Lanthanide-Oligomeric Brush Films: From Luminescence Properties to Structure Resolution.

Lanthanide (Ln) based luminescent materials are experiencing an increasing interest in their applications in several fields. In this study, we report a series of new lanthanide-oligomeric brush films, supported on quartz substrates and prepared using a layer-by-layer method (LbL). Oligomeric brush films are composed of small oligomers from our previously reported coordination polymers [x-EuL] and [x-TbL] (with x = 1, 3, and 5 generations of Ln complexes), which are grown perpendicularly from a carboxylate self-assembled monolayer. Oligomers composed of our previously described helical lanthanide complex LnL (Ln: Eu and Tb) as a luminescent moiety and benzene-1,4-dicarboxylate acid (bdc) used as a linker. Mixed films having the fifth-generation Ln complexes composed of equimolar mixture of Eu and Tb ions were prepared. Oligomeric brush films are highly transparent and exhibited a colored emission under UV irradiation. Pure Ln (Eu or Tb) films showed a strong luminescence from the Ln ions. Their luminescent properties depended on the number of lanthanide layers in the films composed of the first to third generations of lanthanide complexes. Then, the increase of the complex layers induced no difference in the luminescent properties. An energy transfer from Tb to Eu ions in the mixed films indicated a short distance between lanthanide ions of a fifth layer. The structural analysis together with the observed luminescent properties and some previous studies allowed to clarify the disposition of the oligomers in the films.

Introduction

For a few decades, lanthanides (Ln) compounds have been attracting increasing interest in the development of innovative materials for several applications.[1−9] Owing to their specific electronic composition, Ln ions show photoluminescence due to the f−f transitions. Direct sensing of Ln ions is an inefficient process due to the Laporte forbidden 4f–4f transitions. However, Ln ions can be photosensitized by the coordination of π-conjugated organic ligands. In this system, the organic ligand acts as a photoantenna that can transfer, after excitation, energy from the ligand excited triplet state to the excited state of the lanthanide. Then, the relaxation of the energy from the excited level to the fundamental level of the Ln ion produces a long (μs to ms) and sharp luminescence emission that is sparsely affected by the surrounding environment.[10−13] Lanthanide luminescence was used in several applications,[14−24] especially to generate emitting materials by immobilizing monolayer or polylayers of Ln complexes on several substrates.[25−36] In contemporary chemistry, the functionalization of surfaces with hierarchical mono- and polylayers presents a great interest and is used in several technological applications.[37−49]

Controlling the coordination geometry around lanthanides is difficult and results in a complex prediction of the lanthanide coordination sphere. Consequently, the synthesis of lanthanide luminescent films by using supramolecular and coordination chemistry is an important challenge for the research of composite materials and can explain, from our knowledge, the lack of studies of lanthanide coordination polymer brush films.

Previously, we had reported the synthesis of monodimensional coordination polymers with Ln ions in the solid state. The coordination polymers were composed of a helical lanthanide complex EuL or/and TbL connected to a dicarboxylate linker such as benzene-1,4-dicarboxylate acid (bdc) or 2-aminoterephtalte (atpa) (Figure ).[50]

Figure 1

Measured molecular structure and molecular size of (a) LnL complex (Ln = Eu or Tb) and (b) spacers. (c) Schematic representation of LnL polymer with bdc in the solid state.

Eu–Tb-mixed coordination polymers were luminescent, and their properties were dependent on the temperature and polymer composition. Indeed, when bdc was used as a linker, the polymer chains were spaced at 14.4 Å. This distance is too long to observe an energy transfer between Eu and Tb ions. By replacing the linker by atpa, the interaction between polymer chains was increased and modified the packing structure. This modification reduced the distance between the lanthanide ions and allowed an efficient energy transfer between Tb and Eu ions.

Herein, based on our previous study, we report the synthesis and luminescent properties of lanthanide oligomeric brush films with Eu or Tb oligomers. Oligomeric brush films, [x-EuL] and [x-TbL] (with x = 1, 3, and 5 generations of Ln complexes), are composed of small oligomers by linked alternatively of the LnL complex (Ln = Tb or Eu) and bdc. Oligomers are grown perpendicularly from a quartz substrate, forming a brushlike film. The films were prepared on carboxylate self-assembled monolayer (SAM)’s quartz substrates using the layer-by-layer (LbL) method. From the SAM’s substrates, oligomers were grown by alternate chemisorption of LnL complexes (Eu or Tb) and bdc compound. In addition, the mixed films [4-EuL-5Tb/EuL] and [4-TbL-5Tb/EuL], whose fifth-generation Ln complex was composed of an equimolar mixture of Eu and Tb ions, were prepared. All films showed intense luminescence from Ln ions under UV irradiation. The effects of the generation number and the layer composition on luminescent properties were discussed. Finally, structural analysis and luminescence properties were correlated to propose a possible oligomer aggregation into the luminescent films.

