Tuning photoluminescence properties of Au clusters by surface modification and doping: lessons from case studies of icosahedral Au₁₃

Abstract

The photoluminescence (PL) properties of monolayer-protected gold nanoclusters (Au MPCs) have been extensively studied due to their wide range of potential applications, such as photocatalysis, photosensitization, bioimaging, and tumor therapy. The key factor that determines the PL properties of Au MPCs is the Au core size, but other parameters such as surface modifications and doping also significantly affect the PL properties. To understand and highlight the importance of these secondary parameters on the PL properties, this review article focuses on the ubiquitous icosahedral Au₁₃ core as a benchmark and addresses the question of how the PL properties are affected by surface modification with X-type (thiolates, alkynyls) and L-type (phosphines, N-heterocyclic carbenes) ligands and doping with heterometals (Ru, Rh, Ir, Ni, Pd, Pt, Ag, Cd). The PL emission energy of Au₁₃ MPC can be varied in the wide range of 1.1–2.1 eV and PL quantum yields up to 70% can be achieved by appropriate surface modification and doping. Based on the publications available until June 2024, the following empirical rules are derived to tune the PL emission energy and increase the PL quantum yield: (1) The PL emission energy and PL quantum yield can be increased by using L-type ligands and/or doping with small group elements such as Ir, Pt, Ru, and Rh. (2) The PL quantum yield can be enhanced by maximizing the interaction between the ligands through π⋯π or C–H⋯π interaction, chemical bonding, and steric packing. The first rule follows the energy gap law and can be understood qualitatively by simple schemes based on the jellium model. The last rule is due to the suppression of non-radiative decay by the stiffening of the Au core. We hope that this review will help the audience to establish rational guidelines for designed PL properties.

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Dennis Alexander Buschmann

Dr. Dennis Alexander Buschmann studied nano-science at the Eberhard Karls Universität Tübingen, where he obtained his M.Sc. degree in 2017. He received his Dr. rer. nat. degree for research on organo-rare-earth-metal clusters in inorganic chemistry in 2022 under the supervision of Prof. Reiner Anwander. After that, he joined the research group of Prof. Tatsuya Tsukuda as a postdoctoral researcher at The University of Tokyo. His current research interests include the synthesis and photoluminescence properties of NHC-protected gold clusters.

Haru Hirai

Dr. Haru Hirai received a Ph.D. degree in 2023 from The University of Tokyo under the supervision of Prof. Tatsuya Tsukuda. He is currently a postdoctoral researcher at the Department of Chemistry, The University of Tokyo. His research interests are mainly the atomically-precise synthesis and functionalization of ligand-protected gold-based clusters.

Tatsuya Tsukuda

Prof. Tatsuya Tsukuda received his Ph.D. degree in 1994 from The University of Tokyo under the supervision of Prof. Tamotsu Kondow. After postdoctoral research at RIKEN, he was appointed Assistant Professor at The University of Tokyo in 1994 and he became an Associate Professor at the Institute for Molecular Science in 2000. In 2007, he was promoted to Professor at the Catalysis Research Center of Hokkaido University, and in 2011 he moved to The University of Tokyo. His research interests cover the atomically precise synthesis and characterization of stabilized gold clusters and their catalytic applications.

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1. Introduction

For decades, monolayer-protected gold clusters (Au MPCs) have attracted materials science researchers for two main reasons: (1) they exhibit new physicochemical properties not found in the corresponding bulk or nanoparticles (NPs); (2) their properties can be tuned over a wide range by changing the number of constituent atoms.^1–10 The origins for size-specific photophysical^11–22 and catalytic^23–29 properties are the unique atomic packing and quantized electronic structures of the Au cluster cores.

Photoluminescence (PL) is one of the size-specific properties of Au MPCs arising from direct consequences of quantized electronic structures.³⁰ The Au NPs and bulk Au do not exhibit intense PL because non-radiative quenching via strong electron–phonon and electron–electron coupling dominates the relaxation processes of the photoexcited state: the PL quantum yield (QY) for bulk Au is reported to be 10⁻¹⁰%.³¹ In contrast, the energy gap between the quantized orbitals of Au MPCs exceeds the thermal energy so that they behave like conventional molecules and start to show superior PL properties.^11–22 In this regard, Au MPCs are promising materials with superior and tailored PL properties because their electronic structures can be tuned not only by size, but also by shape and composition of the Au core and the nature of the protecting ligands. Their high chemical stability and relatively low cytotoxicity make Au MPCs an attractive tool for medicinal chemistry applications.^32–37 PL emission can be tuned over a wide range, even into the near-infrared (NIR) region, which is important for deep tissue penetration. As a result, Au MPCs are frequently applied as bioimaging agents.^38–41 Recently, Au MPCs have also been used in tumor therapy.^42–44 Au MPCs are actively targeted to cancer cells by surface modification with tumor targeting agents, which are then used to destroy tumor cells via photothermal therapy and reactive oxygen species generated.

In addition, Au MPCs provide us with an ideal opportunity to develop rational design principles of the PL properties for the following reasons: (1) there is an increasing number of Au MPCs whose structures have been determined by single-crystal X-ray diffraction (SCXRD) analysis. Such structural information allows us to determine how the PL properties are affected by each structural parameter, (2) the electronic structure of Au MPCs is much simpler than that of other transition metals. The valence electrons in Au MPCs are accommodated in discrete orbitals labelled 1S, 1P, 1D, 2S, 2P, … distributed over a jellium-like potential (Fig. 1).^45–53 The chemical formula [Au_xM_y]^z (M: ligand) allows us to calculate the formal number of valence electrons (n*) according to the following equation proposed by Häkkinen:⁴⁶

n* = Ax + By − z (1)

where A and B represent the number of valence electrons supplied by a single metal atom and the ligand M, respectively. Specifically, A is 1 for Au, but we should apply suitable A values, depending on the group in the periodic table as shown in Table 1 when other elements are doped onto Au MPCs. Meanwhile, B is 0 for L-type ligands, such as mono-/diphosphines (PR₃/d-PR₂) and mono-/bis-N-heterocyclic carbenes (NHC/bis-NHC), while it is −1 for X-type ligands, such as halide (X), thiolate (SR), and alkynyl (C≡CR) due to their electron-withdrawing nature⁴⁶ as shown in Table 1.

Fig. 1 Schematic representation of the electronic structure of Au₁₃ MPC including the relevant superatomic orbitals. Reprinted with permission from ref. 37. Copyright 2019 Royal Society of Chemistry.

Table 1 Formal charge state and preferential location of dopant

Group (element)	Formal charge (A in eqn (1))	Doping location
Group 8 (Ru)	−2	Center
Group 9 (Rh, Ir)	–1	Center
Group 10 (Ni, Pd, Pt)	0	Center
Group 11 (Cu, Ag)	+1	Center/surface
Group 12 (Cd, Hg)	+2	Surface

