Self-Assembled Dinuclear Lanthanide Helicates: Substantial Luminescence Enhancement upon Replacing Terminal Benzimidazole Groups by Carboxamide Binding Units.

The segmental ligands bis{1-alkyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}methane (alkyl = methyl (L(5)), ethyl (L(6))) react with lanthanide perchlorates (Ln = La, Eu, Gd, Tb) in acetonitrile to yield the f-f dinuclear homotopic triple-stranded helicates Ln(2)(L(i)())(3) (i = 5, 6) under thermodynamic control. The crystal structure of Tb(2)(L(6))(3)(3)(MeCN)(2)(THF)(0.5)(EtOH)(0.5) (11a, C(124)H(145)N(26)O(31)Cl(6)Tb(2), triclinic, P&onemacr;, 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 Å. (1)H-NMR and ES-MS data establish that the triple helical 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) (log beta(22) = 19-20). Quantum yield determination in acetonitrile shows that the terminal N,N-diethylcarboxamide groups in L(5) favor efficient intramolecular L(5) --> Eu(III) energy transfers leading to strong Eu-centered red luminescence, 50 times as intense as the luminescence observed when the carboxamide groups are replaced by substituted benzimidazole units in Eu(2)(L(4))(3). Resistance toward hydrolysis also results from the use of carboxamide groups, and no quenching of luminescence is observed for Eu(2)(L(5))(3) in moist acetonitrile up to 2.5 M water. The crucial role played by carboxamide groups for the control of structural, electronic, and photophysical properties is discussed. Replacing perchlorates by triflates allows the isolation of the dinuclear double-stranded helicate Eu(2)(L(6))(2)(CF(3)SO(3))(4)(H(2)O(2))(2)(2)(MeOH)(2)(H(2)O)(5)(.5), whose crystal structure (13a, C(85)H(106)Eu(2)F(18)N(16)O(30)S(6), 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.