Finding polymorphs (structural variants of the name molecule) is big business in the drug industry where polymorphs can be responsible for unwanted side-effects and for variable products. Structural polymorphs can also be found in other organic molecules, of course, where their existence can be a boon or unwanted. In the area of organic semiconductors, the existence of a polymorphic change that occurred when the system was formed under strain contributed to a significantly higher mobility. The existence of a metastable polymorph under strain was an advantageous change that improved the electrical charge transport characteristics of the material (Bao et al., Nature 2011).<br/> <br/>We have recently investigated this system of substituted pentacene molecules (specifically, TIPS-pentacene) to explore the nature of the energy landscape as we vary the structural parameters of the unit cell (a, b, g). We find a rich landscape consisting of many, deep energy wells (“metabasins”) with kinetically accessible barriers between them. The depth of the energy wells are close on the order of tens or hundreds of meV and can readily be accessed depending on the processing conditions of temperature and/or strain. We were able to match the characteristics of the five lowest energy structures, one-to-one, against the five polymorphs that were found by our Stanford collaborators in the Bao group at Stanford.  However, we reiterate that there are many more metabasins that are traversed so readily that experiments cannot find them. The molecular-scale modeling of the molecules was able to uncover the molecular motions that are involved as we vary the system’s structural constraints (the nature of the unit cell). These mechanisms involve a coordinated rotation of the bulky isopropyl groups, the wagging of this larger head group and the twisting and bending of the acene backbone.  We will show an example of the advantage of using a Bayesian optimization search to speed up the identification of these polymorphs.

Finding polymorphs of organic semiconductors

249th National Meeting (2015)

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