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Tetraphenylethylene-based phosphine: tuneable emission and carbon dioxide fixation† Jianyong Zhang,* Qiuli Yang, Yixuan Zhu, Haoliang Liu, Zhenguo Chi and Cheng-Yong Su A tetraphenylethylene-based phosphine, 1,1,2,2-tetrakis((4-diphenylphosphino)phenyl)ethylene (TPE-P4),
Received 17th June 2014, Accepted 18th August 2014
was synthesized and showed novel aggregation-induced and mechano-responsive emission. A mixture of
DOI: 10.1039/c4dt01808g
TPE-P4 and Ag+ could fix atmospheric CO2 in situ as carbonate ions in neutral solution to yield a rare 3D metal–organic framework with zeolite-like SOD topology, [Ag2(TPE-P4)CO3]x . nH2O (Ag-TPE-P4).
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Ag-TPE-P4 showed turn-on luminescence of TPE-P4, emitting bright bluish green light in the solid state.
Introduction CO2 is the current primary anthropogenic greenhouse gas and elevated CO2 levels in the atmosphere have a discernible influence on global warming and climate change. The chemistry of CO2 fixation and the exploiting of CO2 as a C1 feedstock for conversion into higher carbon-containing compounds have evoked intense interest.1 Fixation of atmospheric CO2 as carbonates is one important strategy. CO2 molecules from the atmosphere have been captured and fixed as CO32− ions by various metal complexes, including Ag,2 Cd,3 Mn,4 Ni,5 Zn,6 Cu,7 Ln8 and other metal complexes.9 Among these discrete metal complexes, transition metal complexes with dinuclear motifs represent a simple model to efficiently encapsulate carbonate ions.5,7 Meanwhile, sedimentary carbonates represent a huge quantity of carbon in the Earth’s lithosphere. Recently Ishihara et al. reported that some minerals (e.g. hydrotalcite-like clay minerals) containing carbonate in their extended frameworks may dynamically participate in the global carbon cycle.10 Thus extended polymeric structures may be important in CO2 fixation. In our continuous efforts to develop phosphine-based polymeric structures,11 we wish to report herein that an extended phosphine-based framework with dinuclear motifs is efficient for the fixation of atmospheric CO2. The framework is based on a tetraphenylethylene-based phosphine, 1,1,2,2-tetrakis((4-diphenylphosphino)phenyl)ethylene (denoted TPE-P4), and Ag+ (Fig. 1).
KLGHEI of Environment and Energy Chemistry, MOE Key Laboratory of Polymeric Composite and Functional Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Tel: +86 20 8411 0539 † Electronic supplementary information (ESI) available: Experimental details, additional spectral data, fluorescence lifetime and quantum yields, TG curve, XRPD patterns and X-ray crystallographic data. CCDC 1006156. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01808g
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Fig. 1 CO2 fixation by a TPE-P4 and Ag+ mixture to form an extended Ag-phosphine framework, Ag-TPE-P4.
The tetraphenylethylene-based phosphine, TPE-P4, was chosen on the basis of the following considerations: (a) TPE-P4 is a bulky phosphine ligand for an individual phosphorous donor, which may show high catalytic activity. Phosphines have been widely used in homogeneous catalysis and coordination chemistry due to their tuneable electronic and steric properties. Their metal compounds show promising luminescent, electronic and optoelectronic properties.12 (b) TPE-P4 is a rigid bridging ligand, which may lead to extended
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structures.11,13 (c) TPE and its derivatives have an unusual fluorescence property of aggregation-induced emission (AIE). AIE materials, firstly reported by Tang et al., emit more efficiently in the aggregated state than in the solvated form.14 TPE and its derivatives are typical AIE compounds and show potential application in organic light-emitting diodes, sensors, biological probes, and so on.15 To the best of our knowledge, this is the first time that an AIE moiety (TPE) has been incorporated into a phosphine ligand, and this incorporation endows TPE-P4 and its metal complex with tuneable emission.
Results and discussion TPE-P4 was synthesized from its corresponding fluoride, 1,1,2,2-tetrakis(4-fluorophenyl)ethylene, by reaction with KPPh2 in THF (Scheme S1†). Its 31P{1H} NMR spectrum showed a single resonance at −6.1 ppm. TPE-P4 emits bright bluish green light with a peak maximum at 450 nm (excitation at 397 nm) in the solid state at room temperature, showing AIE features as expected for TPE derivatives (Fig. S1†). The AIE behavior of TPE-P4 was investigated by monitoring the changes in photoluminescence (PL) intensity with the addition of increasing amounts of water, in a THF–water solvent mixture. THF acts as the solvent and water acts as the anti-solvent. The UV-vis absorption spectra measured in various THF–water mixtures show peaks at around 300 and 340 nm related to the π–π* transitions of the TPE chromophore (Fig. S2†). Because TPE-P4 is insoluble in water, it was expected to aggregate in THF–water mixtures with high water content. From 70% v/v water content, the maximum peaks are red-shifted with leveloff tailing. The spectral changes indicate that nanoparticle suspensions were obtained due to the Mie effect of the nanoparticles.16 The formation of nanoparticles was also evidenced by the Tyndall effect (Fig. S3†). TEM further revealed that the nanoparticles were around 50–100 nm in diameter (Fig. S4†). When TPE-P4 aggregated with the increasing water concentration, the PL intensity increased, reaching a maximum at 90% v/v water content (Fig. 2). The PL intensities in low water
Fig. 2 Photoluminescence (PL) spectra of TPE-P4 in THF–water mixtures with different water fractions and in pure THF (c = 10−5 mol L−1; λex = 350 nm).
