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

Abstract

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 ∼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.

Remote Electrostatic Repulsion Trigged by Excited State Antiaromaticity Relief as Origin of Photoacidity in an Organic Dye

“…[1][2][3] This interesting feature has led to the development of numerous lightcontrolled applications using photoacids. [4][5][6] For instance, metastable merocyanine-based photoacids are capable to modulate the pH of the medium [7][8][9] and initiate polymerization reactions. 10 Polymerization reactions can also be initiated by photoacid generators, 11 which are species that deprotonate irreversibly in the excited state, thereby distinguishing them from true photoacids.…”

Light-Triggered Reversible Change in the Electronic Structure of MoO3 Nanosheets via an Excited-State Proton Transfer Mechanism

“…The use of an acid is not reversible, and there is a need to add a base to induce its reversibility; the use of a photoacid is reversible, i.e., in the dark the driving force is toward reprotonation. It is also important to note that due to the immediate reprotonation of RO – in the ground state in an aqueous solution, the use of such photoacids does not result in a stable pH jump of the solution, which is different from the case for other types of photoacids, such as the merocyanine–spiropyran system that can result in a stable pH jump . Hence, to use the HPTS photoacid to control pH-dependent processes, the terminal proton acceptor has to be the system or molecule in question.…”