Reversible trans-to-cis photoisomerization and irreversible photocyclization reactions of a Co-coordinated stilbene derivative on chiral di-β-diketonate lanthanide complexes

Abstract

Six lanthanide complexes constructed from two chiral β-diketonates (D/L-fbc = 3-heptafluorobutyryl-(+)/(−)-camphorate), the stilbene derivative (E)-N′,N′-bis(pyridin-2-ylmethyl)-4-styrylbenzoyl hydrazide (L), a trifluoroacetate anion (CF₃CO₂⁻), and one water molecule, namely [Ln(D/L-fbc)₂(L)(CF₃CO₂)]·H₂O (LnC₅₇H₅₄F₁₇N₄O₈, Ln = La (1, D-fbc), La (2, L-fbc), Sm (3, D-fbc), Eu (4, D-fbc), Eu (5, L-fbc), and Tb (6, D-fbc), were synthesized and characterized by single-crystal X-ray diffraction, ¹H-NMR, elemental analysis, IR and UV-vis spectroscopy, and thermal gravimetric analysis. The photoisomerization reactions of these complexes were systematically studied by means of experimental and theoretical calculations. Crystals of complexes 1, 2, 3, and 4 were obtained and belong to the monoclinic crystal system and the C2 chiral space group. The Λ- and Δ-diastereomers coexist in their crystals and no apparent bisignate couplets are observed in their ECD spectra. Among the complexes, the photocyclization reaction is followed by the trans-to-cis photoisomerization reaction and competes with the trans-to-cis photoisomerization, then the photocyclization reaction continues. The photocyclization reaction is irreversible in this stilbene derivative and is delayed in the lanthanide complexes. These results provide a viable strategy for the design of promising new stilbene-attached dual-functional lanthanide-based optical-switching materials.