Fluorescence studies and semiempirical calculations on alkali ion indicators

Two newly synthesized cryptands act as sensitive Na(+)- and K(+)-selective indicators for cation concentrations above 20 μM. The fluorescence properties change markedly upon cation binding. In addition, the free ligands exhibit a pronounced sensitivity to pH, which is considerably lower for the cation complexes. Time resolved fluorescence is characterized by a decay time of about 5 ns that is attributed to the diprotonated protolytic state of the uncomplexed ligands. Semiempirical calculations show the systematic influence of the nitrogen lone pairs or the N-H bond on the stability of the system. The cause of the strong fluorescence intensity increase observed upon protonation of the fluorescent cryptands may be attributed to an increase in the S1-T x energy gap as a consequence of bridgehead nitrogen protonation.