modes and consequently different electronic properties. Whereas the conformational changes are very small between I and II and the hydrogen bonding networks in the structures are identical, the adjacent BTBT cores are differently shifted and oriented with respect to each other. It is shown by density functional theory that polymorph I having unfavorable electronic properties is slightly more stable than polymorph II. This is caused by the much more attractive cross stacking between BTBT cores and C5OH chains in I than in II. The stacking in II originates rather from electrostatic interactions between the BTBT cores. The differences and similarities with the packing modes of the dibutanol and dihexanol derivatives are discussed. Both polymorphic forms I and II display negative uniaxial thermal expansion, but in different directions with respect to the packing of the molecules. Different scanning calorimetry measurements suggest that more than two polymorphs exist