Circularly polarized luminescence activity in the near infrared spectral region of a water-soluble ytterbium(iii) complex containing a conjugated chromophoric ligand

Abstract

Two cationic enantiomeric complexes [(R,R)-[YbL]Cl and (S,S)-[YbL]Cl with L = N,N′-bis(2-pyridylmethyl)-1,2-(R,R or S,S)-cyclohexanediamine functionalized at sp³ N with picolinate antennae] have been synthesized and spectroscopically characterized in polar protic solvents, such as water and methanol. The Yb(III) luminescence at about 980 nm is efficiently sensitized upon excitation of the picolinate antenna in the UV spectral region around 330 nm. The complexes exhibit good CPL activity for the ²F_5/2 → ²F_7/2 magnetic dipole (MD) allowed transition (g_lum values of |0.02| and |0.04| at 984 nm and 1021 nm, respectively) and high solubility in aqueous solution. The YbL species is largely predominant at physiological pH (7.4), given its high thermodynamic stability (log β_YbL = 20.98). As elucidated by DFT calculations, among the possible isomeric species the one characterized by the cis-O, O–N, N geometry was found to be dominant. Furthermore, one solvent molecule is bound to Ln(III) in water, giving rise to 9-fold coordination at the metal ion. The enantiomeric (R,R)-[YbL]Cl and (S,S)-[YbL]Cl complexes can be considered promising candidates as NIR-to-NIR chiroptical bioprobes also for in vivo experiments.