Millimeter-Scale Zn(3-ptz)₂ Metal–Organic Framework Single Crystals: Self-Assembly Mechanism and Growth Kinetics

Abstract

The solvothermal synthesis of metal−organic frameworks (MOFs) often proceeds through competing crystallization pathways, and only partial control over the crystal nucleation and growth rates is possible. It challenges the use of MOFs as functional devices in freespace optics, where bulk single crystals of millimeter dimensions and high optical quality are needed. We develop a synthetic protocol to control the solvothermal growth of the MOF [Zn(3-ptz) 2 ] n (MIRO-101), to obtain large single crystals with projected surface areas of up to 25 mm 2 in 24 h, in a single reaction with in situ ligand formation. No additional cooling and growth steps are necessary. We propose a viable reaction mechanism for the formation of MIRO-101 crystals under acidic conditions, by isolating intermediate crystal structures that directly connect with the target MOF and reversibly interconverting between them. We also study the nucleation and growth kinetics of MIRO-101 using ex situ crystal image analysis. The synthesis parameters that control the size and morphology of our target MOF crystal are discussed. Our work deepens our understanding of MOF growth processes in solution and demonstrates the possibility of building MOF-based devices for future applications in optics.

Metal–organic frameworks in pursuit of size: the development of macroscopic single crystals

“…Other outstanding systems have also been proposed with less common linkers. 82,83 In contrast, Zr-based materials (UiO family) are more difficult to crystalize in big size, although the previous specifications would help. Reproducibility may be the Achilles heel in this context, as large variability in the characteristics of the products has been reported from batch to batch.…”

Anisotropic Band-Edge Absorption of Millimeter-Sized Zn(3-ptz)2 Single-Crystal Metal–Organic Frameworks

“…The Zn(3-ptz) 2 crystals were synthesized based on the methodology previously reported. 37 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 50 mL glass bottle with the pH value adjusted to 2.7 using HNO 3 (70%). The glass bottle was introduced into a tube furnace at 113 °C for 40 h (Nabertherm, model 50-250/11) using a horizontal operation and filling the furnace with alumina bulk fiber.…”

“…54,55 When synthesis conditions favor high nucleation rates and fast crystal growth, producing small polycrystalline samples, the dynamics of the crystal packing of MIRO-101 is under a kinetic control. 37 Our slow crystallization rate conditions suppress the kinetic control of the self-assembly process, producing large single crystals in the most stable thermodynamic state (cell A). Although other cell A-like domains can be produced during the crystallization process in the single crystal (three pyridyl groups oriented inward and one outward), our spectroscopic observations and theoretical calculations support a net microscopic orientation.…”

“…We have recently reported a method to grow large single crystals of the MOF Zn(3-ptz) 2 . 37 This MOF shows a noncentrosymmetric unit cell, tetrahedral d 10 coordination geometry, and tetrazole push–pull ligands [3-ptz is 5-(3-pyridyl)-1 H -tetrazolate]. 39 In this work, we study the absorption of large individual Zn(3-ptz) 2 single crystals across different crystal planes and discuss the size dependence of the anisotropy in the absorption spectrum with crystal samples of millimeter dimensions, thus extending previous work on the dependence of MOFs’ electronic properties with size over submicrometer dimensions.…”

“… 31 , 32 According to the vast MOF literature, only a few reports of single-crystal MOFs grown beyond 1 mm can be found. 33 − 37 Although efforts to understand the dynamics of the self-assembly process in MOFs under solvothermal conditions are underway, 38 a general understanding of this process is still elusive, limiting the available strategies for the growth of single crystals for optical devices.…”