Merocyanine dyes: dimeric aggregation in solution, film structure and pH sensing

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 DeltaG degrees = -20.5 +/- 0.4 kJ mol(-1) respectively in chloroform at 20 degreesC. The aggregating molecules probably adopt an antiparallel arrangement in solution but, nonetheless, form non-centrosymmetric Langmuir-Blodgett (LB) structures. The films are purple with lambda (max) = 580 nm but, in an acidic environment, reversibly change to yellow with lambda (max) = 410 nm. The real and imaginary components of the relative permittivity at 632.8 nm are epsilon (r) = 3.0 and epsilon (i) = 1.0 (merocyanine dye) and epsilon (r) = 2.1 and epsilon (i) = 0.0 (protonated form), these data corresponding to a refractive index change of Deltan approximate to 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.