It is indisputable that chemical purity is a key factor to achieve reproducible intrinsic electronic mobilities in organic semiconducors. However, it has been also shown that in these systems charge transport also dramatically depends on the polymorphic phase of the compound. In fact, different polymorphs may correspond to very different molecular packings in the crystal unit cell and thus to altogether different intermolecular orbital overlaps of the π systems responsible for the charge carrier properties. Furthermore, the possible coexistence of more solid phases in a sample is an intrinsic source of disorder. Raman microscopy in the lattice phonon region and X-ray diffraction have been used to study the polymorphism in crystals and nano-crystals of the organic semiconductor 9,10-diphenylanthracene[1] obtained by various methods. While solution grown specimens all display the well known monoclinic structure widely reported in the literature, by varying the growth conditions two more polymorphs can be obtained either from the melt or by sublimation. One of these was finally obtained in a pure form in the shape of micro-ribbons by injecting water as a non-solvent in a DPA solution[2,3]. Lattice energy calculations have allowed us to assess the relative thermodynamic stability of the polymorphs and verify that the energies of the different phases are very sensitive to the details of the molecular geometry adopted in the solid state.
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H. Zhang, H. Y. Yang, and Q. Zhang, J. Phys. Chem. C , 115, 7924-7927 (2011) [3] X. Zhang, G. Yuan, Q. Li, B. Wang, X.Zhang,R. Zhang, J. C. Chang,, C. Lee, and S. Lee,Chem. Mater., 20, 6945-6950,(2008)