Nucleation and Growth of Metastable Polymorphs on Siloxane Monolayer Templates

The development of methods aimed at polymorph screening and effecting phase control is of broad interest to the field of molecular materials. The present work is the most extensive systematic combinatorial study of molecular crystal growth in the presence of modified siloxane templates. 1,3-Bis(m-nitrophenyl)urea (MNPU) is known to crystallize from solution in three polymorphic forms (alpha, beta, delta) as well as a hydrate (gamma), typically concomitantly. Previous growth studies of MNPU on gold-thiol self-assembled monolayers (SAMS) (J. Am. Chem. Soc. 2005, 127, 18321-18327) demonstrated phase selectivity and preferred orientations which could be rationalized on the basis of thermodynamic stabilization at the SAM/crystal interface. Crystallization studies of MNPU on 11 different siloxanes in 3 unique solvents showed that in general siloxane SAMS tended to favor the nucleation and growth of metastable phases (beta, delta, gamma) and completely suppress the growth of the most stable alpha phase. A new epsilon-MNPU phase not previously observed in either solution or template-directed studies was also identified. Differences in the preferred orientations and phase selectivity relative to Au-S SAMS growth studies suggest that kinetic factors play a more pronounced role during interfacial stabilization on siloxane SAMS. This method serves as an effective complement to existing strategies used in the discovery and reliable generation of polymorphs, and may have useful applications in the polymorph screening of other molecular crystal systems.