Optical and electrochemical cations recognition and release from N-azacrown carbazoles

Two azacrown derivatives of carbazole have been prepared and their dipole m oments in the excited state were estimated from solvatochromism. Their comp lexation constant K with calcium and sodium cations have been determined fr om the absorption spectra, the fluorescence spectra and, independently. fro m the first oxidation potential shift, as a function of the concentration i n sodium and calcium perchlorates. when K is lower than 10(5). The fluoresc ence spectra of the calcium and sodium complexes in acetonitrile show two b ands different from the fluorescence of the free probes, one from the excit ed complex similar to the fluorescence of the protonated probes and the oth er one from the nitrogen decoordinated cation complexes. These results are typical for moderatly polar crowned merocyanines with the ionophore acting as an electron-donor substituent. The stepwise decoordination rate of the c alcium and sodium cations from the nitrogen atom of the fluorophore is comp etitive with the rate constant of fluorescence which accounts for the multi ple fluorescence but does not allow a distant diffusion of the cations, From the study of the longer lived radical-cation of the azacrown carbazole s, generated electrochemically, the metal cations are completely released f rom the azacrown receptor as revealed by the unsensitivity of the second ox idation potential to the perchlorate salts. From the probe-cation pair with the higher association constant (log K > 6) the oxidation peak for the com plex allows to estimate the decrease of K upon the oxidation of the probe t o be a value of K.+/K = 8 x 10(-4).