Circularly polarized activity from two photon excitable europium and samarium chiral bioprobes

Abstract

In this contribution, two couples of cationic enantiomeric complexes [(R,R)-[LnL]Cl and (S,S)-[LnL]Cl, with Ln = Sm and Eu and L = N,N′-bis(2-pyridylmethyl)-1,2-(R,R or S,S)-cyclohexanediamine functionalized at sp³ N with the picolinate antennae2] have been synthesized and spectroscopically characterized in polar protic solvents, such as water and methanol. The good sensitization of Sm(III) and Eu(III) luminescence by the picolinate antenna is documented, upon both a one photon absorption process (at about 330 nm) and a two photon absorption process (at 720 nm). The complexes exhibit good CPL activity, in particular for the magnetic dipole (MD) allowed transitions, which are the ⁴G_5/2 → ⁶H_5/2 (564 nm) of Sm(III) and the ⁵D₀ → ⁷F₁ (593 nm) of Eu(III). Both complexes are highly stable in aqueous solution (log K = 20.13 for the EuL species chosen as the representative) and only one main species is present at physiological pH (7.4). Among the three possible isomeric complexes, DFT calculations on the Y(III) counterpart reveal the highest stability of the C₁-symmetric cis–O, O–N, N isomer, in agreement with the presence of only one Eu(III) emitting species in solution. Furthermore, one solvent molecule is bound to the metal ions, giving rise to 9-fold coordinated complexes. Preliminary biphotonic imaging experiments on the (S,S)-[EuL]Cl complex reveal that it can be easily internalized in two different cell lines (namely 293T cancer cells and THP-1 macrophages) with perinuclear diffuse localisation in the cytosol. Both Sm(III) and Eu(III) complexes can be considered promising candidates as NIR-to-RED in cellulo chiroptical bioprobes and a possible extension to the in vivo experiment will be further investigated.