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Luminescent Metal Nanoclusters with Aggregation-Induced Emission Nirmal Goswami, Qiaofeng Yao, Zhentao Luo, Jingguo Li, Tiankai Chen, and Jianping Xie J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02765 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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The Journal of Physical Chemistry Letters

Luminescent Metal Nanoclusters with Aggregation-Induced Emission

Nirmal Goswami, Qiaofeng Yao, Zhentao Luo, Jingguo Li, Tiankai Chen and Jianping Xie*

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585; Email: [email protected]

TOC

Metal Nanoclusters (Au, Ag, Cu etc.)

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ACS Paragon Plus Environment


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Abstract Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials due to their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in biomedical field due to their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspectives, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics, and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

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Noble metal nanoparticles (NPs) have fascinated scientists for decades in many areas of research and applications including optoelectronics, catalysis, chemical sensing, biomedical and environmental science.1-7 The physical and chemical properties of metal NPs are largely dictated by their size, shape and composition.7-13 Based on their distinctly different properties, metal NPs can be roughly classified into two categories: plasmonic NPs and molecular-like NPs. These two types of NPs are in two different size regions. While plasmonic metal NPs are larger than 2 nm and exhibit semi-continuous electronic structures, metal NPs with sizes below 2 nm show discrete electronic states due to the strong quantum confinement effects.14 Some molecular-like properties such as HOMO-LUMO transition, photoluminescence (PL), redox behavior, intrinsic magnetism, and molecular chirality, are therefore emerged in ultrasmall metal NPs in sub-2-nm size region; which also make them a unique family of functional NPs, known as molecular-like metal NPs or nanoclusters (NCs).14-23 Among the various molecular-like properties of metal NCs, PL is the most attractive as the PL properties of metal NCs provide a new avenue to design promising luminescent probes for biomedical and environmental applications.24-41 In particular, an increasing interest in luminescent metal NCs is to explore their use in biomedical field, similar as semiconductor quantum dots (QDs), which are one of the most widely explored luminescent materials.42-47 In fact, akin to QDs, luminescent metal NCs are also highly photostable and exhibit size- and/or composition-dependent PL properties.48-56 In addition, unlike the conventional QDs, luminescent metal NCs do not possess toxic heavy metals, which may offer them much better biocompatibility in the practical settings.57-59 However, it should be noted that the PL quantum yields (QYs) of metal NCs are still not competitive when compared to QDs. One primary reason is the lack of understanding on the structures of luminescent metal NCs, as well as on their photophysical fundamentals, especially at the molecular level. Therefore, further explorations of the PL properties of metal NCs are pivotal to the NC community, not only to achieve an in-depth understanding of the PL fundamentals, but also to fully realize their use in practical applications, particularly in the field of biomedical and environmental science. As the bare metal NCs are not stable in solution, they are typically protected by organic ligands. Among the various organic ligands that are used to protect metal NCs, thiolate ligands have received the most recent attention due to the high affinity of thiols to metal surface.60-63 3



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More importantly, recent advances in the synthetic chemistry of thiolated metal NCs have made possible the preparation of metal NCs with molecular purity. Some examples include Au18(SR)14, Au25(SR)18, Au36(SR)24, Au38(SR)24, and Ag44(SR)30, etc.64-73 The growing availability of these atomically precise metal NCs also stimulates researchers to investigate their size/compositionproperty relationship. Moreover, recent successes in elucidating the cluster structure (largely prompted by the advances in crystallization and self-assembly chemistry of clusters) of these metal NCs also provide a good platform to study their structure-property relationship.64, 66, 69, 73-81 Thiolated metal NCs can be described as Mn(SR)m, where M denotes metal atom and SR is thiolate ligand. Mn(SR)m clusters typically feature with a core-shell structure, where a highly symmetric M(0) core is capped by a shell of staple-like motifs, SR-[M(I)-SR]x. Some molecular structures of thiolated Au NCs are highlighted in Scheme 1.

a

b

c

Scheme 1. Cluster structure of (a) Au25(SR)18, (b) Au38(SR)24, and (c) Au102(SR)44. For clarity, only Au and S atoms are shown. Reproduced with permission.82 Copyright 2012, Royal Society of Chemistry.

With the recognition of a core-shell structure of thiolated metal NCs, a fundamental question regarding to the PL properties of metal NCs may arise: that is, whether the M(0) core or the M(I)-SR shell of thiolated metal NCs is the dominant contributor to their PL. In the early stage of cluster PL research, the behavior of electrons restricted in the small sized M(0) core was well accepted as an important source of PL of metal NCs. In particular, researchers have studied how the charge and valence state of the metal core affect the PL of metal NCs.83, 84 More recently, in addition to the contribution from the M(0) core, the role of M(I)-SR shell in cluster PL has also gained increasing attention, essentially due to the high surface to volume ratio (or high thiolateto-metal ratio) of thiolated metal NCs, as well as the strong interaction between thiolate ligands

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and metal atoms. For example, the type of thiolate ligands has been demonstrated to be efficient to affect the PL of thiolated Au25 NCs.83 In this Perspective, we first discuss current understanding and the ensuing research related to the PL properties of thiolated metal NCs. A technical challenge in this field is that the PL of thiolated metal NCs is often weak, and their QYs rarely exceed 0.1%.17 We therefore attempt to provide some insights into efficient solutions to address these hurdles. In particular, we exemplify the implementation of the recently discovered aggregation-induced emission (AIE) phenomenon to the NC field for the design of highly luminescent metal NCs. The AIE phenomenon has received recent interest in the community to design highly luminescent materials, and the AIE effect is considered to be constructive as the photoemission of the corresponding molecules could be significantly enhanced upon aggregation. The understanding of the AIE mechanism and the design principles of the AIE-type luminescent materials has enabled the synthesis of highly luminescent thiolated metal NCs with QYs of ~5 to 20%.

