A Mono-Cuboctahedral Series of Gold Nanoclusters

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A Mono-Cuboctahedral Series of Gold Nanoclusters: Photoluminescence Origin, Large Enhancement, Wide Tunability and Structure-Property Correlation Qi Li, Meng Zhou, Woong Young So, Jingchun Huang, Mingxing Li, Douglas R. Kauffman, Mircea Cotlet, Tatsuya Higaki, Linda A. Peteanu, Zhengzhong Shao, and Rongchao Jin J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13558 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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Journal of the American Chemical Society

A Mono-Cuboctahedral Series of Gold Nanoclusters: Photoluminescence Origin, Large Enhancement, Wide Tunability and Structure-Property Correlation Qi Li,1 Meng Zhou,1 Woong Young So,1 Jingchun Huang,2 Mingxing Li,3 Douglas R. Kauffman,4 Mircea Cotlet,3 Tatsuya Higaki,1 Linda A. Peteanu,1 Zhengzhong Shao2 and Rongchao Jin*1 1

Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, United States.

2

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science,

Fudan University, Shanghai 200438, China. 3

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United

States. 4

National Energy Technology Laboratory (NETL), Department of Energy, Pittsburgh, PA 15236, United

States. KEYWORDS: Gold Nanoclusters; Photoluminescence; Kernel Emission; Surface Modulation ABSTRACT The origin of the near-infrared (NIR) photoluminescence (PL) from thiolate-protected gold nanoclusters (Au NCs, < 2 nm) has long been controversial, and the exact mechanism for the enhancement of quantum yield (QY) in many works remains elusive. Meanwhile, based upon the sole steady-state PL analysis, it is still a major challenge for researchers to map out a definitive relationship between the atomic structure and the PL property and understand how the Au(0) kernel and Au(I)-S surface contribute to the PL of Au NCs. Herein, we provide a paradigm study to address the above critical issues. By using a correlated series of “mono-cuboctahedral kernel” Au NCs and combined analyses of steady-state, temperature-dependence, femtosecond transient absorption, and Stark spectroscopy measurements, we have explicitly mapped out a kernel-origin mechanism and clearly elucidate the surface-structure effect, which establishes a definitive atomic-level structure-emission relationship. A ~100-fold enhancement of QY is realized via suppressing two effects: i) the ultrafast kernel-relaxation, and ii) the surface vibrations. The new insights into the PL origin, QY enhancement, wavelength tunability and structure-property relation constitute a major step toward the fundamental understanding and structural-tailoring-based modulation and enhancement of PL from Au NCs.

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1. Introduction Gold nanoclusters (NCs) have recently emerged as a new class of luminescent material and hold great promise in sensing, bioimaging and theranostic applications.1-13 Compared to CdSe quantum dots and organic dyes, Au NCs possess several intrinsic merits, such as low toxicity, excellent biocompatibility, high stability, near-infrared emission, large Stokes shift and long PL lifetime (up to ten microseconds).1-12 The study of the photoluminescence (PL) of Au NCs can be dated back to the early 2000s,13-15 and the recent years have witnessed significant progress in this topic.16-30 For example, the PL quantum yield (QY) of Au NCs has been largely improved by several stratagies16-18 (e.g. heterometal doping, surface-rigidifying), and the surface effect has been studied by Lee16 and Millstone19 groups. An aggregation-induction-emission strategy has been introduced by Xie’s group,20 and aggregation-related phosphorescence from metal NCs and crystallization-enhanced PL have also been observed by other groups.21,22 In addition, a combination of Au NCs with biomolecules for biomedical or other applications has been widely explored by different groups.9,23-30 Although significant progress has been made in the studies of PL from Au NCs,16-30 some issues of fundamental importance still remain unaddressed. First of all, the PL origin of the Au NCs is still controversial.1-3,12-15 After several years of research, scientists have achieved a general consensus that, for the absorption of Au NPs, the low energy transitions (e.g. the HOMO-LUMO) mainly occur within the Au(0) kernel, while the high-energy transitions show significant involvement of the surface Au(I) and S components.1-3,31 However, for the PL of Au NCs, whether it arises from the Au(0) kernel or the Au(I)ligand surface, is still controversial. Several mechanisms, such as the surface Au(I)-S triplet state and ligand to metal charge transfer (LMCT), have been widely proposed to explain the NIR PL from Au NCs with long lifetimes (~102 ns to 10 μs).16,17,32,33 Recently, Aiken’s group31,34 proposed a “kernel-origin” PL mechanism based on their computational analysis, which reports that the kernel-structure relaxation, rather than the kernel-to-shell relaxation (i.e., Au(0) to Au(I)) or the intersystem crossing (i.e., singlet to triplet), leads to the NIR emission with large Stokes shift and long lifetime. Experimentally, it should be pointed out that the sole steady-state PL analysis is hard to map out the exact origin of PL of Au NCs and understand how the Au(0) kernel and Au(I)-S surface contribute to the PL. Due to the limited understanding of the PL origin, the mechanism for the enhancement of quantum yield (QY) in many works still remains elusive. Furthermore, a major goal in the research of luminescent Au NCs is to map out the definitive relationship between the atomic structure and the PL property; this unfortunately has not yet been realized due to several obstacles. First of all, many luminescent Au NC samples may not be atomically precise and molecularly pure. The byproducts (e.g., other hidden sizes of Au NCs or Au(I) complexes) in the samples may exhibit critical influences on the overall PL properties as the PL is much more sensitive to impurities than the optical absorption. Second, many luminescent Au NCs have not been structurally characterized 2 Environment ACS Paragon Plus

