ICYMS 1: MOFs and More with Dr. Joe Zhou

Welcome to the first installment of In Case You Missed Seminar! This time, we'll be talking about the MOF-centered research of Dr. Hong-Cai (Joe) Zhou, a professor of chemistry at Texas A&M University.

MOFs - Don't I Put Those on My Ears?

Cartoon depicting copper hexamine formation (in
the box) with simple MOF formation (outside box).
The product MOF is shown in 2-D for clarity.

Dr. Zhou cares a lot about metal organic frameworks, or MOFs for short. We've addressed MOFs briefly in this blog before. If you aren't familiar with MOFs, it might be helpful to think of them as bigger, more complicated versions of simple inorganic complexes. For example, when copper ions (Lewis acidic) react with ammonia (Lewis basic), they make a very simple copper hexamine complex. To make the basic ingredients for a MOF, we can replace the ammonia with a different ligand - one with a rigid shape and at least two Lewis basic sites. This new ligand, called a linker, can in principle bind multiple copper ions. However, the copper ions can also bind multiple linkers, so extended 3-D frameworks can form (see above).

For an analogy, picture a 3-D molecular net; where the ropes cross, a copper ion sits, and the ropes in-between are the linker molecules.

The resulting MOF is a crystalline solid compound. Because of its net-like structure, most of the MOF is empty space - and that makes MOFs very useful as a class of substances. Dr. Zhou's group and many others in the scientific community are investigating MOFs towards applications such as hydrogen storage for alternative energy, carbon dioxide storage for emissions reduction, and gas purification. Due to their high porosity and surface area, MOFs might also make excellent heterogeneous catalysts, gas-phase sensors, and drug delivery agents as well.

The Latest: Light-up MOFs

The most recent paper ($) from Dr. Zhou's group, published in the Journal of the American Chemical Society, details still another application for MOFs: fluorescence emitters, or fluorophores. Chances are good that you're experiencing the benefits of efficient fluorophores as you read this article off of your computer or phone screen.

Crash course on radiative vs. non-
radiative decay modes following the
absorption of light by an active
molecule. Green means go - the authors
want lots of radiative decay!

The central focus of the paper is to investigate the optical absorption spectra and fluorescence spectra of the molecule H₄ETTC and also of the group's new MOF PCN-94, which uses H₄ETTC as a linker. H₄ETTC itself is a highly-conjugated ethylene-based aromatic system featuring four terminal carboxylic acids. In solution, its fluorescence profile is unremarkable, with a quantum yield (photons emitted divided by photons absorbed) of only 30% in air. That means that 70% of the photons are dissipated non-radiatively, probably as wasted heat. A display made only out of H₄ETTC wouldn't be very bright.

However, when H₄ETTC is used to make a MOF with zirconium, the fluorescence picture changes drastically. Optical absorptions attributed to the H₄ETTC linker move higher in energy, as does the fluorescence resulting from those absorptions. Fluorescence from PCN-94 is in the blue region of the spectrum, which is a sweet spot for technological applications. The quantum yield also increases drastically to 76% in air. When the air (which contains oxygen, a common inhibitor of fluorescence) is displaced with argon gas, the quantum yield reaches effectively 100%.

Wait, but Why?

The explanation for the improved fluorescence properties of the PCN-94 MOF over the free H₄ETTC ligand comes down to a very important concept in physical organic chemistry: conjugation. Conjugation is a stabilizing effect by which free p-orbitals in unsaturated atoms can share electron density. Conjugated systems tend to absorb lower energies of light because the extended ("delocalized") p-orbital network spreads out the excited state over a wider area. Only planar molecules can be conjugated. In PCN-94, the arms of the H₄ETTC molecule must twist in order to bind appropriately to 4 zirconium atoms. The twisting action disrupts the planarity of the molecule, which in turn breaks the conjugation. So, the excited state energy is higher, and the fluorescence is of higher energy ("blue-shifted") as well.

The twisted conformation of H₄ETTC in PCN-94 explains the higher quantum yield as well. The MOF freezes the molecule in place, which forces the excited state to decay through fluorescence rather than by vibrational (non-radiative) relaxation. Another effect mentioned by the authors is called concentration quenching, where fluorescence in one H₄ETTC molecule can be shut off by another nearby H₄ETTC. Concentration quenching is prevented in the MOF structure because each H₄ETTC is frozen in place relatively far away from the nearest-neighbor H₄ETTC.

MOFs Are for Everyone!

Dr. Zhou's group's most recent paper showcases an important stride forward in research into the technological applications of MOFs. These versatile structures are a hot research topic in chemistry. With more developments like this, MOFs may break out of the lab and into our daily lives (or, if they make it into our phone screens, our minute-ly lives).

The most striking part of Dr. Zhou's research, and research into MOFs in general, is its interdisciplinarity. The skills required to publish the paper outlined above are spread - delocalized, if you will - among organic, inorganic, physical, and materials chemistry. There's something in there for every chemist.