Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Mechanism of Photoluminescence in Ag Nanoclusters: MetalCentered Emission versus Synergistic Effect in Ligand-Centered Emission Taiqun Yang,†,‡,∥ Shan Dai,†,∥ Hao Tan,† Yuxin Zong,‡ Yangyi Liu,† Jinquan Chen,† Kun Zhang,*,‡ Peng Wu,‡ Sanjun Zhang,*,†,§,⊥ Jianhua Xu,† and Yang Tian†,‡
Downloaded via GUILFORD COLG on July 18, 2019 at 00:02:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
State Key Laboratory of Precision Spectroscopy and ‡Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, No. 3663, North Zhongshan Road, Shanghai 200062, China § Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China ⊥ NYU-ECNU Institute of Physics at NYU Shanghai, No. 3663, North Zhongshan Road, Shanghai 200062, China S Supporting Information *
ABSTRACT: It remains unclear whether the emission center of ligand-encapsulated metal nanoclusters (MNCs) is the surface ligands or the metal core. In this paper, we simultaneously observed metal-centered and ligand-centered emissions in Ag nanoclusters. The contributions of the surface ligands and the metal core were individually investigated to understand the nature of AgNC photoemission. A new ligand synergistic emission effect was observed. The amino correlated nπ* state provides a pivot to bridge the carboxyl correlated ππ* and nπ* states to enhance the charge transfer efficiency between different surface electronic states. Consequently, the photoluminescence quantum yields were significantly improved (∼1 to ∼10%). Transient absorption studies revealed that decreasing the pH could expand the potential energy curve and generate a conical intersection. This would facilitate the charge transfer and relaxation of excited electrons via a radiative pathway, thereby enhancing the emission intensity. These new insights into the photoemission mechanisms of MNCs should stimulate additional experimental and theoretical studies and could benefit the molecular-level design of luminescent MNCs for optoelectronics and other applications.
1. INTRODUCTION Organic ligand-encapsulated metal nanoclusters (MNCs) have attracted tremendous research interest due to their ultrasmall sizes, well-defined structures, and molecule-like properties.1 Luminescent metal nanoclusters (LMNCs) in particular have drawn considerable attention due to their wide applications in single-molecule studies, sensing, biolabeling, and biofluorescence imaging.2−6 However, a fundamental understanding about the origin of photoluminescence remains unclear particularly whether it is the metal core or the surface ligands. It has been widely accepted that the optical properties (energy levels) of MNCs are decided by the number and configuration of metal atoms.1,7 In this case, the emission center is the metal core. According to the Jellium model, the © XXXX American Chemical Society
transition energies of true free-electron metals should scale with inverse cluster radius.2,8 For instance, strong sizedependent photoluminescence has been observed for poly(amidoamine) dendrimer encapsulated AuNCs. The sizes and emission energies of AuNCs fit the scaling relation of EFermi/ N1/3.9 Two different emission wavelengths have been reported in mercaptosuccinic acid (H2MSA)-protected Ag 7 and Ag8NCs: a silver atom difference could shift the emission color from blue to red.10 However, many novel emission phenomena have been observed in MNCs that are Received: April 29, 2019 Revised: June 19, 2019 Published: June 28, 2019 A
DOI: 10.1021/acs.jpcc.9b04034 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
red-brown solution was formed. The as-synthesized DPAcapped AgNCs were stored at 4 °C in a refrigerator for further use. 2.3. Characterization. Absorption (or extinction) spectra were collected with a two-beam UV−vis spectrometer (PERSEE TU-1901, China). Fluorescence was measured by using a FluoroMax-4 fluorimeter (Horiba, Japan). HR-TEM images of NCs were collected with a JEOL JEM 2010 microscope operating at 200 kV. DLS was measured by a Zetasizer Nano ZS90 (Malvern Instruments Limited). Fluorescence lifetime was measured with a homebuilt timecorrelated single photon counting (TCSPC) system with a time resolution of sub-100 ps. Phosphorescence lifetime was excited with a μF2 lamp and measured with an FLS 980 spectrofluorimeter (Edinburgh Instruments). The quantum yield (QY) was determined by the absolute method with a FluoroMax-4 by using an integrating sphere (Quanta-φ, Horiba) with 150 mm size and open hole (less than 1.9%) using polytetrafluoroethylene (90% reflectivity) as an internal reflection material. Rhodamine B was used as the standard sample to calibrate the measurement system. The transient absorption (TA) measurements were conducted in a femtosecond transient absorption spectrometer (Helios Fire, Ultrafast System) with pump probe beams generated with a Ti:sapphire laser system (Astrella, 800 nm, 100 fs, 7 mJ/pulse, and 1 kHz repetition rate, Coherent Inc.). A fraction of the fundamental beam was focused into the sapphire to generate a white light continuum probe beam from 420 to 800 nm. Another fraction of the fundamental beam was used to produce pump beams via an optical parametric amplifier (OPerA Solo, Coherent Inc.), and the power was adjusted to ∼0.3 mW by a neutral-density filter wheel. All experiments were carried out at room temperature. 2 mm cuvettes were used for all spectroscopy measurements.
