Lanthanide-based luminescence resonance energy transfer (LRET) can be used as a tool to enhance lanthanide emission for time-resolved cellular imaging applications. By shortening lanthanide emission lifetimes whilst providing an alternative radiative pathway to the formally forbidden, weak lanthanide-only emission, the photon flux of such systems is increased. With this aim in mind, we investigated energy transfer in differently spaced donor-acceptor terbium-rhodamine pairs with the LRET "on" (low pH) and LRET "off" (high pH). Results informed the design, preparation and characterisation of a compound containing terbium, a spectrally-matched pH-responsive fluorophore and a receptor-targeting group. By combining these elements, we observed switchable LRET, where the targeting group sensitises lanthanide emission, resulting in an energy transfer to the rhodamine dye with an efficiency of E = 0.53. This strategy can be used to increase lanthanide emission rates for brighter optical probes.
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