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    Application of Super Photoacids in Controlling Dynamic Processes: Light-Triggering the Self-Propulsion of Oil Droplets

    Year: 2022

    Journal: J. Phys. Chem. B, Volume 126, AUG 25, page 6331–6337

    Authors: Yucknovsky, Anna; Rich, Benjamin B.; Gutkin, Sara; Variyam, Ambili Ramanthrikkovil; Shabat, Doron; Pokroy, Boaz; Amdursky, Nadav

    Organizations: Ariane de Rothschild fellowship; US - Israel Binational Science Foundation [2018239]; Ministry of Science and Technology [3-16243]; Russel Berrie Nanotechnology Institute (RBNI); Grand Technion Energy Program (GTEP)

    The dynamic control of pH-responsive systems is at the heart of many natural and artificial processes. Here, we use photoacids, molecules that dissociate only in their excited state and transfer their proton to nearby proton acceptors, for the dynamic control of processes. A problem arises when there is a need to protonate highly acidic acceptors. We solve this problem using super photoacids that have an excited-state pK(a) of -8, thus enabling them to protonate very weak proton acceptors. The process that we target is the light-triggered self-propulsion of droplets, initiated by an excited-state proton transfer (ESPT) from a super photoacid donor to a surfactant acceptor situated on the surface of the droplet with a pK(a), of similar to 0. We first confirm using steady-state and time-resolved spectroscopy that a super photoacid can undergo ESPT to the acidic surfactant, whereas a regular photoacid cannot. Next, we show self-propulsion of the droplet upon irradiating the solvated super photoacid. We further confirm the protonation of the surfactant on the surface of the droplet using transient surface tension measurements. Our system is the first example of the application of super photoacids to control dynamic processes and opens new possibilities in the field of light-triggered dynamic systems.
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