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

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Electrostatic properties of gas-liquid interfaces are crucial for microdroplet chemistry.
• Applications span atmospheric chemistry, organic reactivity, and drug delivery.

Purpose of the Study:

• To probe the local electrostatic environment at gas-liquid interfaces at the nanoscale.
• To investigate the extent and nature of interfacial effects in microdroplets.

Main Methods:

• Full-spectral confocal fluorescence microscopy (CFM) was employed.
• A solvatochromic and electrochromic fluorophore (4-Di-1-ASP) was used.
• Density functional theory (DFT) calculations were performed.

Main Results:

• A significant 18 nm emission shift of 4-Di-1-ASP was observed at the gas-liquid interface.
• The interfacial region influencing the dye extends approximately 1 μm into the droplet.
• The interface exhibits a lower dielectric constant than bulk water, equivalent to a high electric field.

Conclusions:

• The gas-liquid interface is characterized by a rigid environment.
• Preferential dipole alignment of the dye and/or water molecules contributes to the observed effects.
• Water presence is essential for the emission shift of solvatochromic dyes at the interface.