Anthracene-induced formation of highly twisted metallacycle and its crystal structure and tunable assembly behaviors

Significance

Macroscale organisms maintain a high degree of orderliness on an extremely small scale. Inspired by natural assemblies, anthracene-based organoplatinum metallacycles were designed as building blocks. Instead of the expected planar rectangles, a highly twisted structure was generated due to strong π···π stacking interactions and steric hindrance between the two anthracene ligands. Enhanced emission of the metallacycle in solution was observed due to restrictions on the conformational change of anthracenes introduced by the intramolecular twist and aggregation of metallacycle. The as-prepared twisted metallacycle was used as a building block to construct suprastructures. The optical properties and self-assembly behavior of the metallacycle in different solvent systems were systematically investigated. This study facilitates the rational design of metallacycle-based emissive multifunctional materials.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) continue to attract increasing interest with respect to their applications as luminescent materials. The ordered structure of the metal−organic complex facilitates the selective integration of PAHs that can be tuned to function cooperatively. Here, a unique highly twisted anthracene-based organoplatinum metallacycle was prepared via coordination-driven self-assembly. Single-crystal X-ray diffraction analysis revealed that the metallacycle was twisted through the cooperation of strong π···π stacking interactions and steric hindrance between two anthracene-based ligands. Notably, the intramolecular twist and aggregation behavior introduced restrictions to the conformational change of anthracenes, which resulted in increased emission intensity of the metallacycle in solution. The emission behaviors and suprastructures based on the highly twisted metallacycle can be modulated by the introduction of different solvents. This study demonstrates that this metallacycle with highly twisted structure is a promising candidate for sensing and bioimaging applications.

Weak intermolecular interactions play an essential role in the construction of well-defined hierarchical architectures with high complexity and functionality (1–4). In nature, a complex interplay of units forms highly organized suprastructures (5). From biofilms to cells, tissues, and organs, orderliness starts with the lipid bilayer and ultimately results in the formation of a macroscopically visible specific shape through multilevel assembly (6). Structurally, any macroscale organism maintains a high degree of orderliness on an extremely small scale, and this multilevel orderliness is carried through to the macroscale (7). DNA and histones coassemble to form chromatids, and chromatids further assemble to form chromosomes (8). Inspired by assemblies in the natural world, researchers have created diverse suprastructures with distinct spatial arrangements via weak interactions, such as hydrogen bonds, aromatic stacking, and hydrophobic interactions (9–19). Among these, coordination-driven self-assembly (CDSA) provides a powerful strategy for producing metal organic complexes (MOCs) with various sizes, shapes, and metal/ligand stoichiometries (20–26). To date, diverse MOCs have been developed by coordination between metal ions and organic ligands, and these exhibit superiority over their precursors and can be used in emission units, sensors, and hosts as well as in applications in biomedical diagnosis and catalysis (27–34). Platinum-based architectures are the most extensively studied of these structures and have undergone the most rapid development, due to their facile synthesis (35–39).

Platinum-based metallacycles are desirable candidates for the construction of luminophores because of their structural versatility. However, most luminophores always suffer from aggregation-induced quenching at high concentrations or in the solid state. Aggregation-induced emission has been employed in the fields of sensors, bioimaging, and drug delivery, due to strong emissions in the aggregated state (40, 41). However, to date, a tetraphenylethylene moiety is required in the construction of metallacycles with emission (42) Anthracene (ANTHR) is an attractive moiety that endows suprastructures with unique optical properties due to its extended π-surface (43, 44), which results in the formation of promising candidates for light emitters (45), photocatalysts (46), and data storage materials (47). For example, Zr and Fe have been incorporated with ANTHR into coordination complexes to perform catalysis (48) and to mimic the functional behavior of enzymes (49). In addition, ANTHR-based rectangles have been proven to provide near-infrared emission for application in ammonia detection (50), and ANTHR-based hexagons have been used for the capture and release of singlet oxygen (51). However, most reported ANTHR-based metallacycles have planar structures. Considering the relationship between structure and function of these metallacycles, the development of more structures with new steric features will lead to the emergence of new functions. This study provides an ANTHR-based metallacycle with a highly twisted conformation among ligands, and the twist angle between ANTHR is close to 90°. Benefiting from this highly twisted structure induced by intramolecular restriction, ANTHR-containing metallacycle with aggregation-induced emission was observed.

