Blue Shift in Electrochromic Dye Emission Indicates Distinct Electrostatic Environment at Air-Water Interfaces

The electrostatic properties of gas-liquid interfaces are thought to play essential roles in microdroplet chemistry with applications ranging from atmospheric chemistry to organic reactivity and aerosolized drug delivery. In this study, we use full-spectral confocal fluorescence microscopy (CFM) to probe the local electrostatic environment at gas-liquid interfaces at the hundreds of nanometers length scale. CFM images show that the solvatochromic and electrochromic fluorophore, 4-Di-1-ASP, exhibits an 18 nm (484 cm^-1) shift in emission at the gas-liquid interface vs the center of a water droplet. The observed interfacial region extends ∼1 μm into the droplet, revealing that interfacial effects extend farther into droplets than previously thought. Solvatochromic measurements and density functional theory calculations show that the shift in emission maximum indicates that the interfacial region has a dielectric constant notably lower than that of water. The same shift would be observed if an electric field of 4.7-10. MV cm^-1 were applied along the x-axis of the molecule. Measurements in a range of organic solvents demonstrate that water must be present to observe a shift in the emission maxima of solvatochromic dyes at the gas-liquid interface. Based on these results, we propose that the interfacial environment is rigid, with 4-Di-1-ASP and/or water likely preferentially aligning their dipoles perpendicular to the air-water interface. This rigid environment leads to a regime of lower dielectric constant or, equivalently, an effective difference in internal electric field. These results set the stage for molecular-level control at gas-liquid interfaces.