Self-assembly of highly luminescent heteronuclear coordination cages

Abstract

Exo-functionalized Pd₂L₄ cage compounds with attached Ru(II) pyridine complexes were prepared via coordination-driven self-assembly. Unlike most of the previously reported palladium(II) cages, one of these metallocages exhibits an exceptionally high quantum yield of 66%. The presented approach is promising to obtain luminescent coordination complexes for various applications.

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Metal-mediated self-assembly is a useful tool to design discrete two- and three-dimensional supramolecular coordination complexes (SCCs) with precise geometries and cavities.¹ These metal-based entities have attracted much attention for a variety of applications in molecular recognition,² catalysis³ and medicinal applications⁴ due to their interesting chemical–physical properties and guest-binding abilities. Especially, the development of luminescent SCCs for potential applications in chemosensing,⁵ material science^6,7 and biological imaging^8,9 has gained increasing attention during the last years,¹⁰ although it is still less explored. Despite the existence of some highly fluorescent coordination complexes,^6,11 the majority of metal-based self-assemblies are little- or non-emissive due to the quenching effect of heavy metal ions.¹²

An interesting research field of SCCs is the self-assembly of M₂L₄ (M = metal, L = ligand) cages because of their simple and highly symmetric structures.¹³ In addition, the cages’ properties can be easily altered by functionalizing the ligand framework.¹⁴ Emissive properties of M₂L₄ metallocages have been discussed, yet examples of highly emissive Pd₂L₄ cages are rare.¹⁵ The incorporation of luminescent groups, such as anthracene^16,17 and ruthenium pyridine complexes,¹⁸ into the ligand framework resulted in palladium cages displaying low emission so far.

Nevertheless, these results generate an increasing interest in tailored design of highly luminescent coordination cages. In this work, an approach is presented to increase the photo-physical properties of palladium cages by separating the luminescent tag from the emissive ligand coordinated to palladium ions. Inspired by previous investigations,¹⁷ two Pd₂L₄ cage compounds ligated by bis(pyridyl) systems coupled to ruthenium complexes were synthesized and their photo-physical properties were investigated. A comparison is made between the Ru terpyridine ligand L1 having no spacer and the ruthenium bipyridine ligand L2 featuring an alkyl bridge as spacer between two emissive moieties.

First, the rigid bis(pyridyl) ligands L1 and L2 coupled to Ru(II) terpyridine and Ru(II) bipyridine, respectively, were synthesized via an amide bond formation (Scheme 1).

Scheme 1 Coupling of the ligand L-NH2 with Ru(II) complexes R1/R2 using the reagent CMPI, followed by precipitation with NaBF₄ to obtain Ru(II)-based ligands L1/L2.

The amine-based ligand L-NH₂ was coupled to [Ru(terpy)(terpy-4-COOH)](PF₆)₂R1 and [Ru(bipy)₂(bipy-4′-CH₃-4-(CH₂)₂-COOH)](PF₆)₂R2 using the coupling reagent 2-chloro-1-methylpyridinium iodide (CMPI) and DMAP as a base. After purification by column chromatography, the Ru(II) complexes L1 and L2 were precipitated by NaBF₄ in 68% yield as red solid and in 56% yield as orange solid, respectively. The complexes were characterized by ¹H, ¹³C, ¹¹B, ¹⁹F, and DOSY NMR spectroscopy, ESI-MS and X-ray crystallography (for details see ESI†).

The coordination cages C1/C2 were self-assembled by mixing the bidentate Ru(II)-based ligands L1/L2 and the palladium precursor [Pd(NCCH₃)₄](BF₄)₂ in a 2 : 1 ligand : metal ratio in DMSO at room temperature for one hour (Scheme 2). Additionally, the self-assembly of the previously described cage C-NH₂ ⁸ is depicted in Scheme 2, in order to evaluate the synthesis and photo-physical properties of the cage compounds C1 and C2 compared to the amine-based cage. Notably, the bulky ruthenium complexes have no effect on the self-assembly reaction.

