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

Two azacrown derivatives of carbazole have been prepared and their dipole moments in the excited state were estimated from solvatochromism. Their complexation constant K with calcium and sodium cations have been determined from the absorption spectra, the fluorescence spectra and, independently, from the first oxidation potential shift, as a function of the concentration in sodium and calcium perchlorates, when K is lower than 105. The fluorescence spectra of the calcium and sodium complexes in acetonitrile show two bands different from the fluorescence of the free probes, one from the excited complex similar to the fluorescence of the protonated probes and the other 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 calcium and sodium cations from the nitrogen atom of the fluorophore is competitive with the rate constant of fluorescence which accounts for the multiple fluorescence but does not allow a distant diffusion of the cations. From the study of the longer lived radical-cation of the azacrown carbazoles, generated electrochemically, the metal cations are completely released from the azacrown receptor as revealed by the unsensitivity of the second oxidation potential to the perchlorate salts. From the probe-cation pair with the higher association constant (log K > 6) the oxidation peak for the complex allows to estimate the decrease of K upon the oxidation of the probe to be a value of K •+/K = 8 × 10-4.