Solid state and solution dynamics of pyridine based tetraaza-macrocyclic lanthanide chelates possessing phosphonate ligating functionality (Ln-PCTMB): Effect on relaxometry and optical properties

The macrocyclic ligand 3,6,9-tris(methylenebutyl phosphonic acid)-3,6,9-15-tetraazabicyclo [9.3.1]pentadeca-1(15),11,13-triene (PCTMB) was synthesized and complexes of Eu³⁺, Tb³⁺, and Gd ³⁺ studied by X-ray crystallography, luminescence, and relaxometry. In the crystal these complexes are dimeric and possess 8-coordinate Ln ³⁺ centers that are linked by bridging phosphonates. The rigidity introduced by the pyridyl nucleus forces the EuPCTMB and TbPCTMB to adopt a twisted snub disphenoid (TSD) coordination geometry. Examination of the ⁵D₀ → ⁷F₀ luminescent transition of EuPCTMB in the solid state confirmed the existence of a single distinct Eu³⁺ coordination environment, whereas two Eu³⁺ coordination environments were observed in aqueous solution. Lifetime analysis of aqueous TbPCTMB solutions determined that q = 0.1 and q = 1.0 for the two coordination environments and Stern-Volmer quenching constants (K _SV^Tau;= 1101 M^-1, K_SV ^Φ= 40780 M^-1) support the presence of a complicated monomer/dimer equilibrium. Relaxivity studies of GdPCTMB in H ₂O/CH₃OH exhibited a concentration dependency (0.02 mM-10.00 mM) ranging from r₁ = 7.0 mM^-1 s^-1 to 4.0 mM^-1 s^-1 consistent with the trend observed by luminescence.