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 Angstrom. We have studied the crystal structure of the 14.1 and 14.5 Angstrom d-spacing morphologies grown by vapour transport and from solution. We rnd a close correspondence between the 14.1 Angstrom structure reported by Holmes, Kumaraswamy, Matzeger & Vollhardt [Chem. Eur. J. (1999), 5, 3399-3412] and the 14.5 Angstrom structure reported by Campbell, Monteath Robertson & Trotter [Acta Cryst. (1961), 14, 705-711]. Single crystals commonly adopt the 14.1 Angstrom 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 Angstrom d-spacing morphology, with a slightly smaller unit-cell volume.