Zirconium in the Spotlight: New Pathways to Photoluminescent Molecules

Carsten Milsmann

C. Eugene Bennett Department of Chemistry, West Virginia University

The efficient utilization of sunlight as a carbon-neutral, renewable energy source remains a challenge for scientists across many disciplines. The enormous scope of solar energy conversion on a global scale requires the development of photoactive materials from readily available and economically viable resources. Photoluminescent complexes based on earth-abundant early transition metals or main group elements present an attractive low-cost, low-toxicity alternative to precious metal photosensitizers commonly used in photochemical processes and solar energy conversion. However, the fundamentally different electronic structures of these elements requires the development of new approaches to light-induced charge separation and electron transfer.

This presentation will highlight the Milsmann group’s efforts to establish design principles for the generation of luminescent early transition metal complexes that can undergo photo-induced single electron transfer (SET) reactions upon irradiation with visible light. A particular focus will be on phosphorescent zirconium complexes that exhibit exceptionally long triplet excited state lifetimes. Our results show that these complexes can not only replace precious metal photosensitizers in photoredox catalytic reactions, but may exhibit optical properties that complement those of traditional late metal compounds. In addition, research towards the development of metal-free phosphorescent molecules based on silicon will be discussed.

Abstract:

The structure of graphene oxide (G-O) is still a challenging topic for developing useful and novel applications, where a compatibility, affinity or even covalent linkage is required at the interface. There is still a no consensus about the essential details, especially relating to the epoxy groups as main complexes in the basal plane, as well as the simultaneous presence of G-O sheets and oxidative debris (OD), with large difference in their oxygen content between both entities. The present seminar  deeps on this topic, through the characterization the base-washed G-O (bwGO) sheets, the OD and the humic fraction of the OD obtained by base digestion, when the parent G-O was previously dried or not, and previously sonicated or not. It was found that the presence of lactols at graphene edges as the dominant surface complexes agrees with all the characterization techniques, and also explains the high decrease of oxygen surface coverage in bwGO, where no carboxylic group removal was observed. These findings suggest that the Hummers-Offeman reaction produces a chemical scissor-effect during the water/hydrogen peroxide quenching step, yielding a broad size distribution of G-O sheets, with little in-plane oxidation, and the vast majority of edges being oxidized to form 7-ring lactol-type heterocyclic functionalities. Therefore, OD consists essentially of the very low sheet-size fractions, highly oxidized poly aromatic hydrocarbons, with a very high oxygen:carbon ratio due to the very high edge to weight ratio. 

Abstract:

Biological macromolecules function in dense, crowded cellular environments.

Early studies of crowding effects have emphasized volume exclusion effects, but it is becoming clear that frequent non-specific interactions between proteins, nucleic acids, and metabolites may be the more important factor in modulating the structure and dynamics of biomolecules. Computer simulation studies at different scales of a series of models ranging from concentrated homogeneous protein solutions to models of bacterial cytoplasms are presented to explore the effects of non-specific quinary protein-protein interactions on protein stability and dynamics.  One focus is on the formation of transient clusters that determine diffusive properties and lead to liquid-liquid phase transitions. The computational results are related to existing experimental data and the challenges and opportunities to expand the current studies to whole-cell modeling in molecular detail are discussed.

Abstract:

When an aqueous solution freezes at atmospheric conditions, essentially pure crystals of hexagonal ice are formed, and the solutes are threaded between the ice grains in ice boundary grooves or in puddles formed on the surface.

Certainly, there are several questions to ask about this process: What is the microstructure of ice with brine like? What is the chemical state and immediate environment of the solutes there? And, most importantly, how is the reactivity of the compounds influenced by freezing?

I would like to invite you to search for the answers with me, and I will be very pleased with your interest. So far, I have applied environmental scanning electron microscopy and optical spectroscopies in seeking indications of the aggregation, pH jumps, and electrical “freezing potential” formed at the interfaces of ice grains upon freezing. My goal is to explain the (photo) reactivity of compounds in environmental ices and during laboratory-based and industrial freezing procedures. I am currently a visiting scholar on sabbatical here at the University of Kentucky, and would appreciate any suggestions on facilities or methods available here.

