Enantioresolution Makes Gold-Silver Clusters Circularly Polarized Emitters

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 together with the largest luminescence dissymmetry factor (|g_lum| = 0.008) among the clusters tested, and incorporating this cluster into a composite device with a cholesteric liquid crystal raised the measured device-level |g_lum| to 1.25.

Materials that emit circularly polarized luminescence (CPL), light whose electric field rotates in a preferred handed 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 (|g_lum|), 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 because these clusters were racemic they showed no CPL. It had remained unclear whether enantioresolution, separating a racemic mixture into its two mirror-image forms, using chiral oxygen-donor ligands instead of building chirality in from the start, could both preserve the strong emission and produce a sizable |g_lum|.

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, then characterized the resulting enantiopure clusters using single-crystal X-ray diffraction, mass spectrometry, NMR, and solution/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 |g_lum| (0.008) of the three ligand types, versus PLQY 0.37/0.36 and |g_lum| 0.003 for a carboxylate-protected pair (R/S-3) and PLQY 0.21/0.21 and |g_lum| 0.002 for a sulfonate-protected pair (R/S-7); measured decay rate constants showed higher radiative rates for the fully protected Au₆Ag₆ clusters than for the Au₆Ag₅ clusters, and progressively lower non-radiative rates from carboxylate to phosphinate to phosphate ligand, consistent with the observed PLQY trend. In all three enantiomer pairs, solid-state |g_lum| (0.002-0.003) was substantially lower than in solution or in polymer (PMMA) film, indicating the surrounding environment strongly affects the degree of circular polarization achieved. CD spectra showed mirror-image signals between R and S enantiomers in solution and film, confirming chirality transfer from ligand to cluster, though solid-state CD signals were faint, especially for the sulfonate pair; DFT calculations found a smaller HOMO–LUMO energy gap for R-7 (3.33 eV) than for R-3 (3.71 eV) or R-5 (3.88 eV), attributed to an elevated highest occupied molecular orbital energy. Time-dependent DFT modeling further attributed R-5's strong CPL to a low-energy electronic transition with a large calculated rotatory strength localized on the chiral metal core rather than the ligand. Combining the phosphate-protected cluster with a cholesteric liquid crystal in a composite device raised the measured |g_lum| to 1.25, versus 0.89 for an achiral cluster analog in the same device architecture, and more than 150-fold above the value for the cluster film alone.

Taken together, these results show that enantioresolution of highly luminescent Au–Ag clusters is a straightforward route to bright CPL-active emitters. The phosphate-protected pair highlights oxygen-donor ligands as a practical handle for tuning emission efficiency and chiroptical response. Integration with cholesteric liquid crystals further amplified CPL at the device level and enabled polarization-dependent information display. Further optimization of ligand, cluster, and device design could extend this approach toward advanced photonic, display, and information-security technologies.

Information of the paper

Authors: Wen‐Ting Liu, Pei Zhao, Zi‐Ang Nan, Wen‐Jing Hu, Jia‐Jing Lan, Qiao‐Hua Wei, Zhen Lei, Masahiro Ehara, Quan‐Ming Wang

Journal: Advanced Optical Materials

Article Title: "Enantioresolution of Well‐Defined Au–Ag Clusters With Near‐Unity Phosphorescent Quantum Yields for Brilliant Circularly Polarized Luminescence"

DOI: 10.1002/adom.71553

Funders

National Natural Science Foundation of China

22371044, 22531004, 92361301

JSPS KAKENHI, Grant-in-Aid for Transformative Research Areas (A)

JP22H05133

JSPS KAKENHI, Grant-in-Aid for Early-Career Scientists

JP24K17663

Research Center for Computational Science, Okazaki, Japan

25-IMS-C224