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Aggregation-induced Fluorescence-toPhosphorescence Switching of Molecular Gold Clusters Mizuho Sugiuchi, Junichi Maeba, Nobuyuki Okubo, Munetaka Iwamura, Koichi Nozaki, and Katsuaki Konishi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017
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Aggregation-induced Fluorescence-to-Phosphorescence Switching of Molecular Gold Clusters Mizuho Sugiuchi,† Junichi Maeba,¶ Nobuyuki Okubo,¶ Munetaka Iwamura,¶ Koichi Nozaki¶ and Katsuaki Konishi†,‡* †
Graduate School of Environmental Science, Hokkaido University, North 10 West 5, Sapporo 060-0810 (Japan).
‡
Faculty of Environmental Earth Science, Hokkaido University, North 10 West 5, Sapporo 060-0810 (Japan).
¶
Graduate School of Science and Engineering, Toyama University, Gofuku, Toyama 930-8355, Japan
Supporting Information Placeholder ABSTRACT: Aggregation-induced optical responses are ubiquitous among a wide range of organic and inorganic compounds. Here, we demonstrate an unprecedented effect of aggregation on the photoluminescence (PL) profiles of [core+exo]-type [Au8]4+ clusters, which displayed a change in the dominant PL emission mode from fluorescence to phosphorescence-type upon aggregation. In solvents in which cluster molecules are highly soluble and exist as monomers, they displayed single PL bands at ~600 nm at ambient temperatures. On the other hand, in solvents in which cluster molecules are less soluble and cluster aggregation is induced, a new PL band at ~700 nm also emerged. Lifetime measurements revealed that the PL emissions at ~600 and ~700 nm had fluorescence and phosphorescence characters, respectively. Studies of the excitation spectra suggested that organized cluster assemblies were responsible for the lower-energy emission at ~700 nm and had exceptionally high emission activity. Accordingly, intense phosphorescence-type emissions were observed in the solid state, in which the quantum efficiencies were higher by two orders of magnitude than those of the corresponding monomeric forms in solution. This work provides an example of the critical effects of cluster aggregation events on their optical properties and shows the potential of such effects in the design of cluster-based materials with unique functions and properties.
Small ligand-protected gold clusters containing partially oxidized gold units have recently attracted attention because of the unique structures and properties associated with the residual 6s electrons.1-4 One prominent feature of such cluster compounds is the unique optical absorption properties associated with their molecule-like discrete electronic structures, which make them distinct from conventional nanoscale metal species. It has also been reported recently that some clusters exhibit photoluminescence (PL), which expands the scope of the fundamental interest and practical applications of this class of compounds.5-15 Thus far, research has mostly focused on the phenomenological aspects of the optical / electronic properties of single molecular objects. For example, current reports have demonstrated that their surface environments, such as the states of surrounding organic ligands, metal complex moieties, and counterions, notably affect their intrinsic quantum efficiencies.8-16
It is well known that aggregation events cause changes in the optical properties of various compounds.17-20 Luminescent molecules often display distinctive decreases in PL intensity, which are known as aggregation-caused quenching.21 In contrast, several organic chromophores and metal complexes exhibit the opposite behavior, which is generally referred to as aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE).21-27 The basic mechanism underlying these phenomena involves a simple decrease/increase in the probability of an emissive pathway, and, in most cases, the emission wavelengths (energies) remain virtually unchanged. It is also known that aggregation events can induce the generation of alternative excited states that modify the dominant emissive pathways, which leads to changes in emission wavelengths,28-30 but examples of this phenomenon have been rare to date. On the other hand, with respect to gold clusters, Xie et al. reported the “aggregation-induced emission” of thiolate-protected gold clusters. However, these examples demonstrated the effect of the “aggregation of gold-thiolate moieties” onto the surface of a gold core,8,9,15 which are thus basically associated with “single-cluster events” rather than “cluster aggregation.” In this paper, we report that the aggregation of [core+exo]-type [Au8]4+ clusters causes notable red shifts in the PL of the clusters at ambient temperatures as a result of switching of the dominant radiative mode from fluorescence to phosphorescence-type. Monomeric clusters exhibited a visible fluorescence emission, but upon aggregation they displayed a phosphorescence-type emission in the near-IR region. Excitation spectral studies demonstrate that the emission originated from exciton-coupled cluster assemblies. We also demonstrate that the cluster assemblies displayed much higher emission activities than the cluster monomers, which was demonstrated by the high quantum efficiencies found for the clusters in the solid state.
