Metallacycle-cored supramolecular assemblies with tunable fluorescence including white-light emission

Significance

Light-emitting materials have been widely studied for their promising applications in chemical and biological science. Here we report three phenanthrene-21-crown-7 functionalized fluorescent Pt(II) rhomboidal metallacycles which were then converted into fluorescent supramolecular oligomers by reacting with a bis-ammonium linker. One of these constructs shows concentration-dependent fluorescence in a wide color range, where orange emission at high concentration and blue emission at low concentration was observed. Moreover, at a concentration of 29 µM the same ensemble emits white light that has emerged from the integration of the complementary orange and blue color from the metallacycles and linker, respectively. This study shows how light-emitting materials can be obtained by the proper implementation of multiple orthogonal interactions in a single process.

Abstract

Control over the fluorescence of supramolecular assemblies is crucial for the development of chemosensors and light-emitting materials. Consequently, the postsynthetic modification of supramolecular structures via host–guest interactions has emerged as an efficient strategy in recent years that allows the facile tuning of the photophysical properties without requiring a tedious chemical synthesis. Herein, we used a phenanthrene-21-crown-7 (P21C7)-based 60° diplatinum(II) acceptor 8 in the construction of three exohedral P21C7 functionalized rhomboidal metallacycles 1–3 which display orange, cyan, and green emission colors, respectively. Although these colors originate from the dipyridyl precursors 10–12, containing triphenylamine-, tetraphenylethene-, and pyrene-based fluorophores, respectively, the metal–ligand coordination strongly influences their emission properties. The metallacycles were further linked into emissive supramolecular oligomers by the addition of a fluorescent bis-ammonium linker 4 that forms complementary host–guest interactions with the pendant P21C7 units. Notably, the final ensemble derived from a 1:1 mixture of 1 and 4 displays a concentration-dependent emission. At low concentration, i.e., <25 µM, it emits a blue color, whereas an orange emission was observed when the concentration exceeds >5 mM. Moreover, white-light emission was observed from the same sample at a concentration of 29 µM, representing a pathway to construct supramolecular assemblies with tunable fluorescence properties.

Fluorescent supramolecular assemblies have received much attention due to their broad applications as biological and chemical sensors (1–5), bioimaging agents (6–10), light-emitting materials (11–15), etc. One method used to prepare such assemblies is coordination-driven self-assembly (16–29), because the incorporation of metal coordination not only endows the resultant assemblies with good stabilities but also at times provides unique emissions that are different from the fluorescent ligands and/or metal ions. By the incorporation of tetraphenylene derivatives into supramolecular coordination complexes (SCC) platforms, our group has prepared a series of highly emissive metal–organic assemblies that are capable of sensing nitroaromatics, cell imaging, etc. (30–34), although the precursors are weakly- or nonemissive. Similarly, Yoshizawa and coworkers prepared a series of M₂L₄ (M = Zn, Cu, Pt, Ni, Co, and Mn) capsules among which only the Zn(II) capsule emits blue fluorescence with a quantum yield of 80%, whereas the other capsules are nonemissive (35).

Fluorophores with aggregation-induced emission (AIE) (36–39) properties are good candidates for the construction of fluorescent supramolecular polymers because both the formation of supramolecular polymers and the accomplishment of a reasonable emission from the AIE cores need high concentrations. However, supramolecular oligomers/polymers with tunable emission have been rarely reported, although some progress has recently been made on the construction of discrete fluorescent supramolecular assemblies (29). The introduction of additional fluorophores via noncovalent interactions provides an alternative approach to further tune the emission of a given SCC (40, 41), leading to the formation of fluorescent supramolecular polymers. Herein we report three emissive metallacycles with pendant crown ether units by the incorporation of three different fluorescent dipyridyl ligands. Upon the addition of a fluorescent bis-ammonium linker, supramolecular oligomers are formed and the emission properties of these metallacycles were further tuned. Moreover, using the orange-emissive metallacycle 1 with AIE properties and the blue-emissive linker 4 with aggregation-caused quenching (ACQ) properties (39), the emission of the resultant supramolecular oligomers can be tuned from orange to blue as the concentration decreases. Interestingly, the same sample emits white-light emission at a concentration of 29 µM. This is an example where white-light emission was achieved by the integration of complementary emission properties (AIE and ACQ), complementary colors (orange and blue), and complementary host–guest interactions (crown ether and ammonium salts).