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content (0%–60%) were very weak. However, significant PL enhancements were observed when the water fraction increases up to 70 and 90%. The PL intensity was 4 (a.u.) in pure THF and was elevated to 155 in the THF–water mixture with a water fraction of 90%. The fluorescence quantum yields were determined (Fig. S5†). In pure THF, TPE-P4 exhibited a low fluorescence quantum yield (Φ = 0.35%). An obvious and drastic increase in the Φ values was observed when the water fraction of the THF–water mixture was increased to 90%. The Φ value was 4.39% at this composition. This dramatic increase in fluorescence efficiency is attributed to the aggregation of TPE-P4. These results indicate that TPE-P4 has a significant AIE. In addition, TPE-P4 exhibits a mechano-responsive emission.17 As shown above, as-synthesized TPE-P4 exhibited strong bluish green emissions with peak centering at 450 nm, upon exciting with UV light (Fig. S1†). After grinding treatment using a pestle and a mortar, TPE-P4 changed from a white to a pale yellow solid, with strong green emission and with peak centering at 499 nm (Fig. 3 and 4). The variation scope in the emission wavelength of TPE-P4 before and after grinding is 49 nm. After exposing to methanol vapor for 12 h, the ground
Fig. 3 Photos of TPE-P4, (a) taken under ambient light and (b) taken under UV light (365 nm) (samples: left, sample exposed to MeOH vapor; right, ground sample).
Fig. 4 Normalized photoluminescence spectra of TPE-P4: (a) ground sample, (b) sample exposed to MeOH vapor, (c) re-ground sample, (d) sample re-exposed to MeOH vapor (λex = 397 nm) (the inset depicts the reversible changes of PL wavelength through grinding and methanol vapor treatments).
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sample was converted into a new blue emissive state. The emission peak was located at 468 nm, which is red-shifted from that obtained for the as-synthesized sample. After regrinding the sample, the emission peak was centered at 499 nm again. Re-exposing to methanol vapor converted the re-ground sample to the second blue emissive state (λem = 468 nm) again. The results indicate reversible interconversion between blue-emitting (468 nm) and green-emitting (λem = 499 nm) phases on grinding and exposing to methanol vapor (Fig. 3b). The color of the luminescence is switched by the grinding and methanol vapor treatments. The time-resolved emission-decay behaviors of the ground and MeOH-treated samples were investigated and show that there are two relaxation pathways in their fluorescence decays (Table S1†). The weighted mean lifetime of the ground sample (3.06 ns) is longer than that of the exposed sample (2.18 ns). The prolonged lifetime may be attributed to increasing π–π interactions after grinding. X-ray powder diffraction (XRPD) measurements were conducted to further reveal the mechanism of the mechanoresponsive emission of TPE-P4 (Fig. S6†). The as-synthesized and MeOH-treated samples show intense and sharp reflection peaks, indicative of long-range order in the material. Moreover the peaks of the as-synthesized sample are more intense and sharper than those of MeOH-treated samples, suggesting the original crystallinity was only partly restored after the exposure to MeOH vapor. This is consistent with the above emission results. In contrast, the XRPD patterns of the ground samples exhibit weak and broad peaks, characteristic of amorphous materials. These results indicate that the long-range order was destroyed after grinding and that grinding converted the crystalline phase to the amorphous phase. Exposure to MeOH converted the amorphous phase back to the crystalline phase. Similar phase transition could also be triggered by ethanol and tetrahydrofuran. Thus reversible inter-conversion of mechanochromism was achieved and the mechano-responsive emission is related to the phase transition between crystalline and amorphous phases. It is worth mentioning that the emissions are similar in the solution aggregate and in the ground solid (λex = 503 and 499 nm), suggesting that they have similar amorphous features. Interestingly, a mixture of TPE-P4 and AgBF4 in toluene– acetonitrile could fix atmospheric CO2 to yield an extended framework, [Ag2(TPE-P4)CO3]x . nH2O (denoted Ag-TPE-P4). Single-crystal X-ray diffraction analysis revealed that Ag-TPE-P4 ˉc.‡ There is one cryscrystallizes in the trigonal space group R3 tallographically independent Ag atom, half a TPE-P4 ligand and half a CO32− in the asymmetric unit. The carbon atom
‡ Crystallographic data for Ag-TPE-P4: C75H56Ag2O9.22P4 Mw = 1444.37, trigonal, ˉc, a = b = 36.6284(4) Å, c = 27.5127(5) Å, α = β = 90°, γ = 120°, V = 31 966.8(7) R3 Å3, Z = 18, D = 1.351 g cm−3, μ = 5.715 mm−1, λ = 1.54178 Å, T = 150(2) K, F(000) = 13 208, 38 595 reflections were collected (7131 were unique) for 3.50 < θ < 74.01, R(int) = 0.0515, R = 0.1076, wR2 = 0.3131 [I > 2σ(I)], R = 0.1345, wR2 = 0.3455 (all data) for 409 parameters with no restraint, GOF = 1.357. CCDC-1006156.
