Shape-persistent macrocycles as ligands and sensitizers of Nd3+ ions

Two shape-persistent macrocycles with a hexagonal backbone incorporating two bpy units and two nonchromophoric methoxytetrahydropyran units (M), or two coumarin 2 dyes (C2-M-C2) are highly absorbing and emitting species in dichloromethane solution (ε max = 1.90×10 5 M -1cm -1 at 308 nm and 1.92×10 5 M -1cm -1 at 309 nm; φ em = 0.60 and 0.73, τ = 0.76 and 3.6 ns for M and C2-M-C2, respectively). Time-resolved fluorescence anisotropy measurements of M in dichloromethane has given information on the hydrodynamic volume of the free macrocycle in solution, which is close to the molecular volume measured by X-ray crystallographic analysis. M and C2-M-C2 are very good ligands for lanthanide ions (Nd 3+ and Gd 3+): they form complexes with 1:2, 1:1 and 2:1 (clearly evident only in the case of M) metal to ligand stoichiometry. These complexes are characterised by very high association constants in dichloromethane solution, as demonstrated by significant changes to the photophysical properties of the macrocycles. In the 1:2 and 1:1 complexes, the lanthanide ion is likely sandwiched between two macrocycles and coordinated by two bpy units. The efficiency of energy transfer from the ligand to the lanthanide ion is quite high (73% in the case of the [Nd 2M] 6+ complex), and it is not sensitive to the presence of oxygen. For the Nd 3+ complexes with the C2-M-C2 macrocycle, sensitisation of the lanthanide NIR emission takes place with lower efficiency (ca. 50%), due to a competitive electron transfer process from the coumarin to the complexed macrocycle. Lastly, upon addition of an excess of a competitive ligand, all the absorption and emission spectroscopic changes are completely reversible.