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by X-ray crystallography, which precludes the atomic-scale structure-property correlation. Third, the photophysics of Au NCs (in particular, Aun(SR)m, SR = thiolate) is much more complicated than QDs and dyes.35-37 Finally, even with molecularly pure NCs and the known structure, as well as careful photophysics studies, the exact PL mechanism may still be somewhat ambiguous because of the lack of a correlated series of relevant NCs that can mutually verify/corroborate the mechanism.38 Therefore, a structurally related series of NCs is highly desirable for in-depth studies of the PL mechanism. The creation of such a series of NCs is quite challenging, as the vast majority of NCs reported in the literature unfortunately do not form a correlated series.1-3 Recently, a desired series of NCs has fortunately become available,39-43 which offers us an excellent opportunity to perform an anatomy of how the different motifs of a NC contribute to its PL properties (e.g., quantum yield, PL wavelength), much like the well-defined organic dyes (such as the rhodamine and BODIPY series)44,45 and the Au(I)/(III) complex series46-49, by which organic and inorganic chemists have systematically studied the structure-PL correlations. In this work, we report a comprehensive optical study by combining steady-state, temperaturedependence, femtosecond transient absorption, and Stark spectroscopy measurements on a “monocuboctahedron kernel” series of gold NCs. Our systematic studies explicitly reveal that the surface is critical for the room-temperature (r.t.) PL quantum yield (QY), but the fundamental origin of PL is indeed in the kernel emission, rather than the surface emission or charge transfer mechanism for this series of NCs. Our results demonstrate that, to enhance the QY of Au NCs at r.t., one needs to suppress i) the ultrafast kernel-relaxation; and ii) the surface vibrations. Guided by these, we have attempted to enhance the PL of mono-cuboctahedron gold NCs and indeed successfully achieved a 100-fold enhancement of QY. Overall, this work not only offers a model study to unravel the fundamental origin of PL based on the correlated series of Au NCs through multi-technique analyses, but also provides a solid basis for rational design of the atomic-structures of Au NCs to enhance their QYs and tailor the PL wavelengths for specific applications of imaging and sensing.

2. Results and Discussion 2.1. Structures of the “Mono-cuboctahedral Kernel” Nanoclusters Figure 1 shows the X-ray structures of the five “mono-cuboctahedral kernel” (MCK for short hereafter) series of Au NCs. Figure 1A displays the X-ray structures of the first three NCs: [Au23(SR)16]− (R = cyclohexyl), [Au21(SR)12(PCP)]+ (where, PCP = PPh2-CH2-PPh2), and [Au19Cd2(SR)16]−.39-41 These three NCs have the same kernel structure (displayed in space-filled mode), i.e., a Au13 cuboctahedron plus two “hub” atoms. On a note, compared with [Au23(SR)16]− and [Au21(SR)12(PCP)]+, the [Au19Cd2(SR)16]− NC shows an only difference in that the two “hub” Au atoms are substituted by two Cd hub atoms. The surface structures of these three “13+2” MCK NCs are, however, significantly different. Compared with

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the surface motifs of [Au23(SR)16]−, the two S-Au-S motifs are replaced by the P-C-P motifs in [Au21(SR)12(PCP)]+; and in [Au19Cd2(SR)16]−, each of the two Au3S4 motifs is replaced by two S-Au-S motifs. For the additional NCs in the MCK series, Figure 1B shows the atomic structure of the Au21(SR’)15 (R’=tert-butyl).42 Compared with the three “13+2” NCs, the kernel of the Au21(SR’)15 is composed of a Au13 cuboctahedron plus only one “hub” atom (i.e., 13+1). For the fifth NC, Figure 1C shows the atomic structure of the Au21(SR’’)15 (R’’=1-admentathiol).43 It can be observed that the kernel of the Au21(SR’’)15 consists of a Au13 cuboctahedron with one gold atom missing, plus two “hub” atoms (i.e., 13-1+2).

