Abstract

(E)-N-Hexadecyl-4-[2-(4-octadecyloxynaphthyl)ethenyl]quinolinium bromide, which has a wide-bodied chromophore and terminal n-alkyl groups, adopts a U-shape when spread at the air–water interface but a stretched conformation when compressed to ca. 35 mN m⁻¹. The high-pressure phase has a narrow stability range prior to collapse but may be extended from 40 to 60 mN m⁻¹ by co-spreading the dye in a 1 ∶ 1 ratio with docosanoic acid. The mixed Langmuir–Blodgett (LB) film has a monolayer thickness of 4.6 ± 0.2 nm which decreases to 2.5 ± 0.1 nm layer⁻¹ in the bulk, the reduction arising from an interdigitating layer arrangement, both top and bottom. It is the first example of LB-Lego^® and, in addition, represents the only fully interdigitating structure with non-centrosymmetrically aligned chromophores. They are tilted 38° from the substrate normal. The second-harmonic intensity increases quadratically with the number of layers, i.e. as I^2ω_(N) = I^2ω₍₁₎N², with a second-order susceptibility of χ⁽²⁾_zzz = 30 pm V⁻¹ at 1064 nm for refractive indices of n^ω = 1.55 and n^2ω = 1.73, d = 2.5 nm layer⁻¹ and ϕ = 38°. Angle resolved X-ray photoelectron spectra (XPS) of these films provide no evidence of the bromide counterion, which suggests that it is replaced by OH⁻ or HCO₃⁻, which occur naturally in the aqueous subphase, or C₂₁H₄₃COO⁻ from the co-deposited fatty acid. This probably applies to all cationic dyes deposited by the LB technique.