Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Luminescence Enhancement of cis-[Ru(bpy)2(py)2]2+ via Confinement within a Metal−Organic Framework Daniel Micheroni,† Zekai Lin,† Yu-Sheng Chen,‡ and Wenbin Lin*,† †
Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States ChemMatCARS, The University of Chicago, Chicago, Illinois 60637, United States
‡
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S Supporting Information *
charged species by using a lattice of the opposite charge, resulting in a densely packed system of a species of interest.21−24 Herein we report the use of an anionic Zn− oxalate MOF to entrap the photolabile [Ru(bpy)2(py)2]2+ compound, thereby allowing detailed photophysical characterization of the encapsulated [Ru(bpy)2(py)2]2+ species. By shutting down the ligand exchange process, encapsulation of [Ru(bpy)2(py)2]2+ in MOF cavities significantly increases the lifetimes of emissive states to drastically enhance its luminescence quantum yield. [Ru(bpy)2(py)2]2+ encapsulated Zn−oxalate MOF (Ru@ MOF) was synthesized in a solvothermal reaction. A mixture of [Ru(bpy)2(py)2][PF6], oxalic acid, Zn(NO3)2·6H2O, and hydrochloric acid in nitrogen saturated dimethylformamide (DMF) was heated at 70 °C under nitrogen for 24 h to yield a highly crystalline product of the formula [Ru(bpy)2(py)2][Zn2(C2O4)3]. Ru@MOF crystallizes in the cubic space group P4132. Zn2+ ions are coordinated with six O atoms from three oxalates with each oxalate bridging two Zn2+ ions to form an infinite 3-D anionic framework (Figure 1a).11,25 The [Ru(bpy)2(py)2]2+ cations are encapsulated in the cavities of the anionic Zn−oxalate framework to afford a neutral host−guest system. Experimental powder X-ray diffraction (PXRD) patterns of Ru@MOF matched that simulated from the single crystal structure very well (Figure 1c), indicating phase purity of Ru@MOF samples. Space-filling models indicate a tight fit between the steric bulk of [Ru(bpy)2(py)2]2+ and the cavity in the Zn2(C2O4)32− framework (Figure 1b and Figure S1, Supporting Information), leading to a high calculated density of 1.79 g/cm3 and leaving no room for trapping solvent molecules. The tight fit between the host framework and guest [Ru(bpy)2(py)2]2+ cations is supported by the thermogravimetric analysis (TGA) result, which showed negligible weight loss between room temperature and the decomposition of Ru@MOF at ∼390 °C (Figure S3a). TGA showed a weight loss of 68.9% at 360−390 °C, matching well with the calculated weight loss of 67.4% for the complete conversion of Ru@MOF to RuO2 and ZnO. Nitrogen sorption studies indicated that Ru@MOF is nonporous with a negligible Brunauer−Emmett−Teller (BET) surface area of 5.76 m2·g−1 (Figure S3), further supporting the tight fit between the host framework and gust [Ru(bpy)2(py)2]2+ cations in Ru@MOF. Ru@MOF was next characterized by steady-state absorption and emission spectroscopies. The absorption spectrum of
ABSTRACT: We report the synthesis, characterization, and photophysical and photochemical properties of [Ru(bpy)2(py)2]2+@Zn−oxalate metal−organic framework (Ru@MOF; bpy is 2,2′-bipyridine and py is pyridine). In Ru@MOF, the cavities of the anionic Zn− oxalate MOF tightly encapsulate [Ru(bpy)2(py)2]2+ complexes, thereby altering the vibrational and electronic states of [Ru(bpy)2(py)2]2+ and preventing photosubstitution of py ligands. [Ru(bpy)2(py)2]2+ in Ru@MOF exhibits significantly increased photoluminescence lifetime and quantum yield, likely through destabilizing the dd state and enhancing photochemical stability.
