Probing Optical Anisotropy and Polymorph-Dependent Photoluminescence in [Ln2] Complexes by Hyperspectral Imaging on Single Crystals

Two homodinuclear and one heterodinuclear lanthanide (Ln)-based complexes of the general formula [Ln₂(bpm)(tfaa)₆] (Ln=Eu (1), Tb (2), Eu-Tb (3), bpm=2,2’-bipyrimidine, tfaa-=1,1,1-trifluoroacetylacetonate) were synthesized and characterized by single-crystal photoluminescence spectroscopy and hyperspectral imaging. Complexes 1 and 2 crystallize in two polymorphic structures, while three polymorphs were isolated for 3, namely having needle-, plate-, and block-like morphologies. Single-crystal photoluminescence spectroscopy and imaging on Eu³⁺-containing 1 and 3 revealed polymorph-dependent J-splitting of the hypersensitive ⁵D₀→⁷F₂ Eu³⁺ transition as well as electric-tomagnetic dipole emission intensity ratios. According to these observations, the lowest symmetry chemical environment was attributed to the Eu³⁺ ions present in the needlelike polymorph, also in agreement with single-crystal X-ray diffraction analysis. More importantly, hyperspectral imaging on all three single-crystal polymorphs of 3 exhibits optical anisotropy with photoluminescence enhancement at specific crystallographic faces. This behavior was ascribed to the distinct molecular packing of the Ln-Ln dimers in each polymorphic crystal as well as to face-specific local symmetry of the Eu³⁺ centers. Overall, opto-structural relationships of three Ln-Ln dimers and their single-crystal polymorphs were established as a particularly promising avenue for control of photoluminescence by chemical crystal engineering.