This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Chiral metal clusters that emit circularly polarized light often face a trade-off between photoluminescence efficiency and luminescence dissymmetry. A study published in the journal Advanced Optical Materials tested whether highly luminescent racemic carbon-centered gold(I)-silver(I) clusters could be converted into bright CPL emitters by enantioresolution with chiral donor ligands.
Researchers have shown that treating highly luminescent but racemic carbon-centered gold(I)-silver(I) clusters with chiral oxygen-donor ligands can separate them into mirror-image forms (enantiomers) that retain strong photoluminescence while gaining the ability to emit circularly polarized light. A phosphate-protected enantiomer pair achieved a photoluminescence quantum yield of 0.92/0.93 and the largest luminescence dissymmetry factor (|glum| = 0.008) among the clusters tested. Incorporating the cluster into a composite device with a cholesteric liquid crystal raised the measured device-level |glum| to 1.25.
Materials that emit circularly polarized luminescence (CPL)—light whose electric field rotates in a preferred direction—are of interest for optical applications. But such materials generally must combine two properties that are difficult to achieve together: a high photoluminescence quantum yield (PLQY, the fraction of absorbed photons re-emitted as light) and a high luminescence dissymmetry factor (|glum|), which quantifies how strongly the emitted light is circularly polarized. Chiral metal clusters are one candidate platform, but conventional bottom-up asymmetric synthesis has struggled to deliver both properties simultaneously.
Earlier work produced racemic (mixed mirror-image) carbon-centered Au6Ag6 and Au6Ag5 clusters with near-unity PLQYs of up to 0.92, but the clusters showed no CPL because they were racemic. It had remained unclear whether using chiral oxygen-donor ligands to separate a racemic mixture into its two mirror-image forms (enantioresolution), rather than building chirality in from the start, could preserve the strong emission and produce a sizable |glum|. The researchers synthesized carbon-centered Au6Ag6 and Au6Ag5 clusters by combining a gold-phosphine precursor with silver ions and chiral or achiral carboxylic, phosphinic, phosphoric or sulfonic acids.
They characterized the resulting enantiopure clusters using single-crystal X-ray diffraction, mass spectrometry, NMR, and solution and solid-state photoluminescence, circular dichroism (CD) and CPL spectroscopy, supported by TD-DFT computational modeling. They also built proof-of-concept devices combining cluster-doped polymer films with cholesteric liquid crystals to test whether the CPL signal could be amplified. In dichloromethane solution, the phosphate-protected cluster pair (R/S-5) reached the highest PLQY (0.92/0.93) and highest |glum| (0.008) of the three ligand types.
The carboxylate-protected pair (R/S-3) had a PLQY of 0.37/0.36 and |glum| of 0.003, while the sulfonate-protected pair (R/S-7) had a PLQY of 0.21/0.21 and |glum| of 0.002. Measured decay rate constants showed higher radiative rates for the fully protected Au6Ag6 clusters than for the Au6Ag5 clusters, and progressively lower nonradiative rates from carboxylate to phosphinate to phosphate ligand, consistent with the observed PLQY trend. Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. d research that matter—daily or weekly.
Extract — continue reading at the source.
