Notably different interfacial behaviors of two molecules with similar structures revealed by second harmonic generation (SHG) scattering and reflection

To probe the subtle structural evolution of molecules on interfaces is both important and technically challenging. Second-order nonlinear spectroscopic methods with unique interfacial selectivity, including sum frequency generation and second harmonic generation (SHG), have been generally applied to obtain such interfacial information. Recently, we investigated the structural evolution of an amphiphilic dye molecule (D289) on the surfaces of vesicles composed of phospholipids or surfactants with the combination of SHG scattering and two-photon fluorescence (TPF) methods. Here, we investigate the interfacial behavior of another amphiphilic molecule (SD289), which has a similar structure to D289 with minor differences. By combining SHG scattering and SHG reflection, a notably different structural evolution of SD289 on the interface composed of mixed surfactants, including SDS (sodium dodecyl sulfate) and CTAB (hexadecyl trimethyl ammonium bromide), was revealed. It was observed that SHG emission from interfacial SD289 was more than 2 orders of magnitude lower than that from D289, which was induced by the difference in the averaged tilting angles (∼88° vs ∼40°) of the two molecules. The structural evolution of SD289 on the vesicle surface under various temperatures was further investigated by TPF emission from SD289. The enhancement of TPF efficiency of SD289 was also interpreted by its structural evolution on the interface. The results further demonstrate the capability of the combined spectroscopic methods in analyzing the detailed structural evolution of molecules on interfaces, which may shed light on the investigations of molecular structures and kinetics on the membrane surface in general.