Dimerisation processes of triaryl pyrylium salts

The present paper deals with dialkylamino 2,4,6-triaryl pyrylium tetrafluoroborates studied in solution at room temperature. It is intended to clarify the dimerisation process described previously. The photophysical properties of the didodecylamino derivative, the synthesis of which is reported for the first time, are practically identical to those of the methyldodecylamino analogue, but its solubility in non-polar solvents is higher by two orders of magnitude allowing a better investigation of the dimers. The absorption spectra of both compounds, recorded as a function of concentration in solvents of low dielectric constant, exhibit isosbestic points. Different equilibria, involving the cationic chromophore and its counterion, are discussed. Two-dimensional H-1-NMR experiments performed by off-resonance ROESY (Rotating frame nuclear Overhauser Effect Spectroscopy) confirm dimerisation. Electrical conductivity measurements show that in the monomer-dimer equilibrium related to the isosbestic point, the ''monomer'' corresponds to an ion-pair and the ''dimer'' is composed of two ion-pairs.