The combination of one contact and three pseudo-contact contributions to the NMR hyperfine paramagnetic shift of each proton in the triple-stranded helicates [Ln₃(L1)₃]⁹⁺(Ln = Ce–Yb except Pm, Gd) produce intractable ¹H NMR spectra whose assignment is limited by the large electronic contribution to the nuclear relaxation processes. The detailed analysis of the NMR spectra for the diamagnetic complexes [Ln₃(L1)₃]⁹⁺(Ln = La, Lu, Y) shows that the triple-helical structure found in the solid state is maintained in solution. Extension of the classical one-nucleus crystal-field dependent model-free method for paramagnetic D₃-symmetrical homotrimetallic lanthanide complexes possessing two different metallic sites (i.e. two second-rank crystal-field parameters: B20^central and B20^terminal) allows (i) the complete interpretation of the paramagnetic signals for Ln = Ce–Yb and (ii) the detection of a concomitant abrupt change of the contact terms F_i and of the pseudo-contact terms S_i=B20^centralG1i+B20^terminal(G2i+G3i) occurring near the middle of the lanthanide series. The derivation and application of a novel three-nuclei crystal-field independent method eventually demonstrates that the helicates [Ln₃(L1)₃]⁹⁺ adopt a single D₃-symmetrical structure along the complete lanthanide series in solution, which ascribes the discontinuity observed for S_i to a concomitant decrease of the two crystal-field parameters. Comparison with structural models is limited by the extreme sensitivity of the structural factors C_ikl and D_ikl to minor geometrical variations affecting the wrapping of the ligand strands, but calculations of the geometrical factors Gmi(m= 1–3) for [Ln₃(L1)₃]⁹⁺ in solution confirm the formation of a regular triple-helical structure. Generalization of this novel three-nuclei method for addressing the solution structure of rhombic lanthanide complexes is discussed.

Lipophilic linear semirigid side arms containing two or three successive phenyl rings separated by carboxylate spacers have been connected to the 5 or 6 positions of bent aromatic terdentate 2,6-bis(benzimidazol-2-yl)pyridine binding units to give extended V-shaped (L11) and I-shaped receptors (L12, L12b, and L13). The carboxylate spacers limit the flexibility of the side arms and provide crossed arrangements of the successive aromatic rings in the crystal structure of L12b (C₆₃H₆₁N₅O₁₀; triclinic, P_↑, Z = 2) in agreement with semiempirical calculations performed on optimized gas-phase geometries. Moreover, the carboxylate spacers in L11−L13 prevent efficient electronic delocalization between the connected aromatic rings and act as weak Ï€ acceptors producing a slight increase of the energy of the ¹Ï€Ï€* and³Ï€Ï€* levels centered on the terdentate binding unit. Intermolecular Ï€-stacking interactions observed in the crystal of L12b are invoked to rationalize (i) the peculiar excimer emission ofL11 in the solid state and (ii) the rich and varied calamitic (I-shaped L12, L12b, and L13) and columnar (V-shaped L11) mesomorphism observed at high temperature. The Col_R mesophase detected for L11 demonstrates that V-shaped bent terdentate binding units are compatible with liquid-crystalline behavior. Complexation of L11 with lanthanide(III) produces I-shaped complexes [Ln(L11)(NO₃)₃] (Ln = La, Eu, Gd, Tb, and Lu) possessing a large axial anisometry as found in the crystal structure of [Lu(L11)(CF₃CO₂)₃(H₂O)] (LuC₈₁H₈₇N₅O₁₇F₉; triclinic, P_↑,Z = 2), which exists in the solid state as H-bonded dimers. No mesomorphism is detected for the complexes as a result of the large perpendicular expansion brought by the metallic coordination site, but the high energy of the ligand-centered ³Ï€Ï€* prevents Eu(⁵D₀) → L11back transfer in the Eu(III) complex, which thus exhibits sizable red luminescence at room temperature, a crucial point for the design of luminescent materials.

The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)₆](BF₄)₂ was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar^± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed.