Optical characterization of oligo(phenylene-ethynylene) self-assembled monolayers on gold

Vibrationally resonant sum-frequency generation (VR-SFG) and spectroscopic ellipsometry (SE) have been used to characterize self-assembled monolayer films of unsubstituted and mononitro-substituted oligo(phenylene-ethynylene) molecules on vapor-deposited An substrates. When combined with quantum chemical calculations of the relevant transition moment directions, orientation distributions and electronic excitation spectra are obtained. The orientation distribution from VR-SFG is in good agreement with previous IR reflection studies, indicating both molecules are tilted from the surface normal by similar to30degrees. The calculated resonant hyperpolarizabilities are in good agreement with experimental spectra. The optical polarizability extracted from SE suggests strong intermolecular interactions, consistent with molecular exciton theory.