The segmental ligand bis{1-ethyl-2-[6'-carboxy)pyridin-2'-yl]benzimidazol-5-yl}methane (L-C) reacts with the entire series of Ln(III) ions to yield a new class of neutral lanthanide carboxylates, the triple-stranded dimetallic helicates [Ln(2)(L-C-2H)(3)], which are stable in water in the pH range 4-13. A competitive titration with 1,4,7,10-tetraazacyclododecane-N,N',N",N'''-tetraacetic acid (dota) shows the Eu-III helicate to have a stability comparable to that of [Eu(dota)](-). According to the X-ray structures of isotypical [Ln(2)(L-C-2H)(3)].20.5H(2)O (Ln = Eu, Tb) (C(93)H(72)Ln(2)N(18)O(12).20.5H(2)O, monoclinic, P2(1)/c, Z = 8), the helicates are comprised of two types of slightly different molecules (A, B) within the asymmetric unit. The three ligand strands are helically wrapped around the two nine-coordinate Ln(III) ions, leading to pseudo-D-3 symmetries around the metals. The Ln(III) distances lie in the ranges 8.81--8.83 Angstrom for molecules A and 9.04-9.07 Angstrom for molecules B, while the helical pitch amounts to 23.7--23.8 Angstrom (A) and 24.5-24.7 Angstrom (B). In solution, the helicates retain time-averaged D-3 symmetries along the entire Ln(III) series, as shown by H-1 and C-13 NMR. The derivation of equations adapted to heterodimetallic complexes allows the separation of contact and dipolar contributions to the isotropic paramagnetic shift. Somewhat different structures are evidenced between larger (Ln = Ce-Tb) and smaller (Ln = Er--Yb) Ln(III) ions, attributed to a lengthening of the helix on going toward heavier lanthanides. The photophysical properties of the helicates depend on the nature of the Ln(III) ion, and LC is revealed to be a good sensitizer of Eu-III. The flexible route used to synthesize LC opens interesting perspectives for the fine-tuning of the chemical and photophysical properties of lanthanide-containing homodimetallic helicates with potential applications in labeling and sensing technologies.