Synthesis and Second Harmonic Generation Studies of Langmuir-Blodgett Monolayers of 4-(N-Docosyl-N-methylamino)-β-nitrostyrene

The nonlinear optical chromophore 4-amino-β-nitrostyrene was derivatized to yield a Langmuir-Blodgett (LB) film forming amphiphillic molecule 4-(N-docosyl-N-methylamino)-β-nitrostyrene (DMANST). The Z-type LB monolayers of DMANST was transferred onto hydrophilic silica substrates and characterized by optical second harmonic generation (SHG) measurements. The SHG results reveal that the LB film of pure DMANST deposited onto a silica substrate consists of molecular inhomogeneities on a scale larger than a wavelength. The presence of a strong SHG signal for both the p- and s-polarized fundamental beams, at the normal incidence, suggests the lack of in-plane symmetry in the monolayer of pure DMANST on the silica surface. The LB film of DMANST mixed with docosanoic acid shows a SHG behavior that is pertaining to the C„ν symmetry. This indicates that the symmetry in the film plane is restored in monolayers of DMANST mixed with docosanoic acid. The chromophore tilt angle of DMANST mixed with docosanoic acid is calculated to be 42° with respect to the film normal.