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(C38) and oxygen atom (O2) of the CO32− moiety lie on a rotoinversion axis. Each TPE-P4 ligand binds four Ag+ ions, using all four P donors. The Ag+ ion is tricoordinate with a trigonal CuP2O coordination geometry. Every Ag+ ion coordinates two P donors from two different ligands and one O donor from CO32− (Ag–P = 2.443(2), 2.457(2) Å, Ag–O = 2.407(7) Å). Each CO32− ion bridges two AgP2 moieties in a butterfly manner and the carbonate anion acts as a η1:η1 bridging ligand (Fig. 5a). The four bulky TPE-P4 ligands stretch in different directions to help release the steric tension. The ∠C– O–C angles in the carbonate unit are close to 120° (122.6(6), 114.9(12) and 109.2(6)°), and the bond distances are 1.221(15), 1.279(10) and 1.279(10) Å. The structure can be considered to consist of large L3Ag6(CO32−)3 triangular planar subunits, which are formed by three bridging TPE-P4 ligands, six Ag+ ions and three CO32− ions (Fig. 5b). Two adjacent triangular subunits are linked by L2Ag4(CO32−)2 rings (Fig. 5c). The adjacent triangular subunits are not located in the same plane, thus stretching into a three-
Fig. 5 X-ray structures of (a) CO32− ion bridging two AgP2 moieties in a butterfly manner, (b) L3Ag6(CO32−)3 triangular planar subunit, (c) two adjacent L3Ag6(CO32−)3 triangular subunits, (d, e) connected triangular subunits of L3Ag6(CO32−)3 in a 3D framework as viewed along the c-axis, (f ) (42·64) zeolitic SOD topological net of Ag-TPE-P4.
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dimensional structure (Fig. 5d and 5e). Considering the Ag+ ions as three-connecting nodes and the ligand TPE-P4 as fourconnecting nodes, the overall structure of Ag-TPE-P4 is threedimensional and topologically possesses a two-nodal (3,4)-connected net with stoichiometry (3-c)2(4-c). The point (Schläfli) symbol is (4·92)2(42·94), calculated using TOPOS.18 The net topology is xbl, if denoted by a bold lowercase three-letter symbol. If both the Ag2(μ-CO32−) units and the ligands are simplified as 4-connecting nodes, the net topological analysis of Ag-TPE-P4 reveals a 3D two-nodal 4-connected net (Fig. 5f ). Its point symbol is (42·64) and vertex symbol is 4.4.6.6.6.6. The net has a zeolitic SOD topology.19 The structure contains 1D channels through the L3Ag6(CO32−)3 triangular rings along the c-axis, which are lined with CO32− ions. The channels are significantly narrowed due to ridges formed by CO32− ions and the PPh2 phenyl rings. The solvent molecules are disordered within the channels. The solvent accessible volume of Ag-TPE-P4 is 3019.3 Å3 per unit cell volume and the pore volume ratio is 9.4% as calculated by PLATON.20 The phase purity of Ag-TPE-P4 was proven using the XRPD pattern, which is consistent with the simulated pattern calculated on the basis of the single-crystal X-ray diffraction data (Fig. S7†). The most salient feature of Ag-TPE-P4 is the exotic use of carbonate ions as bridging motifs. The carbonate ions should be generated by fixation of atmospheric CO2, because no carbonate ions were included in the starting materials. This was confirmed by conducting a control experiment under argon gas, which resulted in no formation of the product. The mechanism of CO2 fixation is proposed as follows. Firstly, the bulkiness of TPE-P4 is effective in producing coordinately unsaturated AgP2 motifs, rather than AgP3 or AgP4 motifs. Additionally, there are only weakly coordinating BF4− anions in the system. Therefore, the TPE-P4 + Ag+ system may readily capture traces of CO32− ions, that are formed from hydration of atmospheric CO2, as coordinating counteranions in order to form a stable structure (Fig. 1). As shown above, two adjacent coordinately unsaturated AgP2 motifs surrounded by bulky TPE-P4 ligands create a cavity to fix CO2, like the binding of CO2 in discrete dinuclear complexes.5,7 Further interconnection then leads to the formation of the three-dimensional framework. To the best of our knowledge, this represents a unique example of incorporation of dinuclear binding motifs for CO2 in a polymeric framework. Additionally this represents a unique way to fix CO2 in neutral solutions. The present formation mechanism is remarkably different from previously reported Zn(II)/Cu(II) MOFs where the ligand contains amine groups that trap CO2.21 In the previous studies, CO2 fixation occurred under basic conditions. The existence of carbonate anions was further confirmed by solid-state CP-MAS NMR (Fig. S8†). The 13C{1H} CP-MAS NMR spectrum of Ag-TPE-P4 shows a peak at 194.9 ppm, which is attributed to the CO3− anions. The 31P{1H} CP-MAS NMR spectrum shows broad peaks at around 13.2 ppm, indicating the formation of a Ag–phosphine coordination bond. But it is not possible to obtain a value of 1J (Ag,P). The signal broadening