Figure 1. Structures of cationic moieties of core+exo type Au8 clusters.
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The Au8 clusters ([Au8(dppp)4L2]X2, 1–3) used in this study comprise a bitetrahedral Au6 core with two additional gold atoms (“[core+exo]-type”), which are decorated by four 1,3-bis(diphenylphosphino)propane (dppp) ligands and two anionic chloride or acetylide ligands (L) (Figure 1).13,31-34 As was reported previously, 1 when dissolved in CH2Cl2 (25 µM) displayed an isolated visible absorption band at 510 nm, which is due to a HOMO–LUMO transition, and a PL emission band at 596 nm upon excitation of the absorption band (Figure 2a(i)).32,34 Time-resolved PL measurements showed a fast single-exponential decay with a lifetime of 55 ps (Table 1, entry 1; Figure S1a [dotted line]),35 which implied that the PL emission had fluorescence character. On the other hand, in MeOH, in which 1 has relatively low solubility, the absorption spectrum was almost identical but the PL profile was significantly different. The PL spectrum displayed a bimodal pattern, with a near-IR emission at 708 nm together with a band at 596 nm (Figure 2a(ii) [dotted line]), the latter of which was almost identical to that found in CH2Cl2 (i). Interestingly, the photophysical characters of these two PL emissions were rather different. The PL emission at 596 nm behaved in a similar way to the fluorescence found in CH2Cl2 and displayed a single-exponential decay with a lifetime of 42 ps (Table 1, entry 2; Figure S1a [solid line]). In contrast, the lower-energy emission at 708 nm decayed double-exponentially with microsecond-order lifetimes of 3.7 (35%) and 0.68 (65%) µs (Table 1, entry 3; Figure S1b), which indicated a preference for much slower emissive processes. Therefore, the emission at 596 nm was characterized as fluorescence, whereas the lower-energy emission (708 nm) had phosphorescence character. The above-mentioned phosphorescence-type emission of 1 in MeOH is considered to be a result of cluster aggregation. Although the solution appeared to be homogeneous and its absorption spectra showed negligible scattering due to the presence of large particles (Figure 2a(ii), solid line), a clear Tyndall effect was observed when light from a laser pointer was passed through the solution (Figure. S2b). Furthermore, dynamic light scattering (DLS) measurements showed the presence of nanoscale objects with sizes of ~150 nm, but gave no responses around 1–2 nm, which corresponds to the size of the cluster monomers. In contrast, the CH2Cl2 solution gave DLS signals exclusively around 1–2 nm (Figure S2a).
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Table 1. Absorption and photoluminescence spectral data of Au8 clusters (1 – 3).a Entry Cluster Medium 1
CH2Cl2
λem (nm)
510
596
2 3
b
λabs (nm)
596 1
MeOH
510
Lifetimec Q.Y.% τ1 (s)
τ2 (s) -11
-
-11
-
5.5 × 10
4.2 × 10
1.2 × 10 (0.45)
9.0 e
-
0.03
515
710
3.4 × 10 (0.55)
5
CH2Cl2
510
576
< 1.5 × 10
Solid
515
687
1.5 × 10 (0.35)
CH2Cl2
505
581
< 1.5 × 10
Solid
513
678
8.5 × 10 (0.44)
7 8
-11 f
-6
3
-11 f
-7
a
d
-6
Solid
2
0.27
6.8 × 10 (0.65)
3.7 × 10 (0.35)
4
6
d
-6
708
-7
0.10
-6
-7
4.7 × 10 (0.65)
18
-
0.02 -7
1.9 × 10 (0.56)
d
e
d
7.0 e
b
Unless otherwise stated, at 20 °C under aerobic conditions. From steady-state photoluminescence spectra upon excitation at λabs. c Picosecond emission data were obtained at 25 °C and λex = 501 nm and the spectra were monitored at λem. Nanosecond emission data were obtained at 25 °C and λex = 532 nm and the spectra were monitored at 700 nm (entry 3), 680 nm (entry 4), and λem (entries 6 and 8). The numbers in parentheses are the ratios of the two decay components, which were determined by curve fitting. d By relative methods using anthracene as a standard. e By absolute methods. f Too fast to be determined accurately.
