Luminescence properties and solution dynamics of lanthanide complexes composed by a macrocycle hosting site and naphthalene or quinoline appended chromophore

The two-component ligand systems 1 and 2 which contain 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) as the hosting unit for the lanthanide cations, and naphthalene (which is devoid of any chelating ability) or quinoline units, respectively, as chromophores, were synthesized. The 1:1 complexes with Gd3+, Eu3+ and Tb3+ have been studied in aqueous solution. Relaxometric properties for Gd · 1 indicate that two water molecules (q = 2) are in the first coordination sphere of the metal ion whereas for Gd · 2, q < 2 is found. For Gd · 1, these results indicate that the naphthalene unit is not coordinated to the metal centre; for the case of quinoline, reasons for the lower hydration state are discussed. In case of Eu · 1, Tb · 1, Eu · 2, and Tb · 2 the absorption and luminescence spectra, the overall luminescence efficiencies, and the metal-centred lifetimes, were obtained both in water and deuterated water. The coordination features of these complexes were explored by comparing their luminescence properties, resulting in hydration state q = 2 and 1.4 for the cases of the complexes of 1 and 2, respectively. Use of the photophysical parameters in air-equilibrated water allowed the determination of the ligand-to-cation energy transfer efficiency, ΦEnT, leading to the overall emission sensitization process. For Eu · 1, and Eu · 2, we found ΦEnT = 0.034 and 0.078, respectively, supporting that also a non-coordinating chromophore like naphthalene, case of ligand 1, can transfer excitation energy to the metal centre.

Graphical abstractPhotoluminescence and relaxomtetric properties of lanthanide complexes containing coordinating (quinoline) and non coordinating (naphthalene) appended chromophores are described in some detail.Figure optionsDownload full-size imageDownload as PowerPoint slide