The basic concept of allosteric cooperativity used in biology, chemistry and physics states that any change in the intermolecular host-guest interactions operating in multisite receptors can be assigned to intersite interactions. Using lanthanide metals as guests and linear multi-tridentate linear oligomers of variable lengths and geometries as hosts, this work shows that the quantitative modeling of metal loadings requires the consideration of a novel phenomenon originating from solvation processes. It stepwise modulates the intrinsic affinity of each isolated site in multisite receptors, and this without resorting to allosteric cooperativity. An easy-to-handle additive model predicts a negative power law dependence of the intrinsic affinity on the length of the linear metallopolymer. Applied to lanthanidopolymers, the latter common analysis overestimates cooperativity factors by more than two orders of magnitude.

This work illustrates the use of basic statistical mechanics for rationalizing the loading of linear multitridentate polymers with trivalent lanthanides, Ln(III), and identifies the specific ionic sizes of europium and yttrium as promising candidates for the further design of organized heterometallic f–f′ materials. Using [Ln(hfac)3] (hfac = hexafluoroacetylacetonate) as lanthanide carriers, the thermodynamically controlled formation of Wolf type-II lanthanidopolymers [{Ln(hfac)3}m(L4)] is modeled with the help of two simple microscopic descriptors: (i) the intrinsic affinity of Ln(III) for the tridentate binding sites fN3(Ln) and (ii) the intermetallic interactions ΔE1–2(Ln,Ln) operating between two occupied adjacent sites. Selective complexation (fN3La << fN3Eu > fN3(Y)) modulated by anticooperative interactions (ΔE1–2(La,La) ≃ ΔE1–2(Eu,Eu) > ΔE1–2(Y,Y) ≈ 0) favors the fixation of Eu(III) in semiorganized lanthanidopolymers [{Eu(hfac)3}m(L4)] displaying exploitable light-downshifting.

This work demonstrates how the thermodynamic loading of monodisperse polymeric single-stranded multi-tridentate receptors of variable lengths is controlled by the nature of the metallic carrier Ln(hfac)3 (Ln is La, Eu or Y, and hfac is hexafluoroacetylacetonate). Whereas the intrinsic affinity of the tridentate binding site is maximum for medium-sized Eu(3+) and decreases for Y(3+), the contraction of the hydrodynamic radius of the polymer upon metal loading induces positive allosteric cooperativity for the smaller cations. The origin of this behaviour is rationalized within the frame of intermetallic dipole-dipole interactions modulated by the solvation potential of dipolar solutes in dielectric materials. Positive cooperativity produces local high-density of metal ions along the ligand strands (metal clustering) with potential interest in energy migration and sensing processes.

This work demonstrates how minor structural and electronic changes between Ln(NO 3 ) 3 and Ln(hfac) 3 lanthanide carriers (Ln ¼ trivalent lanthanide, hfac ¼ hexafluoroacetylacetonate) lead to opposite thermodynamic protocols for the metal loading of luminescent polynuclear single-stranded oligomers. Whereas metal clustering is relevant for Ln(hfac) 3 , the successive fixation of Ln(NO 3 ) 3 provides stable microspecies with an alternated occupancy of the binding sites. Partial anion dissociation and anion/ ligand bi-exchange processes occur in polar aprotic solvents, which contribute to delay the unambiguous choice of a well-behaved neutral lanthanide carrier for the selective complexation of different trivalent lanthanides along a single ligand strand. Clues for further improvement along this stepwise strategy are discussed.