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etal−polypyridyl compounds, in particular those of Ru and Ir, are among the most studied molecular photosensitizers because they absorb strongly in the visible spectrum and undergo both oxidative and reductive chemistry from their long-lived triplet metal-to-ligand charge transfer (3MLCT) excited states.1 As a result of their unique photophysical and photochemical properties, these compounds have been extensively studied for photocatalysis,2,3 photoelectrochemistry,4 dye-sensitized solar cells,5,6 artificial photosynthesis,7 and other applications.8,9 The tremendous utility of metal−polypyridyl compounds has in turn sparked numerous studies on the fundamentals of their photoexcited states. One of the main factors limiting photophysical and photochemical properties of Ru-polypyridyl compounds is the thermally accessible transition to the dd state from the singlet metal-to-ligand charge transfer (1MLCT) state. The dd state or metal centered excited state is nonemissive and provides thermal decomposition pathways in the excited state, thereby limiting the overall quantum yield and photostability of the ruthenium species. In particular, the [Ru(bpy)2(py)2]2+ complex (where bpy = 2,2′-bipyridine and py = pyridine) undergoes a rapid transition to the dd state at a rate of up to 8.7 × 106 s−1 when irradiated,10 leading to the cleavage of the metal−pyridine bonds and photosubstitution by coordinating solvents and other ligands. As a result, the photophysical property of [Ru(bpy)2(py)2]2+ is inherently difficult to study due to rapid photosubstitution of py ligands to result in a mixture of different Ru compounds. Metal−organic frameworks (MOFs) have been recently shown to provide a highly tunable platform to study photophysical phenomena11−19 and reactive photocatalytic intermediates.20 Additionally, MOFs can drive the uptake of © XXXX American Chemical Society
Received: February 10, 2019
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DOI: 10.1021/acs.inorgchem.9b00396 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 2. (a) Emission spectra of Ru@MOF and [Ru(bpy)2(py)2]2+ at room temperature. (b) Temperature-dependent PL spectra of Ru@ MOF in an alcoholic suspension (ethanol/methanol 4/1 v/v) upon excitation at 450 nm. (c,d) Time-resolved fluorescence of a DMF dispersion of Ru@MOF (c) and a DMF solution of [Ru(bpy)2(py)2](PF6)2 (d) when excited at 455 nm.
Figure 1. (a) Structure of Ru@MOF showing encapsulation of [Ru(bpy)2(py)2]2+ in anionic 3D [Zn2(ox)3]2− framework. (b) Space filling model showing tight fit between [Ru(bpy)2(py)2]2+ and the anionic cage of 3D [Zn2(ox)3]2− framework. Red = oxygen, blue = nitrogen, gray = carbon, orange = ruthenium, green = zinc, and white = hydrogen. (c) PXRD pattern of Ru@MOF matches that simulated from the single crystal structure.
593 ± 50, 641 ± 75, and 690 ± 100 nm were probed. As shown in Table 1, lower energy emissions have much longer
entrapped [Ru(bpy)2(py)2]2+ in Ru@MOF resembles that of the solution species with slight red shifts for the absorption maxima (Figure S4). A broad peak with a maximum at 491 nm is attributed to the 1MLCT to the bpy ligands, while another strong peak at 369 nm corresponds to the 1MLCT to the py ligands. Steady-state emission spectra of Ru@MOF show three distinct emission maxima at 585, 639, and 682 nm, which are not visible in the emission spectra of the [Ru(bpy)2(py)2]2+ species in solution (Figures 2). These peaks are attributed to the vibrational progressions of the [Ru(bpy)2(py)2]2+ molecule encapsulated in the MOF.11 The MOF framework prevents coupling of vibrational modes to solvent molecules and limits rotations, resulting in sharpening of the vibrational progressions. The observed wavelengths of the vibrational modes correlate well with those from theoretical analysis based on Franck−Condon fitting (Figure S5).10 We next measured the emission spectra of Ru@MOF at decreasing temperatures down to 140 K (Figure 2b). It is known that at around 200 K the transition from the 3MLCT state to the dd state becomes thermally inaccessible, thereby greatly increasing the quantum yield of [Ru(bpy)2(py)2]2+.26 Indeed, the emission intensity of [Ru(bpy)2(py)2]2+ in Ru@ MOF increased as the temperature decreased. Furthermore, as the temperature decreases, the overall and relative intensities of the second vibrational progression (650 nm) greatly increase relative to the peaks at 585 and 690 nm. It was previously hypothesized that the dd state is preferentially entered from this vibrational level and the metal centered state is primarily responsible for the nonemissive deactivation of [Ru(bpy)2(py)2]2+.10,27 The emissive state of [Ru(bpy)2(py)2]2+ encapsulated in Ru@MOF was further analyzed by time-resolved photoluminescence (PL) spectroscopy. At an excitation wavelength of 450 nm, the emission lifetimes of the three PL maxima at
Table 1. Photoluminescence Lifetimes at Varying Emission Wavelengths for Ru@MOF and [Ru(bpy)2(py)2]2+ in DMF Ru@MOF excitation λ (nm) 450 450 450