Results and Discussion

SAM’s Quartz Preparation

Carboxylate SAM’s quartz functionalization was followed by water contact angle measurement (Figure ).

Figure 2

Values of the water contact angle of modified quartz substrates in each step.

Hydrophilic or hydrophobic behavior of the surface influences the water contact angle of a substrate. After the hydroxylation of commercial quartz (Quartz-OH), the water contact angle decreased drastically due to the presence of hydroxyl groups, highly hydrophilic, on the quartz surface. The addition of a propyl chain and an amine group increased the hydrophobic behavior of the quartz (Quartz-NH2), which is associated with the increase of the water contact angle. Functionalization with a hydrophilic carboxylic acid (Quartz-COOH) and its transformation into sodium carboxylate (Quartz-COONa) decreased the water contact angle, pointing out an increase of the hydrophilic behavior of the quartz substrate.

Lanthanide Films Preparation

Lanthanide films were prepared from the carboxylate SAM’s quartz substrate using the LbL method (Figures and S1). The detailed procedure is given in Materials and Methods.

Figure 3

Schematic representation of the layer-by-layer process for the preparation of pure lanthanide brush films and representation of the (a) first-, (b) third-, and (c) fifth-generation films.

X-ray photoelectron spectroscopy (XPS) was performed to analyze the atomic composition of the films (Figures S2–S5). All EuL films showed the existence of the binding energy band of the Eu 3d XPS band and confirmed the good chemisorption of EuL on the quartz substrate. The increase of the generation number or the addition of the bdc spacer induced no differences in the binding energy of the Eu 3d band (Figure S2). Addition of a bdc linker [1-EuL-bdc] was associated to the disappearance of the component corresponding to the nitrate ion and the apparition of a new component with a bigger intensity in the N 1s band (Figure S3). This new component at 402 eV was associated with the N 1s energy binding of the protonated Et3NH+, which acted as a counteranion of the bdc linker. The coordination of a new EuL complex provoked its disappearance and the recurrence of the band corresponding to the nitrate ion as observed for [3-EuL] and [5-EuL] films (Figure S3). Similar observations were made for Tb films. The binding energy of the Tb 4d orbital was observed in every Tb film (Figure S4), indicating the chemisorption of the TbL complex. Again, the addition of bdc in the [1-TbL-bdc] film was correlated with the disappearance of the nitrate component in the N 1s around 401 eV and apparition of the N 1s assigned to Et3NH+ appeared at a higher side than that of nitrate . This band disappeared in the [3-TbL] and [5-TbL] films (Figure S5). Finally, the presence of the binding energy of Tb 4d and Eu 3d in the XPS analysis of [4-EuL-5Tb/EuL] and [4-TbL-5Tb/EuL] was in correlation with the composition of the fifth-generation films.

AFM Analysis

Figure shows AFM images of the commercial quartz substrate, carboxylate SAM’s quartz, and EuL films (more detailed images and TbL films images are given in the Figure S6).

Figure 4

AFM images and surface profile of quartz (a), carboxylate SAM’s quartz (b), [1-EuL] (c), [1-EuL-bdc] (d), [3-EuL] (e), and [5-EuL] (f).

As observed in the AFM images, the commercial quartz plate had a rough surface with several nanoscratches. After SAM’s functionalization (Figure b), the rough surface parameter (Ra) decreased drastically due to the leveling of the surface by several acidic treatments during the SAM’s preparation. The increase of the Ra parameter for the [1-EuL] film (Figure c) indicated a slightly rougher surface. No change in the roughness was observed with the addition of the bdc linker (Figure d). Again, the augmentation of Ra parameters for the [3-EuL] (Figure e) and [5-EuL] films (Figure f) suggested the increase of the surface roughness with the growth of the film. The average maximum high (Rz) increased with the number of layers. This augmentation is in association with the increase of the size of oligomers, confirming the growth of the films with the number of dipping cycles.

