Stepwise quantum chemical optimization reveals the origin of layered herringbone packing and polymorphism in polyacenes

Polyacenes are archetypal π-conjugated systems with important applications in organic semiconductors. This series exhibits four distinct types of layered packing motifs that significantly affect semiconducting properties, yet their structural origins remain elusive. Here, we show that stepwise quantum-chemical optimization applied to a structural model with a minimal and rational set of variables enables systematic reproduction of all the polymorphs. The model starts with a monomolecular layer composed of molecular pairs, where the short axes are arbitrarily positioned and oriented under glide symmetry, while the long axes remain parallel and unshifted. By introducing long-axis inclinations, torsions, and interlayer displacements, we obtain locally stable structures that reproduce the geometries and relative stabilities of all the polymorphs. Potential energy map analysis reveals distinct stabilizing mechanisms specific to each form. These findings clarify the origin of polymorphism in polyacenes and provide insight into how molecular packing influences charge transport. This work establishes a predictive first-principles framework for understanding and designing functional crystalline molecular materials.