From 2D to 3D: Bridging Self-Assembled Monolayers to a Substrate-Induced Polymorph in a Molecular Semiconductor

In
this study, a new bottom-up approach is proposed to predict
the crystal structure of the substrate-induced polymorph (SIP) of
an archetypal molecular semiconductor. In spite of intense efforts,
the formation mechanism of SIPs is still not fully understood, and
predicting their crystal structure is a very delicate task. Here,
we selected lead phthalocyanine (PbPc) as a prototypical molecular
material because it is a highly symmetrical yet nonplanar molecule
and we demonstrate that the growth and crystal structure of the PbPc
SIPs can be templated by the corresponding physisorbed self-assembled
molecular networks (SAMNs). Starting from SAMNs of PbPc formed at
the solution/graphite interface, the structural and energetic aspects
of the assembly were studied by a combination of in situ scanning
tunneling microscopy and multiscale computational chemistry approach.
Then, the growth of a PbPc SIP on top of the physisorbed monolayer was modeled without prior
experimental knowledge, from which the crystal structure of the SIP
was predicted. The theoretical prediction of the SIP was verified
by determining the crystal structure of PbPc thin films using X-ray
diffraction techniques, revealing the formation of a new polymorph
of PbPc on the graphite substrate. This study clearly illustrates
the correlation between the SAMNs and SIPs, which are traditionally
considered as two separate but conceptually connected research areas.
This approach is applicable to molecular materials in general to predict
the crystal structure of their SIPs.