Abstract

4-(N-Hexadecylquinolinium-4-ylmethylideneamino)-2,6-dichlorophenolate (1a) aggregates in low polarity solvents with an equilibrium constant for dimerisation and Gibbs free energy of K = 4500 ± 700 M⁻¹ and ΔG° = −20.5 ± 0.4 kJ mol⁻¹ respectively in chloroform at 20 °C. The aggregating molecules probably adopt an antiparallel arrangement in solution but, nonetheless, form non-centrosymmetric Langmuir–Blodgett (LB) structures. The films are purple with λ_max = 580 nm but, in an acidic environment, reversibly change to yellow with λ_max = 410 nm. The real and imaginary components of the relative permittivity at 632.8 nm are ε_r = 3.0 and ε_i = 1.0 (merocyanine dye) and ε_r = 2.1 and ε_i = 0.0 (protonated form), these data corresponding to a refractive index change of Δn ≈ 0.3. Consequently, the films may be used as the active component of a surface plasmon resonance device to detect either ammonia or acidic gases at ppm concentrations. The properties are compared with those of the pyridinium analogue, 4-(N-hexadecylpyridinium-4-ylmethylideneamino)-2,6-dichlorophenolate (2a) as well as the dibromo analogues of each. The four dyes form dimeric solution aggregates.