‘Molecular zips’: evidence of the interdigitation of layers in Langmuir-Blodgett films of an optically non-linear dye and a compatible spacer

Abstract

Second-harmonic generation (SHG) from alternate-layer Langmuir–Blodgett (LB) films of (E)-4-[2-(4-methoxynaphthyl)-ethenyl]-N-docosylquinolinium bromide (dye) and N-octadecyl-4-methylquinolinium bromide (spacer) increases quadratically with the number of bilayers with a susceptibility of χ_zzz⁽²⁾= 20 pm V^–1 at 1.064 µm. The lower layers show significant variation of the second-harmonic intensity across the surface whereas the thicker films provide evidence of self-healing with a maximum variation of ± 10% across 90% of the area studied. The thickness and the real and imaginary parts of the dielectric permittivity from the surface plasmon resonance (SPR) at λ= 532 nm are l= 31.8 Å, ε_r= 2.44 and ε_i= 0.23 for monolayer films of the dye and l= 42.9 Å, ε_r= 2.85 and ε_i= 0.29 for the dye–spacer bilayer. The dye and spacer have compatible interlocking geometries and the diminished depth of the spacer layer, 11 Å by difference, may be explained by interdigitation of the alkyl chains at the interface. The bilayer thickness of 42.9 Å from SPR is in agreement with the d-spacing of 42 ±Å from X-ray synchrotron diffraction.