| Polymorphism is the ability of a molecule to adopt multiple, unique solid-state packing arrangements. Polymorphs are therefore composed of the same material but have unique lattice energies which ultimately affects their physical properties such as melting point, solubility, etc. Many materials (like pharmaceuticals) require precise physical properties but multiple phases often crystallize together; therefore, methodologies must be developed which will consistently produce a single polymorph. Two-dimensional surfaces covered in monomers oriented perpendicular to the surface, such as self-assembled monolayers (SAMs), have emerged as a possible strategy for selectively growing polymorphic materials. SAMs can promote crystal growth by lowering the nucleation barrier for a particular polymorph through (1) direct chemical interactions between the terminal functionality of the substrate and solute and (2) through lattice matching, or epitaxy, between the substrate and crystalline solute. This research has investigated the ability of two types of SAMs (gold-thiol and a developed siloxane system) to selectively template the growth of polymorphic compounds.;A series of 24 conformationally flexible meta-X,Y-substituted phenyl urea molecules (mXYPU) were synthesized (where X=H, CH3, NO2, OH, CN, Cl, Br, I and Y=H, CH3, X). After solution screening to determine which were polymorphic, six compound were selected (mNPU, mCyPU, mOPU, mBrHPU, mMeHPU, PU) and grown from both gold-4'-X-biphenyl thiol SAMs (Au-BP-X) and 3-Y-propyl siloxane SAMs (Si-Pr-Y) where X and Y are a variety of chemical functionalities. X-ray diffraction (XRD) techniques were used to determine the phase identity of the resulting crystalline material. Oriented powder XRD and x-ray gonionmetry were used to identify which crystal faces contacted the SAM so as to extrapolate possible chemical interactions which may facilitate the observed growth.;Single polymorphs were successfully grown under selective SAM-solvent conditions. Growth conditions which yielded minor phases were also expanded with the use of a SAM template. Two previously unknown metastable polymorphs (epsilon-mNPU, beta-mCyPU) were discovered on SAMs. Ultimately, the body of evidence reveals extensive growth from siloxanes SAMs but suggests a preference for nucleation of metastable phases without strong chemical matching to the siloxane template. Growth from gold-biphenyl thiol SAMs is more limited but dominated by rational chemical matching. |