Scheme 2 Synthesis of the palladium(II) cages C-NH₂,⁸C1 and C2via self-assembly using the bidentate ligands L-NH₂, L1 and L2 and the precursor [Pd(NCCH₃)₄](BF₄)₂.

¹H NMR spectroscopy confirms the formation of the cage compounds. In ¹H NMR spectra (Fig. 1), the pyridyl protons H_a–H_d are significantly downfield shifted, particularly the signals of H_a and H_b experienced a shift of ca. 0.9 ppm. The terpyridine and bipyridine proton resonances of the attached ruthenium complexes are not influenced by the Pd–N coordination.

Fig. 1 Stacked ¹H NMR spectra (400 MHz, CD₃CN) of ligand L1 (a), cage C1 (b), ligand L2 (c) and cage C2 (d).

Additional proof of the successful self-assembly in solution is given by diffusion-disordered NMR spectroscopy (DOSY), since all proton signals of the cages reveal the same diffusion coefficient. The diffusion coefficients (D) of the ligands L1 and L2 and of the cages C1 and C2 in acetonitrile are approximately 6.9 × 10⁻¹⁰ m² s⁻¹ and 3.3 × 10⁻¹⁰ m² s⁻¹, respectively (see Table S1, ESI†). Thus, the ratios of D_ligand/D_cage are approximately 2 : 1, being in accordance with reported Pd₂L₄ systems.^8,19 The hydrodynamic radii r_s of C1 and C2 have been calculated to be 1.5 nm.

The molecular composition of the Pd₂L₄ cages C1 and C2 is further evidenced by ESI mass spectrometry showing isotopically resolved peaks for [C-nBF₄⁻]ⁿ⁺ (n = 4–6). For example, the ESI-MS analysis of cage C2 reveals peaks at m/z = 744.3, 910.6 and 1160.3, which can be assigned to [C2-6BF₄⁻]⁶⁺, [C2-5BF₄⁻]⁵⁺ and [C2-4BF₄⁻]⁴⁺, respectively.

In order to predict the shape and size of the cages, a geometry optimization was performed using semi-empirical methods (PM6). Exemplarily, the molecular model of C2 is depicted in Fig. 2. The optimized structure of C2 exhibit a Pd⋯Pd distance of 1.1 nm, a distance between the opposing inner C-atoms of 1.2 nm and a span of 5.0 nm. The calculated shape and size is in agreement with previously reported Pd₂L₄ cages.^8,17 Suitable single crystals of the metallocages for X-ray diffraction could not be obtained.

Fig. 2 Molecular model of cage C2 (C grey, N blue, O red, Pd turquoise, Ru green).

Both palladium(II) cages are stable under air and light in solution and in solid state. The compounds are soluble in acetonitrile, DMF and DMSO.

In order to assess the photo-physical properties of the metallocages with attached ruthenium(II) moieties, UV-Vis, excitation and emission spectroscopy were carried out on the Ru(II) complexes R1/R2, the ligands L1/L2/L-NH₂ and the cages C1/C2/C-NH₂. The absorption and emission spectra of the compounds are depicted in Fig. 3 and 4, while the photo-physical parameters are presented in Table 1.

Fig. 3 UV-Vis spectra of ligands and cage compounds in DMSO (c = 10⁻⁵–10⁻⁶ M). Insets: Photographs of DMSO solutions of the cages.

Fig. 4 Emission spectra of ligands and cage compounds in DMSO (c = 10⁻⁵ M, λ_ex = 260 nm). Insets: Photographs of solutions of the cages in DMSO under UV light irradiation (λ_ex = 365 nm).

