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. Here we show that the mechanical bond can be used to connect the antenna to the emitter, providing 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 [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 high selectivity toward Cu(II) ions, acting as an efficient turn-off sensor. This study validates the mechanical bond as a conjugation method between antennas and emitters, yielding otherwise hard-to-access and beneficial features to the resulting molecular systems.