Artificial photosynthesis is a potentially economic, green strategy for converting sunlight energy into useable energy. I will report on my progress toward mimicking nature in function, by harvesting light to drive endergonic proton transfer across a membrane, consequently converting photon energy into ionic current. The initial model system utilized conical nanopore scaffolds functionalized with photoacid dye molecules. Water soluble, pyrenol photoacid dyes with terminal ammonium groups were synthesized. Spectrophotometric titration curves showed that the dye had a hydroxyl pKa value of 5.8; photoluminescence measurements indicated that the hydroxyl excited-state pKa value, pKa*, was –0.7. Therefore, the maximum possible photovoltage from proton pumps using the dye was estimated to be ~380 mV (59 mV x ΔpKa). Films of poly(ethylene terephthalate) (PET) were bombarded with swift heavy ions and subsequently treated with alkaline etchant to produce conical nanopores with fixed anionic carboxylate groups. The nanopores were asymmetrically functionalized to generate a region containing covalently bonded cationic photoacid dyes adjacent to unmodified carboxylate groups. The tailored region of fixed charges was intended as a model for an ion-exchange membrane recently shown to exhibit photovoltaic action. Atypical measurements were used to probe the most confined region of single nanopores; the region contained effectively ~100 mM dyes in ~800 zeptoliters. Electrochemical measurements suggested that the dye pKa value (2.4-5.2) was pore-size dependent, was attached via two out of three ammonium groups, and had a binding density of 0.3 dyes/nm2. Two-photon fluorescence microscopy of modified films further verified that the dye was covalently attached in nanopores; however, the apparent pKa* was ~1.4. Calculations using known values suggested a maximum proton-photocurrent of 10 fA, which is challenging to detect. I am currently expanding this research to other dyes and polymeric materials in hopes of better understanding mechanistic details of nanoconfined photoacid molecules in electrostatically-complicated regions.

Alteration of photoacidic behavior of pyrenol dyes covalently bound in the nanoconfined, electrostatically-complicated tips of asymmetric nanopores

253rd National Meeting (2017)

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