Polymeric Metal Complexes. Metallobiomaterials for Medicine and Biotechnology. Bio-inspired and Sustainable Design. Responsive Nanoscale Assemblies.
Research in the Fraser Lab is concerned with the synthesis, properties and applications of metal complexes with polymeric ligands. Like metalloproteins, polymeric metal complexes feature site-isolated metal centers in well-defined macromolecular environments. These multifunctional targets are prepared via combination of coordination chemistry with living polymerization methodologies and offer many opportunities for further modification. The development of new synthetic methodologies involving main group, transition metal and lanthanide complexes with bipyridine and diketonate macroligands has paved the way for the discovery of some fascinating and unexpected properties and reactivity. The resulting hybrid materials can function as soluble agents, films, nanoparticles, or bulk materials, and block copolymers can form higher order nanoscale assemblies. Presently, we are exploring uses for these metallobiomaterials as optical imaging agents, oxygen sensors, drug delivery systems, and responsive materials in biomedicine and sustainable design.
For example, difluoroboron dibenzoylmethane polylactide, BF2dbmPLA, is a multi-emissive material exhibiting both intense blue fluorescence and unusual long lived, room and body temperature green phosphorescence, the latter of which serves as the basis for optical oxygen sensing via a quenching mechanism. When fabricated as nanoparticles and tested in cell, tissue and in vivo mouse models, 2-photon absorbing, fluorescent boron systems are very stable to photobleaching compared to other common imaging agents such as green fluorescent protein or FITC and very bright compared to quantum dots. Given high sensitivity in low oxygen environments, boron biomaterials may serve as hypoxia or anoxia sensors in tumors, vascular blockage relevant to strokes and heart disease, organ transplantation, and many other medical contexts. Emission color tuning is achievable via modification of the diketonate ligand, polymer composition, or molecular weight. Efforts are underway to better understand the properties of boron diketonate compounds and fully exploit the potential of these impressive, single-component, multi-emissive biomaterials for a wide range of imaging and sensing applications.
We have also explored polymeric iron, ruthenium and europium complexes, including block copolymer systems. Multifunctional iron dibenzoylmethane initiators also serve as catalysts for lactide ring opening polymerization and introduce acid responsive features into the resulting biomaterial. Entirely unexpected air sensitivity is noted for iron tris(bipyridine) complexes in poly(ethylene glycol) environments and other iron bipyridine polymers are reversibly thermochromic. Akin to biomineralization processes in nature, hierarchically structured iron block copolymer films function as both metal delivery systems and nanoscale templates for controlled iron cluster formation upon annealing. Coupling triggered changes at the metal center with structural transformations in ordered films in lanthanide systems introduces unique dynamic features into nanoscale materials. Luminescent ruthenium systems have been tested as gene delivery vectors. Because the properties and stabilities of metal complexes are highly tunable through variation of the metal ion, the ligand set, and other molecular parameters, this makes the coordinate bond a remarkably versatile platform for macromolecular assembly.
Aromatic difluoroboron β-diketonate complexes: effects of π-conjugation and media on optical properties. Xu S, Evans RE, Liu T, Zhang G, Demas JN, Trindle CO, Fraser CL. Inorg Chem. 52:3597-610 (2013).
Laser phosphoroscope and applications to room-temperature phosphorescence. Payne SJ, Zhang G, Demas JN, Fraser CL, Degraff BA. Appl Spectrosc. 65:1321-4 (2011).
An easy method to monitor lactide polymerization with a boron fluorescent probe. Zhang G, Xu S, Zestos AG, Evans RE, Lu J, Fraser CL. ACS Appl Mater Interfaces. 2, 3069-74 (2010).
Stereocomplexed Poly(lactic acid)-Poly(ethylene glycol) Nanoparticles with Dual-Emissive Boron Dyes for Tumor Accumulation. Kersey FR, Zhang G, Palmer GM, Dewhirst MW, Fraser CL. ACS Nano. 4:4989- 4996(2010).
Mechanochromic Luminescence Quenching: Force-Enhanced Singlet-to-Triplet Intersystem Crossing for Iodide-Substituted Difluoroboron-Dibenzoylmethane-Dodecane in the Solid State. Zhang G, Lu J, Fraser CL. Inorg Chem. in press (2010).
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