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-beta-(N-hexadecyl-4-quinolinium)-alpha-cyano-4-styryldicyanomethanide (C(16)H(33)-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 x 10(-5) M in CH(2)Cl(2), the resultant Langmuir-Blodgett (LB) films are purple, lambda(max) 563 +/- 3 nm. They have an optimum effective second-order non-linear optical susceptibility of 75 pm V(-1) at 1.064 mum 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 (phi-CH(2)-Q(3)CNQ). Its films are turquoise, lambda(max) 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.