Millimeter-Sized Metal-Organic Framework Single Crystals without Inversion Symmetry: Controlled Growth and Self-Assembly Mechanism

The controllable growth of non-centrosymmetric metal organic framework (MOF) beyond the conventional micrometer crystal dimensions would represent an enabling step in the development of MOF-based devices for coherent nonlinear optics. This goal has been elusive so far, as MOF crystal typical self-assemble under metastable synthesis conditions that have several competing crystallization pathways open, and only a modest amount of external control over the crystal nucleation and growth rates is currently possible. We overcome some of these issues and achieve the controlled growth of large single crystals of the non-centrosymmetric MOF Zn(3-ptz)₂, with surface areas of up to 25 mm² in 24 hours, in a single solvothermal reaction with in-situ ligand formation. No additional growth steps are necessary. We carry out a mechanistic study to unravel the reaction steps leading to the self-assembly of Zn(3-ptz)₂ crystals, by identifying and isolating several intermediate crystal structures that directly connect with the target MOF, and reversibly interconverting between them. We identify the synthesis parameters that control the size and morphology of our target MOF crystal and model its nucleation and growth kinetics using ex-situ image processing data. Our work is a step forward is understanding and controlling the factors that stabilize the growth of high-quality MOF crystals with sizes that are relevant for coherent optics, thus untapping possible applications of metal-organic frameworks in classical and quantum communication technology.