Linear and non-linear optical properties of the different Langmuir-Blodgett phases of CnH2n+1-Q3CNQ

(Z)-α-cyano-β-(N-hexadecylquinolin-4-ylium)-4- styryldicyanomethanide, C₁₆H₃₃-Q3CNQ, has two Langmuir-Blodgett (LB) phases that differ in thickness, tilt angle and second-order susceptibility. For phase I, I = 2.2nm layer^-1, φ = 8° and χ_zzz⁽²⁾ = 180 pm V^-1 at 1.064 μm, whereas for phase II, I = 1.6 nm layer^-1, φ = 24° and χ_zzz⁽²⁾ = 100 pm V^-1. The susceptibility of phase I is the highest so far reported for an LB multilayer; the structure is non-centrosymmetric (Z-type) and, as predicted by theory, the second-harmonic intensity varies as I_2ω(N) = I_2ω(1)N ² as N increases from 1 to 200 layers. In contrast, C ₁₀H₂₁-Q3CNQ has a low susceptibility (χ _zzz⁽²⁾ = 6 pm V^-1) and neutron reflectivity studies have indicated an antiparallel alignment of chromophores at the air/water interface.