Identification of emissive lanthanide complexes suitable for cellular imaging that resist quenching by endogenous anti-oxidants

Excited state quenching by urate and ascorbate of selected europium and terbium(iii) macrocyclic complexes has been assessed and related to the ease of complex visualisation by optical microscopy inside various living cells, e.g. CHO, COS and NIH 3T3. It is the relative insensitivity of certain sterically encumbered complexes to dynamic quenching by urate that favours their usage for in cellulo applications. Non-covalent binding of the complex by protein also shields the excited lanthanide(iii) ion from collisional quenching; this effect is most marked for a cationic triamide complex, [Ln.1]³⁺, consistent with its ease of visualisation by luminescence microscopy.