Photoswitching the donating and catalytic properties of N-heterocyclic carbenes and the design of functional co-polymers for stabilization of iron oxide nanoparticles
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In an effort to develop broadly applicable photoswitchable catalysts, we have reported a method for modulating N-heterocyclic carbene (NHC) donicity using light by incorporating a photochromic diarylethene (DAE) into the backbone of a NHC scaffold. UV irradiation of 4,5-dithienylimidazolone or an analogous NHC-Ir(CO)₂Cl complex effected a photocyclization between the two thiophene rings, which led to a change in the electron donating ability of the NHC scaffold. Subsequent exposure to visible light reversed the photocyclization reaction. The concept of photo-modulating NHC donicity in this manner enabled photoswitchable NHC organocatalysis. The catalytic activity of a DAE-annulated imidazolium pre-catalyst in transesterification and amidation reactions was successfully switched between the active and nearly inactive states ([kappa]vis/[kappa]UV = 100) upon alternate UV ([lambda]irr = 313 nm) or visible ([lambda]irr > 500 nm) irradiation. The photoswitchable NHC organocatalysis was later extended to facilitating ring-opening polymerizations of cyclic esters, the rates of which were controlled via external light stimuli. Additionally, a photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)-Rh(I) complex was synthesized and enabled photoswitching of the catalytic activity in series of hydroboration reactions. All of the examples demonstrate extremely rare instances of photomodulating a catalyst's activity by tuning its electronic properties. Furthermore, by taking advantage of the versatility of NHCs in both organo- and organometallic catalysis, we have developed novel photoswitchable catalysts for a variety of applicable transformations. Nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery and electromagnetic imaging. We have developed an approach that will facilitate nanopaticle transport through porous media at high salinity by adsorbing or grafting rationally designed co-polymers on platform nanoparticles. Notably, co-polymers of acrylic acid with either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate have been electrostatically adsorbed or covalently grafted onto iron oxide nanoclusters. The presence of sulfonate groups on the iron oxide surface enabled long-term colloidal stability of the particles in extremely concentrated brine (8% wt. NaCl + 2% wt. CaCl₂) at elevated temperatures (90 °C) and minimized their adsorption on model mineral surfaces.