Phase Transition toward a Thermodynamically Less Stable Phase: Cross-Nucleation due to Thin Film Growth of a Benzothieno-benzothiophene Derivative

The molecule 2-decyl-7-phenyl-[1]­benzothieno­[3,2-b]­[1]­benzothiophene is an organic semiconductor, with outstanding
properties in terms of molecular packing and its use in organic electronics.
The asymmetric shape of the molecule causes a double layer crystal
structure at room temperature. In this work we report its thin film
growth by physical vapor deposition starting from the monolayer regime
up to thick films. The films are studied in terms of their morphology,
crystallographic properties, and thermal stability by atomic force
microscopy and X-ray diffraction methods. It is found that the bulk
molecular packing of the bilayer is formed at the initial thin film
growth stage. After a thickness of one double layer, a transition
into a new polymorph is observed which is of metastable character.
The new phase represents a single layer phase; the crystal structure
could be solved by a combination of X-ray diffraction and molecular
dynamics simulations. The observed thin film growth is outstanding
in terms of surface crystallization: the formation of a metastable
phase is not associated with the initial thin film growth, since the
first growth stage represents rather the bulk crystal structure of
this molecule. Its formation is associated with cross-nucleation of
one polymorph by another, which explains why a metastable phase can
be formed on top of a thermodynamically more stable phase.