Complementary surface second harmonic generation and molecular dynamics investigation of the orientation of organic dyes at a Liquid/Liquid interface

The second-order nonlinear response of two dyes adsorbed at the dodecane/water interface was investigated by surface second harmonic generation (SSHG). These dyes consist of the same chromophoric unit, 2-pyridinyl-5-phenyloxazole, with an alkyl chain located at the two opposite ends. The analysis of the polarization dependence of the SSHG intensity as usually performed points to similar tilt angles of the two dyes with respect to the interface but does not give information on the absolute direction. Molecular dynamics (MD) simulations reveal that both dyes lie almost flat at the interface but have opposite orientations. A refined SSHG data analysis with the width of the orientational distribution yields tilt angles that are in very satisfactory agreement with the MD simulations.