Science1 publisher3 min readPublished
Sorting bright gold-silver clusters by handedness adds circular polarization at quantum yields above 0.9
Researchers sorted racemic gold-silver clusters into circularly polarized emitters with chiral phosphate ligands, keeping quantum yields of 0.92 and 0.93. The brightness survives the separation intact, but the polarization gained is small and depends heavily on what surrounds the cluster.
The Scientist · Science desk
What happened
- Earlier racemic carbon-centered Au6Ag6 and Au6Ag5 clusters reached quantum yields up to 0.92 but showed no circularly polarized emission because their mirror-image forms were mixed.
- In a device pairing the phosphate cluster with a cholesteric liquid crystal, dissymmetry reached 1.25, against 0.89 for an achiral cluster analog in the same architecture.
- Time-dependent DFT modelling placed the phosphate cluster's circular polarization on a low-energy transition centered on the chiral metal core, away from the ligand.
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Why it matters
- contradiction Most of the 1.25 device figure also appears with an achiral cluster, so it credits the cholesteric liquid crystal and cannot be set beside cluster emitters measured on their own.
- constraint Solid samples keep well under half the phosphate pair's solution polarization, so any solid-state use of these clusters will depend on finding a host matrix that preserves it.
- capability Resolving an already-bright racemate gives chemists a second route to bright circularly polarized emitters, beside the asymmetric synthesis that has struggled to deliver both properties.
- decision Because ligand choice moved brightness and polarization in the same direction across three families, ligand screens can aim at both properties together, though three families are a thin basis.
Chiral metal clusters that emit circularly polarized light usually trade brightness against polarization [1]. According to the phys.org account of the work, published in Advanced Optical Materials, bottom-up asymmetric synthesis, which builds handedness into the cluster from the start, has struggled to deliver both properties at once [2][6]. This team started with clusters already known to glow efficiently. Those clusters were racemic, so they showed no circular polarization at all [5].
The clusters were made from a gold-phosphine precursor, silver ions and a set of chiral or achiral acids [7]. With a chiral oxygen-donor ligand, the mixture separates into its two mirror-image forms [3]. Circular dichroism spectra of the R and S versions were mirror images of each other in solution and in film. The mirror-image spectra show the ligand's handedness passed to the cluster [13].
Brightness survived the sorting. The phosphate pair's quantum yields of 0.92 and 0.93 match the best of the earlier racemic clusters [5]. Across the three ligand families compared in solution, the phosphate pair led on both brightness and polarization and the sulfonate pair trailed on both, with carboxylate in between [4]. Within this series the two properties rose together. Three data points make that a correlation, and the paper explains the two properties separately. Decay measurements link the brightness ordering to nonradiative losses that fell step by step from carboxylate to phosphinate to phosphate [9]. TD-DFT modelling puts the phosphate cluster's polarization on a low-energy transition localized on the chiral metal core [11].
The device result has to be read next to its control. Combined with a cholesteric liquid crystal, the phosphate cluster reached a dissymmetry factor of 1.25. An achiral cluster analog in the same architecture reached 0.89 [12]. So the achiral control gets about 71 percent of the way with no cluster chirality at all [2], and the chiral cluster adds 0.36 on top [1]. The device figure is about 156 times the 0.008 measured in solution [3]. That ratio compares a device with a dilute solution, and most of the device figure is there with an achiral cluster too.
Taking the cluster out of solution costs polarization. In the solid state every pair fell to a dissymmetry factor of 0.002 to 0.003, below its values in solution and in PMMA polymer film [10]. For the phosphate pair, that is a drop of more than 60 percent from 0.008 [6]. Solid-state circular dichroism signals were faint as well, especially for the sulfonate pair [13].
The published summary reports the device's polarization but not its quantum yield [12]. I think the evidence supports the narrower half of the claim. Brightness survived the separation, and for the phosphate ligand it came with the series' strongest polarization, in dichloromethane solution [4].
What to watch
- Quantum yields for the cluster-doped films and the liquid-crystal device, to show whether the brightness survives outside dichloromethane.
- Host matrices that keep solid-state dissymmetry near the 0.008 the phosphate pair reached in solution.
- Other chiral oxygen-donor ligands, including the phosphinates in this series, tested for whether solution dissymmetry can rise above 0.008 without losing yield.