The surface profile of the carboxylate SAM’s quartz shows the formation of small domains about 10 nm in diameter, which were not visible in commercial quartz. Growth of the films did not affect significantly the size of these domains. The average domain size of the fifth-generation films was only slightly increased at 13 nm, indicating no lateral extension during the growth.

Luminescence Properties

Under UV excitation (λex = 315 nm), the EuL films emitted a red emission characteristic of the Eu ions (Figure ). The emission spectra of the EuL films showed six sharp bands at 579, 590, 616, 649.5, and 685 (br) nm, which correspond to the Eu f–f transitions 5D0 → 7F0, 4D0 → 7F1, 4D0 → 7F2, 4D0 → 7F3, and 4D0 → 7F4, respectively. All films showed similar emission shape, indicating a similar complex composition (Figure a–d). The Eu emission from the films was slightly different from the EuL emission from the solid state (5D0 → 7F1); the excitation spectra coincided with the absorption spectra of the organic ligand and confirmed that the Eu emission was exclusively provided by an energy transfer from the organic ligand (all monitored wavelengths are shown in Figure S7).

Figure 5

Excitation (blue line, λmon = 616 nm) and emission spectra (red line, λex = 315 nm) of (a) [1-EuL], (b) [1-EuL-bdc], (c) [3-EuL], and (d) [5-EuL] films.

Under UV excitation (λex = 315 nm), pure Tb films showed a green emission characteristic of the Tb ion (Figure ). The emission spectra of the Tb films were composed of four bands at 488, 542, 583, and 620 nm assigned to the f–f transitions 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 transitions, respectively. No differences in the band position or in the spectra shape were observed between the Tb films due to similar Tb complex composition. As previously observed for Eu films, the emission spectra of the Tb films were slightly different from those of the TbL complex in the solid state[51] and comparable to those observed for the TbL coordination polymers, indicating a similar structure of the Tb complex in the film and the coordination polymer.[50]

Figure 6

Excitation (blue line, λmon = 542 nm) and emission spectra (green line, λex = 315 nm) of (a) [1-TbL], (b) [1-TbL-bdc], (c) [3-TbL], and (d) [5-TbL] films.

Excitation spectra of the TbL films exhibited the same broadband between 300 and 350 nm as the ligand absorption (other wavelengths monitored are included in the Figure S8). Again, this observation confirmed that the emission of Tb was only induced by an energy transfer between the ligand and the Tb ion.

The mixed films [4-EuL-5Tb/EuL] and [4-TbL-5Tb/EuL] showed luminescence under UV excitation (Figure ). Their emission spectra were similar and composed of seven bands from Eu and Tb ion emissions. The bands at 579, 590, 616, 649, and 685 nm were attributed to the Eu f–f, 5D0 → 7F0, 4D0 → 7F1, 4D0 → 7F2, 4D0 → 7F3, and 4D0 → 7F4 transitions, respectively. The two other bands at 488 and 542 nm were attributed to the Tb transition, 5D4 → 7F6 and 5D4 → 7F5; the other Tb transitions 5D4 → 7F4 and 5D4 → 7F3 being confused within the Eu f–f transitions. As observed in the spectra, the Tb emission was more intense in the [4-TbL-5-Tb/EuL] film than in the [4-EuL-5-Tb/EuL] film due to the difference in the Tb composition in the films. Nevertheless, the Eu emission was more intense than the Tb emission even when the Eu ion was in lower proportion as in the [4-TbL-5-Tb/EuL] film.

Figure 7

Excitation spectra (blue line, λmon = 616 nm) and emission spectra (orange line, λex = 315 nm) of (a) [4-TbL-5-Tb/EuL] and (b) [4-EuL-5-Tb/EuL] films.

Excitation spectra of mixed films were analogous to those of the previously described pure films and corresponded to the absorption spectra of the ligand (all wavelength monitored are included in SI (Figures S9 and S10)). Again, this band confirmed the energy transfer between the ligand and the lanthanide ions.

Luminescence Lifetime and Quantum Yield

Absolute luminescence quantum yields QLnL and luminescence lifetimes τobs of the films were measured at room temperature. The values are estimated and given in Table , Figures S11 and S12.