Current Experimental and Theoretical Understanding of the PL Properties of Metal NCs An efficient strategy to understand the photophysical properties of any material is through a combination of synthesis, structural characterization, chemical physics and theoretical studies. In the early stage of NC research, PL of the small bare Au and Ag NCs was detected in a gas matrix, often at low temperature.85-87 The PL was found to vary from UV to NIR region for different sized metal NCs. These results suggest that the PL of metal NCs originates from the metal core, however, no clear size dependency of PL properties of metal NCs was revealed. In the early 2000s, the size dependent PL was suggested by a number of experimental evidences.50, 88 In particular, a series of different sized Au NCs were synthesized by using a water-soluble dendrimer as the protecting ligand. By taking the advantage of the well-defined molecular weight of the dendrimer, polyamidoamine (PAMAM), the molecular formula of a number of different sized Au NCs, including Au5, Au8, Au13, Au23 and Au31, have been successfully determined by electrospray ionization mass spectrometry (ESI-MS); which have also been correlated to the optical properties of PAMAM-protected Au NCs (Figure 1a).50, 89 This experimental evidence sets the groundwork for the realization of a simple scaling relation of EFermi/N1/3 (EFermi is the Fermi energy of bulk gold and N is the number of Au atoms) to unravel the size dependent PL 5



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properties of the PAMAM-protected Au NCs. The proposed scaling law is analogous to the electronic transitions of the alkali metal NCs in gas phase, which also suggests that the electronic structure of the few atom Au NCs is closely defined by the number of free electrons and the cluster size, generally following the free electron model or the Jellium model. Notably, the Jellium EFermi/N1/3 energy scaling law could accurately describe the size-dependent PL properties of the small sized Au NCs. However, with the increasing of the Au number in Au NCs, the Jellium model starts to deviate because of the electronic screening effects and the harmonic distortion in their potential energy well (Figure 1b). The evolution of the energy spacing of the sp bands with the number of Au atoms has been shown in Figure 1c. The small distortion in the potential energy surface from spherical harmonic to Woods-Saxon and eventually to square-well potential, and their corresponding modified free electron models could be used to explain the trend of PL properties of different sized Au NCs.14

(a)

(b)

(c)

Spherical harmonic potential

Woods-Saxon potential

Au3~Au13

Au23~Au38

Square well potential

Large gold nanoparticles

Figure 1. (a) Photoexcitation (dashed line) and photoemission (solid line) spectra of different sized dendrimer-protected Au NCs. The size of Au NCs and their corresponding emission maxima are Au5 / 385 nm, Au8 / 455 nm, Au13 / 510 nm, Au23 / 760 nm, and Au31 / 866 nm. (b) Correlation of the number of Au atoms per cluster (N) with the photoemission energy of Au NCs. (c) Schematic illustration of the sizedependent surface potentials of Au NCs at different size scales. Reproduced with permission.18 Copyright 2012, Royal Society of Chemistry. 6


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Table 1. Examples of Au25 NCs Protected by Different Ligands and/or Synthesized by Different Methods, and Their Corresponding Photophysical Properties. No.

Nanocluster

Ex. (nm)

Em. (nm)

QY

Synthesis method

Ref.

1 2 3 4

Au25@BSA Au25@NLf Au25@pepsin Au25(SG)18

360, 510 380, 510 360 514 535 530 ~514 ~514 ~514

640 450, 650 670 670 700 826 700 ~750 ~750 ~750

6% 6% 3.5% 0.19% 0.1% 0.01% 0.005% 0.002%

NaOH-mediated reduction NaOH-mediated reduction NaOH-mediated reduction NaBH4 reduction Thiol etching NaBH4 reduction Thiol etching NaOH-mediated NaBH4 reduction CO reduction NaBH4 reduction NaBH4 reduction NaBH4 reduction

90

5 6 7

Au25(SC2H4Ph)18 Au25(SC6H13)18 Au25(SC12H25)18

91 92 93 94 95 96 97 98, 99 83 83 83

BSA = Bovine serum albumin; NLf = Lactoferin; -SG = Glutathionate; QY = Quantum yield; Ex. = Excitation wavelength; Em. = Emission wavelength.

The next question is whether the size dependent PL properties of the dendrimer-protected Au NCs can also be applied to Au NCs protected by other ligands. While the PL properties of some protein-protected Au NCs, such as Au25@BSA90 and Au13@pepsin,92 could be fitted well with the free electron model or a modified free electron model, the above scenario has not been demonstrated to be effective for all types of metal NCs, especially for those NCs protected by thiolate ligands. For example, the PL properties of the atomically precise glutathione (GSH)protected Au NCs remain ambiguous with respect to their core size, which is in stark contrast with the dendrimer-protected Au NCs.95 In addition, the existing free electron model could not properly justify why the Au NCs with the same number of core atoms but different protecting ligands show different PL properties; some examples of Au25 NCs protected by different ligands are shown in Table 1. Moreover, the PL properties of the same Au NCs produced by different synthetic methods might also be different (e.g., Au25(SG)18 in Table 1). Thus, it is clear that the metal core of NCs should not be considered as an absolute or only criterion for determining the PL properties of metal NCs. In addition, on the basis of recently resolved cluster structures, one should also consider other components of metal NCs, such as M(I)-SR shell, valence state of the metal atoms, and the nature of thiolate ligands. For example, the type of thiolate ligands has been recently shown to affect the PL properties of thiolated Au25 NCs.83 In particular, it has been recognized that the presence of electron-rich atoms or functional groups in the thiolate ligands can promote the PL of the Au25 NCs. In addition, increasing the electro-positivity of the Au core, 7



