SELF-ASSEMBLY AND PHOTOPHYSICAL PROPERTIES OF LANTHANIDE DINUCLEAR TRIPLE-HELICAL COMPLEXES

The dinucleating ligand dazol-2''-yl]pyrid-2'-yl)benzimidazol-5-yl]met hane (L) reacts with lanthanide perchlorates to give dinuclear 2:3 com plexes [Ln2(L)3]6+ (Ln = La, Eu, Gd, Tb, and Lu). Detailed ES-MS, H-1- NMR, luminescence, and spectrophotometric measurements in acetonitrile show that the cations [Ln2(L)3]6+ are produced by strict self-assembl y and adopt a triple-helical structure in solution (pseudo-D3 symmetry ). The crystal structure of [EU2(L)3](ClO4)6(CH3CN)9 (11, Eu2C195H177N 39O36Cl6, a=17.634-(3) angstrom, b = 21.408(4) angstrom, c = 29.437(7) angstrom, alpha = 82.13(1)degrees, beta = 85.76(1)degrees, gamma = 89 .79(1)degrees, triclinic, P1BAR, Z = 2) shows a dinuclear pseudo-D3 tr iple-helical cation, [EU2(L)3]6+, where the three bis(terdentate) liga nds L are wrapped around the helical axis defined by the europium atom s. The Eu(III) of each site is 9-coordinated by six nitrogen atoms of the benzimidazole units occupying the vertices and three nitrogen atom s of the pyridine units occupying the capping positions of a slightly distorted, tricapped trigonal prism. Luminescence studies of the cryst alline complex [Eu2(L)3]-(ClO4)6.nsolv (n = 9, solv = CH3CN, 11; n = 2 , solv = H2O, 6; n = 9, solv = H2O, 7) confirm the pseudo-D3 symmetry of the Eu(III) sites in 11 and show that secondary interactions with w ater molecules in 6 and 7 destroy the trigonal symmetry. An efficient intramolecular energy transfer between the 3pipi excited state center ed on L and the excited levels of Eu(III) and Tb(III) is observed (ant enna effect) together with a dipole-dipolar Tb-->Eu intramolecular ene rgy transfer in the heterodinuclear-doped Eu-Tb compound. Stability co nstants and H-1 NMR in acetonitrile show that the homodinuclear comple xes [Ln2(L)3]6+ are less stable for the heavier lanthanides Tb and Lu. The origin of this effect is discussed together with the nonstatistic al distribution of the different species observed when stoichiometric quantities of L (3 equiv) are mixed with Ln1(ClO4)3 (1 equiv) and Ln2( ClO4)3 (1 equiv) in solution (Ln1 not-equal Ln2; Ln1 = La, Eu, Tb, and Lu; Ln2 = Tb and Lu).