incompatible with the metal-centered emission (MCE) mechanism. For example, AuNCs with similar particle sizes (approximately 2−3 nm) coated with glutathione (GSH) can emit different colors with wavelengths of 565,11 600,12 or 810 nm.13 With the development of wet chemical synthesis, atomically precise MNCs were achieved for a number of metal nanoclusters (denoted as MnLm, where n and m are the numbers of metal atoms and ligands in the cluster, respectively).14 Luo and co-workers reported a series GSHprotected AuNCs with different atomic compositions (Au29SG27, Au30SG28, Au36SG32, Au39SG35, and Au43SG37) that exhibit the same emission (∼610 nm).15 Moreover, the emission wavelength could be regulated from 600 to 810 nm by fine-tuning of the surface ligand coverage.16 These results indicate that cluster size is not the only critical factor in determining the photoemission properties of LMNCs. It is insufficient to explain the photoemission of MNCs by focusing only on the metal core of the MNCs. Our early work demonstrated the contribution of surface ligands to the photoluminescence of MNCs.17,18 However, the effects of interactions between different functional groups were not discussed, and these two emission mechanisms remain promiscuous and limit a detailed understanding of MNC photoemission. Herein, MCE and ligand-centered emission (LCE) were simultaneously observed in AgNCs. The contributions of the metal core and surface ligands were systematically investigated. The MCE was ligand-independent and has relatively lower photoluminescence quantum yield (QY). In contrast, the LCE was highly ligand-dependent and exhibited a ligand synergistic effect with unique pH-dependent emission. This was due to the expansion of the potential energy curve and conical intersection (CI).
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemical reagents were directly used without any isolation or purification. Silver nitrate (AgNO3) was purchased from Acros Organics. L-Glutathione in the reduced form (GSH), H2MSA, and cysteamine (CTA) were obtained from Sigma-Aldrich. Sodium hydroxide, nitric acid, and trisodium citrate were obtained from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water with a resistivity of 18.2 MΩ·cm was used throughout all experiments. 2.2. Synthesis of Luminescent DPA-AgNCs, MSAAgNCs, and CTA-AgNCs. The D-penicillamine-capped silver nanoclusters (DPA-AgNCs) were synthesized by a modified cyclic reduction−decomposition approach.19 In a typical synthesis, 2.39 mg of DPA and 60 μL of AgNO3 (0.1 M AgNO3) were dissolved in 4 mL of ultrapure water, and then 40 μL of NaBH4 (112 mM, the 8 mL NaBH4 solution was mixed with 43 mg of NaOH, which protects NaBH4 from spoiling) was added into the solution under vigorous stirring. After stirring for 20 min, a wine red solution could be observed. The solution gradually became colorless after incubating at room temperature for 3 h. Subsequently, another 20 μL of 112 mM NaBH4 was added into the above solution under vigorous stirring. A yellow-brown colored solution was obtained after stirring for 30 min. We observed a yellowish green solution emerging after incubating the sample at room temperature for 5 h. This time, the pH of the solution was adjusted to weakly acidic; thereafter, 10 μL of AgNO3 (0.1 M AgNO3) was introduced into the solution. It was further incubated at room temperature for about 15 h. Eventually, a
3. RESULTS AND DISCUSSION D-Penicillamine (DPA)-capped silver nanoclusters (DPAAgNCs) were synthesized by a modified cyclic reduction− decomposition approach,19 as shown in Figure S1. Sizefocused AgNC precursors were formed via a reduction− decomposition−reduction process. AgNO3 was then added into the precursor solution under weakly acidic conditions to light up the AgNCs by structure optimization. With increasing aging time, light red-brown AgNCs formed with strong red emission (Figure S2). The resulting DPA-AgNCs showed three weak absorption peaks at 340, 400, and 550 nm. The first two absorption peaks were correlated to n → π* and π → π* transitions from the assembled carboxyl and amino ligands on the metal surface.18 The assembly of surface ligands, that is, the clustering of carbonyl and amino on the surface of AgNCs, through space electronic interactions, namely, overlap of π and lone pair (n) electrons among carbonyl and amino groups, extends the conjugation of p electrons and thus leads to the corresponding adsorption peaks to be red-shifted to ∼340 and 400 nm. The absorption peak at ∼550 nm was attributed to ligand-to-metal−metal charge transfer from a metal-centered electron state.17 No obvious plasmonic band was observed, suggesting an ultrasmall size for the as-fabricated AgNCs; this was further confirmed by HR-TEM images (Figure S3). Interestingly, when the DPA-AgNC solution was excited at 550 nm, dual photoemission at ∼580 and ∼665 nm was observed (Figure 1a,b and Figure S2b). The intensity of these two emission peaks could be adjusted by controlling the pH value B
