Modulation of Lanthanide Luminescence with the Mechanical Bond: Antenna-Emitter Confinement in a Compact [2]Rotaxane

Luminescent emitters based on lanthanide ions are of ubiquitous importance in the biological sciences, but typically need sensitization from a covalently attached adjacent chromophore – an “antenna” – to have suitable emission intensities. There is a constant requirement for improved methods of combining antennas and emitters, especially to construct dynamic and (bio)responsive systems. Here we show that the mechanical bond can be used to connect the antenna to the emitter, provid-ing unique dynamic features and stimuli-responsiveness to the resulting assemblies. We outline a strategy to synthesize [2]rotaxanes capped with strong chelating groups, and establish that post-functionalization of the interlocked scaffold by lanthanide ion insertion is modular, high-yielding and straightforward. Photophysical studies revealed effective antenna-emitter energy transfer within the interlocked compact [2]rotaxane, and the sensitization mechanism as well as ring-thread dynamics were studied with spectroscopic and computational methods. The rotaxane was shown to have unique selectivity towards Cu(II) ions, acting as an efficient turn-off sensor. This study demonstrates that the mechanical bond is a valuable conjugation method for attaching antennas to emitters, while also providing otherwise hard-to-access and beneficial features to the resulting molecular systems.