Tuning the photophysical properties of lanthanide(iii)/zinc(ii) ‘encapsulated sandwich’ metallacrowns emitting in the near-infrared range

A family of Zn(16)Ln(HA)(16) metallacrowns (MCs; Ln = Yb(III), Er(III), and Nd(III); HA = picoline- (picHA(2−)), pyrazine- (pyzHA(2−)), and quinaldine- (quinHA(2−)) hydroximates) with an ‘encapsulated sandwich’ structure possesses outstanding luminescence properties in the near-infrared (NIR) and suitability for cell imaging. Here, to decipher which parameters affect their functional and photophysical properties and how the nature of the hydroximate ligands can allow their fine tuning, we have completed this Zn(16)Ln(HA)(16) family by synthesizing MCs with two new ligands, naphthyridine- (napHA(2−)) and quinoxaline- (quinoHA(2−)) hydroximates. Zn(16)Ln(napHA)(16) and Zn(16)Ln(quinoHA)(16) exhibit absorption bands extended into the visible range and efficiently sensitize the NIR emissions of Yb(III), Er(III), and Nd(III) upon excitation up to 630 nm. The energies of the lowest singlet (S(1)), triplet (T(1)) and intra-ligand charge transfer (ILCT) states have been determined. Ln(III)-centered total (Q(L)(Ln)) and intrinsic (Q(Ln)(Ln)) quantum yields, sensitization efficiencies (η(sens)), observed (τ(obs)) and radiative (τ(rad)) luminescence lifetimes have been recorded and analyzed in the solid state and in CH(3)OH and CD(3)OD solutions for all Zn(16)Ln(HA)(16). We found that, within the Zn(16)Ln(HA)(16) family, τ(rad) values are not constant for a particular Ln(III). The close in energy positions of T(1) and ILCT states in Zn(16)Ln(picHA)(16) and Zn(16)Ln(quinHA)(16) are preferred for the sensitization of Ln(III) NIR emission and η(sens) values reach 100% for Nd(III). Finally, the highest values of Q(L)(Ln) are observed for Zn(16)Ln(quinHA)(16) in the solid state or in CD(3)OD solutions. With these data at hand, we are now capable of creating MCs with desired properties suitable for NIR optical imaging.