Figure 1. Atomic structures of the “mono-cuboctahedral kernel” series of Au NCs. Overall, the first three “13+2” NCs provide a good model to study how the different surface structures affect the PL of Au NCs with the same kernel structure. Furthermore, a comparison of the three 21-gold-atom nanoclusters (i.e., [Au21(SR)12(PCP)]+, Au21(SR')15 and Au21(SR'')15, Figure 1) offers insights into how a subtle change of the kernel structure influences the PL of the same “size” Au NCs with the surface motifs being similar.

2.2. PL Properties at Room Temperature (r.t.) To ensure the highest purity of the samples, all the Au NCs studied in this work were first crystallized. Then, single crystals were manually picked up and re-dissolved in solutions for subsequent optical measurements. Figure 2 shows the r.t. UV-Vis absorption, PL and time-correlated-single-photoncounting (TCSPC) data of the three “13+2” MCK NCs. All the three “13+2” NCs show very similar UVVis absorption spectral profiles, each with a single prominent absorption peak (ranging from ~570 to 585 nm, Figure 2B). The PL peaks of the three NCs are also similar (~740 to 750 nm, Figure 2C); of note,


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[Au23(SR)16]− shows a long “tail” in the PL spectrum (to be discussed below, section 2.5). The PL excitation spectra of the three NCs are well consistent with their UV-Vis absorption profiles (Figure S1). On the other hand, the r.t. QY of [Au21(SR)12(PCP)]+ is significantly higher than the other two (~4 times that of [Au19Cd2(SR)16]− and 8 times that of [Au23(SR)16]−). The PL lifetimes of the three NCs at r.t. also show significant differences (Figure 2D), see Table S1. Based on these results, we can conclude that the atomic structures and compositions of surface motifs can significantly affect the QY and lifetime at r.t. in this series of Au NCs, but the PL energy is nevertheless determined by the kernel (hence, the same kernel gives rise to the same PL energy).

Figure 2. Spectroscopic study on the three “13+2” MCK NCs at room temperature. (A) Atomic structures of the three “13+2” MCK NCs. Comparison of (B) UV-Vis absorption and (C) PL spectra and (D) TCSPC trajectories of the three NCs in dichloromethane (DCM). Red curves: [Au23(SR)16]-, green curves: [Au19Cd2(SR)16]-, and yellow-green curves: [Au21(SR)12(P–C–P)]+.

2.3. Temperature Dependence of PL Low-temperature optical studies are further conducted on the three “13+2” MCK NCs. Figure 3 shows

the

temperature-dependent

PL

spectra

and

TCSPC

trajectories

of

[Au23(SR)16]−,

[Au21(SR)12(PCP)]+ and [Au19Cd2(SR)16]−. As the temperature decreases from 295 to 80 K, dramatic enhancement of the PL intensity can be observed for all the three NCs (Figure 3A, C, E); note that the 295 K curve is enlarged for visibility, whereas the 180 and 80 K curves show the original intensities. A significant finding is that the PL lifetimes of the three NCs all increase to the same value (~6 µs) with a mono-exponential decay at 80 K (Figure 3B, D, F). Meanwhile, no “splitting” or other PL peaks emerges 5 Environment ACS Paragon Plus


during the temperature decrease; this suggests a single underlying electronic transition contributing to the PL emission for this cuboctahedral series of NCs, which is different from the previous temperaturedependent results on icosahedral Au25 and Au38 NCs.50 The inset pictures in Figure 3A show the photographs of Au23 in DCM solutions under 365 nm UV at r.t. (QY ~0.4%) and 80 K (QY ~70%), respectively. The drastically enhanced QY and increased PL lifetime, together with the mono-exponential decay, suggest the effective suppressions of the thermally activated non-radiative pathways by decreasing the temperature. The profiles of low-temperature absorption and PL excitation spectra of the three NCs are also similar, see Figure S2 to S4.

