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 (λmax= 614 ± 4 nm; HWHM = 37 ± 2 nm) and occupy 28–34 Å2 per molecule at 25 mN m–1. In contrast, C15H31-Q3CNQ to C20H41-Q3CNQ occupy 40–50 Å2 per molecule; their films are purple with λmax= 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.