Results and Discussion

Here, ANTHR-based metallacycle was synthesized using ANTHR-based ligands (9,10-di(pyridin-4-yl)anthracene [DPA]), dicarboxylate moieties (isophthalic acid [IPA²⁻]), and cis-(PEt₃)₂Pt(OTf)₂. The metallacycle was readily prepared using a heteroligation-directed three-component CDSA strategy. As shown in Scheme 1 A and B, metallacycle 1 was formed by the reaction of pyridyl-functionalized ANTHR ligand (2), dicarboxylate ligand (3), and phosphine-capped 90° Pt(II) acceptor (4) at a molar ratio of 1:1:2. Compounds 2 and 3 were dissolved in acetone and water, respectively. The two solutions were then mixed, and 4 in acetone was added. After heating at 60 °C for 12 h, the solvent was evaporated via N₂ flow. The resultant product 1 was obtained after drying under vacuum (SI Appendix). Metallacycle 1 was characterized by ¹H NMR (Scheme 1C and SI Appendix, Fig. S1) and ³¹P{¹H} NMR (Scheme 1D and SI Appendix, Fig. S2) spectroscopy (52). As shown in the ¹H NMR spectrum of 1, the protons of the pyridyl groups are located at δ = 8.68 and 7.14 ppm, which is attributed to the coordination of the N atoms to the platinum centers. The protons corresponding to dicarboxylate ligand 3 are observed at δ = 8.30, 8.09, and 7.46 ppm. The ³¹P{¹H} spectrum also supported the structures. Further characterization by diffusion-ordered NMR spectroscopy (DOSY) was in agreement with the formation of discrete metallacycles (SI Appendix, Fig. S3). In the pseudo two-dimensional (2D) DOSY spectrum of 1, all the proton signals from metallacycle are correlated with the same D of about (10.0 ± 0.4) × 10⁻¹⁰ m²⋅s⁻¹. Light green single crystals of the metallacycle were obtained by slow diffusion in dichloromethane (DCM) and ethyl acetate at room temperature. The ¹H NMR spectrum of 1 at variable temperature (VT NMR) in 1,2,-dichloroethane was determined to investigate the fluxionality of 1 (Fig. 1). As shown in Fig. 1 A and B, the H_2a and H_2c broadened with the temperature increase from 293 K to 323 K, and then coalesced to a single broad peak at 343 K. This might be caused by the faster rotation speed of the pyridinyl groups (H_2a and H_2c) at high temperature, resulting in the peaks becoming broader and even merging to one peak (the green regions on the left side in Fig. 1 A and B). At the same time, the peak of H_2b and H_2d also broadened with the temperature range from 293 K to 343 K (the green regions on the right side in Fig. 1 A and B), again caused by faster rotation at high temperature. The resonance of H_3b and H_3c maintained a similar peak shape and intensity as the temperature increased, which proves the protons of the phenyl groups are less fluxional, due to the metasubstitution in the cycle. Different from the H_3b and H_3c, H_3a shows a downfield shift with the increase in temperature, which might be caused by the shortened distance between the protons and thus influenced by the ANTHR plane (pink region in Fig. 1 A and B). The environmental change of H_4a and H_4b can also be monitored by VT NMR, as shown in the gray region in Fig. 1B, and broader peaks can be observed with the increased temperature, which indicates a faster rotation speed of the σ-bonds. To make sure that these changes were reversible, the proton NMR was retaken after the temperature was cooled to 293 K. As shown in the top spectra in Fig. 1 A and B, all the peaks returned to the initial state at 293 K, indicating that no thermal reaction has taken place when heating from 293 K to 343 K, and no hysteresis is observed. Based on the above results, it is determined that the twist structure is fluxional in solution, especially in good solvents.

Scheme 1.

(A) Chemical structure and self-assembly of metallacycle 1. (B) Top and side views of 1. (C) The ¹H NMR and (D) ³¹P{¹H} NMR of 1.

Fig. 1.

(A and B) Variable-temperature NMR spectra of 1 in 1,2,-dichloroethane; (C) the scheme represents the proton of 1 in different chemical environment.