Figure 3. Temperature-dependent PL spectra and TCSPC trajectories of the three “13+2” MCK NCs: [Au23(SR)16]− (A and B), [Au21(SR)12(PCP)]+ (C and D) and [Au19Cd2(SR)16]− (E and F). In A), C) and E), the 295 K PL spectra are enlarged by a factor of 10, 2, and 5, respectively, so that the curves can be seen. Temperature-dependent optical studies have also been performed on another three “13+2” MCK NCs: [Au23-xCux(SR)16]-, [Au23-xCdx(SR)16]- and newly-synthesized [Au23(SR)16-x(S-t-butyl)x]- (Figures S5-S7). Compared with [Au23(SR)16]-, the surface Au atoms are partly replaced by Cu or Cd in the [Au23xCux(SR)16]

and [Au23-xCdx(SR)16]-.41 The [Au23(SR)16-x(S-t-butyl)x]- is newly synthesized by partial

ligand-exchange (Figures S7-S8). It can be observed that all the six NCs, which share the same “13+2” MCK kernel but have different surfaces, show weak PL at r.t. with largely different QYs (ranging from 0.3 % to 3%) and significantly varied PL lifetimes (average lifetimes ranging from 20 ns to 150 ns, Figure S5-7). However, at 80 K, they show similarly high QYs and especially the same 6 μs PL lifetime. This observation suggests that the significantly varied QYs and lifetimes at r.t. mainly arise from the different 6 Environment ACS Paragon Plus

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thermally-activated non-radiative effects from the different surfaces (including the atomic structure, composition and type of ligands) on the six “13+2” MCK NCs. The origin of the PL for the six NCs is, however, indeed the same, which lies in the “13+2” MCK kernel. This structural insight has not been attained in previous experimental studies on Aun(SR)m NCs.

2.4. PL of Nanoclusters Embedded in Films The ultrahigh quantum yields obtained at 80 K suggest the possibility for achieving ultrabright PL from the Au NCs at r.t. if the thermally-activated non-radiative pathways can be suppressed. One feasible method to suppress the surface vibrations is to embed NCs in a polymer matrix. We thus fabricated Au NC/polymer

hybrid

films

using

commercial

polymers

including

polystyrene

(PS),

poly(methylmethacrylate) (PMMA), and poly(vinylpyridine) (PVP) via simple casting (Figure 4A). Figure 4B shows the measured PL lifetimes of [Au21(SR)12(PCP)]+/ PS (Au21-PS for short) films and Figure 4C shows the PL spectra. In our experiments, all the Au NCs embedded in polymer films show a ~10-fold enhancement (e.g., QY~40% for Au21-PS film),51 which is accompanied with a significant increase in PL lifetime compared with their solution states (black curves in Figure 4). The photographs of the [Au21(SR)12(PCP)]+ in DCM solution and Au21-PS film under UV lamp are shown in Figure 4A.

Figure 4. (A) Large enhancement of the Au NCs’ PL from solution to film at r.t. Insets are the photographs of [Au21(SR)12(PCP)]+ in DCM solution and in films under a 365 nm UV lamp. (B) TCSPC data and (C) PL spectra of [Au21(SR)12(PCP)]+ in films (red curves) compared with [Au21(SR)12(PCP)]+ in DCM solution (black curves).



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Low-temperature studies of Au21-PS film are also conducted (Figure S9). There is a ~2× enhancement of PL (QY~80 % at 80 K) for the Au21-PS film and the PL lifetime increases to the same ~6 µs as the temperature decreases to 80 K. At 80 K, the same QY and lifetime of [Au21(SR)12(PCP)]+ in both DCM and films suggest the nature of the PL in solution and solid states is the same, while the dramatic differences at r.t. arise from the surface vibrations, that is, after the NCs are embedded in polymer matrices the surface vibrations are suppressed, hence, higher QY. Furthermore, as shown in Figure S10, a very similar blue-shift trend can be observed for both the PL peak (748 to 737 nm from 295 to 80 K) and the absorption peak (567 to 555 nm from 295 to 80 K) of [Au21(SR)12(PCP)]+, suggesting the same underlying transition of the PL and the absorption, i.e., between the HOMO and the LUMO. Previous theoretical calculations suggested that the HOMO and LUMO are mainly composed of 6sp orbitals of Au kernel atoms and the influence of the surface sulfur or other atoms is small,31 thus, the PL is mainly from a kernel-dominant state in this MCK series.

