Complexes of lanthanoid salts with macrocyclic ligands. 41. Photophysical properties of lanthanide dinuclear complexes with p-tert-butylcalix[8]arene

Abstract

The macrocyclic octaphenol p-tert-butylcalix[8]arene (LH[sub 8]) was reacted with lanthanide(III) ions in dimethylformamide (DMF) containing triethylamine to obtain both homo- and heterodinuclear neutral complexes of composition [(Ln1:Ln2)LH[sub 2](DMF)[sub 5]](DMF)[sub n], with n = 4 ([alpha]-phase) or 1.5 ([beta]-phase). ICP-AES determination of the Ln(III) content shows a clear selectivity of the ligand for ions in the middle of the lanthanide series. Luminescence measurements at 77 K suggest that the two lanthanide(III) ions encompassed by the ligand are in very similar environments with pseudo C[sub 3h] symmetry. Small differences in the crystal-field potential are evidenced between the two crystalline phases and when a large ion (e.g Nd) is encapsulated by the ligand. The presence of a low-lying metal-to-ligand charge-transfer state (MLCT) in the Eu-containing complexes at ca. 20,000 cm[sup [minus]1] induces unusual spectroscopic properties. Very large absorption probabilities ([approximately]10[sup [minus]6]) have been determined for the Eu(III) transitions, and the Judd-Ofelt theory for f-f transitions fails to explain the very large value of the [Omega][sub 2] parameter (448 x 10[sup [minus]20] cm[sup 2]). In DMF solution, an efficient energy transfer from the ligand to Tb(III) occurs and makes the Tb(III) calixarene complex an interesting luminescent label.