UC Irvine

Photo-initiated ion transport on the nanoscale is relevant to various fields including energy conversion, neuron triggering, and biomimetic processes. To study this phenomenon, we synthesized and evaluated photoacid-modified polymers, which generate hydrated protons in response to visible-light excitation. Pyrenol photoacid dye molecules were covalently bonded within conical nanopores that had been track etched in poly(ethylene terephthalate). The data suggest that ∼90% of the nanopore surfaces were modified with photoacids and that photoacids were, on average, bound through three sulfonamide groups. In comparison to photoacids dissolved in an aqueous solution, photoacids bound specifically to the nanoporous tip region were, on average, weaker acids in their ground and excited states, which we presume is due to differences in surface potential or solvation environment in the confined nanopores. These results suggest that ion transport initiated by nanoconfined photoacids will require careful molecular engineering to enable efficient light-to-ionic energy conversion. A major challenge facing humanity is the availability of clean and potable water for human consumption and agriculture. Over time, this problem is expected to worsen because of population growth and increases in the severity and frequency of global climate disruption. As such, new technologies for low-cost, efficient, and distributed desalination of salt water are desired. Toward this, our group recently reported ion-selective membranes that directly pump ions when illuminated with sunlight. A critical component in the technology is the polymer-bound photoacid dye, which is carefully characterized herein. In our initial materials systems, controlled quantitative measurements of the properties of polymer-bound photoacids were extremely challenging. Using the knowledge gained from studies herein, we are hopeful that the efficiency of our technology can be increased rapidly, to a point where it enables inexpensive and autonomous desalination of distributed saltwater sources. We previously reported photovoltaic action from photoacid-dye-modified ion-exchange membranes. A more controlled model system for those materials are photoacid-modified nanopores in poly(ethylene terephthalate) reported herein. Photoacids bound to the sub-10-nm-sized tips of these nanoporous poly(ethylene terephthalate) materials exhibited decreases in ground-state and excited-state acidity versus the same photoacid dyes dissolved in solution. The data indicate that nano-confinement and local electrostatics are important considerations when designing light-to-ionic energy conversion devices with possible applications in energy conversion and neuron triggering.