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

## TL;DR

A new metal-based structure binds squaraine dyes and transfers energy efficiently, producing bright near-infrared light with large energy shifts.

## Contribution

A novel metallosupramolecular receptor enables ultrafast dark resonance energy transfer with large pseudo-Stokes shifts.

## Key findings

- The metal-organic cage shows high selectivity for squaraine dyes with 2,6-dihydroxyphenyl substituents.
- Ultrafast dark resonance energy transfer results in bright near-infrared emission with pseudo-Stokes shifts up to 440 nm.
- Chirality transfer from a chiral dye to the host was observed via circular dichroism spectroscopy.

## Abstract

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.

A metallosupramolecular receptor binds squaraine dyes with high affinity and enables efficient dark resonance energy transfer (DRET) that generates bright near‐infrared emission with large pseudo‐Stokes shifts.