emission λ (nm)
τ1 (ns)
τ2 (ns)
593 ± 40 10.6 2.06 641 ± 75 111.0 5.17 695 ± 100 169.0 9.71 [Ru(bpy)2(py)2]2+
F-weighted lifetime (ns) 6.0 71.0 133.0
excitation λ (nm)
emission λ (nm)
τ1 (ns)
τ2 (ns)
F-weighted lifetime (ns)
450 450 450
593 ± 40 641 ± 75 695 ± 100
4.99 3.29 3.01
0.036 0.87 1.31
2.53 1.80 1.63
lifetimes, likely due to the occupation of lower energy vibrational modes as a result of energy transfer throughout the Ru@MOF crystal.11,12 Importantly, the lifetimes of the excited states at 641 and 695 nm for the encapsulated [Ru(bpy)2(py)2]2+ are ∼40 and ∼75 times longer than those of [Ru(bpy)2(py)2]2+ in solution and are considerably shorter than the lifetimes of confined [Ru(bpy3)]2+ (Table S2), confirming that the emission is not a result of impurities. The longest observed lifetime component was 1.3 μs for the emission wavelengths at 695 ± 100 nm and can be assigned to phosphorescence from the 3MLCT state. The significant increase in lifetimes observed for [Ru(bpy)2(py)2]2+ in Ru@ MOF is attributed to the confinement effect of the MOF cavities. In a solvent-free matrix with limited rotational, translational, and vibrational freedom, the transition to the dd state is minimized due to a large increase in the preexponential factor of its rate-law,26 minimizing the available nonemissive pathways for the excited state. The destabilization of the dd state due to steric effects results in a lower population B
DOI: 10.1021/acs.inorgchem.9b00396 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry of dark states in Ru@MOF, leading to an 8-fold increase in quantum yield as determined from the integration of the photoluminescence spectra of Ru@MOF and [Ru(bpy)2(py)2]2+ solutions (Figure S6). The long-lived excited state allows us to test the stabilization of the [Ru(bpy)2(py)2]2+ photophore in Ru@MOF by photoX-ray crystallography (photo-XTAL), as the excited state lifetimes are longer than the time interval between photon packets at the ChemMatCARS at Advanced Photon Source, Argonne National Laboratory. Photo-XTAL experiments were conducted using a 450 nm laser at varying powers and the thinnest single crystal of Ru@MOF available. Prior to all experiments, the crystal was kept in the dark for 1 h to minimize the population of photoexcited states. Upon irradiation of [Ru(bpy)2(py)2]2+ in solution, the Ru−N (py) distances were previously calculated to be 2.18 and 2.80 Å for the 3MLCT and dd states, respectively, while the Ru−N (bpy) distances were calculated to be 2.03 and 2.37 Å for the 3MLCT and dd states, respectively.28 The photo-XTAL results of the irradiated Ru@MOF compared to that of the dark scans show no change in the structures.29 As shown in Table 2, the average
Figure 3. (a,b) Absorption spectra of Ru@MOF dispersions (a) and [Ru(bpy)2(py)2]2+ solution (b) after irradiation in the presence of TBACl for different lengths of time. The spectra after 5 and 10 min of irradiation completely overlap with each other. (c,d) Emission spectra of Ru@MOF dispersions (c) and [Ru(bpy)2(py)2]2+ solution (d) after irradiation in the presence of TBACl for different lengths of time. The solvent is ethanol/methanol (4/1 v/v) and excitation wavelength is 455 nm.
Table 2. Ru−N Bond Lengths of Ru@MOF upon Irradiation at 450 nm As Measured by Photo-X-ray Crystallography laser power (mW) 0 2.7 4.5
Ru−N bond length at 100 K Ru−N bond length at 200 K (Å) (Å) 2.068(11) 2.051(6) 2.070(11)
2.070(11) 2.056(6) 2.067(9)
in PL lifetime, quantum yield, and photostability of the [Ru(bpy)2(py)2]2+ complex in Ru@MOF. Other photolabile molecules can be entrapped in MOF cavities to enable their detailed photophysical and photochemical characterization and to improve their luminescence property and photochemical stability.
Ru−N distances are all within experimental errors and significantly shorter than that calculated for the Ru−N (py) in the dd state, indicating the inability for [Ru(bpy)2(py)2]2+ in Ru@MOF to enter its dd state due to the confinement effect. Furthermore, Ru@MOF maintained its single crystalline structure even after 5 h of irradiation with direct laser light at 4.5 mW, attesting to the stabilization of [Ru(bpy)2(py)2]2+ photophore in the MOF cavities. Photosubstitution of [Ru(bpy)2(py)2]2+ in Ru@MOF was determined by measuring its absorbance after irradiation with an intense white light source in the presence of tetrabutylammonium chloride (TBACl) as a chloride source (Figure 3a,b). The photosubstitution of [Ru(bpy)2(py)2]2+ solution in DMF was nearly complete in 1 min and complete in 5 min. In contrast, the absorption of [Ru(bpy)2(py)2]2+ at 480 remained essentially unchanged after Ru@MOF was irradiated under the same conditions for 30 min, with only a slight shoulder emerging at 525 nm, indicating photosubstitution of [Ru(bpy)2 (py) 2 ]2+ on the surface of the MOF material. Luminescence stability under irradiation was also shown by emission spectra. After 5 min of irradiation, Ru@MOF retained 96.1% of its original emission intensity while the [Ru(bpy)2(py)2]2+ solution showed