Abstract:

Detailed understanding of how conformational dynamics orchestrates function in allosteric regulation of recognition and catalysis at atomic resolution remains ambiguous. The three dimensional structure of protein is not always adequate to provide a complete understanding of protein function. We use atomistic molecular dynamics simulations to complement experiments to understand how protein conformational dynamics are coupled to allosteric function. We analyze multi-dimensional simulation trajectories by mapping key dynamical features within individual macrostates as residue-residue contacts. In this talk, we will discuss computational studies on members of a ubiquitous family of enzymes that regulate many sub-cellular processes. The effects of distal mutations and substrate binding are observed at locations far beyond the mutation and binding sites, implying their importance in allostery. The results provide insights into the general interplay between enzyme conformational dynamics and catalysis from an atomistic perspective that have implications for structure based drug design and protein engineering.

Abstract:

Polymerizing monomers into periodic two-dimensional (2D) networks provides structurally precise, layered macromolecular sheets that exhibit desirable mechanical, optoelectrotronic, and molecular transport properties. 2D covalent organic frameworks (COFs) offer broad monomer scope but are generally isolated as powders comprised of aggregated nanometer-scale crystallites. I will discuss 2D COF formation using a two-step procedure, in which monomers are added slowly to pre-formed nanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micron-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticles provides two to three orders of magnitude improvement in signal quality relative to polycrystalline powder samples and suggests exciton diffusion over longer length scales than those obtained through previous approaches. These findings will enable a broad exploration of synthetic 2D polymer structures and properties.

ABSTRACT: What are the thermodynamic driving forces that influence the free energy of membrane protein folding and association in lipid bilayers? For soluble proteins, the burial of hydrophobic groups away from aqueous interfaces is a major driving force, but membrane-embedded proteins cannot experience hydrophobic forces, as the lipid bilayer lacks water. A fundamental conundrum thus arises: how does a greasy protein surface find its greasy protein partner in the greasy lipid bilayer to fold faithfully into its native structure? Recently, a structurally stable and functional monomeric form of the normally homodimeric Cl^-/H⁺ antiporter CLC-ec1 was designed by introducing tryptophan mutations at the dimer interface. We have used this to develop a new model system for studying reversible dimerization in membranes for free energy measurements, which encompasses the thermodynamic properties of protein interactions in the membrane environment. To quantify monomer vs. dimer populations across a wide range of protein densities, we developed a method that quantifies the capture of subunits into liposomes from large equilibrium membranes single-molecule photobleaching by total internal reflection microscopy.  With this, we are able to determine that CLC-ec1 has a free energy of dimerization of -11 kcal/mole in 2:1 POPE/POPG membranes.  We are now investigating why this complex is so stable, dissecting the changes in enthalpy and entropy while varying protein interactions or the composition of the lipid solvent.  The results from this study will provide a physical foundation for the development of informed strategies aimed at correcting protein mis-folding or regulating protein interactions in membranes in physiologically and pathological situations.

Abstract:

Size-selective cryogenic photoelectron spectroscopy (cryoPES) coupled with electrospray ionization source (ESI) has been demonstrated to be a powerful experimental technique to investigate electronic structures and energetics of a wide variety of solution phase species and chemistry in the gas phase. In this talk, I will present the latest results probing various novel molecular clusters ranged from closo-dodecaborate dianions [B₁₂X₁₂]^2- to atmospherically relevant species by employing this technique. Transition state dynamics of unimolecular isomerization and chemical reactions via photodetachment of corresponding precursor anions will be reported as well. Future directions of ESI-cryoPES, leading to high resolution photoelectron imaging spectroscopy and time-resolved pump-probe experiments will also be briefly discussed.

Abstract:

Magnetic particle imaging (MPI) is an emerging imaging modality that enables the direct mapping of iron oxide nanoparticle tracers.  In MPI, the development of tailored magnetic nanoparticle tracers is paramount to achieving high sensitivity and good spatial resolution. This talk will provide a general overview of the progress in MPI tracer development over the past decade, and will also focus on emerging directions and new opportunities for iron oxide-based tracer design and applications. The presentation will cover magnetic nanoparticle relaxation in MPI and discuss key aspects to consider in tailoring tracers for MPI applications. Emphasis will be given on how structural changes (size, composition, shape, surface chemistry) and inter-particle interactions affect the MPI signal generation process. Moreover, the presentation will discuss emerging research directions in color-MPI (cMPI) and MPI-guided hyperthermia (hMPI).