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The “Au core size” is the primary factor that determines various physicochemical properties of Au MPCs. However, the properties of Au MPCs can also be affected by other parameters such as surface modifications and doping. To understand and highlight the importance of these secondary parameters on the PL properties, this review article focuses on the ubiquitous icosahedral Au₁₃ core as a benchmark and addresses the question of how the PL properties are affected by surface modification with ligands (SR,^54–58 C≡CR,^59,60 halogens (X), PR₃/d-PR₂ ^61,62 or NHC/bis-NHC^63,64) and doping of heterometals (Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Cd, Hg).^65–71Fig. 2 shows the geometric structures of the typical examples of Au₁₃ MPCs determined by SCXRD: [Au₂₅(SC₂H₄Ph)₁₈]⁻ (Fig. 2a),^72,73 [Au₂₅(C≡CPh(CF₃)₂)₁₈]⁻ (Fig. 2b),⁷⁴ [Au₁₃(Ph₂PC₂H₄PPh₂)₅Cl₂]³⁺ (Fig. 2c),⁷⁵ and [Au₁₃(Bzim^Bn)₉Cl₃]²⁺ (Fig. 2d).⁷⁶ The interfacial structures around the Au₁₃ core depend on the type of the ligands. In [Au₂₅(SC₂H₄Ph)₁₈]⁻ and [Au₂₅(C≡CPh(CF₃)₂)₁₈]⁻ protected by X-type ligands, the icosahedral Au₁₃ is capped by six bidentate units of Au₂(SR)₃ and Au₂(C≡CR)₃, respectively (Fig. 2a and b).^72–74 In [Au₁₃(Ph₂PC₂H₄PPh₂)₅Cl₂]³⁺ and isostructural [Au₁₃(bis-NHC)₅X₂]³⁺ protected by L-type ligands, the icosahedral Au₁₃ is capped directly by five bidentate phosphines and NHCs, respectively (Fig. 2c).^75,77,78 In [Au₁₃(Bzim^Bn)₉Cl₃]²⁺, the icosahedral Au₁₃ is capped directly by nine monodentate NHC ligands and three halides (Fig. 2d).⁶³ Despite the structural diversity in the ligand layers, the n* values calculated by eqn (1) are 8. According to the theoretical studies, the 1P and 1D superatomic orbitals correspond to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (Fig. 1). These Au₁₃ MPCs with a closed electron configuration (1S)²(1P)⁶ have been considered representative “superatoms”.^{45–53,72–78} The position of the dopant M in MAu₁₂ MPCs depends on the element.^79–92 Group 8–10 elements such as Pd and Pt exclusively occupy the central position (Fig. 2e), whereas group 12 elements such as Cd and Hg prefer the surface position (Fig. 2f). Nevertheless, the n* values calculated by eqn (1) are always 8 regardless of the dopants, indicating that MAu₁₂ also has a closed electron configuration (1S)²(1P)⁶.

Fig. 2 Molecular structures of (a) [Au₂₅(SC₂H₄Ph)₁₈]⁻, (b) [Au₂₅(C≡CPh(CF₃)₂)₁₈]⁻, (c) [Au₁₃(Ph₂PC₂H₄PPh₂)₅Cl₂]³⁺, (d) [Au₁₃(Bzim^Bn)₉Cl₃]²⁺. Color code: Au = yellow, Cl = green, P = orange, N = blue, S = red, F = light green, C = dark grey. Only one ligand each is shown in ball-and-stick representation, other ligands are shown as a wireframe model for improved visualization. Preferential doping site of (e) M = Pd, Pt, Rh, Ir, Ru and (f) M = Cd, Hg in MAu₁₂ core.

The photophysical processes of Au MPCs, like those of conventional fluorophores, can be described as follows. First, the photoexcited state generated by a vertical transition from the ground state (S₀) relaxes to the vibrational ground state of the first electronically excited state (S₁) according to Kasha's rule. Then, three relaxation pathways compete: (1) non-radiative internal conversion (IC) to the vibrational excited state of S₀, (2) radiative decay to the vibrational states in the Franck–Condon region in the S₀ state (fluorescence), (3) intersystem crossing (ISC) to the triplet state (T₁) followed by radiative decay (phosphorescence) to the S₀ state.^93–95 In the presence of other molecules, deactivation of the T₁ state can occur via energy transfer (ET) to oxygen or emitter molecules, followed by generation of singlet oxygen^96–98 or upconverted fluorescence emission,⁹⁹ respectively. Another deactivation pathway for photoexcited Au MPC is via electron transfer (eT), which can be applied to photoredox catalysis.^91,100,101 In most cases, the radiative decay of small Au MPCs is dominated by phosphorescence rather than fluorescence due to the strong spin–orbit coupling due to the heavy atom effect. However, in contrast to the case of conventional organic emitters, the radiative decay mechanism of Au MPCs cannot be distinguished by the PL lifetime alone due to the fast ISC and vibrational quenching, resulting in phosphorescence lifetimes of Au MPCs in the range of 100 ns. Therefore, additional experimental evidence (e.g. by triplet state quenching or transient absorption spectroscopy) is required. An overview of the emission mechanisms in Au MPCs was recently provided by Jin.²²

Emission energy (E_em) and QY are the key PL properties of Au MPCs that should be tuned for the application. In general, the E_em reflects the electronic structures of Au NCs, such as the energy gap (E_HL) between HOMO and LUMO. The E_em is also affected by the relaxation in the electronically excited states, which is determined by the potential energy curves in the S₁ and T₁ states. The PLQY, on the other hand, is also related to the E_HL according to the energy gap law: the rate of non-radiative IC quenching increases inversely proportional to the E_HL.^102,103 Similarly, the luminescence lifetime follows the energy gap law, and increasing the E_HL usually leads to elongated PL lifetimes. Therefore, the E_HL is a primary descriptor to characterize the PL properties. In other words, the PL properties can be tuned by controlling the E_HL.

In this review article, we first show that the E_HL values of Au₁₃/MAu₁₂ MPCs reported so far are distributed over a wide range of 1.2–2.3 eV, although they all share a common electron configuration of (1S)²(1P)⁶. These data indicate that the electronic structures of Au₁₃ MPC can be finely tuned by the choice of protecting ligands and the dopants. Then, we summarize the PL properties (E_em and PLQY) of Au₁₃/MAu₁₂ MPCs and extract empirical rules on how to tune the E_em values and to enhance the PLQY by the surface modification and doping. In addition, we attempted to rationalize these empirical rules based on modified spherical jellium models. Finally, we discuss the challenges for the future development of Au-based MPCs with designed PL properties. We hope that this review article will help the audience to establish the rational guidelines for designed PL properties.

2. Ligand effects

This section will showcase the characteristic PL properties of Au₁₃ MPCs protected by the respective ligands. Fig. 3 and 4 plot the E_em and PLQY values as a function of E_HL, respectively. The data points for Au₁₃ MPCs are indicated by different colors depending on the ligands: thiolates (blue), selenolate (purple), alkynyls (green), phosphines (red), and N-heterocyclic carbenes (orange). The E_HL was determined either by the onset in the optical absorption spectrum or by a voltametric method, while the E_em was determined by the peak position in the PL spectrum. Thus, the estimated optical E_HL values tend to underestimate the true values. All data presented were recorded under ambient atmosphere unless otherwise noted. In the following, the ligands are divided into two categories, X-type and L-type ligands.

Fig. 3 Overview of E_em as a function of E_HL for Au₁₃ MPCs. Color code: thiolate (blue), selenolate (purple), alkynyl (green), phosphine (red), NHC (orange).

Fig. 4 Overview of PLQYs as a function of E_HL for Au₁₃ MPCs protected by X-type ligands (inset) and L-type ligands. Color code: thiolate (blue), alkynyl (green), phosphine (red), NHC (orange).

2.1. X-type ligands

2.1.1. Thiolates. The E_HL and E_em values of Au₁₃ protected by various thiolates (RS, Scheme 1) are in the range of 1.30–1.66 and 1.10–1.24 eV, respectively, depending on the structure of the R group (blue dots in Fig. 3). The PLQY was relatively small and distributed in the range of 0.1–1.7% (blue dots in Fig. 4). More detailed descriptions of the effects of the thiolate structure on the PL properties are given below.

Scheme 1 Structures and abbreviations of the RS ligands used to protect the Au₁₃ core in section 2.1.1.

[Au ₂₅ (GT) ₁₈ ] ⁻ (GT-H = glutathione, Scheme 1): this cluster is the first member of the ubiquitous family of Au₂₅(SR)₁₈, reported by Negishi and Tsukuda in 2005.^104,105 The E_HL and E_em were 1.35 and 1.24 eV,⁹⁴ respectively (Fig. 3), while the PLQY was only 2 × 10⁻³% (Fig. 4). The PL intensity of [Au₂₅(GT)₁₈]⁻ was increased by ∼1.8 times in [Au₂₅(GT)_18−x(SPNA)_x]⁻, where PNA represents a long-chain peptide nucleic acid with a high proportion of electron-rich N and O atoms.¹⁰⁶ This enhancement was attributed to electron donation by electron-rich groups of SPNA (e.g. carboxy and amino groups) to the Au₁₃ core.

