The dinucleating ligand bis[ 1- methyl-2-(6'- [ 1”-(3,5 imethoxybenzyl)benzimidazol-2~'-ylp]y rid-2'-y1)benzimidazol- 5-yllmethane (L) reacts with lanthanide perchlorates to give dinuclear 2:3 complexes [Lnz(L)#+ (Ln = La, Eu, Gd, Tb, and Lu). Detailed ES-MS, lH-NMR, luminescence, and spectrophotometric measurements in acetonitrile show that the cations [Ln2(L)#+ are produced by strict self-assembly and adopt a triple-helical structure in solution (pseudo-D3s ymmetry). The crystal structure of [EU~(L)~] C~O~)~((1C1H, E~UC2NC1)9s~H 177N39036_Cl6,a = 17.634- (3) A, b = 21.408(4) A, c = 29.437(7) A, a = 82.13(1)', /3 = 85.76(1)', y = 89.79(1)', triclinic, P1, Z = 2) shows a dinuclear pseudo-D3 triple- elical cation, [Eu2(L)#+, where the three bis(terdentate) ligands L are wrapped around the helical axis defined by the europium atoms. The Eu(II1) 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 [Eu~(L)~]- (ClO.&nsolv (n = 9, solv = CH,CN, 11; n = 2, solv = H20,6; n = 9, solv = H20,7) confirm the pseudo-D3 symmetry of the Eu(II1) 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 %A* excited state centered on L and the excited levels of Eu(II1) and Tb(II1) is observed (antenna effect) together with a dipole4ipolar Tb - Eu intramolecular energy transfer in the heterodinuclear-doped Eu-Tb compound. Stability constants and 1H NMR in acetonitrile show that the homodinuclear complexes [LI I ~ (L) ~ ]a~re+ l ess 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(C104)3 (1 equiv) and L I I ~ ( C ~ O(1~ e)q~ui v) in solution (Ln1 # Ln2; Ln1 = La, Eu, Tb, and Lu; Ln2 = Tb and Lu).
