Molecular Conformation in Organic Films from Quantum Chemistry ab Initio Calculations and Second Harmonic Spectroscopy

Identifying molecular species and conformations at surfaces is of interest for both catalytic and biointerfacial research. Here, we present an extinction and second harmonic spectroscopy study of films of crystal violet together with quantum chemistry ab initio calculations on several conformation geometries of this dye. By comparing experimental and theoretical data, we identify the molecular species responsible for the spectral resonances, and we ascribe the nonlinear optical generation to pyramidal conformers with nonzero dipolar hyperpolarizability. Our results, which are consistent with recent findings on alcohol solutions of the same dye, help to settle a long-standing debate and demonstrate that nonlinear second harmonic spectroscopy is a valuable tool for identifying and probing molecular conformation in interfacial systems.