Results and Discussion

The synthetic procedures for rhomboidal metallacycles 1, 2, and 3 and a bis-ammonium linker 4 are shown in Fig. 1. A phenanthrene-21-crown-7 (P21C7)-based 60° diplatinum(II) acceptor 8 was synthesized in a three-step pathway starting from 3,6-dibromophenanthrene-9,10-diol 5 (Fig. 1A). The key intermediate 3,6-dibromophenanthrene-21-crown-7 6 was prepared by a potassium-templated cyclization reaction of 5 and hexethylene glycol ditosylate. The oxidative addition of 6 to Pt(PEt₃)₄ at each bromide site furnished 7, whose bromide anions were then exchanged by treating with AgOTf, leading to the formation of 8 in a good yield. The rhomboidal Pt(II) metallacycles 1, 2, and 3 were prepared by stirring the 60° platinum acceptor 8 and the corresponding 120° dipyridyl donor 10, 11, or 12, respectively, in dichloromethane at room temperature for 24 h. The bis-ammonium linker 4 was prepared from commercially available 2,7-dibromo-9,9-dioctyl-9H-fluorene 13 (Fig. 1B). Reaction of 13 with (4-formylphenyl)boronic acid via a Suzuki coupling provided compound 14 with two aldehyde groups, which were further reacted to afford bis-ammonium salt 4.

Fig. 1.

Synthetic routes and chemical structures of compounds. Synthetic routes of 60° diplatinum (II) acceptor 8 (A) and bis-ammonium linker 4 (B). Conditions: a) hexethylene glycol ditosylate, K₂CO₃, CH₃CN, reflux, 72 h; 64%; b) Pt(PEt₃)₄, toluene, 95 °C, 72 h; 68%; c) AgOTf, dry CH₂Cl₂, room temperature, 12 h; 95%; d) (4-formylphenyl)boronic acid, K₂CO₃, Pd(PPh₃)₄, dioxane/water (2:1), 90 °C, 48 h; 63%; e) (i) n-butylamine, CH₃OH, reflux, 12h; (ii) NaBH₄, room temperature, 24 h; (iii) HCl (aq), and (iv) NH₄PF₆ (aq); 27% in four steps.

The formation of metallacycles 1, 2, and 3 was confirmed by multinuclear NMR (³¹P and ¹H) analysis and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). The ³¹P{¹H} NMR spectra of 1, 2, and 3 exhibit sharp singlets with concomitant ¹⁹⁵Pt satellites at 14.17 ppm for 1, 14.69 ppm for 2, and 13.89 ppm for 3 (Fig. 2, spectra B–D), corresponding to a single phosphorous environment, indicating the formation of discrete, highly symmetric metallacycles (31–34). In the ¹H NMR spectra of these metallacycles, downfield chemical shifts were observed for the α-pyridyl protons H_d (from 8.64 to 9.13 and 9.10 ppm), H_k (from 8.59 to 9.07 and 9.05 ppm), and H_m (from 8.64 to 9.22 and 9.15 ppm) and β-pyridyl protons H_e (from 7.69 to 8.24 and 8.15 ppm), H_l (from 7.61 to 8.17 and 8.15 ppm), and H_n (from 7.53 to 8.31 and 8.26 ppm) and both split into two set of signals (Fig. 2, spectra F–K). The aromatic protons of H_a, H_b, and H_c of platinum(II) acceptor also shifted downfield. All these chemical shift changes are similar to their analogous reaction systems (31–34), providing evidence for the formation of rhomboidal metallacycles. ESI-TOF-MS provided evidence for the stoichiometry of formation of discrete rhomboidal Pt(II) metallacycles. Prominent sets of peaks with charge states (from 2+ to 4+) were observed for all of the metallacycles due to the loss of counterions (OTf⁻), and each peak closely matched the corresponding simulated isotope pattern (Fig. 3), supporting the composition of 1–3.

Fig. 2.

NMR characterization ligands 8, 10, 11, and 12 and metallacycles 1, 2, and 3. Partial (A−D) ³¹P and (E−K) ¹H NMR spectra (CD₃COCD₃, 295 K) of platinum acceptor 8 (A and E), ligands 10 (G), 11 (I), and 12 (K), and metallacycles 1 (B and F), 2 (C and H), and 3 (D and J).

Fig. 3.

Mass spectra of metallacycles 1 (A), 2 (B), and 3 (C).

The complexation between crown ether 8 and secondary ammonium salt 9 was first investigated as a model system (SI Appendix, Figs. S34–S36). The association constant (K_a) of 8·9 in acetone was determined by a ¹H NMR titration method, with a value of 9.4 (±1.0) × 10² M⁻¹, which is comparable to that of benzo-21-crown-7·9 (6.2 (±0.4) × 10² M⁻¹) (42) and suitable for the construction of supramolecular assemblies (Fig. 4).