The structure of 1 was determined unambiguously by single-crystal X-ray diffraction, and the X-ray crystallographic analysis revealed the formation of a unique highly twisted metallacycle. As shown in Fig. 2A, the results from the X-ray diffraction analysis showed that the crystal structure of 1 contained a discrete organoplatinum(II) metallacycle, which consisted of two DPAs, two IPA²⁻, four (PEt₃)₂Pt(II) centers, and four triflate anions (OTf⁻). In addition to the coordination bonds, variable noncovalent interactions, including π···π stacking interactions (4.282 Å), C−H···π interactions (2.839 Å) between the ANTHR moieties of DPA, C−H···π interactions (3.471 and 3.475 Å) between the hydrogen atoms on the ANTHR moieties of DPA and the benzene ring of IPA, and C−H···O hydrogen bonds between the hydrogen atoms on the ANTHR ring (C29−H29···O67, 2.516 Å) or the PEt₃ groups (C44A−H44B, 2.560 Å; C56A−H56C, 2.490 Å) and the carboxylate oxygen atoms of IPA²⁻, facilitated the formation of the metallacycles (Fig. 2B). Despite the strong π···π and C−H···π stacking interactions between two ANTHR-based ligands, to minimize the steric effect between two ANTHR groups and enhance the π···π and C−H···π stacking interactions, the metallacycles adopted a twisted rectangular conformation with the ANTHR-based DPA ligands which was twisted by ca. 79.55° to form the metallacycle. Accordingly, the distance between the two ANTHR groups was compressed due to the twisted structure. Furthermore, the crystallographically independent triflate anions were located in two different chemical environments and exhibited different multiple hydrogen-bonding modes.

Fig. 2.

(A) X-ray single-crystal structure of 1 with ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity. (B) Intramolecular π···π stacking interactions and C−H···π interactions in 1. (C) The 2D network constructed from 1 linked by (D) intermolecular C−H···O hydrogen bonds.

The analysis of the crystallographically independent OTf⁻ anions (A) showed four intermolecular C−H···O hydrogen bonds with H···O distances of 2.174 Å to 2.693 Å (Fig. 2D). These hydrogen bonds assembled the adjacent metallacycles into 2D infinite supramolecular networks (Fig. 2C). For the OTf⁻ anion (B), four C−H···O hydrogen bonds (d_H···O: 2.395 Å to 2.687 Å; Fig. 3A) and two C−H···F hydrogen bonds (d_H···F: 2.568 and 2.600 Å; Fig. 3A) formed between the two metallacycles from two adjacent 2D networks and joined the 2D networks to form the final 3D supramolecular architecture with large rectangular channels running along the b axis (SI Appendix, Fig. S4). In addition, multiple π···π stacking interactions (4.337 Å) and C−H···π interactions (2.841 Å to 2.898 Å) were observed between the ANTHR moieties of DPA from the metallacycles in the neighboring 2D networks, and these interactions helped stabilize the final 3D supramolecular structure of 1 (Fig. 2B). Due to the C−H···π interactions between the pyridine rings and the ANTHR moieties on the ligand DPA, the pyridine rings were rotated up to 55.87° and 60.30° with respect to the central ANTHR plane (Fig. 3C).

Fig. 3.

(A) Hydrogen bonds between the metallacycles and OTf⁻ anions. (B) π···π stacking interactions and C−H···π interactions between adjacent metallacycles. (C) Dihedral angles between the pyridine rings and ANTHR moieties.

To reveal the optical properties of the twisted structure, the absorption and emission spectra of both precursor 2 and metallacycle 1 in DCM, acetone, and ethanol were monitored. Both the absorption and emission spectra were normalized for wavelength shift comparison. As shown in the ultraviolet−visible (UV−Vis) spectra in Fig. 4A, in acetone (green dashed line), the absorption peaks at 353 nm (0−2 transition), 372 nm (0−1 transition) and 392 nm (0−0 transition); in ethanol (blue dashed line), the absorption peaks at 353 nm (0−2 transition), 372 nm (0−1 transition) and 391 nm (0−0 transition). However, different from the absorption in acetone and in ethanol, only a broad band with a peak at 404 nm was observed in DCM (orange dashed line). To be sure of the origin of the absorption change in DCM, the absorption of precursor 2 in the three solvents was also recorded. As shown in the UV−Vis spectra in SI Appendix, Fig. S6A, almost identical absorption peaks at 352, 370, and 390 nm were recorded in acetone and ethanol. In DCM, the peaks slightly redshift to 353, 371, and 392 nm. Nevertheless, they had similar absorption bands. Therefore, the unusual broad absorptions of 1 in DCM most likely originate from the twist structure and resultant fast rotation in good solvents. The emission spectra of 1 and the precursor 2 in different solvents are shown in Fig. 4A and SI Appendix, Fig. S6B. It should be noted that the emission of 1 is located at 434 nm, with a small shoulder peak (410 nm) in acetone (green solid line in Fig. 4A). Different from the spectra in acetone, 1 in ethanol (blue solid line) and DCM (orange solid line) shows only two emission peaks at 447 and 466 nm, respectively (Fig. 4A). These observed emission bands in the region with wavelengths of <500 nm are due to a spin-allowed transition, and the emissions are thus due to fluorescence (53, 54). The VT NMR data reveal the fluxionality of the ANTHR in DCM, therefore the different spectra might be caused by the different solubility of 1 in different solvents, which limits the twist of the ANTHR in a specific solvent.

