Nonlinear optical properties of polypeptide derivative

PNPPLG (poly(4-nitrophenylprolin)-L-glutamate) with nonlinear optical (NLO) chromophores in the side-chain, was synthesized by transesterification of PBLG (poly-γ-benzyl-L-glutamate) with NPP (4-nitrophenylprolinol). PNPPLG was oriented parallel to the surface in the magnetic field. The NLO coefficients were measured by second harmonic generation (SHG) direction normal to the film surface. Measured SHG coefficient of PNPPLG film was one tenth of that of quartz, which is much higher than that of PBLG. This result suggests that NPPs attached in the side-chain are inductively oriented by the orientation of main chain (α-helix), which renders them partially non-centrosymmetric. Comparing the PNPPLG film with PNPPLG+30 wt% NPP, the SHG coefficient of the latter was found to be twice of that of the former. This result suggests that free NPP can be oriented more easily than the side-chain NPP whose motion is limited by the bulky main-chain.