2.5. Ultrafast Electron Dynamics For further understanding the underlying mechanisms of the drastic enhancement of PL QYs, we performed femto-nanosecond transient absorption (fs-ns TA) on the [Au23(SR)16]− (QY: 0.4 %, samples dissolved

in

DCM),

[Au21(SR)12(PCP)]+

(QY:

3

%,

samples

dissolved

in

DCM),

and

+

[Au21(SR)12(PCP)] /polystyrene (Au21-PS) solid film (QY: 40 %). The fs/ns-TA spectra are shown in Figure 5 (A-C). Upon excitation with 360 nm laser pulse, ground-state bleach (GSB) at the wavelength corresponding to the NCs’ absorption peak at 570 nm was observed, along with two significant excitedstate absorptions (ESA) at 500 nm and 650 nm for all the three samples. Global fitting results of the femtosecond transient absorption spectra are shown in Figure 5 (D-F) and the corresponding selected kinetic traces/fitting are shown in Figure 5 (G-I). After excitation at ~360 nm, ultrafast decays within the first picosecond were observed in all samples. This fs process is however absent if the sample is excited at 600 nm wavelength (below the HOMO-LUMO gap energy) (Figure S11), suggesting that this fs process is an internal conversion from Sn to S1.52 Interestingly, during the subsequent few-picoseconds, one can clearly observe that the decay at 500 nm is accompanied by a slight rise at 750 nm from kinetic traces of [Au23(SR)16]− and [Au21(SR)12(PCP)]+ (Figure 5 G-H). It should be noted that this few-picosecond component shows a similar spectral feature (Figure 5 D-E) as that of the third ns one. Because no extra excited-state species was formed, the few-picosecond growth observed in both NCs should be ascribed to the structural relaxation which is commonly observed in polymer materials.52 Meanwhile, this fewpicosecond process still exists when the NC is pumped at 600 nm wavelength which can only excite the kernel state (Figure S11), therefore, this structural relaxation is a kernel relaxation. More importantly, we found that the higher QY of the sample, the less amplitude of this ps component in the DAS spectra (Figure 5 D-F). For the [Au23(SR)16]− ⎯ which shows the lowest QY with a long tail in its PL spectrum, 8 Environment ACS Paragon Plus

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this ps component is the most significant. In contrast, for the [Au21(SR)12(PCP)]+ and the Au21-PS film, this ps component is very weak or even no detectible. These results suggest that the ultrafast kernel relaxation ⎯ which involves electron density redistribution and Au−Au bond length change due to the instant expansion of the structure induced by the pump pulse ⎯ down-shifts the excited electron to lowerenergy states and thus decreases the QY of the sample.

Figure 5. (A-C) Femtosecond transient absorption spectra which show ΔA [Au23(SR)16]−, [Au21(SR)12(PCP)]+ and [Au21(SR)12(PCP)]+/Polystyrene film with (D-F) Corresponding global fitting results of the femtosecond probe transient [Au23(SR)16]−, [Au21(SR)12(PCP)]+ and [Au21(SR)12(PCP)]+/Polystyrene film. selected kinetic traces/fitting with excitation at 360 nm.


at all time-delays of excitation at 360 nm. absorption spectra of (G-I) Corresponding


It should be noted that in our TA results, no other state transitions (such as the core to shell relaxation observed in Au25,35 or the intersystem crossing observed in Au(I)/(III) complex,46-49 or other energy transfer process observed in dye-conjugated Au NCs53,54) can be identified. This excludes the possibility that the PL of the MCK Au NCs comes from the charge transfer states or triplet states. In addition, no PL quenching effect was observed when the NC solution was filled with O2, which is a typical sign of phosphorescence (i.e. triplet state emission). Thus, the triplet emission mechanism is further excluded for our samples. On the other hand, the S1 excited state lifetimes of the NCs (monitored at the corresponding groundstate bleaching, Figure 6A upper) are found to be comparable with the PL lifetimes measured by TCSPC (Figure 6A lower) for all the samples. This consistency implies that both the ground state bleaching and the PL signals have a similar origin, that is, the HOMO-LUMO transition. This directly confirms that the PL origin lies in the kernel-dominated LUMO to HOMO transition. Figure 6B depicts the kernel-emission model and summarizes the relaxation diagram and time-constants obtained from transient absorption measurements.

Figure 6. (A) Comparison of the nanosecond TA decay traces at ground-state bleaching peak and the TCSPC decay traces. (B) Relaxation diagram and time-constants obtained from TA measurements.

