OQX(Z)-a-cyano-~-(N-hexadecylquinolin-4-ylium)-4-styryldicyanomethanide, C16H3,-Q3CNQ, has two Langmuir-Blodgett (LB) phases that differ in thickness, tilt angle and second-order susceptibility. For phase I, /=2.2 nm layer-', 4 = 8 " and ~( ~) = 1 8 0 pm V-' at 1.064 pm, whereas for phase II, /=1.6 nm layer-', #=24" and ~( ~) = 1 0 0 pm V-'. The susceptibility of phase I is the highest so far reported for an LB multilayer; the structure is non-centrosymmetric (Ztype) and, as predicted by theory, the second-harmonic intensity varies as /2w(N)=/2w(,)N2 as N increases from 1 to 200 layers. In contrast, C10H2,-Q3CNQ has a low susceptibility (x,z(~)= 6 pm V-') and neutron reflectivity studies have indicated an antiparallel alignment of chromophores at the air/water interface.

The synthesis, Langmuir-Blodgett (LB) deposition and charge-transfer spectra of Z-/3-(l-alkyl-4-quinolinium)-acyano-4styryldicyanomethanide (R-Q3CNQ where R = C&i 1 to CzoH41) and four substituted analogues are described. The deposition is Z-type and the properties are dependent upon the hydrophobic chain length. C6H13-Q3CNQ to C14H,,-Q3CNQ form green LB films (A,,, = 614 4 nm; HWHM = 37 k 2 nm) !nd occupy 2&34 A' per molecule at 25 mN m -" . In contrast, Cl,H3,-Q3CNQ to C2,H,,-Q3CNQ occupy 40-50 A' per molecule; their films are purple with A , , , = 565 k 4 nm and HWHM = 22 & l 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 C, ,H3,-Q3CNQ exhibit strong second-harmonic generation (SHG) and the intensity increases quadratically with the number of LB layers.

Mixed Langmuir-Blodgett films of Z-p-( 1 -hexadecyl-4-pyridinium)-a-cyano-4-styryldicyanomethanide (C16H33-P3CNQ) and Z-p-( 1-hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (C16H33-Q3CNQ) exhibit a single charge-transfer band that may be tuned within the range 495 to 565 nm; the films are photochromic and exhi bit second-harmonic generation.

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