The crystal and molecular structure of pentacene

Abstract

Crystals of pentacene are triclinic, with two molecules in a unit cell of dimensions a = 7.93, b = 6" 14, c= 16.03 A, a= 101.9 °, fl= 112.6 °, y =85.8 °, space group Pi. The structure, which has been determined from projections along the a and b crystal axes, is very similar to the structures of naphthalene, anthracene and tetracene. The two molecules in the unit cell are situated on independent centres of symmetry, the asymmetric unit consisting of two half-molecules. Both molecules are completely planar. Details of the molecular dimensions and of the intermolecular separations have been obtained.

A Polymorph Lost and Found: The High‐Temperature Crystal Structure of Pentacene

“…The pentacene-HT unit cell is consistent with the unit cell first reported by Campbell et al [5] The crystal structure of pentacene-HT is shown in Figure 1, with the low-temperature polymorph (pentacene-LT) superimposed to illustrate the relative molecular displacements. Individual herringbone-type layers are virtually identical and can be superimposed on each other, whereas adjacent layers are shifted in the pentacene-LT crystal system by (0.295, -0.174, 0.033) in fractional coordinates, or by (1.74 Å, -1.23 Å, 0.47 Å) in an orthogonal system with the x-axis parallel to the a-axis, and the z-axis parallel to the c*-axis.…”

“…A recent attempt to reproduce the solution growth of pentacene from trichlorobenzene used a temperature of 450 K to slowly evaporate the solvent. [9] This yielded pentacene-LT, because the growth temperature was below the transformation temperature of 463 K. However, if the crystal growth described by Campbell [5] was carried out at a temperature above 463 K, but below the boiling point of trichlorobenzene of 486 K, pentacene-HT would be more stable, and fast cooling may have allowed pentacene-HT to be quenched to room temperature.…”

“…Pentacene-HT (reduced cell) [22] (478 K) Pentacene (Campbell) [5,6] (reduced cell) [22] Pentacene-LT (reduced cell) [22] (293 K) the ratio of the phases with temperature in between. This phase coexistence may also be a reason for the difficulty in observing the phase transformation in differential scanning calorimetry (DSC) experiments, where data from powders showed inconclusive results.…”

“…[2][3][4] The triclinic structure of crystalline pentacene was first reported in 1961. [5,6] However, this particular structure has not been reproduced in subsequent crystalgrowth experiments. Various structure determinations on single crystals grown in different ways have consistently revealed different triclinic structures [7][8][9] (e.g.…”

“…The different values are thought to be caused by varying tilt angles of the molecules from the surface normal, with the shortest d-spacing of 14.1 Å, consistent with the recent room-temperature single-crystal structure of pentacene, [7][8][9]13] and the 14.5 Å d-spacing consistent with the initial structure reported by Campbell. [5,6] None of the other thinfilm polymorphs of pentacene have been observed in crystals grown by physical vapor-phase transport [8,9] or from solution. [7,9] Single crystals of pentacene grown by physical vaporphase transport, although appearing well crystallized to visual inspection, often show large mosaic spreads.…”