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Synthesis, structure, photophysical and electrochemical properties of a novel metalloporphyrin with a condensed three-dimensional porous open framework
Inorganica Chimica Acta 414 (2014) 1–7
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Synthesis, structure, photophysical and electrochemical properties of a novel metalloporphyrin with a condensed three-dimensional porous open framework Wen-Tong Chen a,b,⇑, Zhi-Gang Luo a, Yin-Feng Wang a, Xian Zhang a, Hong-Ru Fu b a Institute of Applied Chemistry, School of Chemistry and Chemical Engineering, Jiangxi Province Key Laboratory of Coordination Chemistry, Jinggangshan University, Ji’an, Jiangxi 343009, China b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e
i n f o
Article history: Received 1 November 2013 Received in revised form 11 January 2014 Accepted 26 January 2014 Available online 1 February 2014 Keywords: Lanthanide Magnetic Metalloporphyrin Photoluminescence TPPS
a b s t r a c t A novel 4f–3d metalloporphyrin, [EuZn(TPPS)H3O]n (1) (H2TPPS = tetra(4-sulfonatophenyl)porphyrin), has been synthesized via a solvothermal reaction and structurally characterized by single-crystal X-ray diffraction. Compound 1 features a three-dimensional (3-D) structure with two kinds of one-dimensional (1D) infinite chains. Based on the condensed 3-D porous open framework, compound 1 has a large void space of 223 Å3, corresponding to 9.4% of the unit-cell volume. Compound 1 shows high thermal and aqueous stability. Magnetic measurements reveal that compound 1 has an antiferromagnetic behavior. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Porphyrins with plentiful properties are one of the most widely studied chemical systems. Porphyrins have been found their applications in the areas of optical, medicine, catalysis, adsorption, solar energy conversion, and so forth [1–10]. The modification of a porphyrin through inserting a metal ion into the center of a porphyrinic ring, or decorating the periphery of a porphyrinic ring, can provide various building blocks to prepare new porphyrinic compounds, which may have different chemical, electronic or redox properties. More and more efforts have thus far been devoted to synthesize metalloporphyrinic frameworks (MPFs) with reformative features that may allow them to be put into practical applications [11,12]. Nowadays, MPFs have gained increasing interest because of their useful characteristics in structural robustness, reaction catalysis, energy and electron transfer [13,14]. Tetraarylporphyrins like TPyP (meso-tetra(4-pyridyl)porphyrin), TCMOPP (tetrakis(4-(carboxymethyleneoxy)phenyl)porphyrin), TCPP (tetrakis(4-carboxyphenyl) porphyrin), and TDCPP (tetrakis(3,5-dicarboxyphenyl)porp hyrin) ⇑ Corresponding author at: Institute of Applied Chemistry, School of Chemistry and Chemical Engineering, Jiangxi Province Key Laboratory of Coordination Chemistry, Jinggangshan University, Ji’an, Jiangxi 343009, China. Tel./fax: +86 796 8119239. E-mail address: [email protected] (W.-T. Chen). http://dx.doi.org/10.1016/j.ica.2014.01.037 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.
are square planar building units and they have been widely adopted to synthesize novel MPFs [15–18]. However, H2TPPS (tetra(4-sulfonatophenyl)porphyrin) has been rarely used to prepare new MPFs, although it is one of the tetraarylporphyrins. To our knowledge, only two examples of TPPS-containing MPFs have so far been found [19,20]. Actually, the large, rigid and square planar symmetrical H2TPPS is a useful building unit for the construction of new MPFs because H2TPPS can provide a lot of coordination sites: one at the center of the macrocyclic porphyrin ring and several at the oxygen atoms of four sulfonic ligands. However, the crystal structures and physicochemical properties of the TPPS compounds have yet to be explored. Lanthanide compounds have attracted increasing attention because of their abundant properties, such as photoluminescence and magnetism [21,22]. In recent years, a lot of lanthanide compounds with various organic ligands have been prepared [23–25]. The zinc ion (Zn2+), as one of the 3d elements, is particularly attractive for the following reasons: the variety of coordination numbers and geometries provided by its d10 configuration, fluorescence, photoelectric properties, and its essential role in biological systems. Recently, we focus on the study of the 4f–3d TPPS MPFs that possibly have different frameworks and properties like gas adsorption, photoluminescence and magnetism. We report herein the rational preparation, crystal structure and various properties
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of a 4f–3d TPPS metalloporphyrin, [EuZn(TPPS)H3O]n (1) (H2TPPS = tetra(4-sulfonatophenyl)porphyrin), which was obtained via solvothermal reactions and features a condensed 3D porous open framework. It should be pointed out that compound 1 displays not only good thermal stability, but also wonderful aqueous stability; even it has been ultrasonicated for several hours in distilled water. Based on its good bistability and in order to explore its potential applications, we measured its properties as detailed as we can. The title compound was characterized in detail by using single-crystal X-ray diffraction, UV–Vis spectra, FT-IR, fluorescence, quantum yield, luminescence lifetime, CV/DPV, adsorption measurement, TG/DTA and magnetic measurements.
