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 Gd3+ 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 D-5(0) -> F-7(0) 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 Stem-Volmer quenching constants (K-SV(tau) = 1101 M-1, K-SV(Phi) = 40780 M-1) support the presence of a complicated monomer/dimer equilibrium. Relaxivity studies of GdPCTMB in H2O/CH3OH exhibited a concentration dependency (0.02 mM-10 00 mM) ranging from r(1) = 7.0 mM(-1) s(-1) to 4 0 mM(-1) s(-1) consistent with the trend observed by luminescence.