Article pubs.acs.org/IC
Luminescent Cages: Pendant Emissive Units on [Pd2L4]4+ “Click” Cages Anastasia B. S. Elliott, James E. M. Lewis, Holly van der Salm, C. John McAdam, James D. Crowley,* and Keith C. Gordon* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand S Supporting Information *
ABSTRACT: The photophysics of a family of exo-functionalized [Pd2L4]4+ metallo-supramolecular cage architectures constructed from a tripyridyl 1,2,3-triazole backbone are reported. Several spectroscopic techniques are employed including both electronic (steady-state and transient absorption and emission) and vibrational (resonant and nonresonant Raman) methods. These experimental results are interpreted alongside simulated results from density functional theory calculations of the system’s vibrational and electronic properties. The ligands and cages are shown to be essentially insulated from the exo-functionalization. They exhibit electronic transitions in the UV region and excited-state properties that are little affected by formation of the cage. Upon functionalization, characteristic Raman bands, electronic transitions, and emission bands associated with, and confined to, the substituent are observed.
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INTRODUCTION Metallo-supramolecular architecturesdiscrete polyhedral structures assembled from metal ions and ligandshave become a burgeoning area of research in the last couple of decades.1−10 This is due to the wide range of properties exhibited by these species. In particular, their molecular recognition,4,11−19 catalytic,20−28 and biological29−46 properties have been extensively examined. More recently, a range of photophysically interesting architectures have been synthesized and examined. These have included systems that incorporate photophysically active metal ions as structural units within the architecture framework,47−52 as well as those that employ luminescent organic spacer units within the ligand scaffold.53−57 These species offer the potential for a new generation of molecular sensors and photocatalysts, among others. Fujita and co-workers have reported photoinduced electron transfer between encapsulated guest molecules and the electron-deficient triazine-cored ligands of octahedral M6L4 metallo-supramolecular hosts, resulting in oxidation of alkanes58 and anti-Markovnikov hydration of alkynes.59 Furthermore, a tetranuclear bowl assembled from a carbazole-based ligand, synthesized by Duan and co-workers, was found to act as a photosensitizer for an encapsulated hydrogen-evolving catalyst, allowing photocatalytic production of hydrogen.60 While the above synthetic approaches have been successfully exploited to generate photophysically active metallo-supramolecular systems, they have limitations. The direct incorporation of new photoactive organic units into the ligand scaffold © XXXX American Chemical Society
can make the ligands synthetically difficult to access and potentially interfere with either the self-assembly process or the molecular recognition properties of the system. Additionally, coordination of the metal ions to the ligand may result in inhibition of the desired functionality. Yoshizawa and coworkers developed a dipyridyl ligand incorporating two anthracenyl moieties, imbuing it with fluorescent properties. However, coordination of palladium(II) ions to the ligand, generating the desired Pd2L4 cage species, completely quenched the emissive nature of the ligand due to its conjugated structure.61 This attenuation of fluorescence upon binding of metal ions has also been reported by others.54,55,62−67 Many of the metal ions that display desirable photophysical properties are second- or third-row transition metals, which are quite kinetically inert. This may result in low yields of the desired assembly due to kinetically inert metal centers impeding formation of thermodynamically favored products. An alternative methodology is to separately append the requisite functional unit to the ligand framework. There have been reports in the literature of functionalized architectures generated from ligands with functional moieties attached to the exohedral68−73 or endohedral22,74−77 faces. Limited examples have also been reported of the postsynthetic modification of inert metallo-supramolecular species.78−80 As part of our work on metallo-supramolecular cages,81−85 we recently reported on the use of the functional group tolerant Received: December 9, 2015
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DOI: 10.1021/acs.inorgchem.5b02843 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Scheme 1. Synthesisa of the Ligands (tripyr-py and tripyrbpy) and the Metalloligands (tripyr-Rupy(bpy)2 and tripyrRu(bpy)3)
copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) “click” reaction to generate a range of exo-functionalized ligands. These were found to retain the self-assembly ability of the parent system and to form Pd2L4 cage architectures. The cages were able to act as host molecules for cisplatin guests but were endowed with a variety of functionalities.86,87 Author: In this study we show that it is possible to append, using the CuAAC click reaction, triplet-state metal-based emitters, based on [Re(CO)3 Cl] and [Ru(2,2′-bpy) n ]2+ (where bpy = 2,2′-bipyridine) moieties, to these cages and singlet organic emitters, based on phenyl and naphthalimide functional groups, while essentially retaining the electronic properties of the emitter unit (Figure 1). This is manifest in the
a
Reaction conditions (i) either 2-ethynylpyridine or 5-ethynyl-2,2′bipyridine (1 equiv), CuSO4·5H2O (0.5 equiv), sodium ascorbate (1 equiv), DMF, room temperature, 20 h; (ii) (a) cis-[Ru(2,2′-bpy)2Cls] (1 equiv), ethanol, 125 °C, microwave irradiation (200 W), 1 h, (b) NH4PF6 (10 equiv), ethanol, room temperature, 1 h.
