Multifunctional Properties of (Z)-β-(N-Hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide: a Molecular Rectifier, Optically Non-Linear Dye, and Ammonia Sensor

The zwitterionic dye, Z-β-(N-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33-Q3CNQ), forms dimeric solution aggregates, which are probably centric, but adopt a non-centrosymmetric arrangement at the air/water interface. When spread from dilute solution, e.g. 3 × 10-5 M in CH2Cl2, the resultant Langmuir-Blodgett (LB) films are purple, λmax 563 ± 3 nm. They have an optimum effective second-order non-linear optical susceptibility of 75 pm V-1 at 1.064 μm and, when sandwiched between gold electrodes, exhibit asymmetric current-voltage curves, characteristic of molecular rectification. In contrast, when spread at slightly higher concentrations (e.g. 10-4 M), the resultant LB films are polymorphic and have absorption maxima at 563 and 670 nm, the latter being characteristic of the benzyl analogue (φ-CH2-Q3CNQ). Its films are turquoise, λmax 668 ± 3 nm, and both molecular rectification and second-harmonic generation are inhibited. Thus, the red-shifted charge-transfer band probably reflects an altered alignment whereby the molecules adopt a centric antiparallel arrangement within the monolayer. The zwitterionic films readily protonate, with bleaching, and may be used to sense ammonia at concentrations of ca. 1 ppm by monitoring changes in the surface plasmon resonance.