Supramolecular donor-acceptor complexes of dichlorofluorescein and cis- and trans-4,4′-(N,N′-dimethylpyridinium)ethylene

trans-4,4′(N,N′-Dimethylpyridinium)ethylene [trans-(1)] and cis-4,4′(N,N′-dimethylpyridinium)ethylene [cis-1] form with 2,7-dichlorofluorescein [DCF^2-, (2)] donor-acceptor complexes of 1:1 stoichiometry [K(trans-1)=14 000M^-1 and K(cis-1)=300M^-1 in water]. The lower affinity of cis-1 to form the donor-acceptor complex with DCF^2-, (2), is attributed to the non-planar structure of cis-1 (tilt-angle between the pyridinium rings=26°). The solid-state structure of the complex between DCF^2- and trans-1 indicates alternate stacking of donor and acceptor units with an inter-layer spacing of 3-4 Å. Solubilization of the crystalline DCF^2- and trans-1 complex in water or dimethylformamide (DMF) results in the initial formation of a non-symmetric complex where a trans-1 unit is inter-layered between two DCF^2- components, and a second trans-1 unit is located externally to the supramolecular assembly and participates in charge neutralization [(DCF^2-), ⋯trans-1/trans-1]. The primary non-symmetrical complex is thermally transformed to a themodynamically stable symmetric complex where the DCF^2- and trans-1 form a sandwich-type layered assembly [DCF^2- ⋯trans-1]. The structural features of the complexes were characterized by ¹H-NMR spectroscopy. The kinetics of the transformation of the [(DCF^2-)₂⋯trans-1/trans-1] complex to the [DCF^2-⋯trans-1] assembly was spectroscopically characterized in DMF (k=0·22 s^-1 at 322 K; E_a,=20 kcal mol^-1).