2. Experimental 2.1. Materials and instrumentation All reactants of A.R. grade were obtained commercially and used without further purification. Elemental analysis was carried out with an Elementar Vario EL III microanalyzer. The infrared spectra were recorded on a Nicolet 5DX FT-IR spectrophotometer over the frequency range 4000–400 cm 1 using the KBr pellet technique. The solution UV–Vis spectra were recorded at room temperature on a computer-controlled PE Lambda 900 UV–Vis spectrometer. The diffuse reflectance spectrum was recorded at room temperature on a computer-controlled Lambda 35 UV–Vis spectrometer equipped with an integrating sphere in the wavelength range 190/1100 nm. BaSO4 powder wad used as a reference (100% reflectance), on which the ground powder sample was coated. The absorption spectra were calculated from reflection spectra by the Kubelka–Munk function: a/S=(1-R)2/2R, a is the absorption coefficient, S is the scattering coefficient which is practically wavelength independent when the particle size is larger than 5 lm, and R is the reflectance. The band gap energy value was determined by extrapolation from the linear portion of the absorption edge in a (a/S) versus energy plot. The photoluminescent study was conducted on an Edinburgh FLS920 fluorescence spectroscopy instrument under room temperature. Measurement of an emission quantum yield of solution sample was carried out on a Hamamatsu C9920-0X(PMA-12) U6039-05 fluorescence spectrofluorometer with a integrating sphere adapted to a rightangle configuration at room temperature, involving determination of the diffuse reflectance spectra of the sample. The result was corrected for the detector response as a function of wavelength. Photoluminescent lifetime measurement was conducted using a Photon Technology International GL-3300 nitrogen laser with a Photon Technology International GL-302 dye laser and a nitrogen laser/pumped dye laser system equipped with a four-channel digital delay/pulse generator (Standard Research System Inc., model DG535) and a motor driver (Photon Technology International, model MD-5020). The excitation wavelength was 421 nm with use of a POPOP chromophore. Measurements of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed at 298 K using a BAS 100 W electrochemical analyzer in deaerated benzonitrile containing 0.1 M TBAPF6 (tetra-n-butylammonium hexafluorophosphate) as a supporting electrolyte. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to the Ag/AgNO3 (1.0 10 2 M). All electrochemical measurements were carried out under an atmospheric pressure of argon. Thermo-gravimetry (TG) and differential thermal analysis (DTA) were performed on an NETZSCH STA 449C analyzer.
The adsorption measurement of CO2 was conducted on an intelligent gravimetric sorption analyzer (IGA100B). As-synthesized sample was dipped in CH2Cl2 solutions for 24 h and evacuated under a dynamic vacuum (10 3 Torr) at 423 K for 8 h before the adsorption measurement. Variable-temperature magnetic susceptibility and field dependence magnetization measurements on polycrystalline samples were performed on a PPMS 9T Quantum Design SQUID magnetometer. All data were corrected for diamagnetism estimated from Pascal’s constants. 2.2. Synthesis of [EuZn(TPPS)H3O]n (1) The title compound was prepared by mixing EuCl36H2O (0.1 mmol, 37 mg), ZnBr2 (0.1 mmol, 22 mg), H2TPPS (0.1 mmol, 94 mg) and 10 mL distilled water in a 25 mL Teflon-lined stainless steel autoclave and heating the mixture at 473 K for 5 d. After cooling slowly the mixture to room temperature at a rate of 6 K/h, red crystals suitable for X-ray analysis were obtained. The yield was 41% (based on europium). Anal. Calc. for C44H27EuN4O13S4Zn: C, 45.35; H, 2.34; N, 4.81. Found: C, 45.27; H, 2.38; N, 4.87%. Fourier transform IR (KBr, cm 1): 3422(s), 3102(w), 2359(m), 1602(m), 1401(w), 1336(w), 1240(s), 1171(s), 1126(vs), 1052(s), 997(vs), 860(w), 809(m), 745(s), 667(w), and 640(s). 2.3. Crystal structure determination The intensity data set was collected on a Rigaku Mercury CCD X-ray diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å) using a x scan technique. CrystalClear software was used for data reduction and empirical absorption corrections [26]. The structure was solved by the direct method with the Siemens SHELXTL™ Version 5 package of crystallographic software [27]. The difference Fourier maps based on these atomic positions yield the non-hydrogen atoms, while the hydrogen atoms were generated theoretically, allowed to ride on their respective parent atoms and included in the structure factor calculations with assigned isotropic thermal parameters. The structure was refined using a full-matrix least-squares refinement on F2. All atoms except for hydrogen atoms were refined anisotropically. A summary of crystallographic data and structure analysis is listed in Table 1, and selected bond distances and bond angles are given in Table 2.
