Controlling Molecular Crystal Polymorphism with Self-Assembled Monolayer Templates

The control of crystal polymorphism is a long-standing issue in solid-state chemistry, which has
many practical implications for a variety of commercial applications. At least four different crystalline forms
of 1,3-bis(m-nitrophenyl) urea (MNPU), a classic molecular crystal system, are known to crystallize from
solution in various concomitant combinations. Herein we demonstrate that the introduction of gold−thiol
self-assembled monolayers (SAMs) of substituted 4‘-X-mercaptobiphenyls (X = H, I, and Br) into the
crystallization solution can serve as an effective means to selectively template the nucleation and growth
of α-, β-, and γ-MNPU phases, respectively. Polymorph control in the presence of SAM surfaces persists
under a variety of solution conditions and consistently results in crystalline materials with high phase purity.
The observed selectivity is rationalized on the basis of long-range two-dimensional geometric lattice matching
and local complementary chemical interactions at the SAM/crystal interfaces.