PalmitateLuciferin: A Molecular Design for the Second Harmonic Generation Study of Ion Complexation at the Air–Water Interface

A molecular organic chromophore, PalmitateLuciferin, has been synthesized for studying ion complexation at the air–water interface using second harmonic generation (SHG). This molecule was designed through the addition of a long hydrophobic palmitoyl alkyl chain to the aromatic π-electron system of Luciferin. We first demonstrate that this organic chromophore is a potential candidate for SHG studies of ion complexation with the measurement of its first hyperpolarizability in aqueous solutions by hyper Rayleigh scattering (HRS) with and without calcium ions. Then, we characterize the PalmitateLuciferin surfactant properties at the air–water interface combining surface tension measurements with a surface SHG study and Brewster angle imaging. These results allow us to build a molecular description of the chromophore at the interface and observe its molecular reorganization during the monolayer compression leading to the formation of aggregates. Finally, we show that the initial goal of the designing work is achieved since PalmitateLuciferin indeed exhibits a higher SHG response in the presence of calcium ions in the aqueous subphase as expected.