Scheme 1. Mechanism for the large QY enhancement (~ 100-fold) in the MCK series of Au NCs. 10 Environment ACS Paragon Plus

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Based on the above results, it can be concluded that the key for the enhancement of QY lies in the suppression of i) the ultrafast kernel relaxation process and ii) the surface vibrations (Scheme 1). The suppression of these two processes can be realized by tailoring the surface structure and by embedding NCs in matrices. The suppression of the first process explains the QY enhancement from [Au23(SR)16]- to [Au21(SR)12(PCP)]+ (QY: 0.4 % to 3 %), and the suppression of the second process explains the further QY enhancement of [Au21(SR)12(PCP)]+ from solution to film (QY: 3 % to 40 %), with a combined enhancement of ~100 times. Meanwhile, the same ~100-time-enhancement of both the lifetime (33 ns to 3 µs) and QY (0.4% to 40%) from [Au23(SR)16]- to [Au21(SR)12(PCP)]+/PS film further demonstrates the key for the PL enhancement lies in the suppression of non-radiative rate (knr, from 3×107 s-1 to 3×105 s-1), while the radiative rate (kr) almost remains constant (~1.33×105 s-1) calculated by the following equations: QY = kr/(knr + kr) τ = 1/(knr + kr) In previous research, the PL enhancement of Au NCs was realized by rigidifying the surface using surfactant-assembled micelles, nanogels, counterions or by aggregation.16,20,55,56 The general underlying mechanism for all these strategies (including our case of polymer embedding) is similar, i.e., the suppression of surface vibrations. However, a new discovery in our current work is that we have identified a kernel relaxation process; this ultrafast process can be eliminated by tailoring the atomic structures of the surface “staples”, hence, a significant increase of QY at r.t.. More importantly, most previous works assumed the origin of the PL is the ligand-to-metal charge transfer state of the Au(I)-S surface,16,20,32,33,55,56 but here the mono-cuboctahedral series explicitly demonstrate that, although the surface can dictate the QY and lifetime at r.t., the origin of PL is in the kernel of the cuboctahedral NCs, which is evidenced by the low-temperature and ultrafast spectroscopic studies. It is noteworthy that the mechanism of “kernel origin” of PL in Aun(SR)m has been proposed in theoretical calculations on icosahedral NCs31,34,57 but has not been demonstrated convincingly by experiment. Future work should investigate the origin of PL in the icosahedral NCs.

2.6. Ruling out the Charge-Transfer Photoluminescence Mechanism The charge-transfer PL mechanism exists in the icosahedral Au25 NC as previously reported by Knappenberger et al.58 Our femtosecond analysis did not find charge-transfer features in the cuboctahedral NCs. To further probe the possibility of the charge-transfer contribution, Stark spectroscopic analysis is conducted. Stark measurement is a well-known convincing method to verify the charge-transfer effects,59-61 but it has not been conducted in Au NCs. For the electrofluorescence (EF) measurement (Figure 7), excitation was done at the wavelength where the electric field-induced change in absorption intensity was negligible (Figure S12). From the Stark measurement, three kinds of information can be obtained, including the 0th derivative (change of intensity), 1st derivative (change of



polarizability), and 2nd derivative (change of dipole moment). It should be noted that the 2nd derivative component for [Au21(SR)12(PCP)]+ and [Au23(SR)16]− is similar and extremely small (0.1 and 0.17 D). Considering that the average bond length of Au-S is 2.3 Å, a maximum dipole moment value of 11.04 D would be observed if a full electron charge were transferred between Au and S (i.e. Au+1−S-1). This directly reveals that the ligand-to-metal charge transfer contribution is negligible for the MCK series of NCs and further suggests that both [Au21(SR)12(PCP)]+ and [Au23(SR)16]− NCs have a similar origin of PL (kernel-emission). More importantly, both NCs are shown to have field-induced PL enhancement (0th derivative) rather field-induced quenching, which is totally different from conjugated polymers and quantum dots.60,61 This phenomenon could be related with the effective suppression of the thermally activated non-radiative pathways in the low temperature measurements as one proposed mechanism for electric-field quenching is due to the enhanced internal conversion upon external electric field.60,61

Figure 7. Stark spectroscopy measurements on [Au21(SR)12(PCP)]+ (A, C, E) and [Au23(SR)16]− (B, D, F). (A, B) Electrofluorescence (solid line) and emission (dashed line), (C, D) EF (black) and fit (red), and (E, F) the 0th, 1st, and 2nd derivative components of the EF fit. All measurements were done at 77K.


