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. 2019 Jun 5;4(6):9843–9849. doi: 10.1021/acsomega.9b01215

Luminescent Vesicles and Lyotropic Liquid Crystals in Ethylammonium Nitrate from a Partially Amphiphilic Eu Complex

Qingrun Li †, Sijing Yi ‡, Xiao Chen †,*
PMCID: PMC6648430  PMID: 31460075

Abstract

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Soft luminescent materials have attracted much attention because of their self-assembled and controllable properties. To explore their facile and effective fabrication ways, we report here the self-assembling of luminescent vesicles and lyotropic liquid crystals (LLCs) in a protic ionic liquid, ethylammonium nitrate, by a partially amphiphilic europium β-diketonate complex (Eu(III)) with a 1-dodecyl-3-methylimidazolium cation as the counter ion. An interesting result came from the complex-induced vesicle formation of corresponding amphiphile, 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br), which has been rarely reported in the past. It was the interaction between the Eu(III) and imidazolium group that changed the critical packing parameter of [C12mim]Br, which finally resulted in the occurrence of vesicles. The obtained vesicle aggregates exhibited enhanced fluorescence intensity and lifetime compared to those of Eu(III) solution. Meanwhile, a hexagonal LLC phase with better fluorescence properties was found at higher [C12mim]Br concentration. The obtained photophysical data confirmed that the order degree of Eu(III)-containing aggregates could effectively increase the energy transition efficiency of ligands. The better luminescent properties of LLC resulted from the stronger stabilizing and binding effects on Eu(III) in LLC than that in vesicles, which might be caused by closer molecular packing in LLC. The results presented here will not only expand the strategy of constructing lanthanide-containing luminescent soft materials in ionic liquids but also provide reference to better understand the effect of organized aggregates on luminescence properties.

1. Introduction

Because of its unique 4f electronic layer structure, the lanthanides exhibit abundant properties like luminescence, electricity, magnetism, and catalysis, which make them to be praised as “treasure house of new materials”.1 Therefore, the lanthanide-based luminescent materials have been paid much attention and widely used in many fields, such as biological probes and information display.2,3 To solve the problem of poor luminescence efficiency and easy annihilation of lanthanide ions, the organic ligands are often introduced to form complexes. Then, they could present excellent luminescent properties because of the good antenna effect of ligands, which could improve ligand-to-metal energy transfer or sensitization of lanthanide ions luminescence.4 This is also why the lanthanide β-diketonate complexes are much attractive as high-performance luminescent materials because they display good monochromaticity, long lifetimes, and high luminescence quantum yields. However, their intrinsic defects like poor thermal stability or easily degradable under UV irradiation may block their practical applications. To overcome these difficulties, the luminescent hybrid materials by immobilizing or dispersing the lanthanide complexes in certain matrices through covalent or noncovalent interactions have realized some achievements, especially for those in solid matrices.5,6 In addition, the organized soft aggregates like vesicles, lyotropic liquid crystals (LLCs), and gels have also become more and more recognized because of their rich phase structures.7−9 As examples, an amphiphilic Tb3+ complex has been prepared by Liu et al. which could spontaneously self-assemble into vesicles in water. The addition of adenosine triphosphate could significantly amplify the luminescence intensity mainly because of the replacement of water molecules to reduce fluorescence quenching.7 By introducing the europium complexes into an amphiphilic block polymer-based LLC system, we observed their improved luminescent properties because of the specially organized LLC micro-environment and thus induced confinement effect.8 The novel luminescent gels reported by Li et al., however, could be fabricated via reaction of Eu3+-functionalized ionic liquids (ILs) with imidazolium salts.9 Thus, the organized soft aggregates exhibit good potential as matrices to fabricate lanthanide hybrid luminescent materials.

To further improve the luminescence performance and stabilities of lanthanide complexes, the ILs have been chosen more and more as ideal dispersion media because of their excellent properties such as better thermal stability, nonvolatile, noninterference in the visible and near-IR spectral regions.10−12 Meanwhile, ILs are also ideal self-assembling media for organized aggregate fabrication. Therefore, such dual benefits of ILs have promoted the construction of IL-based lanthanide luminescent materials.13−15 By this motivation, we have also prepared IL-based LLCs with doped lanthanide complexes, which exhibited remarkably improved luminescence properties.16,17 However, in most studies for IL-based luminescent materials, the lanthanide salts or complexes were just simply dispersed or doped, which might limit their uses because of poor dispersibility or solubility of dopants in matrices.18−24 Then, whether the lanthanide complexes could directly participate the aggregate formation to increase their dispersion becomes a challenge.