DOI: 10.1021/acs.jpcc.9b04034 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
nm, Figure S3). These two different wavelength emissions were contrastively discussed, as shown in Figure 1a,b. The ∼580 nm emission has a narrow peak width (FWHM ∼30 nm), small Stokes shift (30 nm), and low QY (∼1%). In contrast, the ∼665 nm emission has a broad peak width (FWHM ∼100 nm), large stokes shift (>200 nm), and relatively high QY (∼10%). These distinct differences indicate that there are two different emission channels in the luminescent DPA-AgNCs. IR spectroscopy confirmed that AgNCs were capped by the DPA ligand (Figure S5). Characteristic peaks of DPA were observed, including the νs(COO−) band at ∼1400 cm−1, the νas(COO−) band at ∼1638 cm−1, and N−H and O−H stretching vibrations at 3100−3550 cm−1. The loss of S−H stretching mode at ∼2526 cm−1 indicates the formation of Ag−S bonds.20 Additional control experiments were carried out to ravel out the correlation between ligands and photoluminescence properties of as-synthesized DPA-AgNCs, as shown in Figures S6−S8. The IR spectra indicated that the as-synthesized CTAAgNCs and MSA-AgNCs were well protected by CTA and MSA ligands. The loss of S−H stretching vibration at ∼2526 cm−1 indicates the formation of Ag−S bonds. These two kinds of AgNCs are all small in size (3−5 nm), which was evidenced by HR-TEM images. When cysteamine (CTA), which does not contain carboxyl, was used as the capping ligand, only a narrow emission peak at ∼575 nm was observed. However, when mercaptosuccinic acid (MSA) with the same carboxylate functional groups as DPA was used as the surface anchoring ligand, the MSA-AgNCs exhibited similar optical properties to DPA-AgNCs, including broad red emission at ∼700 nm. The narrow ∼580 nm emission can be achieved in all three of the AgNCs (DPA-AgNCs, CTA-AgNCs, and MSA-AgNCs), but the broad ∼665 nm emission can only be obtained with carboxyl-containing capping ligands (DPA-AgNCs and MSAAgNCs). That is, the narrow ∼580 nm emission was ligand-
Figure 1. Excitation and emission spectra of the narrow ∼580 nm emission (a) and broad ∼665 nm emission (b). Inset images show the corresponding photographs of DPA-AgNCs under room and UV light exposure at λ = 365 nm. (c) Schematic illustration of the metalcentered and ligand-centered emission mechanisms of DPA-capped AgNCs. (d) Time-resolved luminescence decay profiles of DPAAgNCs in water solution measured at 665 and 580 nm (inset), excited at 400 and 550 nm, respectively. The fitted results are summarized in Table S1.
during the structure-focus process (Figure S4). At high pH synthesis conditions, a narrow emission peak at ∼580 nm was obtained. When the synthesis conditions changed to acidic pH, the emission peak at ∼580 nm disappeared, and a new strong emission at ∼665 nm was observed. The principal excitation peak also changed from 550 to 400 and 570 nm. Despite these distinct excitation and emission spectra, the 580 nm and 665 nm emitting AgNCs have almost identical core sizes (2.8 ± 0.5
Figure 2. (a) Photoluminescence spectra of as-synthesized DPA-AgNCs (synthesis pH condition: pH ∼9) placed in different pH conditions. Fluorescence spectra were measured at λex = 505 nm. (b) PL intensity of DPA-AgNCs upon cyclic switching of the pH between 4.6 and 9.7. (c) PL spectra and PL intensity (inset) of DPA-AgNCs with changing pH values. PL spectra were excited at 400 nm; pH value over the range 1.5−12. (d) Schematic illustration of the ligand synergistic effect and pH-induced conical intersections. C
DOI: 10.1021/acs.jpcc.9b04034 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C independent, while the broad ∼665 nm emission was highly ligand-dependent. We thus attribute them to MCE and LCE, respectively. For the MCE, the emission properties are highly dependent on the metal core.9 According to the Jellium model, correlation of nanodot sizes with emission energies fits the simple relation Eemission = EFermi/N1/3, where EFermi is the Fermi energy of bulk metal (5.5 eV for silver) and N is the number of metal atoms per cluster.9 Based on this equation, we can deduce that the number of silver atoms in AgNCs should be 17 when the emission is centered at ∼580 nm, which is far less than 2.8 nm. The MALDI-TOF mass spectrum shows the presence of metal nanoclusters with varied metal numbers, and the signal of the Ag17 cluster was observed, as shown in Figure S9. By elaborately analyzing the spectra of these three different ligand-capped AgNCs, we find that the excitation and emission wavelengths were slightly different (550 and 580 nm for DPAAgNCs, 552 and 575 nm for CTA-AgNCs, 525 and 555 nm for MSA AgNCs). This might be due