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Besides, we have also tested the effects of different solvents by performing the steady-state PL and ultrafast electron dynamics measurements, but no significant solvent-dependent effect was observed (Figure S13 and S14). This solvent-independence also indicates no significant charge-transfer contribution for the emissions observed in this MCK NC series, which is different from previously reported solvent-dependent optical results of Au25 NCs.58,62,63

2.7. Tunability of PL Wavelength The above analyses establish the kernel-emission mechanism in the MCK series of NCs and elucidation of the underlying mechanism for the enhancement of QY. The next issue is to what extent the PL wavelength can be tuned. We demonstrate a ~230 nm tunability (i.e., PL peak from ~750 to ~980 nm) realized by subtle change of the atomic structure of the kernel. Figure 8 shows the absorption and PL spectra of the three 21-gold-atom MCK Au NCs: [Au21(SR)12(PCP)]+, Au21(SR’)15 and Au21(SR’’)15. It can be observed that, although all the three NCs have the same number (21) of gold atoms in total, their PL energies are distinctly different. Compared with the [Au21(SR)12(PCP)]+, the PL peaks of Au21(SR’’)15 (“13-1+2” kernel) and Au21(SR’)15 (“13+1” kernel) red-shift to ~850 nm and ~980 nm, respectively. This red-shift of PLs is consistent with the red-shift of HOMO-LUMO absorption peaks. This result suggests that the detailed atomic structure of the kernel determines the HOMO-LUMO gap and accordingly, the PL energy can be tuned in a quite wide range (~750 nm to ~980 nm) by subtle tailoring of the kernel structure, as the PL originates from the HOMO-LUMO transition. It should be noted that the different charges of the kernels may be another reason for the different energy gaps in some cases, but considering that both Au21(SR’)15 and Au21(SR’’)15 have the Au14 kernel with the same 6e, the primary reason for the difference in the energy gap should be the detailed packing structure of the 14 Au atoms in the kernel.

Figure 8. Spectral tunability of the three MCK Au21 NCs. The PL spectra were measured by excitation at the wavelengths of their lowest-energy absorption peaks. 13 Environment ACS Paragon Plus


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2.8. Discussions Based on the above results, some general conclusions on the PL of the MCK Au NCs can be summarized as follows. 1. PL Origin. A kernel-origin mechanism is clearly mapped out. The underlying transition of the observed PL is the HOMO-LUMO one, which is dominated by the kernel-structure. No surface charge transfer or triplet state emission can be identified in MCK NCs. 2. PL Energy. The Au(0) kernel structure is the primary factor to determine the PL energy. 3. PL Quantum Yields. The Au(I)-S surface is the key to determine the QY at r.t, as the structure and composition of surface are closely correlated with the non-radiative processes. (i.e. surface vibration and ultrafast structural relaxation) We demonstrate effective suppression of the surfacerelated non-radiative relaxation is critical to enhance the QY. 4. PL Lifetime. The slow radiative relaxation of microseconds (e.g. kr~1.33×105 s-1 for [Au23(SR)16]- and [Au21(SR)12(PCP)]+) is an interesting feature of Au NCs. Our experimental results have first excluded the possibility of triplet-state emission as the underlying mechanism in our NC series. It should be noted that in other nanomaterials such as silicon nanoparticles, similar slow PLs with lifetime up to hundreds of µs have also been observed, and this is explained by the

different positions of valence band maximum and conduction band minimum in k-space which results in a low probability of bandgap radiative transition.64-66 Thus, a possible explanation

could

photoexcitation

31,34

be

the

ultrafast

geometrical

and

electronic

changes

after

the

which induce significant changes to the LUMO (e.g. position, atomic orbital

composition). Such differences of LUMO in excited state (in comparion with the ground-state) could slow down the radiative transition probability of LUMO-HOMO, thus resulting in the long lifetime. 5. Structure-PL correlations. It would be interesting to compare the structure-PL correlations of the MCK Au NCs (Figure 9A) with the classical organic dyes such as the BODIPY series44,45 (Figure 9B). The PL energies of BODIPY series are predominantly determined by the structure of the core conjugation part⎯the BODIPY kernel. The role of the BODIPY kernel for this BODIPY series is similar as the cuboctahedral kernel for the MCK Au NC series. It is well known that for dyes, the “surface” carbon tails greatly influence the QYs due to the vibration/rotation-based nonradiative effects (Figure 9B). Similarly, as we have demonstrated, the atomic structure of the surface Au(I)-S staples also strongly affects the QY of Au NCs at r.t..


