Understanding Polymorphism in Organic Semiconductor Thin Films through Nanoconfinement

Understanding
crystal polymorphism is a long-standing challenge
relevant to many fields, such as pharmaceuticals, organic semiconductors,
pigments, food, and explosives. Controlling polymorphism of organic
semiconductors (OSCs) in thin films is particularly important given
that such films form the active layer in most organic electronics
devices and that dramatic changes in the electronic properties can
be induced even by small changes in the molecular packing. However,
there are very few polymorphic OSCs for which the structure–property
relationships have been elucidated so far. The major challenges lie
in the transient nature of metastable forms and the preparation of
phase-pure, highly crystalline thin films for resolving the crystal
structures and evaluating the charge transport properties. Here we
demonstrate that the nanoconfinement effect combined with the flow-enhanced
crystal engineering technique is a powerful and likely material-agnostic
method to identify existing polymorphs in OSC materials and to prepare
the individual pure forms in thin films at ambient conditions. With
this method we prepared high quality crystal polymorphs and resolved
crystal structures of 6,13-bis­(triisopropylsilylethynyl)­pentacene
(TIPS-pentacene), including a new polymorph discovered via in situ
grazing incidence X-ray diffraction and confirmed by molecular mechanic
simulations. We further correlated molecular packing with charge transport
properties using quantum chemical calculations and charge carrier
mobility measurements. In addition, we applied our methodology to
a [1]­benzothieno­[3,2-b]­[1]­1benzothiophene (BTBT)
derivative and successfully stabilized its metastable form.