Table 1 Crystal data and structure refinement details for 1. Formula
C44H27EuN4O13S4Zn
Formula weight Color Crystal size (mm3) crystal system space group a (Å) c (Å) V (Å3) Z 2hmax (°) Reflections collected Independent, observed reflections (Rint) dcalcd (g/cm3) l (mm 1) T (K) F(0 0 0) R1, wR2 S Largest and mean D/r Dq(max, min) (e/Å3)
1165.27 red 0.25 0.15 0.14 tetragonal P4/mcc 15.507(5) 9.930(5) 2387.8(16) 2 50 11467 1040, 997 (0.0499) 1.621 2.045 123.15 1160 0.0481, 0.1224 1.016 0, 0 0.951, 0.584
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3. Results and discussion The FTIR bands of compound 1 are mainly occurred in the range of 640–1602 cm 1. The absorption band at 3422 cm 1 could be ascribed to the mO–H stretching vibration of the water molecule; the bands in the frequency range of 809–1240 cm 1 are probably resulted from the aromatic ring bending modes of the porphyrin. As for the free-base H2TPPS, two FTIR bands locating at around 3321 and 1351 cm 1 could be assigned to the cN–H and dN–H vibration modes of the pyrrole rings [28]. However, these two bands vanished from the FTIR spectrum of 1 because of the deprotonation and metalation of the pyrrole rings of H2TPPS. This is an important evidence for a free-base H2TPPS to be changed into a metalloporphyrin. X-ray diffraction analysis reveals that the crystal structure of 1 features a 3-D motif. The molecular structure of 1 is given as an ORTEP drawing in Fig. 1. Compound 1 consists of neutral [EuZn(TPPS)H3O]n molecules and crystallizes in the tetragonal P4/ mcc space group. The zinc atom is at the crystallographic inversion center of the perfectly coplanar 24-membered macrocyclic porphyrin core, binding to four nitrogen atoms and two oxygen atoms to construct a slightly distorted octahedron. The bond lengths of Zn–N are 2.066(3) Å, which is comparable with those previously reported [17,29]. The bond length of Zn–O is 2.1798(14) Å. The zinc atom is positionally disordered with the other eclipsed one obtained by the mirror plane and the site occupancy of each one is equal to 0.5 because both are crystallographically identical. The distance between the adjacent zinc atoms is 4.9648(24) Å that is short enough to allow a weak intermolecular Zn–O Zn interaction. There are two types of 1D infinite chains existing in the title compound. One is the –Eu–(SO3)4–Eu– chain and the other is –Zn– O–Zn– chain based on the intermolecular Zn–O Zn interaction, as shown in Fig. 2. These two chains are connected by the europium ions. The europium ion coordinates to eight oxygen atoms, forming a distorted square anti-prism with the bond length of Eu–O being 2.405(2) Å that is normal and comparable with that documented before [30–32]. The europium ions are interconnected via eight sulfonate groups to yield a 1D infinite –Eu–(SO3)4–Eu– chain with the Eu Eu distance being 4.9649(23) Å. These two chains connect together to construct a condensed 3D porous open framework with the channels running along the c-axis (Fig. 3). The displacement of the four Npyrrole atoms is 0 Å from their mean N4 plane. With respect to the N4 plane that represents the mean plane of the porphyrin core, all the twist angles of the aryl rings are exactly 90°. As shown in Fig. 3, it should be pointed out that compound 1 has a large void space being 223 Å3 that is 9.4% of the unit-cell volume. 3.1. Thermo-gravimetry (TG) and differential thermal analysis (DTA) As shown in Fig. 4, the thermal stability of compound 1 was studied by TG and DTA in flowing air. TG-DTA result exhibits that compound 1 is thermally stable up to 350 °C, which is among the highest decomposition temperature in polymers. A continuous weight loss from the beginning to 100 °C can be observed from Table 2 Selected bond lengths (Å) and bond angles (°). Eu(1)–O(1) Zn(1)–N(1) Zn(1)–O(1W) O(1)#1–Eu(1)–O(1)#2 O(1)#1–Eu(1)–O(1)#3 O(1)#1–Eu(1)–O(1)#4
2.405(2) 2.066(3) 2.1798(14) 126.07(10) 73.62(6) 88.58(11)
O(1)#2–Eu(1)–O(1)#3 O(1)#2–Eu(1)–O(1)#4 O(1)#3–Eu(1)–O(1)#4 N(1)–Zn(1)–N(1)#5 N(1)–Zn(1)–N(1)#6 N(1)–Zn(1)–O(1W)
Symmetry codes: #1 y, x 1, z; #2 x, y 1, y, z + 1/2; #5 y, x, z; #6 x, y, z.
z + 1/2; #3
x 1,
158.61(9) 115.85(13) 68.46(13) 88.771(8) 163.15(6) 98.42(3) y 1, z; #4
x 1,
Fig. 1. ORTEP drawing of 1 with 50% thermal ellipsoids. Hydrogen atoms and disordered Zn–O moieties were omitted for clarity.