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Figure 9. Comparism of the structure-PL relationship of MCK NCs (A) and BODIPY dyes (B). It should be noted that for other structures of Au NCs (e.g. icosahedral ones), it is still too early to draw a definite, common mechanism of PL due to the lack of a correlated series of NCs. Different structures may cause different interactions between the core and the shell, which may alter the excited electron relaxation process.67,68 Some NCs show dual or multiple emission peaks, which may result from multiple origins.69,70 The icosahedral NCs seem to have a more complicated PL mechanism57,58 and remain to be investigated in future work by creating a correlated series of NCs. For other FCC structured Au NCs that comprise bi- or multi-cuboctahedral kernels, it still lacks a full series for a systematic study. It is worth mentioning that the excited dynamics of a few other FCC nanoclusters are consistent with our current transient absorption results on the MCK series.71

3. Conclusion In summary, our work provides a paradigm study on a correlated series of NCs, which unambiguously unravels their PL mechanism. Based on the correlated structures of the MCK Au NC series and the combined optical analyses of steady-state, temperature-dependence, femtosecond transient absorption, and Stark spectroscopy measurements, we have convincingly demonstrated the kernelemission mechanism in the MCK NCs. Our results clearly elucidate the different roles of Au(0) core and Au(I)-S surface on the PL of Au NCs and present a new underlying strategy to increase the QY of Au



NCs at r.t., that is, by suppression of the ultrafast kernel-relaxation through tailoring the surface structure. This work enhances our knowledge on the structure-PL relationship in Au NCs to the similar level as the classical organic dyes and Au complexes, which constitutes a major advance toward the fundamental understanding and structural-tailoring-based modulation and enhancement of PL from Au NCs.

Experimental Section Synthesis. The preparation of Au23, Au21 and Au19Cd2 nanoclusters followed the methods reported previously.39-43 For the preparation of AuNC–polymer films, generally, 1 – 10 mg Au NCs were

dissolved in ~3 ml DCM and then different polymers (e.g. polystyrene and poly(methyl methacrylate)) were added to the solution with vigorous stirring. After the polymer was dissolved, a simple drop-casting of the solution was done on a glass plate and after drying in air, the composite film was obtained. We did not observe no distinct influence of nanocluster concentrations on the PL properties owing to large Stokes shifts and negligible self-absorption in NCs. Characterization. The UV−vis absorption spectra were recorded in the range of 190−1100 nm using a Hewlett Packard 8543 diode array spectrophotometer. The PL spectra were recorded on a QM 40 spectrophotometer with a 75 W xenon lamp as the light source and InGaAs (500 to 1700 nm) as the detector, or recorded on a Horiba Nanolog Hybrid Fluorometer connected with an Ocean Optics 65000FL spectrograph/CCD (400-1100 nm). The PL lifetimes were measured with a time-correlated single photon counting (TCSPC) technique with a pulsed LED source (376 nm, 1.1 ns) as the excitation source. The absolute quantum yields were measured by a 60 mm K-Sphere Petite integrating sphere. Temperature-dependent PL. Temperature-dependent PL measurements were carried out on a Fluorolog-3 spectrofluorometer (Horiba Jobin Yvon). The fluorometer was coupled with an Optistat DN cryostat (Oxford Instruments), an ITC temperature controller, and a pressure gauge. This homo-assembled system allowed us to conduct the temperature-dependent PL experiments from 298 to 80 K. The vacuum in the cryostat was maintained with a turbo molecular pump. Spectra were taken at different temperatures after a waiting period of 10 min. The Au nanoclusters were dissolved in 2-methyltetrohydrofuran for temperature-dependent PL measurements. Of note, optical absorption measurements before and after temperature-dependent PL measurements showed no change, suggesting that the samples did not change during the measurements.)


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Supporting Information. Details of the femtosecond/nanosecond transient absorption spectroscopy, Stark spectroscopy, and data analysis, supporting Figures (S1−S14), and supporting Table S1. Corresponding Author: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Dr. Shuxin Wang, Ms. Shuang Chen and Mr. Jingsong Chai for providing the Au21(SR')15 and Au21(SR'')15 crystal samples. This work is financially supported by National Science Foundation (DMR1808675).

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A Mono-Cuboctahedral Series of Gold Nanoclusters

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