pH Responsive Chiral Europium Complexes

Abstract

A series of novel emissive europium(III) complexes, varying in their number of antennae, has been synthesised and their photophysical properties studied. These complexes incorporate highly conjugated arylalkynylpyridyl antennae onto the macrocycle 1,4,7-triazacyclononane, and display highly pH responsive luminescence behaviour, with a significant switching-on of the total emission, emission lifetime, and circularly polarised luminescence with lowering pH.
The potential of such complexes as models for in cellulo probes of pH was demonstrated through the successful monitoring of cellular uptake with time (living NIH-3T3 cells) of the brightest complex using confocal microscopy. Additionally, the use of time-gated measurements and optimisation of the acquisition window was shown to allow for enhanced switching on of luminescence.
A further five pH-responsive europium(III) complexes have been thoughtfully designed, with the optimisation of the properties of the model parent complex and introduction of appropriate functionality for bioconjugation. These pH probes were synthesised and the photophysical behaviour analysed in detail.
In a proof-of-concept study, two probe candidates were converted to their benzyl guanine derivatives and the successful labelling of a SNAP-tag functionalised glucagon-like peptide-1 receptor, a targeted receptor for anti-diabetic drugs, was demonstrated.
Further studies were performed on the C3-symmetric parent europium(III) complex, examining the solvent dependence of the absorption and emission behaviour and the kinetics of enantiomeric interconversion, demonstrating that the spectral form, absorption maximum, and half life for racemisation are strongly influenced by local solvent.