For this aim, we herein report the synthesis and self-assembly of a partially amphiphilic europium complex, 1-dodecyl-3-methylimidazolium tetrakis(thenoyltrifluoroacetonato)europate ([C12mim][Eu(TTA)4], denoted as Eu(III)), which contains a hydrophobic long alkyl chain and a hydrophilic alkylimidazolium cation, with an europium β-diketonate complex as the counterion (see Scheme 1). Eu(III) was then co-assembled with the corresponding amphiphilie, 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br), in a protic IL, ethylammonium nitrate (EAN), to form organized aggregates. Here, EAN was employed as the solvent owing to its rich hydrogen-bonded network similar to water, which could offer driving forces for self-assembly.25 The vesicles and LLC aggregates with Eu(III) could be thus obtained by adjusting [C12mim]Br concentration. To our best knowledge, there are rarely studies on luminescent soft aggregates from co-assembly of the lanthanide complex with corresponding amphiphilic ligands. The effect of aggregate morphology on Eu(III) photophysical properties have also been investigated. This will help us to further understand the luminescence mechanism of lanthanide complexes in different soft aggregates and expand their applications.

Scheme 1. Chemical Structure of Synthesized [C12mim][Eu(TTA)4].

Scheme 1

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2. Results and Discussion

2.1. Phase Behavior of Self-Assembled Eu[III] Aggregates in EAN

We characterized first on the phase structure of aggregates formed only by [C12mim]Br itself. As we know, the formation of micelle of [C12mim]Br in aqueous solution has been observed previously.26 Therefore, is it possible to form the micelle also by [C12mim]Br in EAN? At lower [C12mim]Br concentrations, C12 (<70 wt %), all samples were observed indeed to present as homogeneous, transparent, and isotropic solutions. In addition, the dark-field image of the sample under POM observation (see Figure S1a) and only a wide scattering peak in its small-angle X-ray scattering (SAXS) curve (see Figure 1A) were both characteristic for micelle aggregates.27

Figure 1.

Figure 1

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SAXS patterns for the [C12mim]Br–EAN binary system at [C12mim]Br concentrations (wt %) low (A) or high (B).

With increasing C12 to high values of about 70–90 wt %, however, the binary system formed a more organized aggregate structure which displayed the obvious optical birefringence and characteristic fan-like texture under POM for a hexagonal (H1) phase (see Figure S1b). The SAXS curves in Figure 1B for samples at C12 of 75 or 85 wt % also exhibited three Bragg peaks with their relative scattering factor (q) positions of 1:√3:2, which clearly indicated the formation of the H1 phase. Therefore, the preliminary phase diagram of the [C12mim]Br–EAN binary system at 25 °C could be drawn in Figure 2.

Figure 2.

Figure 2

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Phase diagram of the [C12mim]Br–EAN binary system at 25 °C.

In order to study the influence of Eu[III] on the phase structure of aggregates, the SAXS patterns for samples of the [C12mim]Br–EAN–Eu[III] ternary system at different C12 values were measured with the results shown in Figure 3. Interestingly, from Figure 3A, for SAXS curves measured at C12 of 50 or 60 wt %, two scattering peaks with their relative q ratio of 1:2 could be seen, which indicated the appearance of a lamellar phase.28 Compared to that only one scattering peak being observed in the [C12mim]Br–EAN binary system, the existence of Eu(III) should be the reason for such a phase change. To obtain more aggregate information, freeze-fracture transmission electron microscopy (FF-TEM) observation was applied with the typical results shown in Figure 4. Many spherical polydisperse aggregates with their sizes ranging from 50 to 200 nm could be observed. Therefore, the lamellar phase indexing from SAXS might be originated from the formation of vesicles, which existed at C12 from about 40–70 wt %.

Figure 3.

Figure 3

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SAXS patterns for the [C12mim]Br–Eu(III)–EAN system at [C12mim]Br concentrations (wt %) low (A) or high (B).

