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Pentacene crystallizes in a layered structure with a herringbone arrangement within the layers. The electronic properties depend strongly on the stacking of the molecules within the layers (Haddon et al., 2002). We have synthesized four different polymorphs of pentacene, identified by their layer periodicity, d(001): 14.1, 14.4, 15.0 and 15.4 A. Single crystals commonly adopt the 14.1 A structure, whereas all four polymorphs can be synthesized in thin film form, depending on growth conditions. We have identified part of the unit cell parameters of these polymorphs by X-ray and electron diffraction. The 15.0 and 15.4 A polymorphs transform at elevated temperature to the 14.1 and 14.4 A polymorphs, respectively. Using SCLC measurements, we determined the mobility of the 14.1 A polymorph to be 0.2 cm^2/Vs at room temperature., 9 pages, including 6 figures

Thin films of pentacene are known to crystallize in at least four different polymorphs. All polymorphs are layered structures that are characterized by their interlayer spacing d(001). We develop a model that rationalizes the size of the interlayer spacing in terms of intralayer shifts of the pentacene molecules along their long molecular axes. It explains the wide variety of interlayer spacings, without distorting the herringbone pattern that is characteristic of many acenes. Using two simple theoretical models, we attempt to relate the intralayer shifts with the dominant, although weak, interatomic interactions (van der Waals, weak electrostatic, and covalent). For two polymorphs, a consistent picture is found. A full understanding of the other two, substrate-induced, polymorphs probably requires consideration of interlayer interactions.

The aromatic core of perylene bisimides can be twisted by chemical substitution with chlorine in the bay-position. An example for this strategy is 1,6,7,10-tetra-chloro-N,N'-dimethyl-perylene-tetracarboxylic-bisimide, Cl4MePTCDI. This twisting leads to a decrease in directed intermolecular interactions, which causes a decrease in the electronic coupling of the molecules, interesting to be investigated in thin films of this molecular semiconductor. An amorphous solid phase was formed by physical vapor deposition. This amorphous phase showed the tendency to crystallize under ambient conditions as apparent von optical microscopy at the films. The crystallized phase was investigated by atomic force microscopy AFM and optical methods, where a formation of weak excimers was found.

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.

Thin films (100–150 nm) of 1,6,7,10-tetra-chloro-N, N'-dimethyl-perylene-tetracarboxylic- bisimide (Cl4MePTCDI) were grown by physical vapour deposition on glass and characterized as being widely amorphous with a rather small degree of intermolecular electronic coupling. A slow reorganization process was detected upon conditioning the films in air by optical microscopy. Polarized microscopy and scanning force microscopy revealed the formation of crystalline domains. Locally resolved absorption and photoluminescence measurements were used to analyse changes in intermolecular coupling. Individual domains were studied by micro-photoluminescence and temperature-dependent photoluminescence was used to characterize the degree of intermolecular coupling in the crystalline domains. The results are discussed in view of recently proposed applications of such perylene imides with a twisted aromatic core in organic inverters and organic photovoltaic cells.

6,13-Di­hydro­pentacene and pentacene cocrystallize in a ratio of 2:1, i.e. C22H16·0.5C22H14, during vapour transport of commercial pentacene in a gas flow. The crystal structure is monoclinic, space group P21/n, and contains one di­hydro­pentacene mol­ecule and half a pentacene mol­ecule in the asymmetric unit.