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

Abstract

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)<sub>2</sub>, with surface areas of up to 25 mm<sup>2</sup> in 24 hours, in a single solvothermal reaction with <i>in-situ</i> 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)<sub>2</sub> 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 <i>ex-situ</i> 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.

Anisotropic Band-Edge Absorption of Millimeter-Size Zn(3-Ptz)2 Single Crystal Metal-Organic Frameworks

“…We control the size of the crystal samples by controlling the reaction temperature, dilution and concentration of acid modulators. 27,38,48 We measure the absorbance for crystal samples varying in size from micron-size powder grains, to individual single crystals with top surface areas of 37 mm 2 . Figure 4a shows that crystals with surface areas in the few mm 2 develop a low energy shoulder that is not present in polycrystalline samples.…”

“…All reactants were purchased from Sigma-Aldrich and utilized without any further purification, except 3-cyanopyridine, which was sublimated at 60 • C. The Zn(3-ptz) 2 crystals were synthesized based on the methodology previously reported. 38 Large single-crystals were obtained from a mixture of Zn(CH 3 COO) 2 (3.26 mmol), 3-cyanopyridine (6.52 mmol), sodium azide (9.78 mmol) and acetic acid (3.26 mmol) dissolved in 14 mL of distilled water in a glass bottle of 50 mL and adjusting the pH value to 2.7 using HNO 3 (70%). The glass bottle was introduced to a tube furnace at 113 • C for 40 hrs (Nabertherm, model 50-250/11) using a horizontal operation and filling the furnace with alumina bulk fiber.…”

“…31,32 From the vast MOF literature, only a few reports of single-crystals MOFs grown beyond one millimeter can be found. [33][34][35][36] Although efforts to understand the dynamics of self-assembly process in MOFs under solvothermal conditions are underway, 37,38 a general understanding of this process is still elusive, limiting the available strategies for the growth of single-crystals for optical devices.…”

“…We have recently reported a method to grow large single-crystals of MOF Zn(3-ptz) 2 , 38 which has a non-centrosymmetric unit cell, tetrahedral d 10 coordination geometry and tetrazole push-pull ligands [3-ptz = 5-(3-Pyridyl)-1H-tetrazolate]. In this work, we study the polarized absorption of large individual Zn(3-ptz) 2 single-crystals along different crystal planes, and discuss the size dependence on the anisotropic absorption spectrum.…”