Figure 4.

Figure 4

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FF-TEM images of the aggregate formed from the [C12mim]Br–Eu(III)–EAN system at [C12mim]Br concentrations of 50 wt % and low (A) or high (B) resolutions.

Then, how were the previous micelles in [C12mim]Br–EAN binary system transformed into vesicles in the presence of the europium complex? As a single-chain amphiphile, [C12mim]Br usually forms micelles with its critical packing parameter (CPP) being about 0.23. However, Eu(III) could be considered as a quasi-amphiphile with the same alkyl chain as [C12mim]Br but larger counter ion. In their mixture samples, the solvophobic attraction between alkyl chains made Eu(III) and [C12mim]Br packed shoulder to shoulder. Meanwhile, the larger volume of the complex ion might reduce the electrostatic repulsion between the imidazolium head group, which should increase the CPP and therefore induce the vesicle formation.29

At C12 of about 70–90 wt %, the SAXS curves in Figure 3B also exhibited three scattering peaks with q position ratios of 1:√3:2, reflecting the maintenance of similar hexagonal liquid crystalline phase after adding Eu(III). With the increase of C12, because of the closer packing of alkyl chains from [C12mim]Br and Eu[III], and also the relatively low ratio of Eu(III) to [C12mim]Br, the LLC phase was little influenced. Compared to SAXS patterns in Figure 1B, the first scattering peaks in Figure 3B were found not to change even after adding Eu[III]. This was a sign that no expansion occurred in LLC by Eu(III), indicating that the europium complex ions were mainly distributed on the surface of the columnar micelle and the solvent, similar to the situation discussed before on their distribution for vesicle formation.

2.2. Luminescence Properties of [C12mim]Br–Eu(III)–EAN Aggregates

The excitation and emission spectra of the obtained vesicle and LLC samples were measured to explore the aggregate structure effect on the luminescence property of Eu(III). For comparison, we also mapped the fluorescence spectra of the pure europium complex. Figure 5A illustrated the excitation spectra for aggregates formed at different C12 and for pure Eu(III). The broad bands from 200 to 450 nm could be seen for all samples, which were because of the “energy absorption via the TTA ligand”.30 In addition, a small peak at 467 nm was observed for Eu(III) in the solid status, corresponding to the intra-4f transition of Eu3+.31 This peak, however, disappeared when Eu(III) was distributed in the vesicle and LLC aggregates, reflecting the existence of a more efficient energy transfer process between TTA and Eu3+ than that in the solid status.

Figure 5.

Figure 5

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Excitation (A) and emission (B) spectra of [C12mim]Br–Eu(III)–EAN samples in different aggregation status and Eu[III] solid.

From Figure 5A, the main bands were found to locate around 340 nm, corresponding to a charge transfer from ligand to metal charge transfer (LMCT), which reflected the presence of an effective energy transfer in vesicle or LLC aggregates.32 It could be observed also that the excitation spectra were similar for aggregates with the same phase structure, though at different C12. However, it was interesting to observe that the peak intensities at 340 nm for LMCT were basically the same, but those at 275 nm for the ligand to ligand charge transfer (LLCT) were obviously lower in LLC than in the vesicle. Such a difference indicated a more efficient energy transfer between the ligand and Eu3+ in H1, which was mainly due to the stronger binding effect in LLC.

From the emission spectra shown in Figure 5B, all samples could be seen to show five narrow bands at 582, 595, 616, 652, and 705 nm, belonging respectively to the characteristic Eu3+-centered transition bands 5D0 → 7FJ (J = 0–4).33 Among them, the strongest emission band at 616 nm corresponded to 5D0 → 7F2 hypersensitive transition, which was the main reason for the red emission of samples. As an electric-dipole transition, its intensity is closely related to the coordination structure symmetry of central Eu3+. However, the fluorescence intensity of band at 595 nm, corresponding to 5D0 → 7F1 magnetic dipole transition, is independent of coordination environment change. Therefore, their transition intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 (R) can be used as a measure of coordination environment symmetry of Eu3+.34 Generally, a higher R value represents a more asymmetric europium coordination sphere and therefore a stronger interaction between Eu(III) and its host matrix in the situation here.

