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

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 chi(zzz)((2)) pm V-1 at 1.064 mu m. 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 lambda = 532 nm are l = 31.8 Angstrom, epsilon(r) = 2.44 and epsilon(i) = 0.23 for monolayer films of the dye and l = 42.9 Angstrom, epsilon(i) = 2.85 and epsilon(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 Angstrom by difference, may be explained by interdigitation of the alkyl chains at the interface. The bilayer thickness of 42.9 Angstrom from SPR is in agreement with the d-spacing of 42 +/- 1 Angstrom from X-ray synchrotron diffraction.