A ditopic tetradentate pyridyl amine ligand containing an anthracene fragment: fluorescence intensity and ‘closed’ vs. ‘open’ species formation in the presence of Cu2+, as a function of pH

Abstract

The compounds 9,10-bis(2-pyridylmethylaminoethyl)anthracene L¹ and 9-(2-pyridylmethylaminomethyl)anthracene L², based on nitrogen donors of the amine and pyridine type, were prepared and their properties examined in aqueous solution (water–1,4-dioxane 1∶4 v/v). The protonation constants of both have been determined by means of potentiometric titrations and the obtained species vs. pH distribution diagrams superimposed on the fluorescence intensity vs. pH profiles. This revealed an ‘off-on-off’ window behaviour: the fluorescence intensity is high only in the pH range in which both free pyridine and protonated amines exist. Moreover, the formation constants of the complex species which form in the presence of 1 or 0.5 (in the case of L¹ and L², respectively) equivalent of Cu^II have been determined by means of potentiometric titrations. In particular, in the case of the tetradentate ditopic ligand L¹ the ‘closed’ two ligand–two metal species [Cu₂L¹₂]⁴⁺ and [Cu₂L¹₂(OH)]³⁺ form in basic solution, in addition to two ligand–one metal partially protonated ‘open’ complex species. Moreover, a strong variation of the fluorescence intensity of the anthracene fragment signals the formation of the ‘closed’ species with respect to the ‘open’ ones, while a more subtle variation of the fluorescence intensity also allows one to distinguish between [Cu₂L¹₂]⁴⁺ and [Cu₂L¹₂(OH)]³⁺.