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may possibly be due to the high degree of thermal motion in the framework. Thermogravimetric analysis (TGA) showed that Ag-TPE-P4 released the guest molecules from room temperature to ca. 350 °C (Fig. S9†). The framework began to collapse from ca. 400 °C, upon further heating. The thermostability of AgTPE-P4 was further investigated using temperature-dependent XRPD (Fig. S10†). The as-synthesized material maintained its general powder diffraction pattern up to 250 °C. The diffraction pattern became weak from 250 to 350 °C. The diffraction pattern then disappeared at 355 °C suggesting that the longrange order was destroyed. The photoluminescent properties of Ag-TPE-P4 were investigated in the solid state at room temperature. The solid-state UV-vis absorption spectrum of Ag-TPE-P4 shows broad peaks centered at around 260 and 400 nm (Fig. S11†). Ag-TPE-P4 emitted bright bluish green light with a peak maximum at 454 nm (excitation at 366 nm) in the solid state at room temperature (Fig. 6 and 7a,b). The time-resolved emission-decay behavior was also studied and showed that there are two relaxation pathways in the fluorescence decay. Their lifetimes are 1.14 and 2.48 ns with 51% and 49% contribution, respectively. The weighted mean lifetime is 1.80 ns. These values are akin to those observed for solid TPE-P4. The short and long
Fig. 6 Solid-state excitation and emission photoluminescence spectra of Ag-TPE-P4 measured at room temperature (λex = 366 nm, λem = 454 nm).
Fig. 7 Photos of Ag-TPE-P4, (a) taken under ambient light and (b) taken under UV light (365 nm). Photos of TPE-P4 in toluene (left) and a mixture of TPE-P4 and AgBF4 in toluene–MeCN (right, c = 0.0025 mmol L−1), (c) taken under ambient light, (d) taken under UV light (365 nm).
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exponential decay terms may be attributed to monomer and excimer fluorescence lifetimes, respectively. However, inhomogeneous phenyl ring rotation or flipping kinetics for TPE chromophores are also possible causes.22 Ag-TPE-P4 is among only a few metal–organic frameworks with AIE features.22,23 This shows that the TPE fluorescence of TPE-P4 can be turned on by coordination to Ag atoms in an extended rigid framework. To clarify the mechanism, a control experiment was performed and this showed that the luminescence of TPE-P4 was turned on after TPE-P4 and AgBF4 were mixed in a dilute toluene– MeCN solution (Fig. 7c and d). Additionally, the crystal structure of Ag-TPE-P4 shows that there exists edge-to-face π–π interactions between some PPh2 phenyl groups and some TPE phenyl groups (the shortest Cphenyl⋯Cphenyl separation is 3.666 Å). Thus restriction of the intramolecular rotation of TPE (CvC bond twist and phenyl ring torsion) by metal coordination and aggregation in the matrix may be responsible for the luminescence.23,24 In contrast to TPE-P4, no visible mechano-responsive emission was observed for Ag-TPE-P4, because it has an extended framework and no phase transition could occur when a mechanical stimulus was applied.
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Conclusions In summary, a tetraphenylethylene-based phosphine, TPE-P4, has been synthesized, which represents the first reported phosphine with aggregation-induced and mechano-responsive emission. A mixture of TPE-P4 and Ag+ efficiently fixed atmospheric CO2 in situ as CO32− anions to yield an extended 3D zeolite-like SOD metal-phosphine framework, Ag-TPE-P4. The fixation occurred in neutral solution, which may help to develop new efficient catalysts for CO2 fixation. In addition, Ag-TPE-P4 emitted bright bluish green light in the solid state at room temperature, showing that the luminescence of TPE-P4 can be turned on in an extended porous coordination framework. Ag-TPE-P4 thus provides a platform for the development of novel porous fluorescent sensors with AIE features.
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Acknowledgements This work was funded by the 973 Program (2012CB821701), the NSFC (21273007, 21350110212 and 91222201), the Program for New Century Excellent Talents in University (NCET-13-0615), the NSF of Guangdong (S2013030013474) and the Fundamental Research Funds for the Central Universities (14lgpy05).
