π-Stacking and aggregation of pyridinium-substituted indolizines

The tendency for pyridinium-substituted indolizines to aggregate in aqueous and acetonitrile solution is dominated by solute-solvent interactions and by the amphiphilic nature of the cations. In solution, these compounds exhibit an unusual sequence of discrete, red-shifted fluorescence bands whose intensities are a function of concentration over the range 10^-5-10^-3 M. It is proposed that π-stacking of the indolizine residues results in weak electronic interactions (β ∼ 0.13 eV) which are sufficient to result in the delocalization of bound excitonic states over the aggregate. Fluorescence from the aggregate occurs by radiative annihilation of the delocalized exciton. Stepwise dimerization (K₂) and trimerization (K₃) equilibrium constants for the cations calculated from the emission spectra range from about 400 to 65 000. Solution conductivity measurements indicate that cation aggregation does not require ion pairing. However, where significant ion-pairing does occur, K₂ and K₃ are increased by at least an order of magnitude. X-ray crystallography was used to determine the structures of cation dimers which form in the solid state, and these structures were used as working models for the geometry of the solution aggregates. Electrostatic and dispersion interactions calculated on the basis of those geometries account for some of the free energy of aggregate formation, but solvent entropic effects are believed to provide the strongest driving force for indolizine aggregation in solution.