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especially when the cluster core sustains multiple charge states could also enhance the PL intensity of thiolated Au NCs. This work has shed some light on the ligand’s role on the PL of thiolated metal NCs. More recently, a high ligand density and long ligand chain was also demonstrated to be effective to enhance the PL of thiolated Au NCs.7, 100, 101 Another important parameter of thiolated Au NCs that needs to be considered is the valence state of metal atoms in NCs. From the cluster structure of thiolated Au NCs, both Au(I) and Au(0) oxidation states are present. For thiolated Au NCs, the percentage of Au atoms of different oxidation states can be measured by X-ray photoelectron spectroscopy (XPS). A number of studies have reported the presence of high percentage of Au(I) on the surface of thiolated Au NCs that typically showed high PL. For example, the coexistence of Au(I) and Au(0) in the yellow-emitting GSH-protected Au NCs was observed, showing nearly 40% of Au(I).102 In comparison, the large Au NPs have a dominant Au(0) species. These data suggest that the valence states of Au atoms may influence the PL of Au NCs and NPs. A similar study also suggests that the PL of thiolated Au NCs is closely related to the presence of Au(I) on the NC surface.84 In this study, a red-emitting Au NC with a QY of ~1.3% was prepared by using a zwitterionic ligand, D-penicillamine (DPA), as the protecting ligand. However, if the Au NCs were further reduced by NaBH4, a strong reducing agent, the PL intensity of the Au NCs was obviously decreased, which indicates the importance of Au(I) in the thiolated Au NCs for their strong PL. However, it should be noted that only qualitative experimental evidences about the role of valence state of the metal atoms could be obtained in this study, since the size of the Au NCs might also change upon the oxidation or reduction process along with the oxidation state of Au atoms. From the widely studied Au25(PET)18 NCs (PET: 2-phenylethanethiolate), the PL was shown to intensify upon the chemical oxidation of the Au core.83 More recently, one new surface oxidation mechanism was demonstrated to be efficient to increase the QYs of Au NCs from 12% to ~10%.103 Furthermore, the change of core composition could also affect the PL properties of thiolated metal NCs. For example, one recent study has demonstrated that substituting Ag atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. In particular, AgxAu25-x NCs with x = 1-13, obtained through the reaction of triphenylphosphine-protected Au11 clusters with the Ag(I)-thiolate complexes, showed significantly improved PL intensity (a 200-fold PL 8


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intensity enhancement was observed) when compared with the rod-shaped Au25 NCs or weakly luminescent AgxAu25-x species with x = 1-12.104 The effect of core composition on the PL of metal NCs was also seen in 2-3 nm Au/Cu NPs, where the PL properties were tuned by changing the alloy composition.105 The variation of the composition of the alloy NPs from 0% to 100% molar ratio of copper led to a shift in the PL emission wavelength from 947 to 1067 nm. Taken together, the above experimental observations clearly suggest that the PL of thiolated metal NCs is a cumulative effect of all the associated components. In fact, the case of thiolated metal NCs should be treated in a different manner, as the photophysics of the dendrimer-protected Au NCs are unable to explain the PL behavior of thiolated metal NCs. Despite the significant progress on the understanding of the key role of individual components of thiolated metal NCs in their PL properties, the synthesis of highly luminescent thiolated metal NCs has rarely been achieved. An exciting approach to harness their full potential is through the recently discovered PL enhancement strategy for some of the organic molecules by aggregation, since the aggregation phenomenon is also associated with the formation of thiolated metal NCs. Some fluorophores exhibit a dramatic PL enhancement in their solid or aggregated states, which was first discovered by Tang’s group in 2001.106-108 They have designated this novel phenomenon as aggregation-induced emission (AIE). The discovery of AIE effect has spurred many researchers to study, design, investigate and utilize new AIE materials (or AIEgens). The AIE phenomenon has recently emerged as one of the most active research topics, particularly in designing highly luminescent materials. In the next section, we will discuss the fundamental aspects of this AIE phenomenon with some representative examples, followed by discussing how to implement the concept of AIE to the field of luminescent thiolated metal NCs, with a specific focus on the design of luminescent thiolated metal NCs with AIE characteristics (herein this class of thiolated metal NCs are referred to as M(0)@M(I)-thiolate NCs). AIE Phenomenon and its Connection to the Luminescent Thiolated Metal NCs The first example to display the AIE effect is hexaphenylsilole (HPS), an important member of the silole family.109,

110

While the HPS molecules are thought to be luminescent due to their

extensive conjugated electronic structures like many other organic fluorophores, they are nonluminescent in solution. However, the same molecules become highly luminescent when they are 9



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aggregated (Figure 2). Inspired by this fact, Tang’s group developed a variety of silole derivatives, and they have examined their photophysical properties both in solution and in the aggregated state. It is remarkable that all these silole derivatives follow the photophysical behavior of HPS. Many of these silole derivatives and their aggregation behaviors have been highlighted in recent review articles.108, 110-114 It is noteworthy to mention that the aggregation of these molecules could be induced and controlled by a number of chemical, physical and engineering strategies. For example, in a typical chemical strategy, the properties of solvents such as viscosity and polarity are used to construct the aggregated materials. In a typical physical strategy, temperature or pressure is used to induce the aggregation of materials. By using different strategies, the molecular structures of various AIE materials have been successfully engineered to cater the applications in many fields like chemosensors, bioprobes, and solid-state emitters.112, 115, 116 In fact, what causes the aggregation of molecules to induce an intriguing outcome has motivated researchers from various fields to further decipher the AIE mechanisms. Recent studies suggest that the restriction of intramolecular motion (RIM) is responsible for the AIE phenomenon of these molecular rotor systems. In general, the AIE active molecules consist of a number of rotors, which can rotate or vibrate freely in dilute solution. However, rotations and vibrations of these rotors in the aggregated state are largely restricted, leading to the strong AIE effect. For example, a propellershaped luminogen of tetraphenylethene (TPE) and a shell-like luminogen of 10,10´,11,11´tetrahydro-5,5´-bidibenzo[a,d][7]annulenylidene (THBA) are presented in Figure 3. When TPE is dispersed in solution, all its four phenyl rings can rotate freely and the intramolecular rotation serves as a non-radiative relaxing pathway for excited electrons, rendering TPE non-luminescent in solution. However, upon aggregation, the intramolecular rotations of these phenyl rings of TPE are inhibited, leading to the prohibition of the effective non-radiative relaxation of excited electrons, which generates strong PL of TPE. The restriction of intramolecular rotation (RIR) exemplified with TPE is an important mechanism for AIE. Another important mechanism of AIE is the restriction of intramolecular vibration (RIV). As can be seen in the bottom panel of Figure 3, THBA is a molecule with the prohibition of the intramolecular rotation of phenyl rings. However, the butterfly-like vibration of the symmetric wings of THBA could provide an alternative pathway for non-radiative relaxation of electrons. Therefore, the restriction of such intramolecular vibration by the aggregation of THBA could effectively turn on their luminescence. 10


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Guided by the design principles towards AIE, in recent years a dramatic growth has been witnessed in developing more and more AIE active materials, which results in a booming of the library of AIEgens over the years. One important finding is the AIE characteristics of the organometallic compounds. Similar to the organic luminophores, the notorious quenching effect also comes to play in many organometallic luminophores. For instance, tris(2-phenylpyridine) iridium(III) complex, a well-known organometallic phosphor, has a phosphorescence QY of

Figure 2. PL photographs of solutions of HPS in THF/water mixtures with different fractions of water (top panel). The molecular structure and the crystal structure of the HPS are shown in bottom panel. Reproduced with permission.114 Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA.