procedure (Scheme 1).88 The organic precursors (tripyr-py and tripyr-bpy) were irradiated (200 W) with cis-[Ru(2,2′bipyridine)2Cl2] at 125 °C in ethanol for 1 h, followed by anion metathesis through addition of solid NH4PF6 to the reaction mixture. This gave tripyr-Rupy(bpy)2 and tripyrRu(bpy)3 as orange and red solids in good to excellent yields (92% and 72%, respectively). The ligands were characterized by 1H, 1H DOSY, and 13C NMR, IR and UV−vis spectroscopies, mass spectrometry, and elemental analysis. Exclusive binding of the [Ru(2,2′-bpy)2]2+ moiety within the bidentate pockets of the ligands was confirmed in the 1H NMR spectra of tripyr-Rupy(bpy)2 as a downfield shift of the triazole proton signal (Δδ = 0.57 in deuterated dimethyl sulfoxide (d6-DMSO)), with minimal perturbation of the peaks assigned to the tripyridyl core of the ligand framework seen for both ligands. High-resolution electrospray ionization mass spectrometry (ESI-MS) further confirmed the identity of the metalloligands with isotopically resolved peaks consistent with the formulations [tripyrRu(bpy)3]2+ and [tripyr-Rupy(bpy)2]2+ observed at m/z = 426.60 and 465.11, respectively (Supporting Information). X-ray quality crystals of [tripyr-Rupy(bpy)2]2+ and [tripyrRu(bpy)3]2+ (as PF6− salts) were obtained by vapor diffusion of diethyl ether into acetonitrile solutions of the ligands. The molecular structures were found to be as expected, with coordination of the ruthenium(II) ions to two 2,2′-bipyridine ligands, and the remaining sites chelated to the triazolylpyridyl and bipyridyl units of the tripyr ligands, respectively (Figure 2). The dipalladium(II) cages were prepared by simply stirring each of the ligands in acetonitrile with [Pd(CH3CN)4](BF4)2
Figure 1. Ligands (tripy-R) and [(tripy-R)4Pd2]4+ cages examined in this work.
electrochemical, optical, emission, and photophysical behavior of the cages relative to the isolated emitter compounds. This suggests a general strategy for the attachment of emitter systems in which both long-lived triplet-based or short-lived singlet-state emitting units can be appended without deleterious effect on the properties of the emitter; that is, the quantum yield and lifetime of the excited states are retained after incorporation into the cage.
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RESULTS AND DISCUSSION Synthesis and Characterization. The syntheses of ligands tripyr-Ph, -Naph, and -RepyCl(CO)3, and their respective dipalladium(II) cage architectures, have been reported previously.84,87 The organic ligands tripyr-py87 and tripyr-bpy were synthesized from the azide 1 using standard click conditions (see Scheme 1 and Supporting Information).87 The ruthenium(II) metalloligands tripyr-Rupy(bpy)2 and tripyr-Ru(bpy)3 were generated using Schubert’s microwave B
DOI: 10.1021/acs.inorgchem.5b02843 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. Ellipsoid plots of the molecular structures of tripyrRupy(bpy)2 and tripyr-Ru(bpy)3. Counterions and solvent molecules were omitted for clarity. Ellipsoids are shown at 50% probability level. Colors: gray = carbon, blue = nitrogen, teal = ruthenium, white = hydrogen.