Fig. 4.

Cartoon representation of the formation of supramolecular oligomers.

Concentration-dependent ¹H NMR measurements were then carried out to study the formation of the supramolecular oligomers. Therefore, a 1:1 mixture of 1 and 4 was chosen as the onset. The concentration of the sample was increased from 1.0 mM to 40.0 mM, leading to noticeable chemical shift changes for both precursors. Upfield chemical shifts were observed for the benzyl protons H_t and methylene protons H_u of the bis-ammonium linker 4, whereas the ethyl protons H_α and H_β of the metallacycle 1 shifted downfield (Fig. 5). Moreover, the chemical shifts of the linker 4 in the latter ensemble appeared in a similar region to that of 9 in the complex 8·9, indicating the formation of supramolecular oligomers via host–guest complexation. Two-dimensional diffusion-ordered NMR experiments were also carried out to test the size of the supramolecular oligomers in solution. As the concentration of the sample increased from 5.00 to 40.0 mM, the measured weight average diffusion coefficient (D) decreased from 3.19 × 10⁻¹⁰ to 1.16 × 10⁻¹⁰ m²⋅s⁻¹ (SI Appendix, Fig. S37), indicating the formation of high molecular weight supramolecular assemblies (43–46). To investigate the effect of the counterions in the assembly process, the counterions of 4 were also changed from hexafluorophosphate (PF₆) to trifluoromethanesulfonate (OTf) anions and the ¹H NMR spectrum of a mixture of 1 and 4•OTf at 5 mM was measured (SI Appendix, Fig. S38). However, this shows negligible effects compared with the ¹H NMR spectrum of a mixture containing 1 and 4•PF₆ salt, suggesting that the counterions do not influence these assembly processes.

Fig. 5.

Partial ¹H NMR spectra (CD₃COCD₃, 295 K, 400 MHz) of bis-ammonium linker 4 (A), and equal molar 4 and 1 at the concentration of 40 mM (B), 30 mM (C), 20 mM (D), 10 mM (E), 5.0 mM (F), 1.0 mM (G), and metallacycle 1 (H). The acetone peaks are marked with asterisks.

The UV-vis absorption and emission spectra of ligands 10, 11, and 12, rhomboidal metallacycles 1, 2, and 3 and bis-ammonium linker 4 in acetone are shown in Fig. 6. Ligand 10 displays two broad absorption bands centered at 346 and 440 nm with molar absorption coefficients (ε) of 2.28 × 10⁴ M⁻¹⋅cm⁻¹ and 4.09 × 10⁴ M⁻¹⋅cm⁻¹, respectively (Fig. 6, spectrum A). Ligand 11 shows one broad absorption band in the range of 320–370 nm, whereas ligand 12 exhibits one broad absorption band centered at 344 nm with ε = 3.03 × 10⁴ M⁻¹⋅cm⁻¹. Metallacycle 1 exhibits two absorption bands centered at 380 and 436 nm with ε = 9.13 × 10⁴ M⁻¹⋅cm⁻¹ and 8.04 × 10⁴ M⁻¹⋅cm⁻¹, respectively (Fig. 6, spectrum A). There is one absorption band centered at 322 nm for 2, 323 nm for 3, and 331 nm for 4 with ε = 1.16 × 10⁵ M⁻¹⋅cm⁻¹, 1.77 × 10⁵ M⁻¹⋅cm⁻¹, and 3.85 × 10⁴ M⁻¹⋅cm⁻¹, respectively. It is worth noting that the absorption of metallacycles 1, 2, and 3 is greatly increased compared with their precursors 10, 11, and 12 due to the inclusion of multiple ligands in one metallacycle structure.

Fig. 6.

Spectral characterization of ligands 10, 11, 12, metallacycles 1, 2, 3, and bis-ammonium linker 4. (A) UV-vis absorption spectra of ligands 10, 11, 12, metallacycles 1, 2, 3, and bis-ammonium linker 4 in acetone (c = 10 µM). (B) Emission spectra of ligands 10, 11, 12, metallacycles 1, 2, 3, and bis-ammonium linker 4 in acetone (λ_ex = 365 nm, c = 10 µM). (C) Emission spectra of equal molar 1 and 4 at different concentrations; (Inset) photograph of 1, 4, and mixture of equal molar 1 and 4 in acetone upon excitation at 365 nm using a UV lamp at 298 K (c = 29 µM). (D) CIE chromaticity coordinates of equal molar 1 and 4 at different concentrations, according to the fluorescence spectra recorded in C. (E) Emission spectra of equal molar 2 and 4 at different concentrations. (F) Emission spectra of equal molar 3 and 4 at different concentrations.