[Au ₂₅ (PET) ₁₈ ] ⁻ (PET-H = phenylethylthiol, Scheme 1): this cluster is the first member of the Au₂₅(SR)₁₈ family whose structures were theoretically predicted by Grönbeck,¹⁰⁷ and experimentally determined independently by Murray⁷² and Jin⁷³ in 2008. The E_HL and E_em were 1.66 and 1.14 eV, respectively, indicating a large Stokes shift (Fig. 3).¹⁰⁸ Note that this E_HL corresponds to the value at 0 K estimated from temperature-dependent optical spectra based on the Bose–Einstein model and is larger than that estimated from optical onset.¹⁰⁴ The PLQY was only 1 × 10⁻²% (Fig. 4).¹⁰⁶ [Au₂₅(PET)₁₈]⁻ showed a fluorescence lifetime of 0.73 μs. Theoretical calculations by Aikens revealed that the PL is dominated by core-based transitions with a very small contribution from the outer Au atoms in the staple ligands.¹⁰⁹

[Au ₂₅ (DT) ₁₈ ] ⁻ and [Au₂₅(HT)₁₈]⁻ (DT-H = 1-dodecanethiol; HT-H = 1-hexanethiol, Scheme 1): the E_HL and E_em of [Au₂₅(DT)₁₈]⁻ were 1.3 and 1.16 eV, respectively (Fig. 3).^110,111 Wu and Jin reported that the PLQYs of [Au₂₅(DT)₁₈]⁻ and [Au₂₅(HT)₁₈]⁻ were 5 × 10⁻³ and 2 × 10⁻³%, respectively (Fig. 4).¹⁰⁶ The PLQY of [Au₂₅(SR)₁₈]⁻ decreased in the order of SR = PET > DT > HT, which is parallel to the order of the ability of the ligands to donate electronic charge to the Au₁₃ core. The PL lifetimes were 0.42 and 0.64 μs for [Au₂₅(DT)₁₈]⁻ and [Au₂₅(HT)₁₈]⁻, respectively. Theoretical calculations revealed that core-based transitions as well as ligand-core interactions are essential for the PL behavior of [Au₂₅(SR)₁₈]⁻ (R = H, CH₃, C₂H₅, C₃H₇).¹⁰⁹

[Au ₂₅ (PhT) ₁₈ ] ⁻, [Au₂₅(BMT)₁₈]⁻, [Au₂₅(DMBT)₁₈]⁻ (PhT-H = phenylthiol, BMT-H = benzylmethanethiol, DMBT-H = 3,5-dimethylbenzenethiol, Scheme 1): effects of aromatic thiolates on the optical properties of [Au₂₅(SR)₁₈]⁻ have been reported by Jin.¹⁰⁸ The E_HL and E_em of [Au₂₅(SR)₁₈]⁻ (SR = PhT, BMT, DMBT) were comparable to those of [Au₂₅(PET)₁₈]⁻ (Fig. 3), indicating that the aromatic group does not significantly affect the electronic structures. [Au₂₅(SR)₁₈]⁻ (SR = PhT, BMT, DMBT) showed PLQYs of 1.7, 1.5 and 1.3%, respectively (Fig. 4), significantly larger than that of [Au₂₅(PET)₁₈]⁻.

Typically, [Au₂₅(SR)₁₈]⁻ exhibits an E_em in the NIR region and PLQYs of <2%. Overall, the geometric structure of the R group does not significantly affect the electronic structures of [Au₂₅(SR)₁₈]⁻ in terms of the E_HL and E_em, because the HOMO and LUMO are mostly localized in the common Au₁₃ cores. In contrast, the electronic structure of the R group affects the PLQYs. In particular, R groups with electron-rich atoms (e.g., O, N) or functional groups enhance the PLQY by donating electronic charge to the Au₁₃ core.

2.1.2. Selenolates. [Au ₂₅ (PhSe) ₁₈ ] ⁻ (PhSe-H = phenylselenol): PhSe, an analogue of PhS, was used by the group of Wu to protect the Au₁₃ core.¹¹² Square-wave voltammetry of [Au₂₅(PhSe)₁₈]⁻ revealed a E_HL of 1.24 eV, which is significantly smaller than that of [Au₂₅(PhT)₁₈]⁻ (1.6 eV). The E_em of [Au₂₅(PhSe)₁₈]⁻ was 1.13 eV (purple dot in Fig. 3), comparable to that of [Au₂₅(PET)₁₈]⁻ (1.14 eV), while the PLQY was not reported.

2.1.3. Alkynyls. The molecular structures of the alkynyls used are shown in Scheme 2.

Scheme 2 Structures and abbreviations of the alkynyl ligands used to protect the Au₁₃ core in sections 2.1.3 and 2.2.1.

[Au ₂₅ (FPA) ₁₈ ] ⁻ (FPA-H = 3,5-bis(trifluoromethyl)phenyl-acetylene, Scheme 2): Wang synthesized [Au₂₅(FPA)₁₈]⁻ by direct reduction of the precursor gold complex Au(I)-FPA.⁷⁴ While the Au(I)-alkynyl staples have similar structures to Au(I)-SR staples (Fig. 2b), [Au₂₅(FPA)₁₈]⁻ has a chiral D₃ symmetry, due to the higher steric demand of alkynyl ligands than thiolates in [Au₂₅(SR)₁₈]⁻. The E_HL and E_em of [Au₂₅(FPA)₁₈]⁻ were 1.74 (determined electrochemically) and 1.05 eV, respectively (green dot in Fig. 3).⁸⁷ The PLQY was 0.5% (green dot in Fig. 4).

2.2. L-type ligands

In contrast to the formation of short staple motifs in the protection by X-type ligands (Fig. 2a and b), L-type ligands such as phosphines and N-heterocyclic carbenes bind directly to the Au₁₃ core together with auxiliary X-type ligands (Fig. 2c and d).

2.2.1. Phosphines. With few exceptions such as [Au₁₃(PPhMe₂)₁₀Cl₂]³⁺,¹¹³ [Au₁₃(PPh₂Me)₈Cl₄]⁺,¹¹⁴ and [Au₁₃(P(Oct)₃)₈Cl₄]⁺,¹¹⁵ diphosphines (Scheme 3) have been conventionally used as protecting ligands of Au₁₃. The E_HL and E_em values of diphosphine-protected Au₁₃ are in the range of 1.79–1.90 and 1.53–1.57 eV, respectively (red dots in Fig. 3). The PLQY was in the range of 6–17% (red dots in Fig. 4), which is significantly larger than those of Au₁₃ protected by X-type ligands (section 2.1). This section focuses on the effects of diphosphine structures (Scheme 3) on the PL properties.

Scheme 3 Structures and abbreviations of the diphosphine ligands used to protect the Au₁₃ core in section 2.2.1.

[Au ₁₃ (dppe) ₅ X ₂ ] ³⁺ (dppe = 1,2-bis(diphenylphosphino)ethane, X = Cl, Br, I, Scheme 3): [Au₁₃(dppe)₅Cl₂]³⁺ was synthesized by Konishi in 2010 using size focusing (“etching”) with HCl.⁷⁵ The E_HL and E_em were 1.9 and 1.55 eV, respectively (red dots in Fig. 3). The PLQY was 11% (red dot in Fig. 4), with a phosphorescence lifetime of 2.69 μs.¹¹⁶ The E_HL of [Au₁₃(dppe)₅X₂]³⁺ decreased in the order of X = Cl > Br > I from 1.90 to 1.79 eV, while the E_em values were comparable (1.53–1.54 eV) regardless of X.¹¹⁷ Accordingly, the PLQY decreased in the same order by 11% (Cl) > 10.3% (Br) > 6.5% (I) (red dots in Fig. 4).

[Au ₁₃ (dppe) ₅ (PA) ₂ ] ³⁺ (PA-H = phenylacetylene, Scheme 2): Konishi synthesized [Au₁₃(dppe)₅(PA)₂]³⁺via a ligand exchange reaction of [Au₁₃(dppe)₅Cl₂](PF₆)₃ with PA-H.¹¹⁶ The E_HL of [Au₁₃(dppe)₅(PA)₂]³⁺ (1.9 eV) was same as that of [Au₁₃(dppe)₅Cl₂]⁺ (red dot in Fig. 3). DFT calculations showed that the HOMO of [Au₁₃(dppe)₅(PA)₂]³⁺ is extended over the PA units. Likewise, HOMO−1, HOMO−2 and HOMO−3 with superatomic P-like character located on the Au₁₃ core are distributed over the π system of PA. The interaction of the superatomic valence electrons of Au₁₃ with the π-orbitals then leads to the generation of new and unique molecular orbitals. The E_em value of [Au₁₃(dppe)₅(PA)₂]³⁺ (1.55 eV, red dot in Fig. 3) was also comparable to that of [Au₁₃(dppe)₅Cl₂]⁺, the PLQY of the former (16%, red dot in Fig. 4) was higher than that of the latter (11%), which was attributed to a higher quenching effect of Cl in comparison to PA. This trend is consistent with the longer PL lifetime of the former (3.51 μs) compared to the latter (2.69 μs).

