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

The dinucleating ligand bis[I-methyl-2-(6'-[1''-(3,5-dimethoxybenzyl)benzimidazol-2''-yl]pyrid-2'-yl)benzimidazol-5-yl]methane (L) reacts with lanthanide perchlorates to give dinuclear 2:3 complexes [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-assembly and adopt a triple-helical structure in solution (pseudo-D3 symmetry). The crystal structure of [EU2(L)3](ClO4)6(CH3CN)9 (11, Eu2C195H177N39O36Cl6, 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 triple-helical cation, [EU2(L)3]6+, where the three bis(terdentate) ligands L are wrapped around the helical axis defined by the europium atoms. The Eu(III) of each site is 9-coordinated by six nitrogen atoms of the benzimidazole units occupying the vertices and three nitrogen atoms of the pyridine units occupying the capping positions of a slightly distorted, tricapped trigonal prism. Luminescence studies of the crystalline 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 water molecules in 6 and 7 destroy the trigonal symmetry. An efficient intramolecular energy transfer between the 3pipi* excited state centered on L and the excited levels of Eu(III) and Tb(III) is observed (antenna effect) together with a dipole-dipolar Tb-->Eu intramolecular energy transfer in the heterodinuclear-doped Eu-Tb compound. Stability constants and H-1 NMR in acetonitrile show that the homodinuclear complexes [Ln2(L)3]6+ are less stable for the heavier lanthanides Tb and Lu. The origin of this effect is discussed together with the nonstatistical 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).