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Cite this: DOI: 10.1039/c4dt01808g
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Tetraphenylethylene-based phosphine: tuneable emission and carbon dioxide fixation† Jianyong Zhang,* Qiuli Yang, Yixuan Zhu, Haoliang Liu, Zhenguo Chi and Cheng-Yong Su A tetraphenylethylene-based phosphine, 1,1,2,2-tetrakis((4-diphenylphosphino)phenyl)ethylene (TPE-P4),
Received 17th June 2014, Accepted 18th August 2014
was synthesized and showed novel aggregation-induced and mechano-responsive emission. A mixture of
DOI: 10.1039/c4dt01808g
TPE-P4 and Ag+ could fix atmospheric CO2 in situ as carbonate ions in neutral solution to yield a rare 3D metal–organic framework with zeolite-like SOD topology, [Ag2(TPE-P4)CO3]x . nH2O (Ag-TPE-P4).
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Ag-TPE-P4 showed turn-on luminescence of TPE-P4, emitting bright bluish green light in the solid state.
Introduction CO2 is the current primary anthropogenic greenhouse gas and elevated CO2 levels in the atmosphere have a discernible influence on global warming and climate change. The chemistry of CO2 fixation and the exploiting of CO2 as a C1 feedstock for conversion into higher carbon-containing compounds have evoked intense interest.1 Fixation of atmospheric CO2 as carbonates is one important strategy. CO2 molecules from the atmosphere have been captured and fixed as CO32− ions by various metal complexes, including Ag,2 Cd,3 Mn,4 Ni,5 Zn,6 Cu,7 Ln8 and other metal complexes.9 Among these discrete metal complexes, transition metal complexes with dinuclear motifs represent a simple model to efficiently encapsulate carbonate ions.5,7 Meanwhile, sedimentary carbonates represent a huge quantity of carbon in the Earth’s lithosphere. Recently Ishihara et al. reported that some minerals (e.g. hydrotalcite-like clay minerals) containing carbonate in their extended frameworks may dynamically participate in the global carbon cycle.10 Thus extended polymeric structures may be important in CO2 fixation. In our continuous efforts to develop phosphine-based polymeric structures,11 we wish to report herein that an extended phosphine-based framework with dinuclear motifs is efficient for the fixation of atmospheric CO2. The framework is based on a tetraphenylethylene-based phosphine, 1,1,2,2-tetrakis((4-diphenylphosphino)phenyl)ethylene (denoted TPE-P4), and Ag+ (Fig. 1).
KLGHEI of Environment and Energy Chemistry, MOE Key Laboratory of Polymeric Composite and Functional Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Tel: +86 20 8411 0539 † Electronic supplementary information (ESI) available: Experimental details, additional spectral data, fluorescence lifetime and quantum yields, TG curve, XRPD patterns and X-ray crystallographic data. CCDC 1006156. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01808g
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Fig. 1 CO2 fixation by a TPE-P4 and Ag+ mixture to form an extended Ag-phosphine framework, Ag-TPE-P4.
The tetraphenylethylene-based phosphine, TPE-P4, was chosen on the basis of the following considerations: (a) TPE-P4 is a bulky phosphine ligand for an individual phosphorous donor, which may show high catalytic activity. Phosphines have been widely used in homogeneous catalysis and coordination chemistry due to their tuneable electronic and steric properties. Their metal compounds show promising luminescent, electronic and optoelectronic properties.12 (b) TPE-P4 is a rigid bridging ligand, which may lead to extended
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structures.11,13 (c) TPE and its derivatives have an unusual fluorescence property of aggregation-induced emission (AIE). AIE materials, firstly reported by Tang et al., emit more efficiently in the aggregated state than in the solvated form.14 TPE and its derivatives are typical AIE compounds and show potential application in organic light-emitting diodes, sensors, biological probes, and so on.15 To the best of our knowledge, this is the first time that an AIE moiety (TPE) has been incorporated into a phosphine ligand, and this incorporation endows TPE-P4 and its metal complex with tuneable emission.
Results and discussion TPE-P4 was synthesized from its corresponding fluoride, 1,1,2,2-tetrakis(4-fluorophenyl)ethylene, by reaction with KPPh2 in THF (Scheme S1†). Its 31P{1H} NMR spectrum showed a single resonance at −6.1 ppm. TPE-P4 emits bright bluish green light with a peak maximum at 450 nm (excitation at 397 nm) in the solid state at room temperature, showing AIE features as expected for TPE derivatives (Fig. S1†). The AIE behavior of TPE-P4 was investigated by monitoring the changes in photoluminescence (PL) intensity with the addition of increasing amounts of water, in a THF–water solvent mixture. THF acts as the solvent and water acts as the anti-solvent. The UV-vis absorption spectra measured in various THF–water mixtures show peaks at around 300 and 340 nm related to the π–π* transitions of the TPE chromophore (Fig. S2†). Because TPE-P4 is insoluble in water, it was expected to aggregate in THF–water mixtures with high water content. From 70% v/v water content, the maximum peaks are red-shifted with leveloff tailing. The spectral changes indicate that nanoparticle suspensions were obtained due to the Mie effect of the nanoparticles.16 The formation of nanoparticles was also evidenced by the Tyndall effect (Fig. S3†). TEM further revealed that the nanoparticles were around 50–100 nm in diameter (Fig. S4†). When TPE-P4 aggregated with the increasing water concentration, the PL intensity increased, reaching a maximum at 90% v/v water content (Fig. 2). The PL intensities in low water
Fig. 2 Photoluminescence (PL) spectra of TPE-P4 in THF–water mixtures with different water fractions and in pure THF (c = 10−5 mol L−1; λex = 350 nm).