[Au ₁₃ (dppe) ₅ (EPTpy) ₂ ] ³⁺ (EPTpy-H = 4′-(4-ethynylphenyl)-2,2′:6′,2′′-terpyridine, Scheme 2): in 2023, our group successfully obtained [Au₁₃(dppe)₅(EPTpy)₂]³⁺ with two terpyridyl moieties on opposite sides of the Au₁₃ core.¹¹⁸ The E_HL, E_em and PLQY of [Au₁₃(dppe)₅(EPTpy)₂]³⁺ (1.9 eV, 1.59 eV, 17%) were comparable to those of [Au₁₃(dppe)₅(PA)₂]³⁺ (1.9 eV, 1.55 eV, 16%). The PL of [Au₁₃(dppe)₅(EPTpy)₂]³⁺ was completely quenched by coordination of Co²⁺, Ni²⁺ or Cu²⁺ at the terpyridyl moieties, but not by Zn²⁺.¹¹⁸ The PL lifetime of [Au₁₃(dppe)₅(EPTpy)₂]³⁺ decreased from 3.66 μs to 0.02 μs upon coordination of Co²⁺, while it remained unchanged upon coordination of Zn²⁺ (3.41 μs). This PL quenching phenomenon was explained by efficient energy transfer from the photoexcited Au₁₃ core to the d⁹ transition metal cations via electron exchange between them.

[Au ₁₃ (R/S-dipamp) ₅ Cl ₂ ] ³⁺ (R/S-dipamp = (R,R)/(S,S)-2-bis[(2-methoxyphenyl)phenylphosphino]ethane, Scheme 3): Yang reported the synthesis of enantiopure Au₁₃ clusters [Au₁₃(R-dipamp)₅Cl₂]³⁺ and [Au₁₃(S-dipamp)₅Cl₂]³⁺ using the chiral diphosphine R/S-dipamp.¹¹⁹ The E_HL of [Au₁₃(R-dipamp)₅Cl₂]³⁺ (1.8 eV) was slightly smaller than that of [Au₁₃(dppe)₅Cl₂]³⁺ (1.9 eV) (red dots in Fig. 3). Konishi showed that the E_em of [Au₁₃(R-dipamp)₅Cl₂]³⁺ (1.57 eV) was comparable to that of [Au₁₃(dppe)₅Cl₂]³⁺ (1.54 eV) (red dots in Fig. 3) and that the PLQY (15%) was slightly higher than that of [Au₁₃(dppe)₅Cl₂]³⁺ (11%) (red dots in Fig. 4).¹²⁰ This was attributed to an increased core rigidification, as the chiral R/S-dipamp ligands induced additional torsional strain on the icosahedral Au core. [Au₁₃(R-dipamp)₅Cl₂]³⁺ and [Au₁₃(S-dipamp)₅Cl₂]³⁺ exhibited circularly polarized luminescence (CPL) with emission maxima at 1.63 eV and emission anisotropy factors of 2.5 × 10⁻³ and 2.3 × 10⁻³, respectively.¹²⁰

[Au ₁₃ L ₄ Cl ₄ ] ⁺ (L = Ph₂P-(CH₂)_m-PPh₂ with m = 3 (dppp), 4 (dppb), 5 (dpppe), Scheme 3): the chemical formula of Au₁₃ protected by long bis(diphenylphosphino) ligands [Au₁₃L₄Cl₄]⁺ (m = 3–5) differ from [Au₁₃(dppe)₅Cl₂]³⁺ (m = 2).¹¹⁵ Konishi reasonably proposed that [Au₁₃L₄Cl₄]⁺ adapted an icosahedral Au₁₃ core, although SCXRD analysis was not possible. The absorption spectrum profiles of [Au₁₃L₄Cl₄]⁺ (m = 3–5) were similar regardless of m, but were significantly different from that of [Au₁₃(dppe)₅Cl₂]³⁺ (m = 2). While the E_HL and E_em values of [Au₁₃(dppp)₄Cl₄]⁺ (∼2.0 and 1.60 eV, red dots in Fig. 3) were similar to those of [Au₁₃(dppe)₅Cl₂]³⁺ (∼2.0 and 1.55 eV, red dots in Fig. 4), the PLQY of [Au₁₃(dppp)₄Cl₄]⁺ (0.8%) was significantly lower than that of [Au₁₃(dppe)₅Cl₂]³⁺ (11%). This remarkable PL quenching suggests that diphosphines linked by longer alkyl chains cannot rigidify the Au₁₃ and thus cannot suppress non-radiative decay. The PL properties of [Au₁₃L₄Cl₄]⁺ (m = 4, 5) have not been reported.

Overall, the E_HL and E_em values of diphosphine-protected Au₁₃ are larger than those of Au₁₃ protected by X-type ligands as shown in section 2.1. The PLQYs were significantly increased compared to those of thiolate-protected Au₁₃. This trend is in parallel to the energy gap law between PLQY and E_HL.^102,103 The mechanical stress imposed by the bidentate ligation of diphosphines may contribute to the suppression of non-radiative decay by stiffening the Au₁₃ core and to the emergence of CPL by distorting the Au₁₃ core into a chiral structure. Alkynyls as X-type auxiliary ligands largely enhance the E_HL by electronic conjugation, while the effect of halides as X-type auxiliary ligands is relatively small.

2.2.2. Arsines and stibines. In contrast to monodentate phosphines, monodentate arsines and stibines can protect the icosahedral Au₁₃ core.

[Au ₁₃ (AsPh ₃ ) ₈ Cl ₄ ] ⁺ : the [Au₁₃(AsPh₃)₈Cl₄]⁺ reported by Sun exhibited a phosphorescence of E_em = 1.55 eV,¹²¹ which is comparable to that of [Au₁₃(dppe)₅Cl₂]³⁺ (1.54 eV), but smaller than that of [Au₁₃(P(Oct)₃)₈Cl₄]⁺ (E_em = 1.60 eV).¹¹⁵ No other PL properties, including the PL QY, have been reported.

[Au ₁₃ (dpap) ₅ Cl ₂ ] ³⁺ (dpap = Ph₂As-(CH₂)₃-AsPh₂): Konishi reported on the diarsine-protected cluster [Au₁₃(dpap)₅Cl₂]³⁺.¹²² The E_HL was ∼2.0 eV, while the E_em and PLQY were 1.63 eV and 7%, respectively. The E_em and PLQY were slightly larger and smaller than those of [Au₁₃(dppe)₅Cl₂]³⁺ (1.55 eV and 11%, respectively). The difference in their electronic structures was attributed to subtle deformation of the Au₁₃ icosahedron, rather than the coordinating atoms (As or P).

[Au ₁₃ L ₈ Cl ₄ ] ⁺ (L = SbPh₃, Sb(p-tolyl)₃): Leong reported that [Au₁₃L₈Cl₄]⁺ exhibited both fluorescence at E_em = 1.68 eV and phosphorescence at E_em = 1.50 eV due to a heavy metal effect of Sb.¹²³ This dual emission behavior of [Au₁₃L₈Cl₄]⁺ was attributed to facilitated reverse ISC from the T₁ to the S₁ state.

Overall, the E_em values for the phosphorescence increased in the order of [Au₁₃(SbPh₃)₈Cl₄]⁺ < [Au₁₃(AsPh₃)₈Cl₄]⁺ < [Au₁₃(P(Oct)₃)₈Cl₄]⁺.