Figure 3. Schematic illustration of the AIE phenomenon of a propeller shaped luminogen of tetraphenylethene (TPE) through RIR and a shell-like luminogen of 10,10´,11,11´-tetrahydro-5,5´bidibenzo[a,d][7]annulenylidene (THBA) through RIV. Reproduced with permission.114 Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA. 11



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97% in solution, and this value decreases to ~3% when the complexes are aggregated in a film. The ensuing research proved that the newly discovered AIE effect works well to solve this problem. For example, the AIE property was used to design various highly luminescent Ir(III)-complexes (Table 2).117 In particular, a cationic [Ir(ppy)2(L)]PF6 [ppy: 2-phenylpyridine, PF6-: hexafluorophosphate, and L: 4,7-bis(3´,6´-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9´-bi(9Hcarbazol)-9-yl)1,10-phenanthroline] was recently reported to have the AIE properties.118 It should be noted that the AIE of the as-designed Ir(III)-complexes may follow the RIR mechanism, since they consist of a number of free rotors and the intramolecular rotation could be restricted upon aggregation. Similar to the hydrocarbon and heterocarbon based AIEgens, the aggregation of the as-designed cationic Ir(III)-complexes can be induced by the solvents, and the aggregated complexes exhibit excellent on/off PL by organic vapors.118 Other important organometallic compounds such as the Pt-complexes bearing σ-alkynyl ancillary ligands also exhibit intense phosphorescence in fluid and glassy solutions.119 For example, a Pt(II)complex, namely [Pt(CNtBu)2(CN)2], is non-luminescent in solution, but it shows bright green emission in the solid state.120 Since there is no free aromatic rotors in the molecular structure of [Pt(CNtBu)2(CN)2], the mechanism of RIR is unlikely to be the major cause of its AIE effect. Rather, the presence of weak Pt(II)···Pt(II) bond in their crystal structure is considered to be the key factor for the aggregation of molecules as well as for its PL enhancement. The appearance of Pt(II)···Pt(II) bond in the aggregated [Pt(CNtBu)2(CN)2] is due to the weak attractive interaction between electrons in the 5d orbital of Pt. The phenomenon is known as metallophilicity, which often exists in metal atoms with a closed-shell electronic configuration. Since the manifestation of metallophilicity is the propensity of metal center to aggregate, the metallophilicity-induced aggregation has recently turned into an active area of research, and considerable preparative efforts have been witnessed in the organometallic and inorganic chemistry of coinage metals. The phenomenon of metallophilicity also exists in many Au(I) complexes. For example, a bright yellow emission (λem = 552 nm) was observed in the trimeric organometallic complex of Au, i.e., [Au3(CH3N=COCH3)3], where the aggregation was triggered by contacting a small amount of good solvent (e.g., chloroform and dichloromethane) with the polycrystalline sample of the colorless crystals of [Au3(CH3N=COCH3)3] after UV irradiation.121 This process could be repeated if the liquid was rapidly removed from the contact. However, further addition of the solvent resulted in 12


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the reduction of PL intensity. In fact, no yellow emission was observed for the dissolved Au(I) complexes in solution. The authors thus rationalized that the Au(I)···Au(I) interaction (known as aurophilic interaction) might become prominent in the aggregated state, which could trigger the bright emission of Au(I) complexes. Following this report, a number of Au(I) complexes, such as those containing carbene, amine, phosphine and thiolate ligands, were reported to exhibit PL upon aggregation.122-124 Besides, given that crystals are a particular form of aggregation, the crystallized Au(I) complexes might also luminesce via the AIE mechanism. This concept has been successfully demonstrated in a number of studies.125-128 In particular, a series of cyclic trinuclear Au(I) complexes with alkoxy side chains of various lengths were reported to show strong PL in their crystalline phase, but they are non-luminescent in solution. The high structure order also makes these luminescent crystals suitable for X-ray crystallography analysis, which unambiguously suggests that the aurophilic interaction induced aggregation should be responsible for their strong PL. Two PL bands at 463 and 643 nm were seen for the polymeric Au(I)-phenylthiolate complexes.129 The excited state lifetime for the high energy (HE) band was ~5 ns while that of the low energy (LE) band was 1.1 µs. The relatively long lifetime of the 643 nm band was attributed to the triplet metal-centered state while the behavior of the 465 nm band indicates that it arises from a ligand-to-metal charge-transfer-derived state. The presence of weak Au(I)···Au(I) interaction was also evident from the supramolecular structure of the polymeric Au(I)-phenylthiolate complexes. Since the aggregation of the Au(I) complexes can be influenced by several factors, including the types of neighboring ligands, metal ions, solvents, and temperature, the PL properties of Au(I) complexes can also be manipulated through the subsequent change of Au(I)···Au(I) interaction. For instance, a series of Au(I)-alkanethiolates [Au(I)-SRs, R = -(CH2)nH, n = 2-10, 12, 14, 16, and 18] were reported, which remained as a highly ordered layer structure in the solid state.130 The aggregated Au(I)-SRs showed strong PL when n ≥ 4. A broad HE photoemission band at about 410440 nm and a LE photoemission band at about 610-620 nm were observed. Interestingly, the photoemission intensity increases with the increasing of the chain length of alkyl groups. The X-ray diffraction results from the Au(I)-SRs suggest that the Au(I)···Au(I) interaction increases to a certain degree with a long alkyl chain group. The electron donating ability of the alkyl chain increases with the increasing of the length of alkyl ligands, which could affect the strength of the Au(I)···Au(I) interaction and the PL intensity of Au(I) complexes. 13



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Table 2. Examples of Organometallic Complexes with AIE Characteristics and Their Photophysical Properties. No.