Table 1

Luminescence Lifetimes τobs (%) and Absolute Quantum Yields QLnL of the f–f Emission of EuL and TbL Films at Room Temperature

film	λmon [nm]	τobs [ms] (amplitude)
		τ1	τ2	τ3	τ4	τtot	QLnL (+/−)
[1-EuL]	616	0.10 (28%)	0.51 (43%)	1.24 (29%)		0.60	4.3% (0.4)
[1-EuL-bdc]	616	0.02 (59%)	0.23 (30%)	0.89 (11%)		0.18	1.1% (0.1)
[3-EuL]	616	0.03 (49%)	0.30 (32%)	1.04 (19%)		0.31	1.4% (0.1)
[5-EuL]	616	0.04 (47%)	0.29 (35%)	0.97 (18%)		0.29	1.4% (0.1)
[1-TbL]	543	2 × 10–3 (78%)	0.03 (15%)	0.22 (4%)	1.11 (3%)	0.04	0.9% (0.1)
[1-TbL-bdc]	543	2 × 10–3 (77%)	0.03 (14%)	0.27 (5%)	1.06 (4%)	0.06	1.5% (0.2)
[3-TbL]	543	2 × 10–3 (74%)	0.03 (17%)	0.22 (6%)	1.03 (3%)	0.05	1.0% (0.1)
[5-TbL]	543	2 × 10–3 (73%)	0.03 (19%)	0.21 (5%)	0.99 (3%)	0.05	1.0% (0.1)
[4-EuL-5-Tb/EuL]	616	0.13 (26%)	0.55 (44%)	1.27 (30%)		0.66	4.1% (0.3)
	543	1 × 10–3 (85%)	0.01 (12%)	0.09 (2%)	0.73 (1%)	0.01	a
[4-TbL-5-Tb/EuL]	616	0.09 (18%)	0.54 (38%)	1.44 (44%)		0.86	2.4 (0.1)
	543	2 × 10–3 (80%)	0.02 (15%)	0.18 (3%)	0.90 (2%)	0.03	a

Nondetected value is negligible.

The emission decay curves of pure EuL films can be divided into three components, indicating that three Eu sites with different symmetries coexisted in the films. The [1-EuL] film had the best luminescent properties with a global lifetime evaluated at 0.6 ms and the Eu f–f emission efficiency calculated at 4.3%. If no difference was observed in the Eu lifetime composition by increasing the generation number, the addition of the bdc spacer reduced the global lifetime to 0.18 ms and the quantum yield to 1.1%. This lifetime diminution is possibly due to the reduction of the energy transfer efficiency between the ligand and the Eu ions. In the [3-EuL] film, the addition of two generations of complexes increased the lifetime and the quantum yield to 0.3 ms and 1.4%, respectively. Then, the continued expansion of the film affected neither the lifetime nor the quantum yield. The luminescent properties of the [5-EuL] film were similar to the luminescent properties of the [3-EuL] film.

The luminescent properties of the Tb films were different from the properties observed for the Eu films (Table ). The Tb emission can be divided into four components, indicating than four different symmetries of Tb ions coexisted in the films. The [1-TbL] films had a luminescent lifetime of 0.04 ms and a quantum yield lower than 1%, which are inferior to the properties of the [1-EuL] film. As observed in the solid state[51] and in the coordination polymer,[50] TbL showed weaker luminescent properties than EuL, which could be explained by a back-energy transfer between Tb ions and the organic ligands.[52−54] Again, the increase of the generations did not affect the Tb lifetime composition but did affect the lifetime and emission efficiency. In the [1-TbL-bdc] film, the addition of bdc enhanced the global lifetime by 0.06 ms and the quantum yield by 1.5%. As opposed to Eu film, this improvement in luminescence properties could be explained by an increase of the energy transfer efficiency due to the coordination of the bdc linker. In [3-TbL], the expansion of two generations of TbL induced a reduction of the lifetime by 0.05 ms and of the quantum yield by 1%. As observed for EuL films, the continued growth of the film had no effect on the luminescent properties; similar luminescence properties were observed for [5-TbL] and [3-TbL] films.

