A Metallosupramolecular Receptor for Squaraine Dyes Enabling Ultrafast Dark Resonance Energy Transfer

A metal-organic cage was obtained by combining acridone-based dipyridyl ligands with Pd²⁺ ions. The cage acts as a potent receptor for squaraine dyes, with a pronounced preference for guests with 2,6-dihydroxyphenyl substituents. This selectivity profile differs from that of previously reported receptors for squaraine dyes. The acridone-based cage itself is non-emissive. Upon its photoexcitation, ultrafast (sub-ps) dark resonance energy transfer (DRET) to the encapsulated squaraine dyes was observed, resulting in bright, near-infrared guest emission, with pseudo-Stokes shifts of up to 440 nm. Upon binding of a chiral dye, chirality transfer to the host could be evidenced by circular dichroism spectroscopy.