Complex

Ex. (nm)

Em. (nm)

QY

Lifetime

1 2 3 4 5 6 7

[Ir (ppy)2(DBM)] [Ir (ppy)2(SB)] [Pt (CNtBu)2(CN)2] [Pt (ppy)(L1)] [Au-SPh] [Au-SR] [Au-SG]a

365 365 365 365 275, 360 310 330, 375

625 610 534 560 465, 643 420, 620 565, 610

7.6% 7.9% 33% 38% -

8

[Au-SG]b

330, 375

565, 610

-

0.85 µs 157.2 ns, 428.7 ns 5 ns, 1.1 µs 2.93 µs (85%) & 0.455 µs (14%) 2.41 µs (79%) & 0.355 µs (18%)

Ref. 117 117 120 131 129 130

132

132

*ppy = 2-phenylpyridine; DBM = 1,3-diphenyl-1,3-propanedione; SB = 2-(naphthalen-1-yliminomethyl)-phenol); L1 = 2(phenyliminomethyl)-phenol; QY = Quantum yield; Ex. = Excitation wavelength; Em. = Emission wavelength;

a

Solvent-induced

b

aggregation; Cation-induced aggregation

In Table 2, we have highlighted the photophysical properties of some of the organometallic compounds that exhibit AIE properties. The occurrence of long lifetime (generally in the order of µs scale) in the excited state and large Stokes shift suggest that the radiative decays are most likely associated with inter-system crossing, which would give rise to phosphorescence. Therefore, the photoemission of the organometallic compounds is often assigned as phosphorescence while that of the aggregated organic molecules is considered to be fluorescence. The presence of metallic components in the organometallic compounds is the cause of phosphorescence. On the other hand, the occurrence of RIM, which is thought to be the major cause of the AIE phenomenon in the organic molecules, also seems to be effective for some of the organometallic compounds. However, a special attention should be given particularly when the metal atoms have a closed shell electronic configuration, since abundant PL properties often arise from the metallophilic interaction. By a close comparison between organometallic compounds (e.g., Au(I)-SR complexes) and thiolated Au NCs, one can easily profile their structure similarity: the SR-[Au(I)-SR]x motifs in the protecting shell of Aun(SR)m NCs may be considered as a special type of Au(I)-SR complexes. This intrinsic relationship has prompted us to correlate the AIE of Au(I)-SR complexes to the PL of thiolated Au NCs in 2012. Since thiolated Au NCs are usually synthesized by a reductive decomposition of polymeric Au(I)-SR complexes in solution, and the SR-[Au(I)-SR]x motifs are most likely the residues of these polymeric Au(I)-SR complexes, we have begun our investigation on the AIE of Au(I)-SR complexes as follows. First, we noted that many studies reported the AIE of 14


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Au(I)-thiolate complexes in the solid state. To have a better understanding on the effects of aggregation states on PL, we used a water-soluble thiolate ligand, GSH, to prepare water-soluble oligomeric Au(I)-SG complexes in aqueous solution.132 The oligomeric Au(I)-SG complexes do not show PL in solution. However, inducing aggregation of these Au(I)-SG complexes could light their luminescence up. Specifically, the aggregation was induced by two different strategies: solventinduced aggregation and cation-induced aggregation. The former was based on changed solvent polarity by introducing a poor solvent. Ethanol was used as a poor solvent to destabilize the oligomeric Au(I)-SG complexes in water by disrupting the hydration shell of the complexes, which resulted in the charge neutralization and a subsequent aggregation of the complexes. The latter, in another way, relied on the charge neutralization and crosslinking of the Au(I)-SG complexes by a certain divalent cation like Cd(II), which could electrostatically bind to two negatively charged -SG ligands (from the carboxylic groups). In both cases, upon aggregation, non-luminescent Au(I)-SG complexes became highly luminescent at 565 and 610 nm (λex = 365 nm). Also, both the aggregated Au(I)-SG complexes exhibited a large Stokes shift (>200 nm) and long lifetime components in their excited state decay. These two characteristics of phosphorescence suggest that the bright PL from the aggregates of Au(I)-SG complexes is attributed to the phosphorescence from ligand-to-metal charge transfer (LMCT) or ligand-to-metal-metal charge transfer (LMMCT).

Figure 4. (a) Schematic illustration of solvent-induced AIE properties of the oligomeric Au(I)-SG complexes. (b) Digital photos of the Au(I)-SG complexes in mixed solvents of ethanol and water with different fraction of ethanol (fe) under visible (top row) and UV (bottom row) light. (c) UV-vis absorption and (d) photoemission spectra of the Au(I)-SG complexes in mixed solvents with different fe. Reproduced with permission.132 Copyright 2012, American Chemical Society.

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The PL properties of the aggregated Au(I)-SG complexes can be further tuned by adjusting the degree of molecular aggregates. Varying the polarity of the mixed solvent has been proven to be a good means to modulate the degree of aggregation. For example, we have managed to tune the aggregation states of Au(I)-SG complexes in a mixture of water and ethanol by varying the volume fraction of ethanol fe = volethanol/volethanol+water. As can be seen from Figure 4b, the Au(I)-SG complex solution was clear (under normal light) and non-luminescent (under UV light at λex = 365 nm) until fe reached 75%, after which the solution turned cloudy and became highly luminescent. It is intriguing to note that the Au(I)-SG complex aggregates become denser with the increase of fe from 75% to 95% as suggested by the dynamic light scattering (DLS) analysis, and the denser aggregates tend to emit brighter with a blue-shifted emission from 630 (at fe = 75%) to 565 nm (at fe = 95%) (Figure 4d). While the detailed mechanism for the blue shift of photoemission wavelength with increased density of aggregates remains unclear, the intensified Au(I)···Au(I) interactions in denser aggregates are considered to be an important contributor. Towards Highly Luminescent Thiolated Metal NCs with AIE Characteristics Based on the above understanding of the AIE properties of the oligomeric Au(I)-thiolate complexes, we are now able to generate bright PL by controlling the aggregation of Au(I)-SR complexes on the Au(0) core of thiolated Au NCs. In an exemplified synthesis, GSH was used as reducing-cum-protecting ligand; which could on one way form oligomeric Au(I)-SG complexes, and on the other way reduce Au(I)-X and Au(III)-X (X: halogen ligands) species into Au(0) core at elevated temperature (70 oC). The oligomeric Au(I)-SG complexes could then aggregate and anchor on the in-situ generated Au(0) cores to form Au(0)@Au(I)-thiolate NCs. The controlled aggregation of Au(I)-SG complexes on the surface of Au(0) core could then give rise to strong PL via the AIE mechanism. Indeed, the as-prepared Au(0)@Au(I)-thiolate NCs exhibited strong PL at 610 nm (λex = 365 nm) with a QY of ~15% (Figure 5). Some experimental evidences suggest that the AIE of Au(I)-SG complexes contributes predominantly to the PL of these thiolated Au NCs. First, the UVvis absorption spectrum of these luminescent Au(0)@Au(I)-thiolate NCs shows the absorption features (onset at 500 nm and a shoulder peak at ∼400 nm) similar to those of Au(I)-SG complexes, suggesting the presence of Au(I)-SG complexes on the surface of Au(0) core. Second, the envelope of PL spectrum of these Au(0)@Au(I)-thiolate NCs is almost undistinguishable to that of the aggregated Au(I)-SG complexes, strongly suggesting a similar emitting source for their PL. Third, 16


