PHOTOPHYSICAL PROPERTIES OF LANTHANIDE DINUCLEAR COMPLEXES WITH P-TERT-BUTYLCALIX[8]ARENE

The macrocyclic octaphenol p-tert-butylcalix[8]arene (LH8) was reacted with lanthanide(III) ions in dimethyl-formamide (DMF) containing triethylamine to obtain both homo- and heterodinuclear neutral complexes of composition [(Ln1:Ln2)LH2(DMF)5](DMF)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 C3h 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-1 induces unusual spectroscopic properties. Very large absorption probabilities (approximately 10(-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 OMEGA2 parameter (448 X 10(-20) cm2). 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.