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Altering the Polymorphic Accessibility of Polycyclic Aromatic Hydrocarbons with Fluorination
journal contribution
posted on 2017-04-25, 00:00 authored by Anna M. Hiszpanski, Arthur R. Woll, Bumjung Kim, Colin Nuckolls, Yueh-Lin LooSubstituting hydrogen
with fluorine is an extensively employed
strategy to improve the macroscopic properties of compounds for use
in fields as diverse as pharmaceutics and optoelectronics. The role
fluorine substitution plays on polymorphismthe ability of
a compound to adopt more than one crystal structurehas not
been previously studied. Yet, this understanding is important as different
polymorphs of the same compound can result in drastically different
bulk properties (e.g., solubility, absorptivity, and conductivity).
Strategies to either promote or suppress the crystallization of particular
polymorphs are thus desired. Here, we show that substituting hydrogen
with fluorine affects the polymorphic behavior of contorted hexabenzocoronene
(cHBC). A polycyclic aromatic hydrocarbon and molecular semiconductor,
cHBC exhibits two polymorphs (i.e., P21/c crystal structure which we refer to as polymorph
I and a triclinic crystal structure which we refer to as polymorph
II) that are accessible through postdeposition processing of amorphous
films. While the same two polymorphs remain accessible in fluorinated
derivatives of cHBC, fluorination appears to favor the formation of
polymorph I, with progressively smaller energy barrier for transformation
from polymorph II to polymorph I with fluorination.