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content (0%–60%) were very weak. However, significant PL enhancements were observed when the water fraction increases up to 70 and 90%. The PL intensity was 4 (a.u.) in pure THF and was elevated to 155 in the THF–water mixture with a water fraction of 90%. The fluorescence quantum yields were determined (Fig. S5†). In pure THF, TPE-P4 exhibited a low fluorescence quantum yield (Φ = 0.35%). An obvious and drastic increase in the Φ values was observed when the water fraction of the THF–water mixture was increased to 90%. The Φ value was 4.39% at this composition. This dramatic increase in fluorescence efficiency is attributed to the aggregation of TPE-P4. These results indicate that TPE-P4 has a significant AIE. In addition, TPE-P4 exhibits a mechano-responsive emission.17 As shown above, as-synthesized TPE-P4 exhibited strong bluish green emissions with peak centering at 450 nm, upon exciting with UV light (Fig. S1†). After grinding treatment using a pestle and a mortar, TPE-P4 changed from a white to a pale yellow solid, with strong green emission and with peak centering at 499 nm (Fig. 3 and 4). The variation scope in the emission wavelength of TPE-P4 before and after grinding is 49 nm. After exposing to methanol vapor for 12 h, the ground
Fig. 3 Photos of TPE-P4, (a) taken under ambient light and (b) taken under UV light (365 nm) (samples: left, sample exposed to MeOH vapor; right, ground sample).
Fig. 4 Normalized photoluminescence spectra of TPE-P4: (a) ground sample, (b) sample exposed to MeOH vapor, (c) re-ground sample, (d) sample re-exposed to MeOH vapor (λex = 397 nm) (the inset depicts the reversible changes of PL wavelength through grinding and methanol vapor treatments).
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sample was converted into a new blue emissive state. The emission peak was located at 468 nm, which is red-shifted from that obtained for the as-synthesized sample. After regrinding the sample, the emission peak was centered at 499 nm again. Re-exposing to methanol vapor converted the re-ground sample to the second blue emissive state (λem = 468 nm) again. The results indicate reversible interconversion between blue-emitting (468 nm) and green-emitting (λem = 499 nm) phases on grinding and exposing to methanol vapor (Fig. 3b). The color of the luminescence is switched by the grinding and methanol vapor treatments. The time-resolved emission-decay behaviors of the ground and MeOH-treated samples were investigated and show that there are two relaxation pathways in their fluorescence decays (Table S1†). The weighted mean lifetime of the ground sample (3.06 ns) is longer than that of the exposed sample (2.18 ns). The prolonged lifetime may be attributed to increasing π–π interactions after grinding. X-ray powder diffraction (XRPD) measurements were conducted to further reveal the mechanism of the mechanoresponsive emission of TPE-P4 (Fig. S6†). The as-synthesized and MeOH-treated samples show intense and sharp reflection peaks, indicative of long-range order in the material. Moreover the peaks of the as-synthesized sample are more intense and sharper than those of MeOH-treated samples, suggesting the original crystallinity was only partly restored after the exposure to MeOH vapor. This is consistent with the above emission results. In contrast, the XRPD patterns of the ground samples exhibit weak and broad peaks, characteristic of amorphous materials. These results indicate that the long-range order was destroyed after grinding and that grinding converted the crystalline phase to the amorphous phase. Exposure to MeOH converted the amorphous phase back to the crystalline phase. Similar phase transition could also be triggered by ethanol and tetrahydrofuran. Thus reversible inter-conversion of mechanochromism was achieved and the mechano-responsive emission is related to the phase transition between crystalline and amorphous phases. It is worth mentioning that the emissions are similar in the solution aggregate and in the ground solid (λex = 503 and 499 nm), suggesting that they have similar amorphous features. Interestingly, a mixture of TPE-P4 and AgBF4 in toluene– acetonitrile could fix atmospheric CO2 to yield an extended framework, [Ag2(TPE-P4)CO3]x . nH2O (denoted Ag-TPE-P4). Single-crystal X-ray diffraction analysis revealed that Ag-TPE-P4 ˉc.‡ There is one cryscrystallizes in the trigonal space group R3 tallographically independent Ag atom, half a TPE-P4 ligand and half a CO32− in the asymmetric unit. The carbon atom
‡ Crystallographic data for Ag-TPE-P4: C75H56Ag2O9.22P4 Mw = 1444.37, trigonal, ˉc, a = b = 36.6284(4) Å, c = 27.5127(5) Å, α = β = 90°, γ = 120°, V = 31 966.8(7) R3 Å3, Z = 18, D = 1.351 g cm−3, μ = 5.715 mm−1, λ = 1.54178 Å, T = 150(2) K, F(000) = 13 208, 38 595 reflections were collected (7131 were unique) for 3.50 < θ < 74.01, R(int) = 0.0515, R = 0.1076, wR2 = 0.3131 [I > 2σ(I)], R = 0.1345, wR2 = 0.3455 (all data) for 409 parameters with no restraint, GOF = 1.357. CCDC-1006156.