the TG diagram. This weight loss could be just attributed to the escape of the moisture or air adsorbed in the voids or on the surface of the crystals. The TG diagram shows a sudden weight loss with a beginning temperature of about 350 °C and two endothermic peaks centered at 425 and 516 °C, respectively. The total weight loss of 62.1% is close to the calculated value of 62.8% at the oxidative decomposition to obtain Eu2O3 and ZnO. 3.2. Solution UV–Vis spectrum and solid state diffuse reflectance spectrum According to the four orbitals model brought forward by Gouterman [33], a metalloporphyrin generally exhibits two types of intensive absorption bands, namely, a strong B band (or Soret band) at 400 nm with a molar absorption coefficient e being 105 M 1 cm 1 and several weaker Q bands existing at 500– 650 nm with a e being 103–104 M 1 cm 1. Fig. 5 gives the UV– Vis absorption spectrum of 1 measured in benzonitrile under room temperature. The B band occurs at 427 nm with a e being 3.94 105 M 1 cm 1, which is in good agreement with the above conclusion made by Gouterman. The Q bands are observed at 559 and 599 nm. The number of Q bands of 1 is less than that of a free-base porphyrin that generally has more than three Q bands. Such a difference in the number of Q bands could be ascribed to the increase of the molecular symmetry, originating from the metalation of a porphyrin. As shown in Fig. 6, solid state diffuse reflectance spectrum of 1 exhibits the presence of an optical bandgap of 1.18 eV, which suggests that compound 1 is probably a narrow-gap semiconductor. The gradual slope of the optical absorption edge of 1 indicates the existence of an indirect transition [34]. To our knowledge, several highly efficient photovoltaic materials have thus far been prepared, such as CdTe (1.5 eV), GaAs (1.4 eV), CuInS2 (1.55 eV), and CuInSe2 (1.04 eV) [35–37]. The optical bandgap of 1.18 eV of 1 is close to that of CuInSe2. 3.3. Photoluminescence, lifetime and quantum yield Taking into account the excellent photoluminescent property of the europium, the photoluminescence was studied at room temperature. Photoluminescent investigations were carried out in benzonitrile because, to our knowledge, solid state porphyrins
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Fig. 2. The 1D chains in 1. (a) a –Eu–(SO3)4–Eu– chain; (b) a –Zn–O–Zn– chain. Disordered zinc ions were omitted for clarity.
Fig. 3. Space-filling illustration of the condensed 3-D porous open framework of 1.
and their derivatives cannot display emission bands due to their concentration quenching effect. For compound 1, the maximum photoluminescent emission band appears at 612 nm with a shoulder band at 660 nm, upon the excitation of 438 nm, as shown in Fig. 7. These photoluminescent emission bands could be ascribed to the characteristic emissions of the porphyrin. The absence of the characteristic emissions of the europium indicates that TPPS is not an effective antenna to transfer the adsorbed energy to the europium centers. Using a time-correlated single photon counting technique, the photoluminescent lifetime in benzonitrile solution was measured with an excitation wavelength of 421 nm, because this light can be absorbed by the metalloporphyrin. The time-resolved photoluminescent decay profile of 1 is given in Fig. 8. The
time decay curve was fitted as single exponential. The photoluminescent lifetime of compound 1 is 1.57 ms in benzonitrile. The emission quantum yield of the solution sample was also measured. The emission quantum yield of compound 1 is determined to be 1.5%. 3.4. Adsorption measurement of CO2 The adsorption data for CO2 collected at 273 K was shown in Fig. 9. The CO2 adsorption curve of 1 is a typical type-I shape adsorption isotherm and slowly rising, which indicates further high adsorption capacity under increasing pressure, as the case found in the literature [38]. The CO2 adsorption study reveals that
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Fig. 4. TG-DTA diagrams of 1.
2.0
400
427
300 Intensity(cps)
Absorbance (AU)
1.5
1.0
0.5
200
100
559 599
DMF 5 ¦ÌM
0.0
0
400
600
800
400
500
Wavelength (nm) Fig. 5. UV–Vis absorption spectrum measured at room temperature. Molar absorption coefficient (e/M 1 cm 1): e427 nm = 3.94 105.
700
800
Fig. 7. The excitation and emission spectra at room temperature (dashed line: excitation spectra; solid line: emission spectra).
from these MOFs (10 atm). At 273 K, no saturation behavior can be observed till the highest pressure obtainable by the apparatus. Such a CO2 adsorption ability is lower than most of MOFs, but higher than several [42]. In general, small gas molecules whose diameter is smaller than that of pore windows could go through the window and arrive anywhere of the pore of the open framework. However, the CO2 adsorption ability in compound 1 at 273 K is low, although the size of the pore window (5.6 Å 8.5 Å) is much larger than that of CO2 (3.3 Å). This phenomenon is uncommon for MOFs. The reason for that may be ascribed to that some CO2 molecules are strongly adsorbed by the window and block others to further enter the pores.
3
Eg = 1.18 eV
¦Á/S
2
1
3.5. Electrochemical studies
0 0.5
600 Wavelength(nm)
1.0
1.5
Eg (eV) Fig. 6. Diffuse reflectance spectrum of 1.
compound 1 adsorbed 0.8 mmol/g at the pressure up to 0.01 atm where the molar ratio of adsorbed CO2 to porphyrin moiety is 1. The CO2 uptake of 0.8 mmol/g in 1 seems lower than several famous MOFs, such as MIL-53 (ca. 8 mmol/g of CO2 at 10 atm) [39], MIL-96 (4.4 mmol/g of CO2 at 10 atm) [40], MIL-102 (3.1 mmol/g of CO2 at 10 atm) [41], but compound 1 was measured under very low pressure (only 0.01 atm), which is quite different
Kadish pointed out that the important factors mostly affecting the redox potential of a metalloporphyrin are the properties of the solvent, porphyrin itself and supporting electrolyte [43]. The redox potentials could be different up to 1.0 V or even much more, depending on the different substituents of the porphyrin. The measurements of the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) for compound 1 were conducted in benzonitrile and TBAPF6 (0.1 M) under room temperature. As shown in Fig. 10, slow sweep CV of 1 displays one quasi-reversible wave with E1/2 = 0.31 V that is close to the value of DPV (0.30 V). We also measured the CV and DPV for free-base H2TPPS under the same
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Fig. 8. Nanosecond transient spectra of 1 in benzonitrile.
