Abstract

Langmuir–Blodgett (LB) films of (E)-4-[(N-octadecyl-5,6,7,8-tetrahydro-5-isoquinolylidene)methyl]-N,N-dialkylaniline octadecyl sulfate, where the donor group is dimethylamino (1a), diethylamino (1b), dibutylamino (1c) and dihexylamino (1d), have optimum susceptibilities when the dyes are co-deposited in a 1∶1 mole ratio with octadecanoic acid. Alternate-layer structures of the mixed films, interleaved with poly(tert-butyl methacrylate), are non-centrosymmetric and the second-harmonic intensity increases with thickness as to N>100 bilayers. Furthermore, when the dialkylamino group is sufficiently hydrophobic, i.e. for 1c and 1d, the dyes form non-centrosymmetric Z-type structures without interleaving spacer layers. The optimum susceptibilities, chromophore tilt angles, thicknesses and refractive indices of the interleaved and non-interleaved mixed films are as follows: alternate-layer (1b), = 67 pm V⁻¹ at 1.064 µm for φ = 37°, l = 4.08 nm bilayer⁻¹, n^ω = 1.50 and n^2ω = 1.58; Z-type (1c), = 76 pm V⁻¹ for φ = 33°, l = 3.15 nm layer⁻¹, n^ω = 1.52 and n^2ω = 1.58. The moderately high susceptibilities arise from an optimised transparency/efficiency trade-off, the films in this series being transparent at the fundamental wavelength (1.064 µm) and having a very slight absorbance of 5 × 10⁻⁴ per active layer at 532 nm.