This research focuses on superacid perfluoroalkyl(aryl) sulfonimides and their derivatives as a source of stable zwitterionic salts. The strong resonance stabilization of sulfonimides imparts unexpected stability to the zwitterions.
Two types of zwitterions were investigated. The first are symmetrical diaryliodonium zwitterions (DZs) as potential photo acid generators (PAGs) for microlithography. Driven by the demand for smaller feature sizes in the microelectronics industry, microlithography using shorter wavelength light has evolved and new PAGs are of interest. New DZs are potentially useful because they have good thermal stability and may be structurally modified to alter their spectral absorption characteristics. As an extension of earlier research in our group, three new symmetric difunctional DZs (p-PhI+PhSO2N-SO2Rf)2 (Rf = C4F8, or C6F12, or C2F4OC2F4) were synthesized. Similar to earlier monofunctional DZs, they have very good thermal stability and high extinction coefficients.
The second zwitterions were developed for electrochemical applications. In order to incorporate strong acid electrolytes into nanoporous carbon electrodes for Polymer Electrolyte Membrane (PEM) fuel cells, we have synthesized functional diazonium zwitterions (FDZs) based on fluorinated sulfonimides. FDZs provide a means to covalently attach the electrolyte to the carbon substrate as an alternative to physical deposition of an electrolyte from a solution. The typical electrolyte deposition for membrane electrode assembly using solubilized NafionŽ cannot be used for nanoporous carbons due to size exclusion.
Most diazonium salts are unstable at room temperature and some are explosive. Three series of thermally stable FDZs were successively prepared. Monofunctional Diazonium Zwitterions (MFDZs) p-N2+PhSO2N-SO2Rf (Rf = CF3 or C4F9) were the first example and were shown to have surprisingly high decomposition temperatures and good solubility in organic solvents. Then, Difunctional Diazonium Zwitterions (DFDZs) (p-N2+PhSO2N-SO2Rf)2 (Rf =C4F8 or C6F12) were synthesized from the experience of making DZs and MDZs. These salts only dissolve in more polar solvents such as DMSO and exhibited good thermal stability. Finally, Multifunctional Diazonium Zwitterions (MFDZs) p-N2+PhSO2N-SO2RfSO2N(H)SO2CF3 (Rf = C4F8 or C6F12) were prepared with two acidic proton positions on grafting. These salts had good solubility in water and several organic solvents. In principle, all of these salts are composed of acidic protons upon grafting to carbon. The best carbon coating method is still under investigation.
An alternative route to higher ion exchange capacity after attachment to carbon was envisaged via two other types of FDZs. A trifluorvinyl ether (TFVE) of the type p-N2+PhSO2N-SO2PhOCF=CF2 was prepared and can in principle be used to prepare a multifunctional polymer using step-growth polymerization with an appropriate functional TFVE monomer. Similarly, the novel FDZs p-N2+PhSO2N-SO2RfSO2N(H)SO2F (Rf = C4F8 or C6F12) can be used to grow or attach a polymer by condensation reaction with an appropriate amide.