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 Au 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 ∼30°. 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.