Photoluminescence Investigation of Fundamental Charge Transfer Processes in Stable Nitroxyl Radical-Containing Polymers

The electrochemical redox reactions of stable organic polymer materials with unpaired electrons is a relatively new and exciting area of battery research.  Specifically, stable nitroxyl radical-containing polymers have displayed excellent characteristics as the cathode-active material and have presented themselves as low-cost and environmentally friendly alternative to the current Li-ion battery technologies. However, with the advancement of organic-radical battery technology there is a lack of fundamental understanding of the dynamics of charge transfer processes, electronic and ionic, within organic radical-containing polymers.  In order to elucidate the mechanisms further, we have conducted an extensive photoluminescence investigation of the polymers in various oxidation states

Since nitroxyl radicals are known fluorescence quenchers, we probed the charge transport in this system by the incorporation of fluorescent perylene markers into the radical-containing polymer matrix.  We have investigated the interplay between collisional and static quenching processes using steady-state and time-resolved photoluminescence quenching studies of perylene by TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), radical-containing species.  Quenching studies for these model systems, including both free TEMPO monomers and non cross-linked, solution-phase PTMA-nitroxy polymers, (poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) have allowed us to calculate optimal synthetic parameters for construction of polymer cathode systems.  We have determined how such polymer properties as average chain length, spacing between pendant groups, proclivity for polymer folding, and solvent swelling affect both the mechanism of quenching as well as the efficiency of charge transfer. As a function of polymer length, we observe a change in quenching mechanism, while the density of radical moieties has significant affects on quenching/charge transfer efficiency.  The quenching sphere of action and thus the optimal distance between radical moieties and fluorophores for the most efficient charge transfer has also been determined and will be discussed at length.