Fig. 4.

(A) Normalized UV absorption spectra and fluorescence emission spectra of 1 (10 μM) in different solvents. (B) Normalized UV absorption spectra and fluorescence emission spectra of 1 (10 μM) in DCM/EA mixtures with different fractions of EA. (C and D) CIE diagram of 1 in (C) DCM and (D) DCM/EA mixtures with different fractions of EA. (E) STEM image and EDX mapping images of 1-based self-assemblies in a DCM/EA mixture (80% EA).

This solubility-caused spectral change is further demonstrated by the absorption and emission spectra in DCM/ethyl acetate (EA) with a change in EA ratio (Fig. 4B). For 1 in pure DCM, the maximum absorption peak is located at 404 nm. At an EA ratio of 20%, the 404-nm absorption is blue shifted to 395 nm. When the EA ratio is 40%, two new peaks at 375 and 355 nm appear. Moreover, the absorption at 391 nm (0−0 transition) continues to increase. With the EA content increased to 60%, all the peak intensities at 391 nm (0−0 transition), 375 nm (0−1 transition), and 355 nm (0−2 transition) increase (SI Appendix, Fig. S8). The reason for the change of absorption bands and increasing emission intensity is that the addition of EA induces the aggregation of 1 in solution and thus introduces further restrictions to the conformational change of the metallacycle and decreases the fluxionality of 1 in solution (42). Similar wavelength blueshift was also observed in the fluorescence spectra. The emission blue shifted from 466 nm (pure DCM) to 464 nm (20% EA), then to 461 nm (40% EA), and to 459 nm (60% EA). Moreover, a new peak at 409 nm appears, and the intensity increases when the EA content increases (Fig. 4B). As shown in Fig. 4C, the Commission Internationale de l’Éclairage (CIE) diagram shows the color of 1 in DCM and the gradual blueshift with decreases in the DCM content (Fig. 4D). The CIE analysis shows emission colors of (0.129, 0.262) in pure DCM, and the color changes from (0131, 0.239) to (0.135, 0.197) to (0.139, 0.155) as the EA content increases (from 20% to 40%, then to 60%).

To investigate the optical properties systematically, the absorption and emission spectra of 1 in acetone/water mixtures were studied (SI Appendix, Fig. S9). As shown in SI Appendix, Fig. S9A, the absorption spectra of 1 in acetone exhibited three peaks, 353, 372, and 392 nm, and remain unchanged as the water content is increased from 0 to 40%. The disappearance of the shoulder peak was observed in the analysis of the emission spectra in acetone/water. As the water content is changed from 0 to 40%, the two emission peaks at 410 and 434 nm in pure acetone merge into a new peak at 425 nm in a 40% acetone/water mixture (SI Appendix, Fig. S9A). As the water content is changed from 0 to 40%, an obvious increase in the intensity of the emission was observed, due to the aggregation of 1 (SI Appendix, Fig. S9B). The CIE analysis of 1 in acetone/water shows a change in the emission color from (0.148, 0.074) to (0.155, 0.037) as the water content increases (SI Appendix, Fig. S10).

Subsequently, the aggregation behavior of 1 was investigated by transmission electron microscopy (TEM) and scanning TEM (STEM). 1-based assemblies were obtained by the assembly of 1 in DCM/EA. The overall STEM images of the 1-based assemblies in a DCM/EA mixture are shown in SI Appendix, Fig. S11. The images indicate that nanospheres of ∼100 nm were formed. Energy-dispersive X-ray (EDX) mapping analysis confirmed the homogeneous distributions of N, Pt, P, F, S, and O across the spheres and provided evidence showing that the spheres were composed of ANTHR, cis-Pt-(PEt₃)₂(OTf)₂, and carboxylate ligands (Fig. 4E). These results prove the formation of nanospheres by the assembly of ANTHR-based platinum metallacycles. The observation of aggregates further suggested that the twisted cycle and its higher clusters in solution led to fluorescent enhancement. STEM observations obtained after EA was changed to another solvent, n-pentane, revealed the formation of cluster-like assemblies in the DCM/n-pentane mixture with 80% n-pentane (Fig. 5 A and B). The distributions of N, Pt, P, F, S, and O in the assemblies were verified by EDX mapping analysis (Fig. 5C), and the results confirmed that the assemblies were composed of metallacycle 1. The magnification of the assemblies indicated a cluster with uniform size (Fig. 5D and SI Appendix, Fig. S12), and the histograms of the size distribution of these clusters are illustrated in Fig. 5E. As revealed by the histograms, the average diameter and SD were 0.88 ± 0.16 μm. Furthermore, a sheet-based intermediate was observed (Fig. 5F), which demonstrated that sheets were the origin of the resultant clusters. For further investigations, highly polar protic solvent water was added to acetone to induce the self-assembly of 1. The self-assembly behavior of 1 in acetone/water mixtures was studied by TEM, which revealed the formation of irregular ANTHR-based assemblies in acetone/water with 80% water (SI Appendix, Fig. S13).