Figure 3. Electronic absorption and emission spectra of representative tripyr-R ligand (dashed lines) and (tripyr-R)4Pd2 cages (solid lines), (1 × 10−5 mol L−1 DMSO solutions). Note for the emission data the spectra of tripyr-Naph and (tripyr-Naph)4Pd2 are scaled by 0.1, and those of tripyr-Rupy(bpy)2 and [tripyr-Rupy(bpy)2]4Pd2 are scaled by 5.
and subsequently precipitating to give the assemblies as orange to red solids in good yields (60%−95%). The 1H NMR spectra of the assemblies revealed a downfield shift of the terminal pyridine rings relative to the metalloligands, indicating coordination of palladium(II) at these sites, as we have previously observed for related systems (Supporting Information). The formation of larger species was supported by 1 H DOSY NMR experiments, with the palladium(II) complexes displaying significantly smaller diffusion coefficients (consistent with larger hydrodynamic radii) than the ligands (D = 0.82 vs 1.33 and 0.61 vs 1.24 × 10−10 m2 s−1 for tripyr-Rupy(bpy)2 and tripyr-Ru(bpy)3, respectively). The ESI mass spectra confirmed the formation of the desired dipalladium(II), quadruply stranded cages (Supporting Information). A series of isotopically resolved peaks consistent with the formulation [Pd2L4(X−)12−n]n+ (n = 3−7, X− = PF6− and/or BF4−) were observed for both systems. Unfortunately, despite multiple attempts, X-ray quality crystals were unable to be obtained for any of the cages. Spectroscopic and Electrochemical Properties. There are two levels to the perturbation of the electronic properties of the emitter units; the first is the attachment of the unit to the tripyr ligand, and the second is the incorporation into the Pd cage. The electronic absorption spectra (Figure 3) indicate that the cage and emitter unit chromophores are unconnected; the electrochemistry also suggests minimal perturbation of the emitter unit with incorporation into the cage. However, these are ground-state properties; the critical properties for the emitter units are the photophysical parameters, which are presented in Table 1. Ligand and cage components with weakly emissive pendant groups show virtually no emission (tripyr-Ph and (tripyrPh)4Pd2 in Table 1 are representative examples), while R groups that are inherently emissive imbue the relevant ligand and cage with these properties. Although cage formation decreases intensity relative to the free ligand, the emission band is not shifted. Emission observed for tripyr-Naph and (tripyrNaph)4Pd2 occurs at ∼525 nm, agreeing with a previous study of 4-amino-1,8-naphthalimide, which reports a λem value of 522 nm.89 Likewise, the fluorescence from the ligand and complex with [RepyCl(CO)3] functionalization is in good agreement with literature90 at ∼535 nm as are the Ru(II) containing systems, with λem maxima at ∼638 nm.91,92 Table 1 summarizes the wavelengths for the observed fluorescence bands along with
Table 1. Wavelength Maxima (λem), Quantum Yields (ϕ), and Lifetimes (τem) for Emission Signals from the Compounds of Interest in Degassed Dimethylformamide Solution at 25 °C, λexc = 354.7 nm
a
compound
λem, nm
τem, nsa
ϕ × 10−2 a
tripyr-Ph (tripyr-Ph)4Pd2 tripyr-Naph (tripyr-Naph)4Pd2 tripyr-Ru(bpy)3 (tripyr-Ru(bpy)3)4Pd2 tripyr-Rupy(bpy)2 (tripyr-Rupy(bpy)2)4Pd2 tripyr-RepyCl(CO)3 (tripyr-RepyCl(CO)3)4Pd2
362 362 525 525 638 638 620 620 534 542
