Charge-transfer complexes. Part IX. Charge-transfer mechanisms in the quenching of the fluorescence of the N-methylacridinium ion by counter-ions

The fluorescence of the N-methylacridinium cation is quenched by a range of counter ions. The quenching efficiency depends on the nature of the anion and on solvent, the quenching constant being higher both in less-polar solvents and for those anions with low redox potentials. Association constants for complex formation in the ground state, determined from the absorption spectrum, are generally much smaller than the quenching constant. This, together, with the increase of the quenching constant with solvent fluidity, indicates that the quenching occurs by three distinct mechanisms, charge-transfer complex formation in the ground state (static quenching), further complex formation in the excited state, and collisional deactivation.