Polymorphism in pentacene

Pentacene, C₂₂H₁₄, crystallizes in different morphologies characterized by their d(001)-spacings of 14.1, 14.5, 15.0 and 15.4 Å. We have studied the crystal structure of the 14.1 and 14.5 Å d-spacing morphologies grown by vapour transport and from solution. We find a close correspondence between the 14.1 Å structure reported by Holmes, Kumaraswamy, Matzeger & Vollhardt [Chem. Eur. J. (1999), 5, 3399-3412] and the 14.5 Å structure reported by Campbell, Monteath Robertson & Trotter [Acta Cryst. (1961), 14, 705-711]. Single crystals commonly adopt the 14.1 Å d-spacing morphology with an inversion centre on both mol­ecules in the unit cell. Thin films grown on SiO₂ substrates above 350 K preferentially adopt the 14.5 Å d-spacing morphology, with a slightly smaller unit-cell volume.