Table 1 Photo-physical parameters of ruthenium complexes, ligands and palladium cages (DMSO, λ_ex = 260 nm)

Compound	λ max (abs) [nm]	ε max [L mol^−1 cm^−1]	λ max (em) [nm]	Φ [%]
R1	278, 317, 492	58 400	—	—
R2	292, 456	79 400	645	12
L-NH2	293, 305, 360	52 200	430	52
C-NH2	293, 371	152 200	435	17
L1	290, 303, 493	64 600	—	—
C1	289, 311, 494	232 200	—	—
L2	293, 461	134 000	640	88
C2	293, 462	523 900	640	66

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The absorption spectra of the metallocages are dominated by strong π–π* transitions of the highly conjugated ligands showing bands in the range of 250–350 nm. The UV-Vis spectra of the cages with conjugated ruthenium complexes exhibit an additional band in the vis region, C1 (red solution) at 495 nm and C2 (orange solution) at 460 nm. Overall, the cage compounds feature an approximately four-times higher extinction coefficient compared to their corresponding ligands resulting from the M₂L₄ composition.

The metallocages reveal interesting emissive properties, showing that the luminescence can be increased or decreased by altering the molecular structure of the ligand framework. Recently, we investigated the photo-physical properties of bis(pyridyl) ligands coupled to naphthalene and anthracene moieties via an amide bond.¹⁷ These systems possess less emissive properties due to a disruption of the chromophoric system in the excited state by bending the amide bond.

As expected, ligand L1 and the respective cage C1 are not luminescent, although the amine ligand L-NH₂ is highly emissive by itself. Notably, the red solution of R1 is not luminescent at room temperature being in accordance with reports on similar ruthenium(II) terpyridine complexes.²⁰

To avoid the predicted torsion of the amide bond, a spacer, namely an alkyl bridge, was inserted between the bis(pyridyl) ligand and the ruthenium moiety. Upon irradiation at 260 nm, ligand L2 emits strong orange luminescence showing a broad band in the emission spectrum at λ_max = 640 nm with an exceptional high quantum yield of 88%. However, by irradiation at lower energies at 460 or 495 nm the quantum yield is significantly reduced to 6 and 4%, respectively. The amine ligand L-NH₂ shows blue fluorescence at λ_max = 430 nm with a quantum yield of 52%. Interestingly, cage C2 exhibits one of the highest quantum yields (Φ = 66%) at λ_max = 640 nm reported for supramolecular coordination complexes.^6,11a,b The coordination cage C-NH₂ features a fluorescence quantum yield of 17%. In agreement with previous reports, in both cases C2 and C-NH₂ the luminescence is significantly reduced by coordination of the ligand to palladium ions. Notably, cage C2 displays a higher emission compared to the amine-based cage, while cage C1 exhibits lower luminescence.

In summary, two palladium(II) coordination cages coupled to ruthenium(II) pyridine complexes via an amide bond have been synthesized by self-assembly. In order to obtain bright luminescence, the ruthenium complex was separated from the coordinating bis(pyridyl) ligand using an alkyl spacer. The photo-physical properties of the Pd₂L₄ cage coupled to a ruthenium complex with and without spacer were compared. Remarkably, the palladium cage without spacer is non-emissive, while the other one features a quite high quantum yield of 66%, making it one of the highest luminescent metallosupramolecular complexes known to date. The applied approach is promising to further design highly emissive metallocages for potential applications as biological labels and chemosensors, among others.

A. S. and M. H. are grateful for the financial support of the TUM Graduate School of Chemistry. A. C. acknowledges support from Cardiff University and the August–Wilhelm Scheer Visiting Professorship at the Technical University of Munich. Authors acknowledge the support of the Technische Universität München – Institute for Advanced Study, funded by the German Excellence Initiative and the European Union Seventh Framework Program under grant agreement no. 291763. Authors thank the Chinese Scholarship Council for a PhD fellowship to J. H. Authors thank Christian Jandl for measuring the emission spectra. Dr. Alexander Pöthig's support with crystallographic data is greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR spectra and crystallographic details. CCDC 1484108. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt02708c

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