Size-Dependent Thermal Shifts to MOF Nanocrystal Optical Gaps Induced by Dynamic Bonding

Abstract

Conventional semiconductor nanocrystals exhibit wide-ranging optical behavior, whereas the size-dependent photophysical properties of metal−organic framework (MOF) nanocrystals remain an open research frontier. Here, we present sizeand temperature-dependent optical absorption spectra of common MOFs with particle sizes ranging from tens of nanometers to several micrometers. All materials exhibit optical gaps that decrease at elevated temperatures, which we attribute to the dynamic nature of MOF metal−linker bonds. Accordingly, whereas the labile titanium−carboxylate bonds of MIL-125 give rise to bandgaps that red-shift by ∼600 meV over 300 K, the more rigid zinc− imidazolate bonds of ZIF-8 produce a red-shift of only ∼10 meV. Furthermore, smaller particles induce far larger decreases to optical gaps. Taken together, these results suggest MOF bonding becomes more flexible with smaller nanocrystal sizes, offering a powerful tool for manipulating optical behavior through composition, temperature, and dimensionality.

Electrically conductive [Fe4S4]-based organometallic polymers

“…MUV-10, typically exhibit a large temperature dependence in their UV-vis spectra and E g values due to dynamic metal-linker bonding. 45,46 On the other hand, the VT UV-vis spectra of 1 displayed only a slight increase in the intensities upon cooling and the change in the E g was almost negligible. Plotting a difference curve δ F ( R ) T = F ( R ) T − F ( R ) 373 indicates that the intensities between 3.5 and 6.2 eV (200–350 nm) increase at the peak maximum of 5.34 eV (232 nm, Fig.…”

Dynamic metal-linker bonds in metal–organic frameworks

“…Recent evidence was documented in a report on the temperature dependence of archetypical MOFs with particle sizes ranging from 20 nm–4 μm. 20 As determined by VT-DRUVVis, all materials displayed decreasing optical gaps with increasing temperatures. Although conventional semiconductors typically show this “Varshni behaviour”, the effect in MOFs was much more severe.…”

“…While many early reports of MOFs document phase-change behaviour, soft mode behaviour was only introduced into the MOF literature recently. [17][18][19][20][21] In this Frontier article we will highlight recent studies of dynamic ML bonds and their implications for the future of the MOF field.…”

Atomistic Insight into the Interfacial Structuring of ZIF-67 MOF/Polymer Composites and Their Propylene–Propane Adsorption Properties

“…A representative model of ZIF-67 NP was cleaved from the supercell made of 2 × 2 × 2 unit cells of the previously reported crystal structure (Figure S1). The resulting spherical-shaped atomistic model contains 16 cages (cage size of ∼11.5 Å), and its outer surface was terminated by −OH groups and −H considering the dissociative adsorption of water since the standard hydrothermal synthesis of ZIF-67 proceeds in aqueous solution. , The size of our modeled ZIF-67 NP is 53 Å, falling within the range of NP sizes recently obtained for certain MOFs (50–100 Å) using various synthetic routes, thus bridging the gap between experimental and theoretical length scales. The resulting outer surface can be schematically described as a “zigzag” model where the outermost atoms are the NH groups of the imidazole linkers and the innermost atoms are the OH groups directly bonded to the Co atoms.…”