3.6. Magnetic properties
CO2 absorbed (mmol/g)
0.8
0.6
0.4
0.2
0.0 0.000
0.002
0.004
0.006
0.008
0.010
P/P0 Fig. 9. CO2-adsorption isotherm of 1 at 273 K.
conditions as 1. The CV of free-base H2TPPS is characterized by one reversible redox wave with E1/2 = 0.84 V, which is similar to that of DPV ( 0.82 V) (as shown in the inset of Fig. 10).
The temperature dependent magnetic susceptibility of compound 1 was investigated from 300 to 2 K at 5000 Oe on the polycrystalline sample, as shown in Fig. 11a. The value of leff is 2.84 lB at 300 K, which is smaller than the expected value (3.40 lB) in the free-ion approximation for an isolated Eu3+ ion, and decreases with temperature to 0.35 lB at 2 K. When decreasing the temperature, the xM of compound 1 increases gradually from 3.23 10 3 emu/ mol at 300 K to reach 4.81 10 3 emu/mol at 25 K. On further cooling, the xM of compound 1 rises fast to about 9.24 10 3 emu/mol at 2 K. Such a behavior is typically observed for a paramagnetic system that displays a primarily antiferromagnetic interaction [44]. The origin of such an antiferromagnetic behavior of compound 1 could be ascribed to the progressive thermal depopulation of the Stark components of the europium ion, because the zinc ion is diamagnetic. The field dependence of the magnetization for compound 1 conducted at 2 K shows that the magnetization increases slowly with the increasing field. Clearly, the magnetization of compound 1 is unsaturated at 80 kOe with a value of 0.017 Nb, as shown in Fig. 11b.
Fig. 10. CV (green) and DPV (red) profiles for 1 under an argon atmosphere (Inset: free-base H2TPPS). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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1EZ, UK (Fax: +44-1223-336033; email: [email protected] or http://www.ccdc.cam.ac.uk). References
Fig. 11. (a) Thermal dependence of xM and effective magnetic moment of 1. (b) Magnetization vs H of 1.
In summary, by using a hydrothermal reaction, we have prepared a crystalline 4f–3d MPF, [EuZn(TPPS)H3O]n (1), which is characterized by a robust 3D porous open framework. This compound was characterized in detail by single-crystal X-ray diffraction, UV–Vis spectra, FT-IR, fluorescence, quantum yield, luminescence lifetime, CV/DPV, adsorption measurement, TG/DTA and magnetic measurements. Compound 1 displays both good thermal and aqueous stability, which is important for future practical applications. The magnetic measurements revealed that compound 1 possesses antiferromagnetic interactions. Future research in our laboratory will focus on preparing other crystalline 4f–3d MPFs, to gain deeper insights into the synthetic methodology, as well as the crystal structures and properties. Acknowledgments This work was supported by the NSF of China (21361013), the NSF of Jiangxi Province (20132BAB203010), and the open foundation (No. 20130014) of the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Appendix A. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 964911. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CBZ
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Synthesis, structure, photophysical and electrochemical properties of a novel metalloporphyrin with a condensed three-dimensional porous open framework Wen-Tong Chen a,b,⇑, Zhi-Gang Luo a, Yin-Feng Wang a, Xian Zhang a, Hong-Ru Fu b a Institute of Applied Chemistry, School of Chemistry and Chemical Engineering, Jiangxi Province Key Laboratory of Coordination Chemistry, Jinggangshan University, Ji’an, Jiangxi 343009, China b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e
i n f o
Article history: Received 1 November 2013 Received in revised form 11 January 2014 Accepted 26 January 2014 Available online 1 February 2014 Keywords: Lanthanide Magnetic Metalloporphyrin Photoluminescence TPPS
a b s t r a c t A novel 4f–3d metalloporphyrin, [EuZn(TPPS)H3O]n (1) (H2TPPS = tetra(4-sulfonatophenyl)porphyrin), has been synthesized via a solvothermal reaction and structurally characterized by single-crystal X-ray diffraction. Compound 1 features a three-dimensional (3-D) structure with two kinds of one-dimensional (1D) infinite chains. Based on the condensed 3-D porous open framework, compound 1 has a large void space of 223 Å3, corresponding to 9.4% of the unit-cell volume. Compound 1 shows high thermal and aqueous stability. Magnetic measurements reveal that compound 1 has an antiferromagnetic behavior. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Porphyrins with plentiful properties are one of the most widely studied chemical systems. Porphyrins have been found their applications in the areas of optical, medicine, catalysis, adsorption, solar energy conversion, and so forth [1–10]. The modification of a porphyrin through inserting a metal ion into the center of a porphyrinic ring, or decorating the periphery of a porphyrinic ring, can provide various building blocks to prepare new porphyrinic compounds, which may have different chemical, electronic or redox properties. More and more efforts have thus far been devoted to synthesize metalloporphyrinic frameworks (MPFs) with reformative features that may allow them to be put into practical applications [11,12]. Nowadays, MPFs have gained increasing interest because of their useful characteristics in structural robustness, reaction catalysis, energy and electron transfer [13,14]. Tetraarylporphyrins like TPyP (meso-tetra(4-pyridyl)porphyrin), TCMOPP (tetrakis(4-(carboxymethyleneoxy)phenyl)porphyrin), TCPP (tetrakis(4-carboxyphenyl) porphyrin), and TDCPP (tetrakis(3,5-dicarboxyphenyl)porp hyrin) ⇑ Corresponding author at: Institute of Applied Chemistry, School of Chemistry and Chemical Engineering, Jiangxi Province Key Laboratory of Coordination Chemistry, Jinggangshan University, Ji’an, Jiangxi 343009, China. Tel./fax: +86 796 8119239. E-mail address: [email protected] (W.-T. Chen). http://dx.doi.org/10.1016/j.ica.2014.01.037 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.
