Multipolar Contributions to the Second Harmonic Response from Mixed DiA−SDS Molecular Aggregates

Abstract

Polarization-resolved hyper-Rayleigh scattering experiments are reported for mixed suspensions of a constant concentration of 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA), a cationic amphiphilic compound with a strong quadratic hyperpolarizability, and varying concentrations of sodium dodecyl sulfate (SDS), an anionic surfactant with a vanishing quadratic hyperpolarizability. It is observed that the electric dipole contribution of the HRS signal intensity attributed to the free DiA molecules in the solution decreases with an increasing concentration of SDS whereas the electric quadrupole contribution increases simultaneously. These changes are direct indicators of the molecular changes undergone in the solution, and in particular the appearance of centrosymmetric mixed DiA-SDS micelles. A detailed analysis of the HRS signal intensity is performed, allowing the monitoring of the evolution of the organization of these mixed molecular systems. These experimental results are described within a general model based on the centrosymmetrical spatial organization of the DiA probe compounds in the micelles. These results underline the potential use of polarization-resolved hyper-Rayleigh scattering as a noninvasive technique to investigate the molecular organization at the nanometer scale in liquid suspensions.