SHG from a Single Monolayer of the RuPZn Cofactor within the 4-Helix Bundle AP0
Venkata Krishnan, Hongling Zou, Ventsislav Valev, Thierry Verbiest, Inge Asselberghs, Koen Clays, Michael J. Therien & J. Kent Blasie
Extended chromophores can be designed to possess extraordinary non-linear optical properties. When incorporated into designed n-helix bundle peptides, the peptide can control the microscopic properties of the cofactor and allow for the vectorial-orientation of the peptide-cofactor complex at interfaces, including on surfaces. Together, they can provide for the translation of the cofactor’s microscopic property into a potential device application, either single-molecule based or ensemble-based. Here, we demonstrate that the design of an amphiphilic 4-helix bundle peptide AP0 can be used to vectorially incorporate the non-linear optical cofactor RuPZn, based on an ethyne-bridged zinc-porphyrin polypyridyl-ruthenium. The designed amphiphilicity of the peptide has allowed the covalent attachment of the peptide-cofactor complex to the surface of inorganic substrates, e.g., silicon oxide, maintaining the orientation of the chromophore’s long-axis approximately perpendicular to the plane of the surface. As a result, highly efficient second harmonic generation (SHG) from a monolayer ensemble of the complex has now been observed in reflection geometry, preserving the designed large hyperpolarizability of the cofactor as the incoherent sum in the absence of undesirable interactions between cofactors, extending to very high in-plane densities, e.g. up to one chromophore per 100Å2. While these polarized SHG measurements were performed with a 840nm fundamental, hyper-Rayleigh scattering measurements on the detergent-solubilized cofactor-peptide complex have verified that the molecular hyperpolarizabilty of the chormophore is at least 30x larger at the telecommunications relevant wavelength of 1300nm.