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(C38) and oxygen atom (O2) of the CO32− moiety lie on a rotoinversion axis. Each TPE-P4 ligand binds four Ag+ ions, using all four P donors. The Ag+ ion is tricoordinate with a trigonal CuP2O coordination geometry. Every Ag+ ion coordinates two P donors from two different ligands and one O donor from CO32− (Ag–P = 2.443(2), 2.457(2) Å, Ag–O = 2.407(7) Å). Each CO32− ion bridges two AgP2 moieties in a butterfly manner and the carbonate anion acts as a η1:η1 bridging ligand (Fig. 5a). The four bulky TPE-P4 ligands stretch in different directions to help release the steric tension. The ∠C– O–C angles in the carbonate unit are close to 120° (122.6(6), 114.9(12) and 109.2(6)°), and the bond distances are 1.221(15), 1.279(10) and 1.279(10) Å. The structure can be considered to consist of large L3Ag6(CO32−)3 triangular planar subunits, which are formed by three bridging TPE-P4 ligands, six Ag+ ions and three CO32− ions (Fig. 5b). Two adjacent triangular subunits are linked by L2Ag4(CO32−)2 rings (Fig. 5c). The adjacent triangular subunits are not located in the same plane, thus stretching into a three-
Fig. 5 X-ray structures of (a) CO32− ion bridging two AgP2 moieties in a butterfly manner, (b) L3Ag6(CO32−)3 triangular planar subunit, (c) two adjacent L3Ag6(CO32−)3 triangular subunits, (d, e) connected triangular subunits of L3Ag6(CO32−)3 in a 3D framework as viewed along the c-axis, (f ) (42·64) zeolitic SOD topological net of Ag-TPE-P4.
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dimensional structure (Fig. 5d and 5e). Considering the Ag+ ions as three-connecting nodes and the ligand TPE-P4 as fourconnecting nodes, the overall structure of Ag-TPE-P4 is threedimensional and topologically possesses a two-nodal (3,4)-connected net with stoichiometry (3-c)2(4-c). The point (Schläfli) symbol is (4·92)2(42·94), calculated using TOPOS.18 The net topology is xbl, if denoted by a bold lowercase three-letter symbol. If both the Ag2(μ-CO32−) units and the ligands are simplified as 4-connecting nodes, the net topological analysis of Ag-TPE-P4 reveals a 3D two-nodal 4-connected net (Fig. 5f ). Its point symbol is (42·64) and vertex symbol is 4.4.6.6.6.6. The net has a zeolitic SOD topology.19 The structure contains 1D channels through the L3Ag6(CO32−)3 triangular rings along the c-axis, which are lined with CO32− ions. The channels are significantly narrowed due to ridges formed by CO32− ions and the PPh2 phenyl rings. The solvent molecules are disordered within the channels. The solvent accessible volume of Ag-TPE-P4 is 3019.3 Å3 per unit cell volume and the pore volume ratio is 9.4% as calculated by PLATON.20 The phase purity of Ag-TPE-P4 was proven using the XRPD pattern, which is consistent with the simulated pattern calculated on the basis of the single-crystal X-ray diffraction data (Fig. S7†). The most salient feature of Ag-TPE-P4 is the exotic use of carbonate ions as bridging motifs. The carbonate ions should be generated by fixation of atmospheric CO2, because no carbonate ions were included in the starting materials. This was confirmed by conducting a control experiment under argon gas, which resulted in no formation of the product. The mechanism of CO2 fixation is proposed as follows. Firstly, the bulkiness of TPE-P4 is effective in producing coordinately unsaturated AgP2 motifs, rather than AgP3 or AgP4 motifs. Additionally, there are only weakly coordinating BF4− anions in the system. Therefore, the TPE-P4 + Ag+ system may readily capture traces of CO32− ions, that are formed from hydration of atmospheric CO2, as coordinating counteranions in order to form a stable structure (Fig. 1). As shown above, two adjacent coordinately unsaturated AgP2 motifs surrounded by bulky TPE-P4 ligands create a cavity to fix CO2, like the binding of CO2 in discrete dinuclear complexes.5,7 Further interconnection then leads to the formation of the three-dimensional framework. To the best of our knowledge, this represents a unique example of incorporation of dinuclear binding motifs for CO2 in a polymeric framework. Additionally this represents a unique way to fix CO2 in neutral solutions. The present formation mechanism is remarkably different from previously reported Zn(II)/Cu(II) MOFs where the ligand contains amine groups that trap CO2.21 In the previous studies, CO2 fixation occurred under basic conditions. The existence of carbonate anions was further confirmed by solid-state CP-MAS NMR (Fig. S8†). The 13C{1H} CP-MAS NMR spectrum of Ag-TPE-P4 shows a peak at 194.9 ppm, which is attributed to the CO3− anions. The 31P{1H} CP-MAS NMR spectrum shows broad peaks at around 13.2 ppm, indicating the formation of a Ag–phosphine coordination bond. But it is not possible to obtain a value of 1J (Ag,P). The signal broadening
