Abstract

The second-harmonic intensity from Langmuir–Blodgett films of 5-{4-[2-(4-dimethylaminophenyl)vinyl]benzylidene}-2-octadecyl-5,6,7,8-tetrahydroisoquinolinium octadecyl sulfate(1), interleaved with poly(tert-butyl methacrylate), increases quadratically with thickness to more than 100 bilayers. The susceptibility, molecular hyperpolarisability, thickness, refractive indices and chromophore tilt angle, relative to the substrate normal, are χ_zzz⁽²⁾ = 45 ± 5 pm V⁻¹ at 1064 nm, β = 8 × 10⁻³⁸ m⁴ V⁻¹, l = 3.65 ± 0.12 nm bilayer⁻¹, n^ω = 1.53 ± 0.01, n^2ω = 1.56 ± 0.01 and φ = 44 ± 2°. The moderately high susceptibility arises from an optimised transparency/efficiency trade-off, the alternate-layer LB film being transparent at 1064 nm and having a very slight absorbance of 3.5 × 10⁻⁴ bilayer⁻¹ at 532 nm. In contrast, alternate-layer films of the corresponding 7-diethylamino-2-oxo-2H-chromen-3-ylmethylene (2) and 3-(dimethylaminophenyl)prop-2-enylidene (3) derivatives, also reported, have absorbances of 1.1 × 10⁻³ and 1.5 × 10⁻³ bilayer⁻¹ at 532 nm and resonantly enhanced susceptibilities of 60 and 95 pm V⁻¹ respectively.