Reversible electrochemical pH-switching of luminescence in a p- sulfonatothiacalix[4]arene— terbium(3+) system

Abstract

A quinone-hydroquinone system was used as a reversible electrochemical pH switcher of luminescence in the p sulfonatothiacalix[4]arene-terbium(3+) complex in an aqueous medium.Supramolecular systems with photo 1-9 , redox, 10-21 and pH switchable 22-25 properties, in particular, devices with switchable binding and switchable color and lumi nescence, are widely studied. In the earlier created pH switching devices, switching occurs by the addition of acid and base. This results in an increase in the salt con tent with each cycle. Therefore, switching occurs only restrictedly and, hence, this variant of pH switching can not be used for the creation of practically useful devices. In this connection, it is important to develop another method of pH switching when the reversible change in pH would be accompanied by the reversible change in the composition of the solution and functioning of such a system during unrestricted quantity of cycles.It is known 26 that the oxidation of organic compounds is often associated with proton elimination, whereas the reduction is accompanied by the accumulation of protons. At the same time, there are organic redox pairs in which redox processes involving protons are chemically reversible, for instance, pairs quinone-hydroquinone or nitroso compound-hydroxylamine. Therefore, reversible electrochemical switching of the pH of the medium with the reversible change in the composition of the solution during electrolysis is principally possible. The only prob lem is the choice of the pH of the switcher with a set of properties required for each particular case.Water soluble p sulfonatothiacalixarenes bind effi ciently bulky complex cations at the upper sulfonate rim, whereas metal ions are bound at the lower phenolate rim. 2,3 Complexes of this type are used in the creation of devices with redox and pH switching luminescence. 2,3 Now we report the results of investigation of the quinone-hydro quinone redox system as an electrochemical pH switcher of luminescence in the complex of p sulfonatothiacalix [4]arene (TCAS) with the Tb 3+ ion.The Tb 3+ ion itself does not luminesce, and its com plex with TCAS, in which the metal ion is bound at the lower phenolate rim, possesses high luminescence. Bind ing and, therefore, luminescence, are pH dependent. At pH 7 the Tb 3+ ion is bound and the complex luminesces, whereas at pH 3 the metal ion is bound to a lesser extent and the luminescence is quenched. 3,27 Taking into account these data, we stated a problem of electrochemi cal pH switching from 7 to 3 and back in a non buffer aqueous solution containing 0.1 М NaClO 4 as a support ing electrolyte and a system 5•10 -4 М TCAS + 5•10 -4 М Tb 3+ using the quinone-hydroquinone redox pair.The cyclic voltammograms (CV) of quinone reduc tion and hydroquinone oxidation at pH 3.04 and 7.01 in the absence and in the presence of TCAS were prelimi narily detected to determine the region of oxidation and reduction potentials. As it was expected, one peak of