Comparison of the structure property relationships in LB films of zwitterionic TCNQ adducts

Abstract

Substitution reactions of TCNQ (7,7,8,8-tetracyanoquinodimethane) with suitable quinolinium and picolinium electron donor moieties yielded an extensive analogous series of α-bridged quinolinium and picolinium adducts R(2)Q3CNQ and R(2)P3CNQ respectively of which the long chain (Z)-{β-[N-(1-hexadecyl)quinolinium-2-yl]-α-cyano-4-styryl}-dicyanomethanide (C₁₆H₃₃(2)Q3CNQ) is a typical example. These zwitterionic D–π–A materials (where D and A are electron donor and acceptor respectively) exhibit properties such as solvatochromism, molecular rectification and second harmonic generation and were compared and contrasted to the existing γ-bridged adducts. The behaviour of the materials on the water subphase and their resultant Langmuir–Blodgett (LB) film fabrication were studied. The abrupt change in molecular orientation in LB films of the γ-bridged adducts occurring at R = C₁₅H₃₁ and above was not seen in the more flatly orientated α-bridged adducts. Further characterisation of the LB films was performed using reflectance–absorption infra-red spectroscopy (RAIRS) and surface plasmon resonance (SPR). The crystal structure of the long chain adduct, C₁₀H₂₁(2)Q3CNQ, is also reported.