2.2.3. N-heterocyclic carbenes. In contrast to the phosphine ligands, NHC ligands can stabilize the Au₁₃ cores even in monodentate form via strong Au–C bonds.¹²⁴ Further stabilization can be achieved by using bidentate NHC ligands. One of the practical advantages of NHCs is their structural designability for fine-tuning the PL properties of Au₁₃ NCs. Therefore, the effects of the substituent group R at the wingtips and multidentate ligation on the PL properties have been systematically investigated. The molecular structures of the NHCs used are shown in Scheme 4. The E_HL values of NHC-protected Au₁₃ are nearly constant at ∼2.0 eV, while the E_em values vary over a wide range of 1.57–1.94 eV (orange dots in Fig. 3). The PLQYs are comparable to those of diphosphine-protected Au₁₃ in most cases, but it can be as high as 61% when using a specific NHC (orange dots in Fig. 4). The increase in PLQY was attributed to the stiffening of the Au₁₃ core by π⋯π or C–H⋯π interaction between the adjacent NHC ligands. More detailed descriptions of the effects of the NHC structures on the PL properties are given below.

Scheme 4 Structures and abbreviations of the NHC ligands used to protect the Au₁₃ core in section 2.2.3.

[Au ₁₃ (Bzim ^R ) ₉ Cl ₃ ] ²⁺ (Bzim^R = benzyl/naphthyl/pyridyl-benzimidazole for R = Bn, Nph, and Py, respectively, Scheme 4): [Au₁₃(Bzim^R)₉Cl₃]²⁺ is an ideal benchmark system to study how the wingtips of NHC ligands affect the PL properties.^76,77 While the E_HL values were ∼2.0 eV independent of R, the E_em values were 1.63, 1.70 and 1.94 eV (orange dots in Fig. 3) with the PLQYs of 7, 16, and 7.5% (orange dots in Fig. 4), for R = Bn, Nph, and Py, respectively. The highest PLQY for R = NPh was explained by the stiffening of the Au₁₃ core by the π⋯π interaction between the naphthyl wingtips.

[Au ₁₃ (R/S-Bzim^MeBz^Pi) ₈ Br ₄ ] ⁺ (R/S-Bzim^MeBz^Pi = R/S-pyridine-ylmethylbenzylbenzimidazole, Scheme 4): Zang reported the synthesis of enantiopure Au₁₃ clusters [Au₁₃(R-Bzim^MeBz^Pi)₈Br₄]⁺ and [Au₁₃(S-Bzim^MeBz^Pi)₈Br₄]⁺ using chiral benzimidazole ligands.¹²⁵ The E_HL and E_em values were 2.00 and 1.77 eV, respectively (orange dot in Fig. 3). The PLQY was as high as 61% (orange dot in Fig. 4) with a PL lifetime of 3.07 μs. This extraordinary high PLQY was explained by the combination of two factors: rigidification of the Au₁₃ core by multiple interligand interactions such as C–H⋯N, C–H⋯π, C–H⋯Au and π⋯π interactions and higher electron-donating capability of R/S-Bzim^MeBz^Pi than other NHC ligands. [Au₁₃(R-Bzim^MeBz^Pi)₈Br₄]⁺ and [Au₁₃(S-Bzim^MeBz^Pi)₈Br₄]⁺ exhibited CPL with an E_em of 1.63 eV and a dissymmetry factor of 2.65 × 10⁻³ in a frozen DCM solution.

[Au ₁₃ (μ-(CH ₂ ) ₃ -bis-Bzim ^R ) ₅ X ₂ ] ³⁺ (μ-(CH₂)₃-bis-Bzim^R = μ-propyl-di-benzyl/naphthyl/pyridyl/pyrenyl-bis-benzimidazole for R = Bn, Nph, Py, Pyre; X = Cl, Br, Scheme 4): although the PL properties of these MPCs could not be plotted in Fig. 3 and 4 due to the lack of the E_HL values, the remarkable effects of the wingtips on the PL were observed. The E_em values of [Au₁₃(μ-(CH₂)₃-bis-Bzim^Bn)₅Br₂]³⁺ and [Au₁₃(μ-(CH₂)₃-bis-Bzim^Py)₅Br₂]³⁺ were 1.91 and 1.94 eV, respectively, while their PLQYs were both 15%.¹²⁶ Extending the π-conjugation of the R group further improved the PLQY. The PLQY increased from 38% for [Au₁₃(μ-(CH₂)₃-bis-Bzim^Nph)₅Cl₂]³⁺ to 56% for [Au₁₃(μ-(CH₂)₃-bis-Bzim^Pyre)₅Cl₂]³⁺, while their E_em values were both 1.75 eV. Consistent with this enhancement of the PLQY, their PL lifetimes were extended from 2.6 to 3.4 μs. This enhancement of the PLQY is associated with the stiffening of the Au₁₃ core by stronger π⋯π interaction between the adjacent ligands. The PLQYs of [Au₁₃(μ-(CH₂)₃-bis-Bzim^R)₅X₂]³⁺ were higher than those of [Au₁₃(Bzim^Bn)₉Cl₃]²⁺ (7%).^77,126 This enhancement of PLQY was attributed to the suppression of non-radiative decay of the photoexcited state by restricting the motion of the Au₁₃ core by the bidentate coordination of μ-(CH₂)₃-bis-Bzim^R.

[Au ₁₃ (μ-(CH ₂ ) ₃ -bis-Im ^R /Bzim ^R ) _m (tpp) _n X ₂ ] ³⁺ (μ-(CH₂)₃-bis-Im^R = μ-propyl-di-benzyl/iso-propyl-bis(benz)imidazole for R = Bn, iPr; tpp = PPh₃; X = Cl, Br; (m, n) = (2, 4), (3, 3), Scheme 4): the Biffis group reported mixed NHC/phosphine protected Au₁₃ clusters.¹²⁷ All clusters showed the same E_em of 1.63 eV regardless of R or NHC/phosphine ratio, while the E_HL of the clusters was not reported. The PLQYs were significantly different depending on the NHC/phosphine ratio. [Au₁₃(μ-(CH₂)₃-bis-Im^Bn)₂(tpp)₄Cl₂]³⁺ and [Au₁₃(μ-(CH₂)₃-bis-Bzim^Bn)₂(tpp)₄Cl₂]³⁺ and showed QYs of 4 and 3%, respectively, while [Au₁₃(μ-(CH₂)₃-bis-Bzim^Bn)₃(tpp)₃Cl₂]³⁺ showed a high PLQY of 44%. This was attributed to increased rigidity due to a higher number of NHC ligands. PL lifetimes of the clusters were not reported.

[Au ₁₃ (μ-(M/P-o-xylyl)-bis-Bzim ^Bn ) ₅ Cl ₂ ] ³⁺ (μ-(M/P-o-xylyl)-bis-Bzim^Bn = minus/plus-(μ-o-xylyl)-dibenzyl-bisbenzimidazole, Scheme 4): the E_HL and E_em of [Au₁₃(μ-(M/P-o-xylyl)-bis-Bzim^Bn)₅Cl₂]³⁺ were 1.9 and 1.57 eV, respectively (orange dot in Fig. 3).⁷⁸ The PLQY was 23% (orange dot in Fig. 4), which is higher than that of the monodentate counterpart [Au₁₃(Bzim^Bn)₉Cl₃]²⁺ (7%). This led to an increased core rigidification due to both bidentate coordination by the ligand system on the one hand, and the exertion of torsional strain on the Au core on the other hand. These effects prevented vibrational quenching of the excited state. Although CPL properties might be expected due to the ligand-induced torsion of the Au₁₃ core, CPL spectra have not been reported.

[Au ₁₃ (μ-(m-Ph)-bis-Bzim ^R ) ₅ Br ₂ ] ³⁺ (μ-(m-Ph)-bis-Bzim^R = μ-(m-phenylene)-di-methyl/ethyl/3,5-dimethylbenzyl-bis-benzimidazole for R = Me, Et, 3,5-Me₂Bn, Scheme 4): [Au₁₃(μ-(m-Ph)-bis-Bzim^R)₅Br₂]³⁺, reported by Mak, demonstrated the key role of the NHC wingtip R in the relaxation behavior of bis-NHC Au₁₃.¹²⁸ While the E_em value was nearly constant (1.75 eV) regardless of R, the PLQYs increased from 8 to 16% in the order of R = Me < Et < 3,5-Me₂Bn, which was explained by an increased rigidification by the bulkier ligand systems. Counterintuitively, the PL lifetime decreased from 1.03 to 0.80 μs in the same order. The origin of the discrepancy between the PLQY and the PL lifetime is unknown.