are square planar building units and they have been widely adopted to synthesize novel MPFs [15–18]. However, H2TPPS (tetra(4-sulfonatophenyl)porphyrin) has been rarely used to prepare new MPFs, although it is one of the tetraarylporphyrins. To our knowledge, only two examples of TPPS-containing MPFs have so far been found [19,20]. Actually, the large, rigid and square planar symmetrical H2TPPS is a useful building unit for the construction of new MPFs because H2TPPS can provide a lot of coordination sites: one at the center of the macrocyclic porphyrin ring and several at the oxygen atoms of four sulfonic ligands. However, the crystal structures and physicochemical properties of the TPPS compounds have yet to be explored. Lanthanide compounds have attracted increasing attention because of their abundant properties, such as photoluminescence and magnetism [21,22]. In recent years, a lot of lanthanide compounds with various organic ligands have been prepared [23–25]. The zinc ion (Zn2+), as one of the 3d elements, is particularly attractive for the following reasons: the variety of coordination numbers and geometries provided by its d10 configuration, fluorescence, photoelectric properties, and its essential role in biological systems. Recently, we focus on the study of the 4f–3d TPPS MPFs that possibly have different frameworks and properties like gas adsorption, photoluminescence and magnetism. We report herein the rational preparation, crystal structure and various properties
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of a 4f–3d TPPS metalloporphyrin, [EuZn(TPPS)H3O]n (1) (H2TPPS = tetra(4-sulfonatophenyl)porphyrin), which was obtained via solvothermal reactions and features a condensed 3D porous open framework. It should be pointed out that compound 1 displays not only good thermal stability, but also wonderful aqueous stability; even it has been ultrasonicated for several hours in distilled water. Based on its good bistability and in order to explore its potential applications, we measured its properties as detailed as we can. The title compound was characterized in detail by using single-crystal X-ray diffraction, UV–Vis spectra, FT-IR, fluorescence, quantum yield, luminescence lifetime, CV/DPV, adsorption measurement, TG/DTA and magnetic measurements.
2. Experimental 2.1. Materials and instrumentation All reactants of A.R. grade were obtained commercially and used without further purification. Elemental analysis was carried out with an Elementar Vario EL III microanalyzer. The infrared spectra were recorded on a Nicolet 5DX FT-IR spectrophotometer over the frequency range 4000–400 cm 1 using the KBr pellet technique. The solution UV–Vis spectra were recorded at room temperature on a computer-controlled PE Lambda 900 UV–Vis spectrometer. The diffuse reflectance spectrum was recorded at room temperature on a computer-controlled Lambda 35 UV–Vis spectrometer equipped with an integrating sphere in the wavelength range 190/1100 nm. BaSO4 powder wad used as a reference (100% reflectance), on which the ground powder sample was coated. The absorption spectra were calculated from reflection spectra by the Kubelka–Munk function: a/S=(1-R)2/2R, a is the absorption coefficient, S is the scattering coefficient which is practically wavelength independent when the particle size is larger than 5 lm, and R is the reflectance. The band gap energy value was determined by extrapolation from the linear portion of the absorption edge in a (a/S) versus energy plot. The photoluminescent study was conducted on an Edinburgh FLS920 fluorescence spectroscopy instrument under room temperature. Measurement of an emission quantum yield of solution sample was carried out on a Hamamatsu C9920-0X(PMA-12) U6039-05 fluorescence spectrofluorometer with a integrating sphere adapted to a rightangle configuration at room temperature, involving determination of the diffuse reflectance spectra of the sample. The result was corrected for the detector response as a function of wavelength. Photoluminescent lifetime measurement was conducted using a Photon Technology International GL-3300 nitrogen laser with a Photon Technology International GL-302 dye laser and a nitrogen laser/pumped dye laser system equipped with a four-channel digital delay/pulse generator (Standard Research System Inc., model DG535) and a motor driver (Photon Technology International, model MD-5020). The excitation wavelength was 421 nm with use of a POPOP chromophore. Measurements of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed at 298 K using a BAS 100 W electrochemical analyzer in deaerated benzonitrile containing 0.1 M TBAPF6 (tetra-n-butylammonium hexafluorophosphate) as a supporting electrolyte. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to the Ag/AgNO3 (1.0 10 2 M). All electrochemical measurements were carried out under an atmospheric pressure of argon. Thermo-gravimetry (TG) and differential thermal analysis (DTA) were performed on an NETZSCH STA 449C analyzer.