Such R average values calculated from Figure 5B were listed in Table 1, accompanying with other fluorescence parameters for the [C12mim]Br–Eu(III)–EAN vesicle or LLC aggregates and pure europium complex. It can be seen that the R value obtained in LLC (12.53) was higher than that from vesicle (11.33), indicating a higher asymmetry degree of Eu(III) coordination sphere in LLC than in the vesicle. This result could be easily understood because the hydrogen bond formed between C–H from an imidazolium ring and C=O from a diketonate group of TTA was the origin to make the distortion of the local symmetry around the europium ions. The closer packing in LLC helped to form a stronger hydrogen bond, and therefore, a higher coordination asymmetry degree of the Eu(III). In addition, compared with the pure europium complex, the R values in two aggregates were both increased to exhibit improved luminescent monochromaticity because of the increase of the coordination asymmetry degree.

Table 1. Characteristic Luminescence Parameters for Eu(III) in Aggregate and Solid Status.

status R τ (ms) Q (%) kr (ms–1) knr (ms–1)
LLC 12.53 0.535 48.51 0.917 1.055
vesicle 11.33 0.436 29.29 0.672 1.622
solid 7.05 0.953 46.63 0.489 0.561
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Other parameters in Table 1 were derived from the fluorescence intensity decay curves shown in Figure 6 for 5D0 → 7F2 emission for all samples at room temperature. All curves could be fitted to a single exponential function, which indicated only one coordination environment of Eu3+ occupied in both LLC and vesicle phases. Also, such observed single-exponential fluorescence decay lifetimes (τ) indicated that Eu(III) did not decompose in these soft matrices. Meanwhile, as seen in Table 1 and Figure S2, the average lifetimes in both aggregates were higher than that of direct dispersion of Eu(III) in EAN (τ = 0.191 ms) but lower than that of Eu(III) solid, which was due to the high-energy vibrations from nitrate ions in EAN to produce high nonradiative transition inactivation. Compared with that in the vesicle, the Eu(III) complex had a longer fluorescence lifetime in LLC, indicating a more obvious confinement effect in LLC.35 Also, the lifetime of all samples could be simply compared by the intensity of red emission under UV irradiation (λmax = 365 nm) at room temperature (see Figure S3).

Figure 6.

Figure 6

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Luminescence decay curves of Eu(III) in aggregate samples and the solid status under excitation at 340 nm and observed at 616 nm at room temperature.

Based on the emission spectra and lifetime data, the radiative (kr) and nonradiative (knr) rate constants and the internal emission quantum efficiency (Q) of the 5D0 excited state were determined (see the Supporting Information for details), with the results listed also in Table 1. Compared to the vesicle system, a higher kr and a lower knr values could be observed in the LLC sample. A higher Q value in LLC was therefore obtained. Such improved luminescent properties in EAN-based LLC have been ascribed to decreased quenching by effective binding and orderly dispersion of Eu(III).36 Then, only fewer nonradiative transitions were produced in LLC and a better energy transfer efficiency was observed. The abovementioned short fluorescence lifetime for Eu(III) dispersion in EAN, however, further illustrated the role of effective confinement by organized soft aggregates to luminescence enhancement.

2.3. Intermolecular Forces between Eu(III) and LLC or Vesicle Matrices

As a protic IL, a three-dimensional hydrogen-bonding network structure similar to water can be formed in EAN, which is also a key factor to promote surfactant self-assembly.37 Thus, whether the hydrogen bonding in EAN was responsible for the effective confinement of Eu(III) should be clarified. For this purpose, we used infrared spectroscopy to analyze the hydrogen bonding effect both in the vesicle and in LLC phases.

Figure 7 exhibited the FTIR spectra of EAN and [C12mim]Br–Eu(III)–EAN vesicle and LLC samples. In spectra for both soft aggregates, the stretching vibration at 3055 cm–1 and the asymmetric deforming vibration at 1617 cm–1 of N–H bond were shifted to the higher wavenumber region compared to those in pure EAN. This tendency reflected that the presence of the imidazolium cation led to the reconstruction of hydrogen bonding network of EAN and the new hydrogen bond formation between atoms of N in the imidazole head group and H in the ethylammonium cation.38 The changes of N–H stretch and asymmetric deformation vibrational peaks were listed in Table 2, from which the largest shift was observed in the LLC sample, implying the strongest stabilizing effect on Eu(III).