to the slight difference in the metal cores of these three AgNCs. Moreover, they all exhibited small Strokes shifts (20−30 nm) and low QY (∼1%), and their fluorescence signals have monoexponential decay with a lifetime of ∼1.6 ns. The LCE exhibits high selectivity for the surface anchoring ligands. The large Stokes shifts (>200 nm; Figure 1b) and relatively longer lifetime (∼180 ns, Figure 1d and Table S1) suggested that the emission process involves multiexcited states. The functional groups, such as carboxyl and amino, anchored on the surface contribute to the emission process.18,21 The self-assembly-induced clustering by weak interactions such as n → π* interactions, electrostatic interactions, and hydrogen-bond interactions could lead to efficient delocalization of electrons between neighboring functional groups, namely, overlap of π and lone pair (n) electrons among carbonyl or amino groups, and extends the conjugation. The strong interaction between surface ligands and the metal core at the nanoscale interface not only induces rigid conformations of carbonyl or amino groups but also affords a perfect ligand cluster that acts as an exact chromophore of metal NCs, which accounts for the long wavelength emission. Interestingly, without amino groups, the synthesized MSA-AgNCs were weakly luminescent with a QY less than 1%. The introduction of the amino group as a hybridization group could immensely improve the photoluminescence efficiency (from ∼1% for MSA-AgNCs to ∼10% for DPA-AgNCs, see Figure S6d and Figure 1b). A ligand synergistic emission mechanism was proposed to understand this phenomenon as shown in Figure 2d. The red emission at ∼665 nm was ascribed to the radiation relaxation of the carboxyl-correlated surface state (COOH-nπ* state of carbonyl groups anchored on the metal core by electron state relaxation from the COOH-ππ* state).18,22 The large energy gap between these two excited states makes it difficult for state transition (Figure 2d, purple dot line route). The amino-correlated nπ* state acts as an intermediate state and could bridge the ππ* and nπ* states of the carboxyl group. Thus, it largely enhances the transition efficiency between different excited states and improves the QY as demonstrated in Figure 2d (green dot line route). These two emission peaks exhibit a distinct response to the pH value (Figure 2a). The ∼665 nm emission dramatically decreased when the pH increased from 4.6 to 9.6, while the intensity of ∼580 nm emission sustained without any changes. The pH-independent property of the ∼580 nm emission is
consistent with the MCE mechanism, which is mainly dependent on the properties of the metal core. The pHdependent emission has been observed in numerous luminescent metal nanoclusters, such as DPA-CuNCs,23 GSH-CuNCs,24 and CYS-AuNCs.25 The pH effect has been ascribed to a trigger of aggregation-induced emission due to the decrease in solubility of MNCs at low pH conditions.23−25 However, in our case, no obvious precipitates were observed when the pH decreased from 9.7 to 4.6. Even in the solid state, the DPA-AgNCs were not luminescent (Figure S10). This indicated that the pH effect could not be simply attributed to aggregation. The reversible change in PL spectra between pH ∼4.6 and ∼9.7 implies a protonation-correlated photoemission process, as shown in Figure 2d. The protonation of carboxyl groups could lead to expansion of energy curves and could consequently result in conical intersections.26,27 Therefore, the state transfer becomes more efficient and eventually intensifies the photoluminescence. This model was further verified by pH-dependent luminescence lifetime decay (Figure S11 and Table S2). The pH-dependent emission has also been observed in organic chromophores, such as 9-anthracene carboxylic acid28 and pyrene-1-carboxylate.29 The photoluminescence was intensified with increasing acidity. The acidity constants in ground (pKa) and excited (pKa*) states could be crudely estimated by pH-dependent absorption and emission spectra,30 as shown in Figure 2c and Figure S12. The pKa and pKa* of DPA-AgNCs were measured to be 5.6 and 6.5, respectively. The excited state exhibits stronger bond capabilities to the proton, which implies a proton-coupled emission process. The pH-dependent emission was also observed in other carboxyl-capped AgNCs (Figure S13). The acidity constant of AgNCs differed with the change of the surface ligand. The pKa* was roughly estimated to be 5.15 for MSA-AgNCs via the emission spectra, a little lower than DPAAgNCs, exhibiting ligand-dependent properties. However, the pKa was difficult to estimate because the MSA-AgNCs tend to precipitate at low pH. Interestingly, the trends in emission intensities of these two kinds of AgNCs (DPA-AgNCs and MSA-AgNCs) were different when the pH decreased to strong acid conditions (