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the PL of these Au(0)@Au(I)-thiolate NCs is phosphorescence in nature (Stokes shift > 200 nm and lifetime of the dominant component ~1.53 µs), which agrees well with the AIE mechanism of Au(I)-SG complexes. The strong PL of this thiolated Au NC should then be attributed to a large content of Au(I)-SG complexes on the surface of Au(0)@Au(I)-thiolate NCs, which is supported by the molar ratio of Au(I) : Au (0) = 3 : 1, as measured by XPS and thermogravimetric analysis (TGA). We further investigated the size dependence of PL properties of the AIE-type Au(0)@Au(I)thiolate NCs. From the polyacrylamide gel electrophoresis (PAGE) analysis of the as-synthesized luminescent Au(0)@Au(I)-thiolate NCs, five different bands can be observed, which correspond to five different sized Au NCs. Using ESI-MS, the compositions of these five Au(0)@Au(I)-thiolate NCs were assigned to Au29(SG)27, Au30(SG)28, Au36(SG)32, Au39(SG)35, and Au43(SG)37, respectively. We found that for these Au(0)@Au(I)-thiolate NCs, the photoemission maximum was blue-shifted from 620 to 605 nm with the increase of the cluster size. This observation was in striking contrast with the Jellium model, where a larger sized NC should emit at a longer wavelength. However, this blue shift trend could be explained by the AIE mechanism. Considering the fact that the denser aggregates emit at shorter wavelengths (see Figure 4 and the corresponding discussion in the context), larger Au(0)@Au(I)-thiolate NCs, which have a denser aggregation of complexes in their shell, should similarly emit at shorter wavelengths. The AIE properties of the oligomeric Au(I)-SR complexes not only offer a good way to produce highly luminescent Au(0)@Au(I)-thiolate NCs, but also provide an alternative way to understand the cluster PL. More recently, we employed the carbon monoxide (CO)-reduction method to successfully synthesize an atomically precise thiolated Au NC, Au22(SG)18, which luminesces intensely at 665 nm (λex = 520 nm) with a QY of ~8% (Figure 6). It is intriguing to note that Au25(SG)18, which possesses the same ligand number but with 3 more Au atoms, shows much weaker PL at a similar wavelength (λex = 520 nm), with a QY of only ~0.2% (Figure 6). This comparison highlights that a subtle change in size (e.g., 3 Au atoms difference) of thiolated Au NCs

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Figure 5. UV-vis absorption (solid blue line), photoemission (solid red line, λex = 365 nm), and photoexcitation (dotted red line, λem = 610) spectra of the luminescent thiolated Au NCs. (Insets) Digital photos of luminescent thiolated Au NCs in the solid state (top row) and in water (bottom row) under (1) visible and (2) UV light. Reproduced with permission.132 Copyright 2012, American Chemical Society.

Figure 6. (a) UV-vis absorption (black line), photoemission (red line, λex = 520 nm), and photoexcitation (blue line, λem = 665 nm) spectra of the raw red-emitting thiolated Au NCs. (Insets) Digital photos of the raw product dissolved in water under (1) visible and (2) UV light. (b and c) PAGE gels of (lane I) mixsized Aun(SG)m prepared according to a published protocol,95 and (lane II) the raw red-emitting thiolated Au NCs under (b) visible and (c) UV light. Note that the mobility of band 4 (lane II) is similar to that of band 6’ in the reference lane I, which suggests that species 4 and 6’ have a similar charge-to-mass ratio. As reported by Negishi et al.,95 band 6’ corresponds to the Au25(SG)18, whereas band 4 (lane II) was assigned to be Au22(SG)18. (d) Photoemission spectra of 4 (red line) and 6’ (blue line) (λex = 520 nm), and the photoexcitation spectrum of 4 (green line, λem = 665 nm). (Insets) Digital photos of 4 and 6’ under UV light. (e) UV-vis absorption spectra of 4 (red line) and 6’ (blue line). (Insets) Digital photos of 4 and 6’ under visible light. Reproduced with permission.133 Copyright 2014, American Chemical Society.

18


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(a) (b)

Figure 7. (a) ESI mass spectrum of red-emitting Au22(SG)18 NCs. The red line is the simulated isotope pattern of [Au22(SG)180 - 5H]5-. (b) The ball stick structure model of Au22(SR)18 predicted by DFT. The purple balls represent the eight Au atoms in the core, and the yellow balls represent the Au atoms in the staple motifs, while the red balls represent S atoms. All other atoms (carbon, hydrogen) have been omitted for clarity. Reproduced with permission.133 Copyright 2014, American Chemical Society.

could lead to a distinctly different PL property. The relatively larger atomic ratio of SR-to-Au in Au22(SG)18 over Au25(SG)18 (0.82 vs. 0.72) suggests that the strong PL of Au22(SG)18 might originate from the AIE of the relatively longer oligomeric SR-[Au(I)-SR]x motifs on the surface of Au(0) core. The large Stokes shift (~145 nm) and the long lifetime of excitons [1.37 µs (57.4%) and 0.46 µs (35.4%)] agree well with the AIE mechanism. To have more insights into the structural origin of the strong PL in Au22(SG)18, density functional theory (DFT) calculation was used to predict the structure of Au22(SG)18. It should be mentioned that the capability of determining the atomically precise cluster formula of Au22(SG)18 could significantly reduce the calculation expense and make the prediction of cluster structure by DFT practically possible. As can be seen in Figure 7b, the optimized structure of Au22(SG)18 possesses a Au8 core, which is capped by two trimeric (SR-[Au(I)-SR]3) and two tetrameric (SR-[Au(I)-SR]4) motifs in an interlocked fashion. The X-ray absorption fine structure (XAFS) further confirms the presence of relatively longer SR-[Au-SR]x motifs in Au22(SG)18. Of note, a different structural model for Au22(SR)18 was recently proposed, which suggests a cluster structure with a bitetrahedron Au7 kernel surrounded by one unique [Au6(SR)6] and three Au3(SR)4 staple motifs.134 In both models, long Au(I)-thiolate staple motifs were found on the surface of thiolated Au NCs, which support that the AIE of relatively long staple motifs is the most likely cause of the strong PL of Au22(SG)18. In addition to Au22(SG)18, several

