SELF-ASSEMBLED DINUCLEAR LANTHANIDE HELICATES - SUBSTANTIAL LUMINESCENCE ENHANCEMENT UPON REPLACING TERMINAL BENZIMIDAZOLE GROUPS BY CARBOXAMIDE BINDING UNITS

The segmental ligands lcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}metha ne (alkyl methyl (L-5), ethyl (L-6)) react with lanthanide perchlorate s (Ln La, Eu, Gd, Tb) in acetonitrile to yield the f-f dinuclear homot opic triple-stranded helicates [Ln(2)(L-i)(3)](6+) (i = 5, 6) under th ermodynamic control. The crystal structure of -2(L-6)(3)](ClO4)(3)(MeC N)(2)(THF)(0.5)(EtOH)(0.5) (11a, C124H145N26O31Cl6Tb2, triclinic, <P(1 )over bar>, Z = 2) shows the wrapping of the ligands about a pseudo-C- 3 axis passing through the metal ions. The Tb ions are 9-coordinate in facial pseudo-tricapped trigonal prismatic sites and are separated by 9.06 Angstrom. H-1-NMR and ES-MS data establish that the triple helic al structure is maintained in solution. Spectrophotometric titrations (Ln = La, Eu) indicate log beta(23) = 24-25 and the formation of a 2:2 complex [Ln(2)(L-5)(2)](6+) (log beta(22) = 19-20). Quantum yield det ermination in acetonitrile shows that the terminal N,N-diethylcarboxam ide groups in L-5 favor efficient intramolecular L-5 --> Eu(III) energ y transfers leading to strong Eu-centered red luminescence, 50 times a s intense as the luminescence observed when the carboxamide groups are replaced by substituted benzimidazole units in [Eu-2(L-4)(3)](6+). Re sistance toward hydrolysis also results from the use of carboxamide gr oups, and no quenching of luminescence is observed for [Eu-2(L-5)(3)]( 6+) in moist acetonitrile up to 2.5 M water. The crucial role played b y carboxamide groups for the control of structural, electronic, and ph otophysical,properties is discussed. Replacing perchlorates by triflat es allows the isolation of the dinuclear double-stranded helicate 3SO3 )(4)(H2O2)(2)](CF3-SO3)(2)(MeOH)(2)(H2O)(5.5), whose crystal structure (13a, C85H106Eu2F18N16O30S6, monoclinic, C2/m, Z = 2) reveals a side- by-side arrangement of the two strands and 9-coordinate Eu ions linked through hydrogen-bonded water molecules.