Research Interests

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We are an inorganic chemistry group emphasizing synthetic and physical inorganic chemistry. We are primarily interested in the design of molecules with interesting photophysical properties, which we interrogate using a variety of physical techniques.

Deep-Blue Phosphorescence

Efficient and stable blue phosphorescence requires strong σ-donor ligand sets, and we have pioneered the use of “nontraditional” carbene ligands that are even stronger σ-donors than the ubiquitous N-heterocyclic carbenes (NHCs) and can lead to even greater enhancements in efficiency and stability. This work involves several creative synthetic strategies to install these other carbene ligand classes onto blue-phosphorescent compounds and uses in-depth spectroscopic and computational analysis to evaluate their influence on the excited-state dynamics.

Red to Near-Infrared Phosphorescence

In the opposite extreme of the spectrum, there is a persistent challenge designing deep-red and near-infrared emitters with quantum yields as high as those found in other regions of the visible spectrum. We have tackled this challenge and made several advances both in terms of fundamental photophysics and in the performance metrics of these compounds, which can benefit not only electroluminescent applications but also bioimaging, night-vision technology, and data encryption. We have discovered a molecular design strategy to improve the photoluminescence quantum yields of red to near-infrared emitting cyclometalated iridium complexes . The key feature in these compounds is an electron-rich, strongly donating nitrogen-containing ancillary ligand, which provides synthetic control over excited-state spin-orbit coupling as a means of optimizing phosphorescence quantum yields.

Photoredox Catalysis

Our group has designed next-generation photoredox catalysts which are potent excited-state reductants. We have created an extensive library of bis-cyclometalated iridium photosensitizers supported by electron-rich β-diketiminates, all of which are significantly more potent for light-induced electron transfer when compared to current state-of-the-art photocatalysts. We have also shown that these compounds are active catalysts for the photoinduced hydrodebromination of aryl bromide substrates, operating under simpler reaction conditions without the need for wasteful additives, and have improved the reaction conditions and expanded the substrate scope to include alkyl bromides, aryl chlorides, aryl fluorides, alkyl aryl ethers, and ketones. More recent methodological advances include visible-light promoted C–C bond-forming radical-addition reactions using ketone and imine substrates, where the strongly reducing photosensitizer enables facile production of the substrate radical that can then be trapped in a variety of ways.

Earth-Abundant Photosensitizers for Solar Fuels

Large-scale photochemical applications require photosensitizers, charge-transport materials, and photocatalysts crafted from earth-abundant elements. In our group’s newest research effort, we are addressing the challenges of designing copper photosensitizers with long lifetimes and panchromatic visible absorption, capable of photoinduced redox chemistry and charge separation. We have prepared heteroleptic copper(I) complexes Figure 4 involving 1,10-phenanthroline or 2,2ʹ-biquinoline derivatives paired with substituted β-diketiminates. These compounds have spatially-separated frontier orbitals, which allows the orbital energies and associated reduction and oxidation potentials to be tuned independently. These complexes are exceptional light absorbers with tunable charge-transfer absorption bands that can span the entire visible region, arising from low-energy charge-transfer excited states involving donor orbitals on the β-dekiminates and acceptor orbitals on the diimine.

Supramolecular Chemistry of Cyclometalated Iridium

Photochemically active supramolecular constructs, where two or more chromophoric and/or luminescent molecules are tethered through covalent or non-covalent interactions, have been valuable platforms for studying excited-state energy and electron-transfer processes, and many have been used in sensing applications. Despite their well-known photophysical attributes, which include high photoluminescence quantum yields, easy tuning of excited-state energies, and good photostability, the supramolecular chemistry of cyclometalated iridium remains underdeveloped. We are designing supramolecular cyclometalated iridium complexes as platforms to study fundamental aspects excited-state energy transfer, as candidate nonlinear optical materials, and, in the long run, for “host-guest” photochemical applications. Our group has discovered a facile and modular strategy for coordination-driven self-assembly of cyclometalated iridium, using isocyanide-based bridging ligands to drive rapid, clean assembly.

Ratiometric Oxygen Sensing

Accurate, real-time measurement of oxygen concentrations is critical for several biomedical applications, including tumor biology and physiological monitoring in extreme conditions. Whereas any phosphorescent compound can in principle serve as a “turn-off” oxygen sensor, this measurement mode suffers from poor reproducibility due to fluctuations in excitation intensity, detector drift, and scattering in heterogeneous environments. Ratiometric oxygen sensing, in which oxygen is measured as a ratio of two luminescent signals, is much simpler and more reliable. Our group has developed ratiometric sensors prepared by a modular synthetic approach, allowing rapid modulation of the key sensor attributes (spectral profile, sensitivity, dynamic range). They pair cyclometalated iridium complexes, which emit oxygen-sensitive phosphorescence, with organic molecules which produce oxygen-insensitive fluorescence.

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