19



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other thiolated Au NCs also show AIE characteristics in terms of PL, which have been summarized in the top panel of Table 3. The AIE mechanism could also be employed to construct highly luminescent thiolated NCs with other metals. For example, a recent study reported a facile boiling water synthesis protocol for thiolated Ag NCs, which exhibit strong PL at 686 nm (λex = 420 nm).135 In addition, the assynthesized luminescent Ag NCs have a featureless absorption profile and a large Stokes shift in its photoemission spectrum, which are similar to the reported Au(0)@Au(I)-thiolate NCs, indicating that the PL of the as-prepared thiolated Ag NCs is also caused by AIE. In particular, the assynthesized thiolated Ag NCs have a unique core-shell Ag(0)@Ag(I)-thiolate structure, where the long Ag(I)-thiolate complexes are condensed on the Ag(0) core. The prevalence of long PL lifetime in their decay profile is also notable (see bottom panel of Table 3), which further supports that the PL of the as-synthesized Ag NCs is generated from the AIE of Ag(I)-thiolate complexes on the NC surface, mostly like a phosphorescence via the metal-centered triplet states. Table 3. Examples of Highly Luminescent Thiolated Metal NCs with AIE Characteristics and Their Photophysical Properties. Thiolated Au NCs: No Chemical formula 1

Εx. (nm) Em. (nm)

QY

610

15%

2

Au29(SG)27, Au30(SG)28, Au36(SG)32, Au39(SG)35, ~400 & Au43(SG)37 ~520 Au22(SG)18

665

8%

3 4

Aux(MUA)y Aux(SG)y

607 605

13% 4.4%

~330 ~410

Lifetime 1.99 µs (61%), 0.536 µs (29%) & 0.144 µs (8.9%) 1.37 µs (57.4%) & 0.46 µs (35.4%) 3.28 µs (87%) & 14.4 µs (13%)

Thiol/Au

Au(I) content

Ref.

0.9

75%

132

0.82

67%

133

0.78

75% 82%

136 137

Other thiolated metal NCs: No

Chemical formula

Εx. (nm)

Em. (nm)

QY

Lifetime

1.

Ag(0)@Ag(I)-thiolate

~420

686

7.4%

2

Au@Ag-thiolate

~520

667

6.8%

3 4

Aucore-Agshell-thiolate Cux(DPA)y

~385 ~345

565 640

4.6% 14.1%

1.08 µs (39.8%), 0.361 µs (39.4%), 0.086 µs (17.2%), & 14 ns (3.6%) 2.21 µs (56%), 0.641 µs (31.4%), 0.121 µs (10.3%), & 11.8 ns (2.3%) ~12.05 to 24.03 nsa 150.6 µsa

Ref. 135

138

139 140

*MUA = 11-mercaptoundecanoic acid; -SG = Glutathionate; a Average lifetime; DPA = D-penicillamine; QY = Quantum yield; Ex. = Excitation wavelength; Em. = Emission wavelength.

20


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AIE could also be applied to thiolated bimetallic NCs. For example, one recent study reported the synthesis of a highly luminescent Au@Ag-thiolate NC, which is also based on the AIE of SG[Au(I)-SG]x motifs on the surface of Au18(SG)14. It should be noted that Au18(SR)14 has a face-fused Au9 bi-octahedral core capped by a number of oligomeric SR-[Au(I)-SR]x motifs, i.e., one tetrameric (x = 4), one dimeric (x = 2) and three monomeric (x = 1) motifs.62 A dense aggregation of these oligomeric -[Au(I)-SR]x motifs could be achieved by neutralizing their negative charges with Ag+ cations. The as-prepared Au@Ag NCs exhibited red emission at ~667 nm (λex = 520 nm) with a QY of 6.8%, while the pristine Au18(SG)14 showed only weak PL at ~800 nm with a QY of 0.37%. Some photophysical properties of the Au@Ag-thiolate NCs, such as lifetime, photoemission wavelength, together with their corresponding photoexcitation wavelength and QYs, are summarized in Table 3 (bottom panel), which are all in good agreement with the proposed Ag+assisted AIE mechanism. In addition to Au and Ag NCs, AIE could also facilitate the generation of strong PL in thiolated Cu NCs. For example, one efficient method was recently reported to prepare highly luminescent thiolated Cu NCs.140 The as-prepared Cu NCs has a QY of 14.1% in solution. After evaporation of the solvent, the QY of the solid Cu NCs was slightly increased to 16.6%. Some PL characteristics of thiolated Cu NCs including large Stokes shift (295 nm) and long PL lifetime (150.6 µs) also agree well with the AIE mechanism. It should be noted that, the PL efficiency of the above M(0)@M(I)-thiolate NCs is still much lower than that of the hydrocarbon or heterocarbon based AIE materials (QY could reach near unity). The detailed mechanistic studies of the hydrocarbon or heterocarbon based AIE materials highlight the importance of RIM process. While the weak metallophilic interaction in the shell is considered to be a main source for the PL of M(0)@M(I)-thiolate NCs, the rotation and vibration motions of the thiolate ligands in the shell might still remain active in solution. Thus, if one considers inhibiting such type of motions, the PL efficiency of the M(0)@M(I)-thiolate NCs could be significantly improved. To achieve this goal, immobilization of the NCs inside a solid matrix could be an effective approach. There are some successful attempts along this direction. For example, a recent study showed that the localization of the thiolated Au NCs inside a 2D layered double hydroxides (LDHs) nanosheet could significantly improve the QY of Au NCs from 2.69% to 14.11%, in the meantime to prolong their PL lifetime from 1.84 µs to 14.67 µs.141 Since LDHs nanosheets have a positively charged 2D graphene-like ultrathin structure and thiolated Au NCs are 21