As mentioned above, the luminescence properties cannot be improved after the third-generation Ln complex. Several studies have shown that Eu emission can be increased with an energy transfer from Tb ion.[55−58] For this purpose, mixed films [4-EuL-5-Tb/EuL] and [4-TbL-5-Tb/EuL] were prepared. The mixing of Eu and Tb ions in the last generation induced an improvement of the Eu f–f emission (Table ). From the decay curves (Figure S13), the Eu emission was still composed of three components, but the luminescence lifetime reached a value of 0.66 and 0.86 ms for [4-EuL-5-Tb/EuL] and [4-TbL-5-Tb/EuL], respectively, and the luminescence efficiency was evaluated at 4.1 and 2.4%, respectively. The adding Tb ions to the fifth-generation Ln complex allowed to increase the luminescence of the Eu ion to some values superior to the values observed for the pure film [5-EuL] and confirmed the occurrence of an energy transfer between Tb and Eu ions. The through-space energy-transfer efficiency (η) can be estimated by the following equation:where τ is the Tb lifetime in mixed films and τ0 is the lifetime in pure [5-TbL] film.[57] The energy-transfer efficiency from Tb to Eu was calculated to be 80 and 40% in [4-EuL-5-Tb/EuL] and [4-TbL-5-Tb/EuL] films, respectively, using the above equation. Additionally, the quantum yield of Tb ion cannot be detected in these films, demonstrating the occurrence of the energy transfer from Tb to Eu ions.

Structure of Lanthanide-Oligomeric Films

An efficient energy transfer is induced by the short distance between Eu and Tb ions. As mentioned previously, in the solid state, the Tb and Eu ions of the chain-structured complex were too far in the mixed polymer to induce an energy transfer.[50] Optical waveguide spectroscopy of the [1-EuL] (Figure a) and [3-EuL] films (Figure b) was performed to study the structure and disposition of the lanthanide oligomers into the films. The bands in this wavelength region are assigned to the π–π* transitions localized on the coordinating ligand moiety, L. Dp and Ds are the polarized absorption spectra observed with polarized light, with polarizer angles in the perpendicular and horizontal directions, respectively. The maximum absorbance in [1-EuL] of Dp and Ds differs from each other, meaning that the π-electric systems obviously exists with a tilt angle. Furthermore, the polarized absorption bands of [3-EuL] appear at the corresponding positions than in [1-EuL] but with different absorbance, suggesting that the oligomers have no perpendicular growth on the quartz surface.

Figure 8

Optical waveguide spectroscopy of [1-EuL] (a) and [3-EuL] (b) films. Complex orientation into films [1-EuL] (c) and [3-EuL] (d). α was estimated from the equations in the SI.

From the optical waveguide spectra, the average orientation angle of the complex into the films was calculated (Figure c,d, equations are given in Figure S14).[59,60] In the [1-EuL] film, the average angle of the complex with the z-axis was about 80°. By increasing the number of generations, the average angle decreased to 68°, demonstrating that the complexes became more tilted. From there, the arrangement of the oligomers into the film can be supposed. From the carboxylate SAM’s quartz, the oligomers grow perpendicularly and bent with the increasing generation number, forming a domelike structure. The oligomers of the fifth generation could be arranged in two possible fashions, finishing by the blooming or tightening of the complexes (Figure ).

Figure 9

Possible domelike packing structure of the LnL oligomers into the films: (a) blooming arrangement and (b) tightening arrangement.

These structures were confirmed by synchrotron X-ray diffraction using an attachment for small angle.[36] X-ray diffraction patterns of oligomers showed one broadband between 10 and 25° (Figure S15).[61,62] It assimilated to the disorder caused by the bending of the oligomers and the change in the orientation of the complexes with the growth of the film.

As previously mentioned, the luminescent properties of mixed films showed an efficient energy transfer between Tb and Eu ions. This energy transfer follows a Förster-type mechanism, which is directly dependent on the distance between the two lanthanide ions (dmax ≈ 10 Å).[58] Here, the presence of an efficient energy transfer because of the short distance between Tb and Eu ions is more accurate in the tightening arrangement (Figure b).

Conclusions

A series of new lanthanide-oligomeric brush films composed of small oligomers of EuL or TbL were prepared using the LbL method and bcd as a spacer. Preparation and composition of the films were studied by AFM and XPS. All films showed a f–f emission from Eu or Tb ions through an energy transfer from the organic ligand. Analysis of the luminescent properties showed the modification of the luminescent properties until the third generation; no effect on the luminescence properties were observed after the third generation. By mixing Eu and Tb ions, the mixed films [4-EuL-5-Eu/TbL] and [4-TbL-5-Eu/TbL] demonstrated an increase of the Eu luminescence properties caused by an efficient energy transfer between Tb and Eu ions. This property was associated to the optical waveguide spectroscopy and synchrotron X-ray diffraction of the fifth-generation films to conclude that the oligomers are packed in a domelike structure, finishing by the tightening of the complexes in the last layer.

