Optical Tuning of Surface Tension at the Air–Water Interface Using a Merocyanine Photoacid

Merocyanine photoacids (MCH) undergo light-induced proton release followed by cyclization to form spiropyran (SP). While the bulk photochemistry of this system has been extensively characterized, its influence on interfacial thermodynamic properties, such as surface tension, remains less explored. Here, we quantify light-driven changes in the surface tension of aqueous MCH solutions using a Langmuir–Blodgett (LB) trough. Upon illumination by a 430 nm LED, the surface tension decreases rapidly and recovers to its initial value after the light is removed, demonstrating reversible modulation of surface tension. The magnitude of the surface tension decrease is consistent with enhanced interfacial partitioning of the photogenerated SP relative to MCH. To probe the role of interfacial electrostatics on the partitioning, we introduced LB monolayers bearing neutral, positive, or negative headgroups. Because SP is negatively charged, the light-induced surface tension change is amplified in the presence of positively charged monolayers and suppressed with negatively charged and neutral monolayers. These trends support an electrostatic contribution to the interfacial stabilization of the photogenerated SP. Together, these results establish the parameters to tune surface tension with visible light, which may enable spatial and temporal control of physical and chemical processes at the interface.