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protected by a negatively charged thiolate ligand (GSH, which contains carboxylic groups), the authors proposed that the positively charged LDH nanosheets have effectively polarized the charge distribution of localized Au NCs, thus increasing the content of emissive Au(I)-thiolate units in Au NCs. Moreover, the experimental observation was further supported by a DFT study. It was revealed that the thiolated Au NCs were tightly localized and confined in the microenvironment of LDH nanosheets via the host-guest hydrogen bond and the electrostatic interactions. As a result, the vibration and rotation of Au NCs has been inhibited to a certain extent and the corresponding nonradiative process is therefore reduced, which results in a promoted PL performance. Conclusions and Perspectives Because of the great demand of highly luminescent inorganic nanomaterials in many practical applications including biomedical and environment science, the research in the field of luminescent metal NCs is expected to increase significantly in the coming years. A number of thiolated metal NCs with high QYs have been recently designed and successfully synthesized under the guidance of newly discovered AIE phenomenon. For these AIE-type thiolated metal NCs, the QYs are generally higher than that of the conventional thiolated metal NCs. A large Stokes shift and long PL lifetime are typically observed in the PL of the AIE-type metal NCs. In addition, some recent studies have suggested that a high content of long oligomeric SR-[Au(I)-SR]x motifs on the NC surface is conducive for the strong PL of thiolated Au NCs with AIE. Considering the aggregation of Au(I)thiolate motifs is manifested by the aurophilic interaction between Au(I) centers, the metallophilic interaction in the shell of NCs is therefore considered to be a key factor to design a highly luminescent thiolated Au NC. Though advances have clearly been made, more intense research efforts should be devoted into this area. Thiolated Au NCs with AIE characteristics remain as an important platform to study, however, the field is still in its infancy and much are required to be done before our knowledge on gold could be extended to other metals. It is not the aim to simply reproduce the known AIE characteristics of Au(I)-thiolate complexes with other metals (e.g., Ag, Cu and Pt) as well as the subsequent development of several luminescent metal NCs, future research should take advantage of the principle of AIE phenomenon to explore the AIE mechanism in thiolated metal NCs, and to develop a design guideline for highly luminescent metal NCs. The use of appropriate thiolate ligands and the preparation of suitable M(I)-thiolate complexes are thus critical. In addition, it 22


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remains unclear whether metal NCs protected by the non-thiolate ligands also feature with the AIE characteristics. A concerted effort from both experimental and theoretical scientists may help in improving the PL efficiency of these metal NCs. Regarding the AIE phenomenon in thiolated metal NCs, the aggregation through the metallophilic interaction has been predicted however, the effect of RIM, which has been proven as a key factor for the strong PL of hydrocarbon and heterocarbon based materials, has not been considered in the thiolated metal NC system. Continuing and concerted theoretical and experimental research efforts are in demand to clarify the mechanism responsible for the enhanced PL. Finally, structural elucidation of thiolated metal NCs is also very important, since for any types of materials, their properties should be correlated with their structure to complete the picture. As the field moves forward, the cluster structures of metal NCs with different size and composition will certainly help to understand their PL origin as well as to enhance their PL efficiency. When all these elements come together, it will be possible to create a design rule for preparing highly luminescent metal NCs that could function equally well as other well-established luminophores such as inorganic QDs and organic dyes. In the present scenario, these AIE-type metal NCs have already demonstrated their potential as promising luminescent materials; however, more unexpected splendor is yet to come.

Quotes to highlight

1: It appears that a recently discovered “aggregation-induced emission” phenomenon by Tang’s group is also applicable to the M(I)-thiolate complexes (M = Au, Ag, Cu etc.), and the recognition that the spectral features of the aggregated M(I)-thiolate complexes are closely similar to the highly luminescent thiolated metal nanoclusters, highlights the significance of AIE phenomenon to unravel or understand the origin of photoluminescence in thiolated metal nanoclusters.

2: Design criteria – a delicate control of the aggregation of the long M(I)-thiolate complexes onto the ultrasmall metal core can function as an important strategy for constructing highly luminescent thiolated metal NCs. 23



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3: An in-depth understanding of the structures of AIE-type metal NCs will not only allow researchers to understand their structure-property relationship but also provide a good platform to design metal NCs with high PL efficiency that could find increasing acceptance in practical applications.

Author Biographies

Nirmal Goswami is currently a postdoctoral fellow in Prof. Jianping Xie’s group at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He obtained his PhD (2014) in Chemistry from the S. N. Bose National Centre for Basic Sciences, India. He is interested in noble metal nanoclusters and their biomedical applications.

Qiaofeng Yao is currently a postdoctoral fellow in Prof. Jianping Xie’s group at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He obtained his PhD degree from the same department under co-supervision of Prof. Jim Yang Lee and Prof. Jianping Xie. His current research focus is synthesis and self-assembly of thiolated metal nanoclusters.

Zhentao Luo earned his PhD from the Department of Chemical and Biomolecular Engineering, National University of Singapore (NUS) under the guidance of Prof. Jianping Xie. Currently, he is in Duke-NUS Medical School for Medical Doctor Training.

Jingguo Li is currently a Research Engineer at the Department of Chemical and Biomolecular Engineering, National University of Singapore, under the supervision of Prof. Jianping Xie. His research interest is design of nanocomposite membrane for environmental applications.

Tiankai Chen received his BS degree in chemistry from Peking University in 2014. After that, he joined Prof. Jianping Xie’s group as a PhD student at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He is interested in developing metal nanoclusters for environmental applications. 24


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Jianping Xie received his BS and MS degree from the Department of Chemical Engineering in Tsinghua University. He graduated with PhD from the Singapore–MIT Alliance program. He joined the National University of Singapore as an Assistant Professor in 2010 and established the “Noble Metal Nanoclusters” research group. His major research interest is engineering ultrasmall metal nanoclusters for biomedical and environmental applications (http://cheed.nus.edu.sg/stf/chexiej/index.html).

Acknowledgements

This work is financially supported by the Ministry of Education, Singapore, under the Grant of R-279-000-409-112.

Notes The authors declare no competing financial interest.

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