Figure 7.

Figure 7

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FTIR spectra for EAN and [C12mim]Br–Eu(III)–EAN vesicle and LLC materials.

Table 2. FTIR Band Assignments for Eu(III) in Two Aggregates and Solid Status.

EAN/cm–1 solid/cm–1 vesicle/cm–1 LLC/cm–1 assignment
3055   3068 3079 N–H stretch
1617   1625 1633 N–H asymmetric deformation
  3170 3162 3155 C–H stretch
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Besides, the TTA ligands could also form hydrogen bonds with the imidazolium cations, which could be demonstrated by comparing C–H stretching vibrations on the imidazole ring, which was shifted from 3170 in pure Eu(III) to 3162 in the vesicle and 3155 cm–1 in LLC. Such a shift to lower wavenumbers indicated that the imidazolium cations were involved in the formation of hydrogen bonds, where the hydrogen atom on the imidazole ring should act as a donor for hydrogen bonds.39 In addition, the larger shift in the LLC sample implied formation of stronger H-bonding.

Therefore, the imidazolium cations might serve as the linker between the Eu(III) and EAN solvent. The hydrogen bonding between EAN and TTA ligands played a key role for constraining Eu(III) in aggregates. The enhanced hydrogen bond interaction in LLC should be resulted from the denser packing of [C12mim]Br molecules, which could induce a more stable microchemical environment to reduce the nonradiative transition deactivation of Eu(III) to improve the luminescent properties.40

2.4. Stability of [C12mim]Br–Eu(III)–EAN LLC and the Vesicle

Usually, the emission intensity of lanthanide complexes with β-diketonate could be strongly decreased under UV irradiation because of the ligand dissociation. Then, how about the luminescent stability in such fabricated LLC and vesicle? As a comparison, Eu(III) was dissolved respectively in CH3CN and EAN. Then, all prepared Eu(III)-containing luminescent materials were exposed to UV light (6 W) at a wavelength of 365 nm to investigate their UV-resistance. The measured emission intensity changes of 5D0 → 7F2 line under UV light irradiation were plotted in Figure 8 for photostability study. It was shown that the emission intensity was seriously reduced in CH3CN or EAN solution upon UV irradiation as the exposure time increased. After 50 h, the intensity was declined to 22 or 50% of their initial values in EAN or in CH3CN. In the vesicle and LLC samples, however, the intensity attenuation was small. The 50 h exposure only made the reduction of intensity to 74 or 94% of their initial values in the vesicle or in LLC. The experimental results further confirmed that luminescent materials constructed with ordered aggregates have excellent UV light resistance even comparable to solid substrates.41

Figure 8.

Figure 8

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Emission intensity changes for 5D0 → 7F2 transition of Eu(III) under UV light exposure in LLC, vesicle, and solutions of CH3CN or EAN.

3. Conclusions

In this work, we have constructed two stable luminescent soft aggregates in a protic IL (EAN) from europium β-diketonate complex [Eu(III)] by molecular self-assembly. In particular, the partially amphiphilic Eu(III) co-assembled with [C12mim]Br to induce successful construction of vesicles. It was the hydrogen bonding between the imidazolium cation or TTA ligand and EAN that constrain Eu(III) distribution relatively homogeneous in aggregate matrices. Thus, the obtained luminescent vesicle and hexagonal LLC both exhibited excellent luminescent property and stability. Because of the stronger hydrogen bonding effect in LLC and thus induced more ordered microenvironment, higher quantum yield and lifetime were observed. Therefore, using soft aggregate as matrices is an effective way to enhance the luminescence efficiency of the lanthanide, which might expand their applications in the monitor and detection metal ions in medicine.

4. Experimental Section

4.1. Materials

EuCl3·6H2O (99.9%) and 2-thenoyltrifluoroacetone (99%) were purchased from Alfa Aesar and used as received. [C12mim]Br was purchased from the center for Green Chemistry and Catalysis, LICP, CAS. EAN was prepared according to the procedures reported previously.42 A portion of 3 M nitric acid was added to a concentrated solution of ethylamine while stirring and cooling it in an ice bath. The product was evaporated at 50 °C under vacuum to remove most of water and excess ethylamine. Then, the residue was dried at 50 °C under vacuum for 24 h, which was finally freeze-dried for 24 h until the residual water content was below 0.5 wt %, as determined by Karl Fischer titration. 1H NMR, δ (300 MHz, D2O): 1.15 (t, 3H), 2.89 (m, 2H).

