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, Φ_EnT, leading to the overall emission sensitization process. For Eu · 1, and Eu · 2, we found Φ_EnT = 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.