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 Åd‐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 Åd‐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 Åd‐spacing morphology, with a slightly smaller unit‐cell volume.