Control of the Orientational Order and Nonlinear Optical Response of the “Push−Pull” Chromophore RuPZn via Specific Incorporation into Densely Packed Monolayer Ensembles of an Amphiphilic 4-Helix Bundle Peptide: Second Harmonic Generation at High Chromophore Densities

Abstract

The macroscopic nonlinear optical response of the “push−pull” chromophore RuPZn incorporated into a single monolayer of the amphiphilic 4-helix bundle peptide (AP0) covalently attached to a solid substrate at high in-plane density has been measured. The second-order susceptibility, χzzz, was found to be in the range of ∼15 × 10−9 esu, consistent with a coherent sum of the nonlinear contributions from the individual chromophores (β⃡) as previously measured in isotropic solution through hyper-Rayleigh scattering as well as estimated from theoretical calculations. The microscopic hyperpolarizability of the RuPZn chromophore is preserved upon incorporation into the peptide monolayer, suggesting that the chromophore−chromophore interactions in the densely packed ensemble do not substantially affect the first-order molecular hyperpolarizability. The polarization angle dependence of the second harmonic signal reveals that the chromophore is vectorially oriented in the two-dimensional ensemble. Analysis of the order parameter together with information obtained from grazing incidence X-ray diffraction help in determining the chromophore orientation within the AP0−RuPZn monolayer. Taking into account an average pitch angle of ∼20° characterizing the coiled-coil structure of the peptide bundle, the width of the bundle’s tilt angle distribution should be σ ≤ 20°, resulting in a mean value of the tilt angle 23° ≤ θ0 ≤ 37°