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may possibly be due to the high degree of thermal motion in the framework. Thermogravimetric analysis (TGA) showed that Ag-TPE-P4 released the guest molecules from room temperature to ca. 350 °C (Fig. S9†). The framework began to collapse from ca. 400 °C, upon further heating. The thermostability of AgTPE-P4 was further investigated using temperature-dependent XRPD (Fig. S10†). The as-synthesized material maintained its general powder diffraction pattern up to 250 °C. The diffraction pattern became weak from 250 to 350 °C. The diffraction pattern then disappeared at 355 °C suggesting that the longrange order was destroyed. The photoluminescent properties of Ag-TPE-P4 were investigated in the solid state at room temperature. The solid-state UV-vis absorption spectrum of Ag-TPE-P4 shows broad peaks centered at around 260 and 400 nm (Fig. S11†). Ag-TPE-P4 emitted bright bluish green light with a peak maximum at 454 nm (excitation at 366 nm) in the solid state at room temperature (Fig. 6 and 7a,b). The time-resolved emission-decay behavior was also studied and showed that there are two relaxation pathways in the fluorescence decay. Their lifetimes are 1.14 and 2.48 ns with 51% and 49% contribution, respectively. The weighted mean lifetime is 1.80 ns. These values are akin to those observed for solid TPE-P4. The short and long
Fig. 6 Solid-state excitation and emission photoluminescence spectra of Ag-TPE-P4 measured at room temperature (λex = 366 nm, λem = 454 nm).
Fig. 7 Photos of Ag-TPE-P4, (a) taken under ambient light and (b) taken under UV light (365 nm). Photos of TPE-P4 in toluene (left) and a mixture of TPE-P4 and AgBF4 in toluene–MeCN (right, c = 0.0025 mmol L−1), (c) taken under ambient light, (d) taken under UV light (365 nm).
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exponential decay terms may be attributed to monomer and excimer fluorescence lifetimes, respectively. However, inhomogeneous phenyl ring rotation or flipping kinetics for TPE chromophores are also possible causes.22 Ag-TPE-P4 is among only a few metal–organic frameworks with AIE features.22,23 This shows that the TPE fluorescence of TPE-P4 can be turned on by coordination to Ag atoms in an extended rigid framework. To clarify the mechanism, a control experiment was performed and this showed that the luminescence of TPE-P4 was turned on after TPE-P4 and AgBF4 were mixed in a dilute toluene– MeCN solution (Fig. 7c and d). Additionally, the crystal structure of Ag-TPE-P4 shows that there exists edge-to-face π–π interactions between some PPh2 phenyl groups and some TPE phenyl groups (the shortest Cphenyl⋯Cphenyl separation is 3.666 Å). Thus restriction of the intramolecular rotation of TPE (CvC bond twist and phenyl ring torsion) by metal coordination and aggregation in the matrix may be responsible for the luminescence.23,24 In contrast to TPE-P4, no visible mechano-responsive emission was observed for Ag-TPE-P4, because it has an extended framework and no phase transition could occur when a mechanical stimulus was applied.
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Conclusions In summary, a tetraphenylethylene-based phosphine, TPE-P4, has been synthesized, which represents the first reported phosphine with aggregation-induced and mechano-responsive emission. A mixture of TPE-P4 and Ag+ efficiently fixed atmospheric CO2 in situ as CO32− anions to yield an extended 3D zeolite-like SOD metal-phosphine framework, Ag-TPE-P4. The fixation occurred in neutral solution, which may help to develop new efficient catalysts for CO2 fixation. In addition, Ag-TPE-P4 emitted bright bluish green light in the solid state at room temperature, showing that the luminescence of TPE-P4 can be turned on in an extended porous coordination framework. Ag-TPE-P4 thus provides a platform for the development of novel porous fluorescent sensors with AIE features.
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Acknowledgements This work was funded by the 973 Program (2012CB821701), the NSFC (21273007, 21350110212 and 91222201), the Program for New Century Excellent Talents in University (NCET-13-0615), the NSF of Guangdong (S2013030013474) and the Fundamental Research Funds for the Central Universities (14lgpy05).
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