The adsorption measurement of CO2 was conducted on an intelligent gravimetric sorption analyzer (IGA100B). As-synthesized sample was dipped in CH2Cl2 solutions for 24 h and evacuated under a dynamic vacuum (10 3 Torr) at 423 K for 8 h before the adsorption measurement. Variable-temperature magnetic susceptibility and field dependence magnetization measurements on polycrystalline samples were performed on a PPMS 9T Quantum Design SQUID magnetometer. All data were corrected for diamagnetism estimated from Pascal’s constants. 2.2. Synthesis of [EuZn(TPPS)H3O]n (1) The title compound was prepared by mixing EuCl36H2O (0.1 mmol, 37 mg), ZnBr2 (0.1 mmol, 22 mg), H2TPPS (0.1 mmol, 94 mg) and 10 mL distilled water in a 25 mL Teflon-lined stainless steel autoclave and heating the mixture at 473 K for 5 d. After cooling slowly the mixture to room temperature at a rate of 6 K/h, red crystals suitable for X-ray analysis were obtained. The yield was 41% (based on europium). Anal. Calc. for C44H27EuN4O13S4Zn: C, 45.35; H, 2.34; N, 4.81. Found: C, 45.27; H, 2.38; N, 4.87%. Fourier transform IR (KBr, cm 1): 3422(s), 3102(w), 2359(m), 1602(m), 1401(w), 1336(w), 1240(s), 1171(s), 1126(vs), 1052(s), 997(vs), 860(w), 809(m), 745(s), 667(w), and 640(s). 2.3. Crystal structure determination The intensity data set was collected on a Rigaku Mercury CCD X-ray diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å) using a x scan technique. CrystalClear software was used for data reduction and empirical absorption corrections [26]. The structure was solved by the direct method with the Siemens SHELXTL™ Version 5 package of crystallographic software [27]. The difference Fourier maps based on these atomic positions yield the non-hydrogen atoms, while the hydrogen atoms were generated theoretically, allowed to ride on their respective parent atoms and included in the structure factor calculations with assigned isotropic thermal parameters. The structure was refined using a full-matrix least-squares refinement on F2. All atoms except for hydrogen atoms were refined anisotropically. A summary of crystallographic data and structure analysis is listed in Table 1, and selected bond distances and bond angles are given in Table 2.
Table 1 Crystal data and structure refinement details for 1. Formula
C44H27EuN4O13S4Zn
Formula weight Color Crystal size (mm3) crystal system space group a (Å) c (Å) V (Å3) Z 2hmax (°) Reflections collected Independent, observed reflections (Rint) dcalcd (g/cm3) l (mm 1) T (K) F(0 0 0) R1, wR2 S Largest and mean D/r Dq(max, min) (e/Å3)
1165.27 red 0.25 0.15 0.14 tetragonal P4/mcc 15.507(5) 9.930(5) 2387.8(16) 2 50 11467 1040, 997 (0.0499) 1.621 2.045 123.15 1160 0.0481, 0.1224 1.016 0, 0 0.951, 0.584
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3
3. Results and discussion The FTIR bands of compound 1 are mainly occurred in the range of 640–1602 cm 1. The absorption band at 3422 cm 1 could be ascribed to the mO–H stretching vibration of the water molecule; the bands in the frequency range of 809–1240 cm 1 are probably resulted from the aromatic ring bending modes of the porphyrin. As for the free-base H2TPPS, two FTIR bands locating at around 3321 and 1351 cm 1 could be assigned to the cN–H and dN–H vibration modes of the pyrrole rings [28]. However, these two bands vanished from the FTIR spectrum of 1 because of the deprotonation and metalation of the pyrrole rings of H2TPPS. This is an important evidence for a free-base H2TPPS to be changed into a metalloporphyrin. X-ray diffraction analysis reveals that the crystal structure of 1 features a 3-D motif. The molecular structure of 1 is given as an ORTEP drawing in Fig. 1. Compound 1 consists of neutral [EuZn(TPPS)H3O]n molecules and crystallizes in the tetragonal P4/ mcc space group. The zinc atom is at the crystallographic inversion center of the perfectly coplanar 24-membered macrocyclic porphyrin core, binding to four nitrogen atoms and two oxygen atoms to construct a slightly distorted octahedron. The bond lengths of Zn–N are 2.066(3) Å, which is comparable with those previously reported [17,29]. The bond length of Zn–O is 2.1798(14) Å. The zinc atom is positionally disordered with the other eclipsed one obtained by the mirror plane and the site occupancy of each one is equal to 0.5 because both are crystallographically identical. The distance between the adjacent zinc atoms is 4.9648(24) Å that is short enough to allow a weak intermolecular Zn–O Zn interaction. There are two types of 1D infinite chains existing in the title compound. One is the –Eu–(SO3)4–Eu– chain and the other is –Zn– O–Zn– chain based on the intermolecular Zn–O Zn interaction, as shown in Fig. 2. These two chains are connected by the europium ions. The europium ion coordinates to eight oxygen atoms, forming a distorted square anti-prism with the bond length of Eu–O being 2.405(2) Å that is normal and comparable with that documented before [30–32]. The europium ions are interconnected via eight sulfonate groups to yield a 1D infinite –Eu–(SO3)4–Eu– chain with the Eu Eu distance being 4.9649(23) Å. These two chains connect together to construct a condensed 3D porous open framework with the channels running along the c-axis (Fig. 3). The displacement of the four Npyrrole atoms is 0 Å from their mean N4 plane. With respect to the N4 plane that represents the mean plane of the porphyrin core, all the twist angles of the aryl rings are exactly 90°. As shown in Fig. 3, it should be pointed out that compound 1 has a large void space being 223 Å3 that is 9.4% of the unit-cell volume. 3.1. Thermo-gravimetry (TG) and differential thermal analysis (DTA) As shown in Fig. 4, the thermal stability of compound 1 was studied by TG and DTA in flowing air. TG-DTA result exhibits that compound 1 is thermally stable up to 350 °C, which is among the highest decomposition temperature in polymers. A continuous weight loss from the beginning to 100 °C can be observed from Table 2 Selected bond lengths (Å) and bond angles (°). Eu(1)–O(1) Zn(1)–N(1) Zn(1)–O(1W) O(1)#1–Eu(1)–O(1)#2 O(1)#1–Eu(1)–O(1)#3 O(1)#1–Eu(1)–O(1)#4
2.405(2) 2.066(3) 2.1798(14) 126.07(10) 73.62(6) 88.58(11)
O(1)#2–Eu(1)–O(1)#3 O(1)#2–Eu(1)–O(1)#4 O(1)#3–Eu(1)–O(1)#4 N(1)–Zn(1)–N(1)#5 N(1)–Zn(1)–N(1)#6 N(1)–Zn(1)–O(1W)
Symmetry codes: #1 y, x 1, z; #2 x, y 1, y, z + 1/2; #5 y, x, z; #6 x, y, z.
