Langmuir–Blodgett alignment of zwitterionic optically non-linear D–π–A materials

Abstract

The synthesis, Langmuir–Blodgett (LB) deposition and charge-transfer spectra of Z-β-(1-alkyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (R-Q3CNQ where R = C₆H₁₃ to C₂₀H₄₁) and four substituted analogues are described. The deposition is Z-type and the properties are dependent upon the hydrophobic chain length. C₆H₁₃-Q3CNQ to C₁₄H₂₉-Q3CNQ form green LB films (λ_max= 614 ± 4 nm; HWHM = 37 ± 2 nm) and occupy 28–34 Ų per molecule at 25 mN m^–1. In contrast, C₁₅H₃₁-Q3CNQ to C₂₀H₄₁-Q3CNQ occupy 40–50 Ų per molecule; their films are purple with λ_max= 565 ± 4 nm and HWHM = 22 ± 1 nm. The shift is attributed to a change in molecular tilt which causes the transition to alter from an intermolecular to an intramolecular process. Multilayer films of C₁₆H₃₃-Q3CNQ exhibit strong second-harmonic generation (SHG) and the intensity increases quadratically with the number of LB layers.