[Au ₁₃ (μ-(R/S-oxol)-bis-Bzim ^Bn ) ₄ I ₄ ] ⁺ (μ-(R/S-oxol)-bis-Bzim^Bn = μ-(R/S-2,2-dimethyl-1,3-dioxolane)-dibenzyl-bis-benzimidazole, Scheme 4): the number of bis-NHC ligands (4) in this cluster is smaller than that in other Au₁₃ NCs protected by bis-NHC ligands (5) due to the greater steric hindrance of the ligands.¹²⁹ [Au₁₃(μ-(R/S-oxol)-bis-Bzim^Bn)₄I₄]⁺ showed an E_HL of 2.0 eV and an E_em of 1.59 eV (orange dot in Fig. 3). The PLQY was 3% (orange dot in Fig. 4), which is significantly lower compared to the bis-NHC stabilized Au₁₃ NCs mentioned above. This behavior was attributed to less efficient core stiffening due to a smaller number of NHC ligands and confinement of valence electrons to a smaller volume due to a larger number of X-type ligands.

2.3. Qualitative explanation of ligand effects on the PL properties

Overall, Fig. 3 shows that E_HL values of the Au₁₃ core can be tuned over a wide range of 1.2–2.0 eV by surface modification. There is a clear trend that E_HL values for the Au₁₃ NCs protected by L-type ligands (phosphines, NHCs) are much larger than those protected by X-type ligands (thiolates, alkynyls). We previously explained this trend based on the spherical jellium model (Fig. 5). The Au₁₃ protected by X-type ligands is viewed as a formal Au₁₃⁵⁺ core protected by six electronegative surface units (e.g. [Au₂(SR)₃]⁻). In contrast, the Au₁₃ protected by L-type ligands is viewed as a formal Au₁₃⁵⁺ core protected mostly by electroneutral ligands with few electronegative ligands (e.g. halogen, alkynyls). We make a simple assumption that the isotropic siege of the Au₁₃ core by electroneutral ligands leads to the expansion of potential volume for the confinement of valence electrons (step ❶ in Fig. 5), and thereby stabilizing both HOMO and LUMO orbitals, corresponding to 1P and 1D superatomic orbitals, respectively (step ❷ in Fig. 5). If we assume that the 1P orbitals have a larger populated outer region of the core than the 1D orbitals, we can predict that the 1P orbitals are stabilized more than 1D orbitals when reducing the negative charge on the ligand layer by using L-type ligands.⁵³ This qualitatively explains why the E_HL values of Au₁₃ protected by L-type ligands are obviously larger than those of Au₁₃ protected by X-type ligands.

Fig. 5 Schematic jellium potentials for the Au₁₃(8e) core protected by X-type (left) and L-type (right) ligands. The blue and red rectangles represent X-type and L-type ligands, respectively.

Fig. 3 shows that the E_em values for the Au₁₃ NCs protected by L-type ligands (1.5–1.9 eV) are much larger than those by X-type ligands (1.1–1.3 eV). This ligand effect is not only due to larger E_HL values for the former, but also a smaller Stokes shift for the L-type ligation (0–0.4 eV) than X-type ligation (0.1–0.6 eV). The trend in the Stokes shift suggests that the photoexcited state of Au₁₃ NC protected by X-type ligands undergoes larger relaxation, but the reason is not clear at this moment.

Fig. 4 illustrates that the PLQYs for the protection with L-type ligands are much larger than those with X-type ligands. The PL of Au₁₃ protected by L-type ligands is basically assigned to phosphorescence from the T₁ state because the excited lifetime is in the order of μs.⁹⁵ The correlation between the E_HL and PLQY is in line with the energy gap law, in which an expansion of the E_HL can efficiently suppress the non-radiative decay process of photoexcited states.^102,103 Practically, L-type ligands are suitable to enhance the PLQY. Further enhancement in PLQY is expected by proper design of the ligand structures to enhance the noncovalent interaction (π⋯π, C–H⋯π, and hydrogen bonding) between the adjacent ligands. The non-radiative decay of the photoexcited state can be suppressed by the rigidification of the Au₁₃ core by these interligand interactions. Recently, Tsukuda demonstrated the importance of ligand packing on the PLQY. The PLQY of [IrAu₁₂(dppe)₅(PA)₂]⁺ decreased from 53% to 25% upon the removal of one PA ligand by Brønsted acid treatment.¹³⁰ In contrast, the PLQY of the resulting [IrAu₁₂(dppe)₅(PA)]²⁺ dramatically increased up to 70% by introducing an isocyano-adamantane to the vacant site. These results clearly indicated that dense packing in the ligand layers facilitate the rigidification of the Au₁₃ core to improve the PLQY.

3. Doping effects

This section focuses on the effects of heterometal doping on the PL properties of Au₁₃ MPCs. According to the energy gap law,^102,103 broadening of the HOMO–LUMO energy gap is a guiding principle for improving the PLQY. Tuning the electronic structure by heterometal doping is an effective method to change the HOMO/LUMO levels of Au₁₃ MPCs.^65–71 So far, group 8–12 elements have been successfully doped into the icosahedral Au₁₃ core. The formal charge and preferred dopant sites in Au₁₃ are listed in Table 1. Doping a heterometal into the Au₁₃ core modifies the depth and shape of the jellium potential, leading to an upshift or downshift of the superatomic S, P, and D orbitals, thereby changing the HOMO–LUMO gap of the Au₁₃ MPC. Typical examples of the doping effects on the electronic structure and the PL properties are described below. Fig. 6 and 7 show the E_em and PLQY as a function of E_HL, respectively. The data for doped Au₁₃ MPCs are shown in lighter tones depending on the ligands while the data for undoped Au₁₃ MPCs are shown in brighter tones. As described in the introduction, the total charge z of the doped Au₁₃ MPC differs depending on the dopant group to keep the n* value at 8. Therefore, in the following, the z value is omitted for simplicity.

Fig. 6 Overview of E_em as a function of E_HL for doped Au MPCs. Color code for ligands: thiolate (blue), alkynyl (green), phosphine (red). The data for undoped Au₁₃ MPCs (M, M′ = Au) are shown in brighter tones. The arrows indicate the change in E_em and E_HL after doping with the respective heterometal.

Fig. 7 Overview of PLQY as a function of E_HL for doped Au MPCs. Color code for ligands: thiolate (blue), alkynyl (green), phosphine (red). The data points for undoped Au₁₃ MPCs (M, M′ = Au) are painted in brighter tones. The arrows indicate the change in PLQY and E_HL after doping with the respective heterometal.

3.1. X-type ligands

MM′Au ₂₃ (PET) ₁₈ ((M, M′) = (Pt, Au), (Au, Cd), (Pt, Cd)): multi-metal doping can synergistically enhance the PL properties.⁸⁹ PtAu₂₄(PET)₁₈ had a Pt-centered icosahedral Pt@Au₁₂ core.¹³¹ The E_HL and E_em values of PtAu₂₄(PET)₁₈ having a PtAu₁₂(8e) core were 1.40 and 1.37 eV, respectively (light blue dot in Fig. 6), which are significantly larger than those of undoped Au₂₅(PET)₁₈ (1.66 and 1.21 eV): the E_HL value of 1.17 eV estimated from the optical onset was used for comparison rather than that at 0 K (1.66 eV) estimated from temperature-dependent optical spectra based on the Bose–Einstein model.¹⁰⁸ The PLQY of PtAu₂₄(PET)₁₈ was 0.41% (light blue dots in Fig. 7), ∼9 times larger than that of Au₂₅(PET)₁₈ (0.048%),⁸⁸ due to the increase in E_HL. In contrast, the Cd dopant in CdAu₂₄(PET)₁₈ was located on the surface of the Au@Au₁₁Cd core and the E_HL and E_em values were 1.22 and 1.24 eV, respectively (light blue dot in Fig. 6),⁸⁹ comparable to those of undoped Au₂₅(PET)₁₈. The PLQY of CdAu₂₄(PET)₁₈ (0.27%, light blue dot in Fig. 7) was ∼5 times higher than that of Au₂₅(PET)₁₈, due to a larger transition dipole moment from the photoexcited state to the ground state. Interestingly, the Pt- and Cd-codoped cluster PtCdAu₂₃(PET)₁₈ had an icosahedral Pt@Au₁₁Cd core and has the E_HL and E_em values of 1.41 and 1.37 eV, respectively (light blue dot in Fig. 6). The PLQY of PtCdAu₂₃(PET)₁₈ was 2.2%, 46 times higher than that of Au₂₅(PET)₁₈. Since the enhancement factor was expressed by the products of those by Pt (×9) and Cd (×5) doping, the PL enhancement was associated with the synergy of two independent mechanisms by Pt and Cd doping.

