Dual‐Sensitized Luminescent Europium(ΙΙΙ) and Terbium(ΙΙΙ) Complexes as Bioimaging and Light‐Responsive Therapeutic Agents

Abstract

Dual-photosensitized stable Eu^ΙΙΙ and Tb^ΙΙΙ complexes, namely [Eu(dpq)(tfnb)₃ ] (1) and [Tb(dpq)(tfnb)₃ ] (2), in which dpq=dipyrido[3,2-d:2',3'-f]quinoxaline and Htfnb=4,4,4-trifluoro-1-(2-napthyl)-1,3-butanedione, were designed as bioimaging and light-responsive therapeutic agents. Their X-ray structures, photophysical properties, biological interactions, photoinduced DNA damage, photocytotoxicity, and cellular uptake properties were studied. Discrete mononuclear complexes adopt an eight-coordinated {LnN₂ O₆ } distorted square antiprism geometry with bidentate N,N-donor dpq and O,O-donor tfnb ligands. The designed probes have the advantage of dual-sensitizing antennae (dpq, Htfnb) to modulate their desirable optical properties for cellular imaging and light-responsive intracellular damage. The remarkable photostability, absence of inner-sphere water (q<1), and longer excited-state lifetimes of the complexes make them suitable as cellular-imaging probes. The dpq ³ T state is well located energetically to allow efficient energy transfer (ET) to the emissive ⁵ D₀ and ⁵ D₄ states of Eu^ΙΙΙ and Tb^ΙΙΙ . This leads to higher quantum yields (φ=0.15-0.20) in aqueous media and makes these compounds suitable cellular-imaging probes. The complexes display significant binding ability toward DNA and bovine serum albumin (K≈10⁵ m^-1 ). They effectively cleave supercoiled DNA to its nicked circular form at λ=365 nm through photoredox pathways. The cellular internalization studies showed cytosolic and nuclear localization. The remarkable photocytotoxicity of these probes offers a strategy towards developing photoresponsive Ln^ΙΙΙ probes as cellular-imaging and phototherapeutic agents.