The binding of lanthanide containers [Ln(-diketonate)3dig] (dig = 1-methoxy-2-(2methoxyethoxy)ethane) to aromatic tridentate N-donor ligands (L) in dichloromethane produces neutral nine-coordinate heteroleptic [LLn(-diketonate)3] complexes, the equilibrium reaction quotients of which vary with the total concentrations of the reacting partners. This problematic drift prevents the determination of both reliable thermodynamic stability constants and intrinsic host-guest affinities. The classical solution theory assigns this behavior to changes in the activity coefficients of the various partners in non-ideal solutions and a phenomenological approach attempts to quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent and contact-non-innocent contributors to the chemical potential. This assumption eventually predicts an empirical linear dependence of the equilibrium reaction quotient on the concentration of the formed [LLn(-diketonate)3] complexes, a trend experimentally supported in this contribution for various ligands L differing in lipophilicity and nuclearity and for lanthanide containers grafted with diverse beta-diketonate co-ligands. Even if the origin of the latter linear dependence is still the subject of debate, this work demonstrates that this approach can be exploited by experimentalists for Introducing eqn (4) into eqn (2) yields eqn (5) where ,M 1,1,eq Q L is the experimentally accessible reaction quotient, which is (very) often mistaken by coordination chemists for the thermodynamic stability constant ,M 1,1

While the low-absorption cross section of lanthanide-based upconversion systems, in which the trivalent lanthanides (Ln3+) are responsible for converting low- to high-energy photons, has restricted their application to intense incident light, the emergence of a cascade sensitization through an organic dye antenna capable of broadly harvesting near-infrared (NIR) light in upconversion nanoparticles opened new horizons in the field. With the aim of pushing molecular upconversion within the range of practical applications, we show herein how the incorporation of an NIR organic dye antenna into the ligand scaffold of a mononuclear erbium coordination complex boosts the upconversion brightness of the molecule to such an extent that a low-power (0.7 W·cm–2) NIR laser excitation of [L6Er­(hfa)3]+ (hfa = hexafluoroacetylacetonate) at 801 nm results in a measurable visible upconverted signal in a dilute solution (5 × 10–4 M) at room temperature. Connecting the NIR dye antenna to the Er3+ activator in a single discrete molecule cures the inherent low-efficient metal-based excited-state absorption mechanism with a powerful indirect sensitization via an energy transfer upconversion, which drastically improves the molecular-based upconverted Er3+-centered visible emission.

The concept of preorganization is famous in coordination chemistry for having transformed flexible bidentate 2,2'-bipyridine scaffolds into rigid 1,10-phenanthroline platforms. The resulting boosted affinities for d-block cations has successfully paved the way for the design of a wealth of functional complexes, devices and materials for analysis and optics. Its extension toward terdentate homologues adapted for the selective complexation of f-block cations with larger coordination numbers remains more overlooked. The resulting rigidification of 2,6-bis(1-methyl-1H-benzo[d]imidazol-2yl)pyridine ligands (L1-L7) produces the highly preorganized and extended polyaromatic benzo [4',5']tors, which offer some novel and rare opportunities for efficiently complexing trivalent lanthanides with polyaromatic soft terimine ligands. The crystal structures of the stable heteroleptic [LkLn(hfac) 3 ] adducts (Lk=L1, L8, L9; Ln=La, Eu, Gd, Er, Yb, Y; H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-di-one) show a drastic decrease in the LnÀ N bond valences upon replacement of the flexible ligand L1 with its preorganized counterparts L8 and L9. This points to a limited match between the preorganized cavity and the entering [Ln(hfac) 3 ] lanthanide containers. However, thermodynamic studies conducted in dichloromethane reach the opposite conclusion, with an improved affinity, by up to three orders of magnitude for catching Ln(hfac) 3 when L1 is replaced by the preorganized L8-L9 receptors. The key to the enigma lies in the removal of the energy penalty which accompanies the formation of flexible [L1Ln(hfac) 3 ] complexes in solution. This driving force overcomes the poor match between the preorganized terdentate N \ N \ N cavity in L8 and L9 and the size of trivalent lanthanides. As planned, the rigid, planar and extended π-conjugated system found in L8 and L9 shifts the ligand-centered absorption bands by about 5000 cm À 1 toward lower energies, a crucial point if these stable [L8Ln(hfac) 3 ] and [L9Ln(hfac) 3 ] platforms have to be considered for the visible sensitization of luminescent lanthanides in metallopolymers.

The adducts between luminescent lanthanide tris­(β-diketonate)­s and diimine or triimine ligands have been explored exhaustively for their exceptional photophysical properties. Their formation, stability, and structures in solution, together with the design of extended metallopolymers exploiting these building blocks, remain, however, elusive. The systematic peripheral substitution of tridentate 2,6-bis­(benzimidazol-2-yl)­pyridine binding units (Lk = L1–L5), taken as building blocks for linear oligomers and polymers, allows a fine-tuning of their affinity toward neutral [Ln­(hfa)3] (hfa = hexafluoroacetylacetonate) lanthanide containers in the [Lk Ln­(hfa)3] adducts. Two trends emerge with (i) an unusual pronounced thermodynamic selectivity for midrange lanthanides (Ln = Eu) and (ii) an intriguing influence of remote peripheral substitutions of the benzimidazole rings on the affinity of the tridentate unit for [Ln­(hfa)3]. These trends are amplified upon the connection of several tridentate binding units via their benzimidazole rings to give linear segmental dimers (L6) and trimers (L7), which are considered as models for programming linear Wolf-Type II metallopollymers. Modulation of the affinity between the terminal and central binding units in the linear multitridentate ligands deciphers the global decrease of metal–ligand binding strengths with an increase in the length of the receptors (monomer → dimer → trimer → polymer). Application of the site binding model shed light onto the origin of the variation of the thermodynamic affinities: a prerequisite for the programmed loading of a polymer backbone with luminescent lanthanide β-diketonates. Analysis of the crystal structures for these adducts reveals delicate correlations between the chemical bond lengths measured in the solid state (or bond valence parameters) and the metal–ligand affinities operating in solution.

The binding of the terdentate precursor 2,2'-(4-methyl-3,5-divinylpyridine-2,6-diyl)bis(1-allyl-5-bromo-1Hbenzo[d]imidazole) (1) to the lanthanide container [Ln(hfac) 3 ] (Ln=La, Eu, Gd, Y, Er; H-hfac = 1,1,1,5,5,5hexafluoropentane-2,4-dione) ensures the cis-cis orientation of the two adjacent α,α'-diimine units that is required for the successful intramolecular Grubb ring-closing metathesis generating the target rigid 6-methyl-9,11-dihydro-1H,3H-2λ 2 ,10λ 2 -pyrido[2,3-c:6,5-c']bis(azepine) scaffold decorated with two terminal 5-bromo-1Hbenzo [d]imidazole in ligand L7. The bond valence analysis of the crystal structures of the associated ninecoordinate adducts [L7Ln(hfac) 3 ] (Ln=La, Eu, Gd, Er, Y) reveals a satisfying match between the rigid terdentate cavity and the size of the bound lanthanide metal, with a pronounced preference for the largest lanthanum cation. Thermodynamic studies in dichloromethane confirm the formation of [L7Ln(hfac) 3 ] adducts with unprecedented stabilities due to the removal of the energy penalty associated with trans-trans to cis-cis reorganization. The introduction of saturated methylene groups within the polyaromatic ligand backbone breaks extended aromatic delocalization and clears the visible part of the electromagnetic spectrum from emission arising from low-energy ligand-based excited states.