Ligands 10, 11, and 12 display moderate emission bands centered at 465, 472, and 445 nm in acetone, respectively (Fig. 6, spectrum B). Upon the formation of rhomboidal metallacycles, red-shifts were observed for 1 and 2. Specifically, rhomboids 1, 2, and 3 exhibit one emission band centered at 563, 490, and 445 nm, respectively, corresponding to the orange, cyan, and green emission according to the 1931 Commission Internationale de L’Eclairage (CIE) chromaticity diagram. Bis-ammonium linker 4 shows two emission bands centered at 391 and 411 nm, corresponding to the blue emission. Compared with their dipyridyl precursors 10, 11, and 12, the emissions of metallacycles 1 and 3 decrease whereas that of metallacycle 2 increases. The decrease of the emission is due to heavy atom effect which was also observed in other SCC systems (29). Although this effect also influences the emission of 2, the metal coordination inhibits the free rotation of the aromatic rings of the tetraphenylene derivatives, which enhances the emission of 2 relative to 11 (31–34).

The emission properties of the supramolecular assemblies formed by metallacycles (1, 2, or 3) and bis-ammonium 4 were investigated (Fig. 6, spectrum C). Interestingly, the supramolecular assemblies formed by 1 and 4 (1:1 molar ratio) show concentration-dependent fluorescence in a wide color range, where an orange emission at high concentration (>0.5 mM) and a blue emission at low concentration (<25 µM) were observed. This is likely because in the ensemble the metallacycle 1 acts as an AIE fluorophore, whereas the bis-ammonium linker 4 is an ACQ fluorophore (SI Appendix, Figs. S39 and S40). At high concentration, the AIE fluorophore plays a more important role so the sample mainly shows orange emission derived from 1. However, at low concentration, the ACQ fluorophore dominates the fluorescence, thus the ensemble exhibits a blue emission derived from 4. Moreover, the same sample displays white-light emission (CIE chromaticity coordinate: 0.30, 0.34) at a concentration of 29 µM (Fig. 6, spectrum D), which has only been rarely found in such supramolecular assemblies. At 29 µM, both the orange emission and the blue emission are observed and the emission covers the entire visible spectral region (400∼700 nm), endowing the solution with an overall white-light emission. Tunable emission was also observed in the assemblies formed by 2 and 4 or 3 and 4 (Fig. 6, spectra E and F). However, in these two assemblies, the color of the emission cannot be tuned in a wide range because of the strong overlap between the emission of 2 (or 3) and 4. This study provides a simple yet efficient approach for finely tuning the emission of supramolecular assemblies via concentration by the incorporation of both AIE and ACQ fluorophores as their building blocks.

Conclusion

In summary, three Pt(II) rhomboidal metallacycles with orange (1), cyan (2), and green (3) emissions were prepared by the metal-coordination-driven self-assembly of the 60° diplatinum(II) acceptor 8 and 120° dipyridyl donors 10–12. Supramolecular oligomers were further obtained via the host–guest interactions between the P21C7 units of the metallacyles and ammonium salts. The emissions of these supramolecular assemblies as well as their precursors were studied. These supramolecular oligomers show concentration-dependent fluorescence. Notably, the assemblies formed by orange-emissive metallacycle 1 and blue-emissive bis-ammonium linker 4 emit from orange to blue as the concentration decreases, whereas white-light emission was obtained at the concentration of 29 µM. This study provides a strategy to prepare light-emitting metal–organic assemblies by the precise manipulation of the AIE and ACQ properties (39) of the fluorophores.

Materials and Methods

All reagents and deuterated solvents were commercially available and used without further purification. Hexa(ethylene glycol) ditosylate (42), Pt(PEt₃)₄ (47), 5 (48), 9 (49), 10 (31), and 11 (50) were prepared according to the literature procedures. NMR spectra were recorded on a Varian Unity 300- or 400-MHz spectrometer. ¹H and ¹³C NMR chemical shifts are reported relative to residual solvent signals, and ³¹P{¹H} NMR chemical shifts are referenced to an external unlocked sample of 85% H₃PO₄ (δ 0.0 ppm). Mass spectra were recorded on a Micromass Quattro II triple-quadrupole mass spectrometer using electrospray ionization with a MassLynx operating system. The melting points were collected on an SHPSIC WRS-2 automatic melting point apparatus. The UV-vis experiments were conducted on a Hitachi U-4100 absorption spectrophotometer. The fluorescent experiments were conducted on a Hitachi F-7000 fluorescence spectrophotometer. Quantum yields were determined using quinine sulfate at 365 nm as reference (Φ_F = 56%).