4.2. Synthesis of Europium β-Diketonate Complex

The europium complex, Eu[III], was prepared according to the procedures previously reported.43 First, 6 mmol 2-thenoyltrifluoroacetone was dissolved in 10 mL ethanol, which was deprotonated by 1 M NaOH aqueous solution. Then, 1.5 mmol [C12mim]Br (dissolved in 9 mL ethanol) was added dropwise and the mixture was stirred at 50 °C for 0.5 h. After that, the solution with 1 mmol EuCl3·6H2O dissolved in 10 mL water was added dropwise and the mixture was stirred at 50 °C for 2 h. A precipitate was formed and the reaction was then cooled down to room temperature overnight. The product was filtered and carefully washed with ice water. The obtained light yellow powder was dried in a vacuum oven at 50 °C for 48 h and stored in a dry and dark place. The complex composition was characterized by the elemental (C, H, N) analysis with the result measured (calculated) as follows (%): C48H51N2F12O8S4·Eu: C, 44.70 (44.51); H, 3.95 (3.76); N, 2.17 (2.16); S, 9.94 (9.95).

4.3. Preparation of the Aggregate Sample

For a typical Eu[III]-participated aggregate sample preparation, 5 mg of europium complex was first dissolved in EAN. Then, the exact amount of [C12mim]Br was mixed with this Eu[III]-containing IL. The final volume of each obtained sample at different [C12mim]Br weight percentages was kept at 1 mL to keep the same Eu[III] concentration in all samples. These mixtures were homogenized by repeated vortex mixing and centrifugation. Then, they were equilibrated at 25 ± 0.1 °C for four weeks in the darkness before further investigation. For comparison, the Eu[III]-dissolved EAN solution at the same concentration was prepared.

4.4. Characterization

4.4.1. Optical Microscopy

In order to identify the LLC phase, the sample birefringent textures were recorded by a Motic B2 polarized optical microscope (POM) with a charge-coupled device camera (Panasonic Super Dynamic II WV-CP460).

4.4.2. Small-Angle X-ray Scattering

SAXS measurements for aggregate samples were performed on a SAXSess MC2 high flux SAXS instrument (Anton Paar, Austrian) with a Ni-filtered Cu Kα radiation (λ = 0.154 nm), operating at 40 kV and 50 mA. The distance from the sample to the detector was 27.8 cm. The temperature for all samples was controlled at 25 °C and the test time was 15 min.

4.4.3. Fluorescence Spectroscopy

The excitation and emission spectra were measured on a Hitachi F-7000 spectrofluorometer equipped with a xenon lamp (150 W). The luminescence lifetime was measured on an Edinburgh Instruments FLS920 luminescence spectrometer (xenon lamp, 450 W). The luminescence lifetime was measured by monitoring the luminescence intensity decay at the 5D0 → 7F2 transition of Eu3+. The absolute value error of lifetime was about 0.3 μs.

4.4.4. Fourier Transformed Infrared Spectroscopy

FTIR spectra were measured by an ALPHA-T spectrometer (Bruker) with a resolution of 4 cm–1 from 400 to 4000 cm–1. The solid powders were mixed with KBr salt and then compressed into a transparent plate. The LLC or neat IL solution was coated on KBr plates.

4.4.5. Freeze-Fracture Transmission Electron Microscopy (FF-TEM)

To observe Eu(III) aggregate morphology in the solution, the sample was mounted on a specimen holder and frozen by quickly inserting into liquid ethane cooled by liquid nitrogen. Fracturing and replication were carried out on a freeze-fracture apparatus (EM BAF 060, Leica, Germany). The replicas were transferred onto the copper grid and then observed with a Hitachi 100CX-II operating at 100 kV.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (21373127 and 21673129).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01215.

  • POM textures of the [C12mim]Br aggregate in different phases, luminescence decay curves and visual images of all samples under UV light, and calculation methods of radiative (kr) or nonradiative (knr) rate constants and internal quantum efficiency (Qin) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b01215_si_001.pdf (298.4KB, pdf)

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