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

Abstract

A molecular organic chromophore, Palmitate-Luciferin, 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 πelectron 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.

Synthesis and Characterization of a Chromo-Extractant to the Probe Liquid–Liquid Interface in a Solvent Extraction Process

“…This difference may be due to the geometry of the chromophore whose symmetry is more pronounced for azobenzene (carried by the MAD) than it is for the azopyridinium derivative (DiA). Finally, this hyperpolarizability is of the same magnitude as that of palmitateluciferin 100 ± 20 × 10 –30 esu, another molecular design organic chromophore that was not sufficiently lipophilic to be studied at the liquid–liquid interface. A first SHG experiment was thus carried out at the water/methylcyclohexane interface.…”

“…The second harmonic generation (SHG) experiments have shown their potentiality but a limitation is that the technique suffers from a weak signal. , The data analysis depends indeed on several factors that are difficult to determine independently such as the concentration of the various chemical components, their orientation distribution, and their hyperpolarizabilities. It is therefore necessary to choose a chemical probe that has both a fairly strong SHG response compared to those of the other components and the ability to extract specific ions, which make the search for such a molecule an interesting challenge . Molecular systems containing photoresponsive units such as azobenzenes have been the subject of several investigations such as the design of azobenzene photopolymers , or photosurfactants. − However, to the best of our knowledge, there is no system reported in the literature focused on the incorporation of azobenzene into the structure of an extractant molecule.…”

“…It is therefore necessary to choose a chemical probe that has both a fairly strong SHG response compared to those of the other components and the ability to extract specific ions, which make the search for such a molecule an interesting challenge. 22 Molecular systems containing photoresponsive units such as azobenzenes have been the subject of several investigations such as the design of azobenzene photopolymers 23,24 or photosurfactants. 25−27 However, to the best of our knowledge, there is no system reported in the literature focused on the incorporation of azobenzene into the structure of an extractant molecule.…”

Ordering and Nonideality of Air–Ionic Liquid Interfaces in Surface Second Harmonic Generation

“…Figure 1 does not provide the full polarization analysis but already suggests that the orientational distribution is the key factor in the changes observed. To get a complete view of the modifications undergone by the interface, polarization-resolved measurements were performed as reported previously, 3,4 rotating the linear input polarization angle from 0 to 90°and collecting the SH intensities for two different output polarizations, namely the S-out and P-out polarizations. The corresponding polarization plots are shown in Figure 2 In these plots, the trend shown in Figure 1 is also observed, namely a large increase of the P-out intensity at γ = 0°while the S-out plot increases too, but to a lesser extent.…”

“…This sensitivity arises from the cancellation of this process within the bulk of centrosymmetric media like liquids in the electric dipole approximation, whereas it is allowed at the interface where the centrosymmetry is broken. Hence, a noninvasive approach can be designed to investigate both the structure and the dynamics of such surfaces and interfaces. − The SHG technique is therefore well adapted to study the air–ionic liquid (ILs) interfaces.…”