Solid-state supramolecular assembly, luminescence thermometry and solution-state photoisomerization studies in lanthanide polyoxazamacrocycle

Abstract

Macrocyclic ligands provide a modular platform for luminescent lanthanide materials, yet predictive control over supramolecular nuclearity and functional response remains challenging. Here, we study a family of polyoxazamacrocycles bearing phenolate-based pendant arms – phenol (LA₂H₂), 3-ethoxyphenol (LB₂H₂) and 4-(phenylazo)phenol (LC₂H₂) – and assess how pendant-arm identity and lanthanide ion (Dy³⁺, Nd³⁺) influence supramolecular assembly and photophysical behaviour. All metal ion/ligand combinations form inseparable mixtures of predominantly monometallic 1+1 species, nevertheless, the presence of dimetallic 2+2 species cannot be excluded. The assemblies adopt eight- or nine-coordinate lanthanide environments linked by macrocyclic donors, bound solvent and phenolate bridges, while lanthanide contraction does not dictate nuclearity, highlighting the intrinsic ambivalence of the N₃O₂/phenolate motif toward monomer–dimer outcomes. Semiempirical and DFT-level calculations provide insight into the multidimentional energy landscape of the macrocycle and coherent thermodynamic explanation for the coexistence of the 1:1 and 2:2 motifs. Despite such structural heterogeneity, functional solid-state lumienscence thermometry is realized through ligand–metal selection. All LA₂- and LB₂-based Nd³⁺ and Dy³⁺ complexes exhibit solid-state lanthanide-centered emission, with Nd/LA₂ and Dy/LB₂ enabling reliable intensity-ratio thermometry in near-infrared and visible-range, respectively. By contrast, azophenol-functionalized Ln/LC₂ complexes exhibit photoresponsive behaviour exclusively in solution. Guided by design principles established in our earlier published six-membered macrocyclic systems, we assess here the transferability of azobenzene-based photoswitching to a five-donor polyoxazamacrocyclic scaffold. UV excitation induces azobenzene isomerization with partial, acid/base-gated reversibility, while no detectable solid-state emission is observed for these complexes, which indicates that solution-phase photoresponse is tolerant to nuclearity ambiguity but sensitive to macrocyclic framework and excitation window. Together, these observations benchmark the transferability of pendant-arm design strategies across lanthanide macrocyclic platforms.