Ag ₃ Au ₂₂ (PET) ₁₈ : in 2013, Kauffman et al. reported on Ag₃Au₂₂(PET)₁₈, with all 3 Ag atoms occupying positions on the surface to form an Au@Au₉Ag₃ icosahedral core.¹³² Optical absorption spectroscopy revealed an increase in both E_HL (1.45 eV) and E_em (1.24 eV) compared to undoped Au₂₅ (light blue dot in Fig. 6). DFT calculations indicated that the shift was due to an increase in the LUMO energy level, while the HOMO energy level remained unaffected. The PLQYs of Ag₃Au₂₂(PET)₁₈ were not reported.

Ag _x Au _25−x (DT) ₁₈ (x ∼1, ∼4, ∼6, ∼7, ∼9): Negishi succeeded in increasing the number of Ag dopants to Au₂₅(DT)₁₈ with an accuracy of ±1 atom.¹¹¹ There was an overall trend of increasing E_HL and E_em with increasing Ag dopant content. In contrast, PLQY varied in the range of 1–4%, with no clear dependence on x observed.

MAu ₂₄ (FPA) ₁₈ (M = Au, Pd, Pt): the doping of one Pd atom on Au₂₅(FPA)₁₈ decreased the E_HL value from 1.74 to 1.66 eV, while Pt doping increased the E_HL value from 1.74 to 1.83 eV.⁸⁷ The E_em value showed little change from 1.05 to 1.06 eV with Pd doping on Au₂₅(FPA)₁₈, while it increased to 1.20 eV with Pt doping (light green dots in Fig. 6). The Pt-doped cluster has the largest E_HL and shows the highest PLQY of 3% in contrast to the undoped or Pd-doped clusters (0.5% each, light green dots in Fig. 7), which was due to a large increase in the E_HL induced by Pt doping.

3.2. L-type ligands

MAu ₁₂ (dppe) ₅ Cl ₂ (M = Au, Pt, Pd, Rh, Ir): doping Au₁₃(dppe)₅Cl₂ with a group 9 (Rh, Ir) or group 10 (Pt, Pd) increased the E_HL value in the order of 1.9 eV (M = Pd) ≤ 1.9 eV (Au) < 2.0 eV (Rh) < 2.1 eV (Pt) < 2.3 eV (Ir).⁹¹ In accordance with the increase in E_HL, the E_em was shifted toward higher energies (light red dots in Fig. 6): 1.53 eV (M = Pd) < 1.54 eV (Au) < 1.78 eV (Rh) < 1.80 eV (Pt) < 2.00 eV (Ir), and correspondingly, the PLQY increased dramatically (light red dots in Fig. 7): 11% (M = Pd) ∼12% (Au) < 46% (Rh) < 61% (Pt) < 67% (Ir) under Ar atmosphere. The PL lifetime was also increased in the following order: 2.88 μs (M = Au) < 3.07 μs (Pd) < 5.71 μs (Pt) < 6.03 μs (Ir) < 6.96 μs (Rh), which can be explained by the energy gap law. In the presence of O₂, the PLQYs for M = Rh and Ir were significantly reduced to 0.17 and 0.15% of the initial values, respectively, while that of the undoped clusters remained almost unaffected at 11%. This PL quenching by O₂ indicates that phosphorescence is responsible for the PL of Rh- or Ir-doped clusters.

MAu ₁₂ (dppe) ₅ X ₂ (M = Au, Ir; X = Br, I): the E_HL and E_em values of IrAu₁₂(dppe)₅Br₂ were 2.01 and 1.92 eV, while those of IrAu₁₂(dppe)₅I₂ were 2.03 and 1.96 eV, respectively.¹⁰⁰ These values are larger than those for the undoped counterparts (1.85, 1.54 eV) and (1.79, 1.54 eV). Accordingly, the PLQY of IrAu₁₂(dppe)₅Br₂ and IrAu₁₂(dppe)₅I₂ under Ar atmosphere remarkably increased to 75 and 77%, respectively, from those of the undoped counterparts (10.3 and 6.5%). These QYs of IrAu₁₂(dppe)₅X₂ (X = Br, I) are among the highest in MAu₁₂ NCs, but are not plotted in Fig. 6 and 7 because the doping effects were similar to those of Au₁₃(dppe)₅Cl₂. The PL lifetimes of IrAu₁₂(dppe)₅X₂ under Ar atmosphere were in the μs order (6.03, 4.90, and 4.15 μs), while they were reduced to 8.04, 7.16, and 6.65 ns for X = Cl, Br, and I, respectively, under air. The PL quenching of IrAu₁₂(dppe)₅X₂ by O₂ indicates that a phosphorescence from T₁ state, populated by efficient ISC, is responsible for the radiative decay.

MAu ₁₂ (dppe) ₅ (PA) ₂ (M = Au, Ir): the E_HL and E_em of IrAu₁₂(dppe)₅(PA)₂ were 2.3 eV and 2.02 eV, respectively.¹³⁰ The PLQY increased from 16% for Au₁₃(dppe)₅(PA)₂ to 53% for IrAu₁₂(dppe)₅(PA)₂. Again, the data are not plotted in Fig. 6 and 7 for comparison as the doping effect was similar to that in MAu₁₂(dppe)₅Cl₂.

MAu ₁₂ (R/S-dipamp) ₅ Cl ₂ (M = Au, Ir): the E_HL and E_em values of IrAu₁₂(S-dipamp)₅Cl₂ were 2.3 and 2.04 eV, respectively (light red square in Fig. 6), which are significantly larger than those of undoped Au₁₃(R/S-dipamp)₅Cl₂ were 1.9 and 1.57 eV, respectively.¹³³ As expected from the increased E_HL, IrAu₁₂(S-dipamp)₅Cl₂ showed a significantly increased PLQY (66%) and PL lifetime (5.87 μs) than those of undoped Au₁₃(R/S-dipamp)₅Cl₂ (15%, 3.19 μs) (light red square in Fig. 7). IrAu₁₂(R/S-dipamp)₅Cl₂ exhibited CPL properties. Although Ir doping did not appreciably enhance the PL anisotropy factors, the brightness of the CPL of IrAu₁₂(S-dipamp)₅Cl₂ was five times higher than that of undoped Au₁₃(S-dipamp)₅Cl₂. This enhancement in CPL brightness is mainly due to that in the PLQY.