Therefore, the cluster molecules were present in a dispersed monomeric form in CH2Cl2, but as aggregates in MeOH. The involvement of aggregation events in the emergence of phosphorescence-type emission was further supported by concentration effects. Figure. 2b shows concentration-corrected PL spectra of 1 in MeOH in the concentration range of 3–50 µM. At the lowest concentration (3 µM) the solution displayed a single emission at 596 nm, but upon an increase in the cluster concentration a new near-IR emission appeared and developed at ~700 nm, while the original emission band (596 nm) virtually retained its intensity. Unique effects of aggregation events were also observed for the homologous acetylide-modified clusters (2 and 3). These clusters were completely insoluble in MeOH but highly
Figure 2. a) Absorption / diffuse reflectance (solid lines) and PL (dotted lines, λex = 510 nm) spectra of 1 in (i) CH2Cl2 (25 µM), (ii) MeOH (25 µM), and (iii) the solid state at 20 °C. b) Concentration-corrected PL spectra of 1 in MeOH, (λex = 510 nm) at 20 °C. From bottom to top: [1]0 = 3, 6, 12.5, 25 and 50 µM. c) PL excitation spectra of 1 in (i) CH2Cl2 (12.5 µM), (ii) MeOH (12.5 µM), and (iii) the solid state upon monitoring at 596 nm (red lines) and 708 nm (blue lines). The absorption/reflectance spectra are provided (dotted lines) for comparison.
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soluble in MeCN or CH2Cl2. An MeCN/water mixture was therefore used to induce aggregation via hydrophobic interactions. In the solvents in which they were highly soluble, 2 and 3 only exhibited single PL bands at ~580 nm with picosecond-order lifetimes (Table 1, entries 5 and 7; Figure S3(i) [black line] and S4). On the other hand, in MeCN/water (10/90, v/v) they displayed additional lower-energy bands at ~680 nm (Figure. S3(ii)). In relation to the above findings, some luminescent metal compounds are known to exhibit aggregation-induced phosphorescence.25-27 This behavior is suggested to be a result of restrictions on molecular motion, which block non-radiative pathways and open inherent radiative channels from excited triplet states (phosphorescence) in the monomer state.21 This simple AIE-based scheme is potentially a factor that may account for the emergence of phosphorescence-type emission in the Au8 clusters. In order to clarify this point, we performed studies of PL excitation spectra. As shown in Figure 2c(ii), the excitation spectrum of 1 in MeOH upon monitoring the lower-energy emission (708 nm) displayed two bands at 480 and 550 nm (blue line), which did not match the absorption spectrum (dotted line). In contrast, when the fluorescence emission at 596 nm was monitored, the excitation spectrum displayed a unimodal shape (red line), which coincided with the absorption spectrum. Likewise, for 2 and 3 in aqueous MeCN significant differences were observed between the excitation spectra for the two emissions (Figure S3(ii)). Therefore, the species responsible for the fluorescence and phosphorescence-type emissions of 1–3 were electronically different from each other. The fluorescence emissions appeared to arise virtually from the monomeric forms, as evidenced by the coincidence of the excitation and absorption spectra of 1 in CH2Cl2 (Figure 2c(i)), in which the cluster molecules were present in the monomeric form. On the other hand, the phosphorescence-type emissions may specifically arise from intrinsically different species generated by cluster aggregation. In this respect, the close packing of dye molecules leads to red/blue shifts in the absorption bands as a result of the exciton coupling of transition dipoles, which are known as J- and H-type aggregates.17 Therefore, the bimodal nature of the excitation spectra of 1–3 (Figures 2c(ii) and S3(ii) [blue lines]) suggests the involvement of organized cluster assemblies with particular orientations, e.g., J- or H-type. The formation of such ordered assemblies would result in the generation of unique