Dendrimers as Ligands. Formation of a 2:1 Luminescent Complex between a Dendrimer with a 1,4,8,11-Tetraazacyclotetradecane (Cyclam) Core and Zn2+

Abstract

We have investigated the formation of metal complexes between Zn²⁺ and two derivatives, 1 and 2, of the well-known 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand. Compound 1 is 1,4,8,11-tetrakis(naphthylmethyl) cyclam, and compound 2 is a dendrimer consisting of a cyclam core with appended 12 dimethoxybenzene and 16 naphthyl units. Compound 1 exhibits an emission band with a maximum around 480 nm, assigned to the formation of exciplexes between amine and excited naphthyl units. Dendrimer 2 exhibits three types of weak emission bands, assigned to naphthyl localized excited states (λ_max = 337 nm), naphthyl excimers (λ_max ca. 390 nm), and naphthyl-amine exciplexes (λ_max = 480 nm). In CH₃CN−CH₂Cl₂ 1:1 v/v, titration of ligand 1 with Zn²⁺ causes the disappearance of the exciplex emission and the appearance of a strong naphthyl localized fluorescence; the titration plot is linear and reaches a plateau for a 1:1 stoichiometry, showing that a highly stable [Zn(1)]²⁺ complex is formed. In the case of 2, titration with Zn²⁺ causes the disappearance of the exciplex band, with a concomitant increase in the excimer and naphthyl localized emissions; the titration plot is again linear, but in this case it reaches a plateau for a 2:1 stoichiometric ratio, showing the unexpected formation of a [Zn(2)₂]²⁺ complex. Such an unexpected stoichiometry for the complex of the dendritic ligand has been fully confirmed by ¹H NMR titrations. The results obtained show that the dendrimer branches not only do not hinder, but in fact favor coordination of cyclam to Zn²⁺.