Fig. 5.

(A and B) STEM images, (C) corresponding element distribution, (D and F) magnification of TEM images, and (E) histogram of the size distribution of 1-based self-assemblies in a DCM/n-pentane mixture (80% n-pentane).

Then, to get further insight into the self-assembly behaviors of 1, dynamic light scattering (DLS) was used to monitor the aggregate size. Given that the size of 1 calculated from the crystal structure is ca. 1.5 nm (SI Appendix, Fig. S14 A and B), the DLS results indicate that most of the metallacycles assemble into large aggregates in solution. For metallacycle 1 in the DCM/EA system, the average hydrodynamic diameter of aggregates increased from 51 nm to 218 nm with an increasing fraction of EA from 20 to 80% (SI Appendix, Fig. S14C), demonstrating that the size of the nanospheres increased when the EA content increased. In addition, the size observed is in good agreement with the aggregates in Fig. 4E. Similar results were observed in DCM/n-pentane mixtures. The increasing n-pentane fraction induces the formation of larger self-assembly, with a mean size of 274 nm (SI Appendix, Fig. S14D). This is in accordance with the TEM observation that the cluster-like assemblies originate from sheet-based intermediates. In acetone/water mixture with 20% water, bimodal signals were detected with mean sizes of 85 and 319 nm (SI Appendix, Fig. S14E). Upon the addition of water, the smaller aggregates further assembled to form the larger aggregates, and the bimodal peak fused to form a unimodal one.

As mentioned above, this metallacycle with a twisted structure shows both tunable emissions and self-assembly behaviors. Considering the highly twisted structure revealed in this study, in the future, by overcoming the strong π···π stacking interactions between ANTHR groups, we will focus on the design of ligands that can induce inversion of the twist, making them a unique candidate for sensing and bioimaging applications.

Conclusion

This study reveals the importance of the interaction between rigid aromatic moieties in the formation of highly organized suprastructures. In addition, it provides a strategy for the fabrication of ANTHR-containing highly twisted emissive metallacycle stabilized by π···π stacking interactions. Meanwhile, the fluorescence emission and aggregate morphology of the resultant metallacycle can be systematically tuned by changing the binary solvent systems. As mentioned above, this work will open up avenues for developing emissive ANTH-based multifunctional materials via weak intermolecular interactions.

Materials and Methods

All reagents were commercially available and used as supplied without further purification. Deuterated solvents were purchased from Cambridge Isotope Laboratory. Compounds 2, 3, and 4 were prepared according to modified procedures detailed in the literature. NMR spectra were recorded using a spectrometer at room temperature, and ¹H NMR spectra were recorded in the designated solvents using a Varian Inova 400-MHz spectrometer. The DOSY NMR spectra and variable-temperature NMR spectra were recorded on a Varian Inova 500-MHz spectrometer. The ³¹P{¹H} NMR spectra were obtained using on a Varian Unity 300 NMR spectrometer, and the ³¹P{¹H} NMR chemical shifts were referenced to an external unlocked sample of 85% H₃PO₄ (δ 0.0 ppm). To minimize the interpretation of solid particulates to the optical characterization, the data measured from turbid solutions were not recorded. The TEM investigations were performed with a JEM-2100EX instrument. For TEM, dispersions of the assemblies were dried on carbon-coated copper support grids. High resolution TEM images were obtained using a Tecnai G2 F30 S-TWIN instrument. A Zeiss Supra55 field-emission scanning electron microscope was used to investigate the assemblies. For SEM, dispersions of the assemblies were dried on silicon wafers. Size distribution was measured by a ZetaPALS analyzer (Brookhaven).

Data Availability

All study data are included in the article and SI Appendix.