electronic structures via intercluster interactions, which may be responsible for the appearance of the phosphorescence-type emission. From the above observations, the aggregation-induced phosphorescence-type emission of the Au8 clusters (1–3) appears primarily due to the static formation of closely packed cluster assemblies, rather than the simple AIE mechanism involving the restriction of molecular motions to enhance the phosphorescence emitted by the monomer. It should also be noted that the absorption spectra of 1 in MeOH (Figure 2c(ii) [dotted line]) displayed no signs of the bimodal spectral pattern associated with the phosphorescence-type emission. Therefore, the proportion of cluster molecules that contributed to the lower-energy emissions seemed to be limited, and the remaining majority of cluster molecules, even in the aggregates, seemed to behave as monomers. Figure 3 shows a schematic illustration of a possible arrangement of the cluster aggregates. Small fractions of the cluster molecules are densely packed and form phosphorescent domains, which are
surrounded by a shell that contains most of the cluster molecules, counterions, and solvent molecules. Within this shell, the clusters are loosely aggregated and remain virtually dispersed, and thus display only the fluorescence characteristic of monomers. This scheme agrees well with the behavior of 1 in MeOH at various concentrations (Figure 2b), where the relative intensities of the fluorescence due to monomers remained nearly unchanged in spite of the development of the emission at 700 nm. On the other hand, only marginal effects of molecular oxygen (air), which are well known to quench the phosphorescence emissions, were observed (Figure S5). This observation can be explained in terms of shielding by the shell that encapsulates the cluster assembly domains having phosphorescent character.
Figure 3. Schematic illustration of a possible aggregated state of the Au8 clusters (1–3) in solution. Some clusters form assemblies that give phosphorescence-type emissions, whereas the rest of the cluster molecules dispersed in the shell, together with counter-anion and solvent molecules, only exhibit weak fluorescence.
It is also noteworthy that the intensities of the fluorescence and phosphorescence-type emissions of the solutions of 1 in MeOH ([1]0 ≥ 12.5 µM) (Figure 2b) were comparable to each other, although most of the cluster molecules should contribute to the fluorescence emission, as was mentioned previously. Therefore, the cluster aggregates should have significantly higher quantum efficiency than the monomeric clusters. If this is the case, clusters in the solid state should exhibit strong phosphorescence emissions. In fact, crystals of 1–3, which represent the ultimate aggregates of cluster molecules, displayed intense PL emissions at ~700 nm. For example, 1 exhibited an emission band at 710 nm (Figure 2a(iii) [dotted line]), which almost coincides with the lower-energy emission observed from the corresponding cluster aggregates in solution (ii), and implied that the emission in the crystalline state had phosphorescence character. Accordingly, the solid-state emissions of 1–3 behaved in a similar way to those in solution. They all displayed double-exponential decay comprising two components with microsecond-order lifetimes (Table 1, entries 4, 6, and 8; Figure S6). Negligible emissions were observed at ~600 nm, which indicates that 1–3 exhibited very weak, if any, fluorescence emissions below the detection limit. Consequently, the solid clusters only displayed phosphorescence-type emissions, which contrasts with the
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fluorescence-only activity of the monomeric clusters. The quantum yields of the emissions of crystalline 1–3 were in the range of 7–18% (Table 1, entries 4, 6, and 8), which corresponds to an increase of approximately two orders of magnitude in quantum yield in comparison with the emissions from the monomers (