MAu ₁₂ (dppm) ₆ (M = Au, Ru, Rh, Ir): undoped Au₁₃(dppm)₆ had a significantly distorted Au₁₃ core,¹³⁴ whereas MAu₁₂(dppm)₆ (M = Ru, Rh, Ir) had an icosahedral M@Au₁₂ core.⁹⁰ The E_HL value increased in the order of 1.47 eV (M = Rh) ≤ 1.69 eV (Ir) < 1.95 eV (Ru). Although Au₁₃(dppm)₆ was non-emissive, MAu₁₂(dppm)₆ exhibited PL behavior. Consistent with the increase in E_HL, the E_em was shifted to higher energies (light red triangles in Fig. 6): 1.48 eV (M = Rh) < 1.70 eV (Ir) < 1.72 eV (Ru). The PLQY increased dramatically in the order of 1.3% (M = Rh) < 13.5% (Ir) < 37% (Ru) (light red triangles in Fig. 7). The PL lifetime of RuAu₁₂(dppm)₆ at room temperature decreased from 5.7 μs under Ar to 0.22 μs under ambient conditions, indicating that phosphorescence is responsible for the PL. Interestingly, the PL mechanism of RuAu₁₂(dppm)₆ changed to fluorescence at low temperature (<130 K). This temperature dependence was attributed to an energy barrier between the S₁ and T₁ states, which could not be overcome at low temperatures due to the lack of thermal energy.⁹⁰

3.3. Qualitative explanation of doping effects on the PL properties

Overall, Fig. 6 shows that single heterometal atom doping can tune the E_em values of the Au₁₃ core over the range of 1.1–2.1 eV, a wider range than that achievable by surface modification (Fig. 3). In contrast, the Stokes shift is not affected by heterometal doping, as evidenced by an almost equal increase in both E_em and E_HL. Fig. 7 illustrates that the PLQY can be dramatically enhanced to as high as >70% by single heterometal atom doping. We can clearly see in Fig. 7 that the correlation between the PLQY and E_HL follows the energy gap law:^102,103 the PLQY increases with increase in E_HL. Practically, doping of smaller group element (lower valence element) such as Ir is very effective to enhance the PLQY. Previously, we qualitatively explained this trend by a two-step spherical jellium model,⁵³ which assumes that the host (Au) and dopant atoms provide different background potentials for the confinement of valence electrons. The potential provided by the dopant with lower valency is shallower than that by the Au host element, and vice versa (Table 1 and Fig. 8). When the smaller group dopant is located at the center of the icosahedral Au₁₃ core, the central portion of the jellium potential is upshifted (step ❶ in Fig. 8). According to this simple criterion, the 1P orbitals are destabilized by doping a smaller group element in the same period or a smaller period element in the same group with smaller electronegativity. If we assume that the 1D orbitals has larger population near the core center than the 1P orbitals,¹³⁵ we can predict that the 1D orbitals are destabilized more than 1P orbitals when upshifting the central portion of the jellium potential by doping a smaller period element (step ❷ in Fig. 8). This qualitatively explains why the E_HL value of Ir@Au₁₂ MPC is obviously larger than that of Au₁₃ MPC.

Fig. 8 Two-step jellium potential for doping lower valent atoms to Au₁₃.

4. Conclusion and perspective

In summary, we show that the PL emission energy of Au₁₃ MPC can be varied in the range of 1.1–2.1 eV and PL quantum yields (PLQYs) as high as 70% can be realized by proper surface modification and heterometal doping. We can draw the following empirical rules to tune and improve the PL properties of Au₁₃ MPC:

(1) The emission wavelength can be blue shifted by using L-type ligands (phosphines, N-heterocyclic carbenes) and/or doping smaller group element such as Ir, Pt, Ru, and Rh.

(2) The PLQY can be enhanced by using L-type ligands (phosphines, N-heterocyclic carbenes) and/or doping smaller group element such as Ir, Pt, Ru, and Rh.

(3) The PLQY can be enhanced by maximizing the interaction between the ligands via π⋯π or C–H⋯π interaction, chemical bonding, and steric packing.

(4) Circularly polarized luminescence (CPL) emerged by the structural deformation of the Au₁₃ core by chiral ligands.

The first two rules are associated with the expansion of the energy gap between HOMO and LUMO and can be understood qualitatively with a simple scheme based on the jellium model (Fig. 5 and 8). However, a more sophisticated theoretical explanation is needed. The third rule is due to the suppression of non-radiative decay by rigidifying the Au core. When the proper interactions between ligands are at work, the independent ligands forming the protective layer act as if they were a single multidentate ligand, thereby limiting the freedom of movement of the Au₁₃ core. When the Au₁₃ core is in such a protective state, its vibrational frequency increases, reducing the density of vibrational states. As a result, its ability to act as a heat bath for radiation-free relaxation from electronically excited states is reduced. In order to enhance the brightness of CPL, not only PLQY, but also the dissymmetry factor should be increased by structural deformation by chiral ligands.

Given that the PL quantum yield is governed by the energy gap law,^102,103 the obvious challenge is how to achieve high PL quantum yields in the long wavelength region. In particular, for bioimaging applications, there is a strong need for phosphors that emit efficiently in the NIR-I or NIR-II region, where the penetration into deep tissue is high. So far, Au₁₃ clusters emitting in the NIR-II region have been limited to those protected by X-type ligands with very low PLQYs, as illustrated in the upper panel of Fig. 9.

Fig. 9 Examples of MPCs showing PL in the NIR I and II region. Color code: Au = yellow, Ag = silver, Ru = turquoise, Pt = light blue, Cd = purple, S = red, Cl = green, P = orange, N = blue, F = light green, C = dark gray. Ligand structures are omitted in the lower panel for simplicity.

However, the PL properties in the NIR-I or NIR-II region can be tuned and improved by controlling other structural parameters such as the nuclearity, morphology, and element of the metal core of MPCs. Actually, there are several examples of non-Au₁₃ MPCs with superior PL properties in the NIR-I or NIR-II region, as listed in the lower panel of Fig. 9. [Au₂₅(tpp)₁₀(SC_nH_2n+1)₅Cl₂]²⁺ (n = 2–18) with a bi-icosahedral Au₂₅ core¹³⁶ had an E_em of 1.24 eV and PLQY of 0.1%.¹³⁷ The PL of [Au₂₅(tpp)₁₀(SC_nH_2n+1)₅Cl₂]²⁺ was assigned to phosphorescence.¹³⁸ [Au₂₅(tpp)₁₀(PET)₅Cl₂]⁺ had a PLQY of ∼8%,¹³⁹ while [Au₃₇(tpp)₁₀(PET)₁₀Cl₂]⁺ with a tri-icosahedral Au₃₇ core¹⁴⁰ had an E_em and PLQY of 0.80 eV and 0.1%, respectively.¹³⁹ Au₄₂(PET)₃₂ with a rod-like Au₂₀ core with an aspect ratio of 6.2 also exhibited dual emission with E_em of 1.41 eV and 1.19 eV. These PLs correspond to fluorescence and phosphorescence with PLQYs of 3.2% and 8.7%, respectively.¹⁴¹ When this Au MPC was embedded in polystyrene films, the PLQY of phosphorescence increased to 20.3%, while the PLQY of fluorescence decreased to 1.1%. The PLQYs of Au₅₂(SR)₃₂ ranged from 4 to 18% depending on the steric demand of the ligands:¹⁴² Au₅₂(p-MBT)₃₂ (p-MBT: p-methylbenzenethiolate) had an E_em of 1.38 eV with a PLQY of 18%. Recently, Wang reported that Au₁₆Cu₆(^tBuPA)₁₈ (^tBuPA = ^tBuPhC≡C) exhibited remarkably bright PL in the NIR-I region.¹⁴³ The E_em and PLQY were 1.72 eV and 95%, respectively, under ambient conditions. Impressively, the PLQY reached >99% under deaerated conditions. The PLQY of Au₁₆Cu₆(^tBuPA)₁₈ was significantly higher than that of undoped Au₂₂(^tBuPA)₁₈ (9% under air), while the E_em value was comparable to that of undoped Au₂₂(^tBuPA)₁₈ (E_em = 1.79 eV). The authors concluded that Cu doping slowed down the non-radiative decay by ∼60 times and accelerated the ISC process by ∼300 times. A NIR-emitting Ag MPC was reported by Nakashima.¹⁴⁴ The modification of [Ag₂₉(BDT)₁₂(tpp)₄]³⁻ (BDT: 1,3-benzenedithiol) with Ag(I)-PyDPP complexes (PyDPP: pyridyl-diphenylphosphine) resulted in the formation of Ag₃₃(BDT)₁₂(PyDPP)₄. This modification reduced the E_em from 1.82 to 1.61 eV and increased the PLQY from 1.4 to 39%. The authors proposed that the involvement of a triplet state is responsible for the intense PL. These examples suggest that bright PL in the NIR I and II regions can be realized by (1) increasing the Au core size, (2) doping more Ag and Cu into the Au core, and/or (3) making the morphology of Au MPCs anisotropic.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is affiliated with the Carbon to Metal Coating Institute at Queen's University and was partially funded by the NFRF-T program (#NFRFT-2020-00573). This research was supported financially by JST, CREST (grant no. JPMJCR20B2).

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