The final comparison which can be made is of the simulated powder diffraction pattern from the structure solved from the 300 K thin film with the experimentally determined powder diffraction pattern from the bulk powder used for the deposition of the films. As it is known that new polymorphs can form when a material crystallizes at a substrate surface, this is an important check to see if the structure obtained here is the bulk polymorph or a surface-mediated polymorph.9 The diffraction patterns of the bulk EPI powder and the simulated powder diffraction pattern of the structure solved from the thin film are shown in Fig. 7. There is a clear difference in the two powder diffraction patterns, suggesting that the structure solved from the thin film is actually a surface-mediated polymorph and not the bulk structure present in the powder. It should be noted, that it was not possible to index the diffraction pattern of the bulk EPI powder due to the significant anisotropic peak broadening caused by crystal defects and particle size effects (compare, for example, peaks shapes of the bulk powder with the calculated diffraction pattern of the thin film polymorph in Fig. 7). Therefore, from the data presented here, it is not possible to offer a crystal structure solution for the bulk EPI polymorph. As previously stated, it has been observed that growth of large, defect free crystals of EPI and the closely related family of quinacridone hydrogen-bonded pigments is challenging;11,12 this is also the likely reason that a crystal structure for EPI has not, until now, been solved. As no known bulk unit cell parameters are available, a direct comparison between the thin film diffraction pattern (see Fig. 4) and the bulk structure cannot be made. However, from visual comparison of the bulk EPI powder diffraction pattern and the simulated powder diffraction pattern of the structure solved from the thin film, it is clear that a different polymorph is present, adding a further level of complexity to the crystallization behavior of epindolidiones. The presence of a surface-mediated polymorph of EPI is significant, as it is likely this structure, and not the bulk polymorph, which dictates the properties of the high stability OFETs and organic light-emitting diodes (OLEDs) previously produced using this material.11