Substrate-induced polymorphs (SIPs), the polymorphs that exist only near the substrate, of organic molecules have drawn increasing attention since they often possess distinct physical properties compared with the bulk phases. However, the formation mechanism of SIPs is still not fully understood, and predicting their crystal structure is a very delicate task.
Here, a new bottom-up approach is proposed to predict the crystal structure of the SIP of an archetypal molecular semiconductor material: lead phthalocyanine (PbPc). We select this material because it is a highly-symmetrical yet non-planar molecule and we demonstrate that the growth and crystal structure of the PbPc SIPs can be templated by the corresponding physisorbed monolayers. First, the structure and stability of those monolayers were studied by a combination of in-situ scanning tunneling microscopy (STM) and multi-scale computational chemistry. Then, the growth of a PbPc SIP on top of the physisorbed monolayer was modelled 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.
Our work clearly illustrates the connection between the first physisorbed monolayer and SIPs. The molecules first assemble on the substrate with an organization that does not exist in the bulk polymorph. Following this, the new molecular organization could template the subsequent crystal growth leading to the formation of a SIP. Such a sequence may represent a typical growth mechanism for the SIPs.
We believe that the bottom-up approach we propose here shows strong potentials for predicting SIPs of other organic molecules. Such approach could be further generalized as a design strategy in crystal engineering of organic semiconducting materials. This is of prime importance for the technological applications of those materials.

Comparison of crystal parameters between the newly discovered SIP and known bulk phases of PbPc

Yves Geerts

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