Cation complexation with functionalized 9‐arylacridinium ions: Possible applications in the development of cation‐selective optical probes

Absorptive and emissive properties of 9‐arylacridinium ions 2‐6 have been studied and shown to be dependent on the electron‐donating properties of the aryl group. While for 2 and 3 the first excited state remains largely localized on the acridinium chromophore, interaction of this chromophore with the electron‐donor‐substituted aryl group in 4‐6 produces a low‐lying intramolecular charge‐transfer state, resulting in a state inversion. This state inversion leads to the appearance of a new long‐wavelength absorption in the visible region and to complete quenching of the acridinium fluorescence. Protonation of the amino function in 4‐6 cancels this state inversion, thereby causing a dramatic chromofluoroionophoric effect, i.e. cation‐induced change of both colour and fluorescence properties. Replacing the amino function by an aza crown ether resulted in system 7, which displays similar effects, not only upon protonation but also upon metal ion complexation.