Pentacene is one of the lab rats of organic electronics. It has been widely studied in the solid state, grown into thin films, and derivatized for improved solubility or different packing or more interesting electronic properties. The structure that is tattooed on my back is not surprising, crystallographically—there are plenty of organic compounds that crystallize in triclinic space groups and exhibit herringbone packing motifs. It is not even the only crystal structure reported for this molecule. Like many compounds, pentacene exhibits several polymorphs; this one is observed in single crystals.
This structure is not my favorite because of the molecule itself, but because of all the research it has enabled—including my own PhD investigations. Crystal structures provide great insight into the physical properties of organic semiconductors. We can use the information in a .cif file to calculate electronic structure or predict crystal habit. Beyond single crystal structure determination, diffraction data are useful in exploring interesting physical properties such as negative thermal expansion (which pentacene exhibits along its a axis) and determining how well ordered thin films are. In the right hands, a crystal structure is a very useful tool.—Jes Sherman