Photochromic Rare-Earth Complexes with Chiral Macrocycle Ligands for Magnetic and Luminescent Regulations

The construction of chiral photochromic lanthanide complexes represents a topic of interest in photoresponsive materials for the potential applications of chiroptical photoswitches. Here, the combined employment of chiral hexa-azamacrocycle and photochromic dithienylethene-containing carboxylic ligand, 2,2″,5,5″-tetramethyl-[3,2':3',3″-terthiophene]-5'-carboxylic acid (3ThCOOH), results in the assembly of a pair of chiral lanthanide macrocyclic complexes, RRRR/SSSS-[Ln(L^N6)(3ThCOO)₂](BPh₄) (R/S-Ln, Ln = Eu, Dy, Y; L^N6 derived from the condensation of pyridine-2,6-dicarbaldehyde and (1R, 2R)/(1S, 2S)-1,2-diphenylethyl-enediamine), which were characterized by single-crystal XRD and CD spectra. The spectroscopic comparisons before and after photoirradiation revealed the occurrence of photochromic behaviors in both solution and solid states arising from the complementary effects of two different photoresponsive mechanisms. In solution, the photocyclization takes place in dithienylethene moieties upon photoirradiation and leads to luminescence modulation. By contrast, the measurements including spectra, electron paramagnetic resonance (EPR), and static magnetism suggest a different photochromic mechanism operating in solid states. Here, the photoinduced electron transfer (PET) from BPh₄^- to a macrocyclic ligand generates radicals and further modulates the luminescence and magnetism. This represents a new strategy to construct photochromic lanthanide complexes for functional regulations.