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

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.

Keywords: fluorescence; host–guest interactions; orthogonal interactions; supramolecular metallacycles; white-light emission.

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.

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).

Fig. 4.

Cartoon representation of the formation of supramolecular oligomers.

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.

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.