Materials and Methods

Functionalization of Quartz Substrates and Complexes Synthesis

The carboxylate SAM’s quartz[63,64] and lanthanides complexes[51] were synthesized from an adapted procedure reported previously (more details of the process are given in the SI).

Carboxylate SAM’s Quartz Preparation

Quartz plate was dipped, under sonication, in a mixture of HCl and methanol (1:1) for 30 min and then in H2SO4 (6 N) aqueous solution for 1 h. Fresh hydrolyzed quartz plates were dipped in a solution of 3-aminopro-pyltrihydroxysilane (0.1 M in ethanol) and heated at 60 °C for 3.5 h. Followed by immersion for 9 h, in glutaric anhydride solution (0.3 M in dimethylformamide) at room temperature. Finally, the quartz plate was dipped in an aqueous solution of NaOH (0.01 M) for 2 h and rinsed with ultrapure water. Carboxylate SAM’s quartz was then dried with N2 to remove all physisorbed impurities.

Synthesis of Hexadentate Ligand[51]

The hexadentate ligand was prepared by the addition of ethylenediamine (0.26 mL, 3.91 mmol, 0.5 equiv) in a solution of bipyridine-6-aldehyde (1.45 g, 7.87 mmol, 1 equiv) in methanol (5 mL). After 30 min, the precipitate was filtrated and recrystallized with methanol to yield the pure compound as a white solid (1.03 g, 2.62 mmol, 67%).

General Procedure for Complex Synthesis

In a solution of the hexadentate ligand (100 mg, 0.25 mmol, 1 equiv) in methanol (60 mL) was added Ln3+·6H2O (0.25 mmol, 1 equiv). After 30 min, NH4PF6 (41.5 mg, 0.25 mmol, 1 equiv) was added and the reaction mixture was stirred for another 3 h. The white precipitate was filtrated and washed with cold methanol to yield the Ln complex as a white solid.

Lanthanide Film Preparation

Freshly prepared carboxylate SAM’s quartz substrate was cyclically dipped in LnL (Ln = Eu or Tb) solution (1 × 10–3 M + 1 equiv of Et3N in acetonitrile) and in bdc solution (1 × 10–3 M + 2 equiv of Et3N in ethanol). Each immersion step was intercalated with several rinses and sonication for 5 min in a spectroscopic grade solvent. Then, the films were dried with N2 gas to remove the maximum physisorbed impurities. [1-LnL], [3-LnL], and [5-LnL] films were obtained after 1, 3, and 5 cycles, respectively. The preparation of mixed lanthanide films [4-EuL-5Tb/EuL] and [4-TbL-5Tb/EuL] followed a similar process. The four first-generation films were prepared with a pure LnL solution (Eu or Tb) as previously described. A fifth layer on [4-EuL] or [4-TbL] was prepared with an equimolar solution of EuL and TbL complexes (1 × 10–3 M + 1 equiv of Et3N in acetonitrile).

Apparatus

Contact angles were observed with CA-X (Kyowa Interface Science Co. Ltd.). Synchrotron XRPD patterns were recorded on a large Debye–Scherrer camera installed at SPring-8 BL02B2 (JASRI/SPring-8) using an imaging plate as a detector. Atomic force microscope SPM-8100FM (Shimadzu) was used for the measurements of FM-AFM. Electronic absorption spectra in the solid state were observed from the diffused reflection method with the conversion of the y-axis by UV-3600 (Shimadzu). Polarized electronic absorption spectra using an optical waveguide system was obtained using a modified SIS-5100 attached with a Glan–Taylor polarizer prism (System Instruments Co.). Luminescence and excitation spectra were recorded on a Fluorolog 3-22 (Horiba Jobin Yvon). Absolute luminescence quantum yields and luminescence lifetimes were determined using an absolute luminescence quantum yield C9920-02 spectrometer (Hamamatsu Photonics K. K.) and a Quantaurus-Tau C11367-12 spectrometer (Hamamatsu Photonics K. K.), respectively, with pulsed excitation light sources.