Metallacycles 1, 2, and 3 were synthesized by stirring 8 with 10, 11, or 12 in a 1:1 molar ratio at room temperature for 12 h. Then diethyl ether was added to the homogeneous solution to give the corresponding metallacycles as precipitates. Bis-ammonium salt 4 was synthesized by the reaction of compound 14 with n-butylamine, followed by reduction, protonation, and ion exchange. The supramolecular assemblies were prepared by mixing metallacycles (1, 2, or 3) with 4 in a 1:1 molar ratio in acetone at room temperature.

Metallacycle 1: ¹H NMR (400 MHz, CD₃COCD₃, 295 K): 9.11 (dd, J₁ = 12.1 Hz, J₂ = 5.4 Hz, 8H), 8.70 (s, 4H), 8.18 (d, J = 5.6 Hz, 8H), 8.01–8.23 (m, 18H), 7.82 (d, J = 8.5 Hz, 4H), 7.49 (d, J = 7.8 Hz, 8H), 7.29 (d, J = 7.8 Hz, 4H), 4.37–4.48 (m, 8H), 3.97–4.07 (m, 8H), 3.74–3.83 (m, 8H), 3.69–3.74 (m, 8H), 3.56–3.68 (m, 16H), 1.41–1.66 (m, 48H), 1.15–1.34 (m, 72H). ³¹P{¹H} NMR (121.4 MHz, CD₃COCD₃, 295 K): 14.17 ppm (s, ¹⁹⁵Pt satellites, ¹J_Pt–P = 2,674.2 Hz). ESI-TOF-MS: m/z 896.3092 [1 – 4OTf]⁴⁺, 1244.7423 [1 – 3OTf]³⁺, 1,941.5898 [1 – 2OTf]²⁺.

Metallacycle 2: ¹H NMR (400 MHz, CD₃COCD₃, 295 K): 9.06 (t, J = 6.2 Hz, 8H), 8.68 (s, 4H), 8.15 (dd, J₁ = 16.7 Hz, J₂ = 5.3 Hz, 8H), 8.04 (d, J = 8.4 Hz, 4H), 7.86 (d, J = 8.2 Hz, 8H), 7.82 (d, J = 8.6 Hz, 4H), 7.33 (d, J = 8.2 Hz, 8H), 7.10–7.29 (m, 20H), 4.37–4.52 (m, 8H), 3.98–4.08 (m, 8H), 3.75–3.82 (m, 8H), 3.68–3.75 (m, 8H), 3.61–3.68 (m, 16H), 1.41–1.62 (m, 48H), 1.14–1.30 (m, 72H). ³¹P{¹H} NMR (121.4 MHz, CD₃COCD₃, 295 K): 14.69 ppm (s, ¹⁹⁵Pt satellites, ¹J_Pt–P = 2,784.6 Hz). ESI-TOF-MS: m/z 901.3514 [2 – 4OTf]⁴⁺, 1,252.1193 [2 – 3OTf]³⁺, 1,952.6388 [2 – 2OTf]²⁺.

Metallacycle 3: ¹H NMR (400 MHz, CD₃COCD₃, 295 K): 9.09–9.30 (m, 8H), 8.69 (s, 4H), 8.21–8.47 (m, 12H), 7.94–8.18 (m, 24H), 7.79 (d, J = 7.0 Hz, 4H), 4.35–4.50 (m, 8H), 3.97–4.07 (m, 8H), 3.74–3.82 (m, 8H), 3.67–3.74 (m, 8H), 3.57–3.67 (m, 16H), 1.36–1.61 (m, 48H), 1.07–1.31 (m, 72H). ³¹P{¹H} NMR (121.4 MHz, CD₃COCD₃, 295 K): 13.89 ppm (s, ¹⁹⁵Pt satellites, ¹J_Pt–P = 2655.5 Hz). ESI-TOF-MS: m/z 898.3780 [3 – 4OTf]⁴⁺, 1,247.4213 [3 – 3OTf]³⁺, 1,945.6079 [3 – 2OTf]²⁺.