z + 1/2; #3
x 1,
158.61(9) 115.85(13) 68.46(13) 88.771(8) 163.15(6) 98.42(3) y 1, z; #4
x 1,
Fig. 1. ORTEP drawing of 1 with 50% thermal ellipsoids. Hydrogen atoms and disordered Zn–O moieties were omitted for clarity.
the TG diagram. This weight loss could be just attributed to the escape of the moisture or air adsorbed in the voids or on the surface of the crystals. The TG diagram shows a sudden weight loss with a beginning temperature of about 350 °C and two endothermic peaks centered at 425 and 516 °C, respectively. The total weight loss of 62.1% is close to the calculated value of 62.8% at the oxidative decomposition to obtain Eu2O3 and ZnO. 3.2. Solution UV–Vis spectrum and solid state diffuse reflectance spectrum According to the four orbitals model brought forward by Gouterman [33], a metalloporphyrin generally exhibits two types of intensive absorption bands, namely, a strong B band (or Soret band) at 400 nm with a molar absorption coefficient e being 105 M 1 cm 1 and several weaker Q bands existing at 500– 650 nm with a e being 103–104 M 1 cm 1. Fig. 5 gives the UV– Vis absorption spectrum of 1 measured in benzonitrile under room temperature. The B band occurs at 427 nm with a e being 3.94 105 M 1 cm 1, which is in good agreement with the above conclusion made by Gouterman. The Q bands are observed at 559 and 599 nm. The number of Q bands of 1 is less than that of a free-base porphyrin that generally has more than three Q bands. Such a difference in the number of Q bands could be ascribed to the increase of the molecular symmetry, originating from the metalation of a porphyrin. As shown in Fig. 6, solid state diffuse reflectance spectrum of 1 exhibits the presence of an optical bandgap of 1.18 eV, which suggests that compound 1 is probably a narrow-gap semiconductor. The gradual slope of the optical absorption edge of 1 indicates the existence of an indirect transition [34]. To our knowledge, several highly efficient photovoltaic materials have thus far been prepared, such as CdTe (1.5 eV), GaAs (1.4 eV), CuInS2 (1.55 eV), and CuInSe2 (1.04 eV) [35–37]. The optical bandgap of 1.18 eV of 1 is close to that of CuInSe2. 3.3. Photoluminescence, lifetime and quantum yield Taking into account the excellent photoluminescent property of the europium, the photoluminescence was studied at room temperature. Photoluminescent investigations were carried out in benzonitrile because, to our knowledge, solid state porphyrins
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Fig. 2. The 1D chains in 1. (a) a –Eu–(SO3)4–Eu– chain; (b) a –Zn–O–Zn– chain. Disordered zinc ions were omitted for clarity.
Fig. 3. Space-filling illustration of the condensed 3-D porous open framework of 1.
and their derivatives cannot display emission bands due to their concentration quenching effect. For compound 1, the maximum photoluminescent emission band appears at 612 nm with a shoulder band at 660 nm, upon the excitation of 438 nm, as shown in Fig. 7. These photoluminescent emission bands could be ascribed to the characteristic emissions of the porphyrin. The absence of the characteristic emissions of the europium indicates that TPPS is not an effective antenna to transfer the adsorbed energy to the europium centers. Using a time-correlated single photon counting technique, the photoluminescent lifetime in benzonitrile solution was measured with an excitation wavelength of 421 nm, because this light can be absorbed by the metalloporphyrin. The time-resolved photoluminescent decay profile of 1 is given in Fig. 8. The
time decay curve was fitted as single exponential. The photoluminescent lifetime of compound 1 is 1.57 ms in benzonitrile. The emission quantum yield of the solution sample was also measured. The emission quantum yield of compound 1 is determined to be 1.5%. 3.4. Adsorption measurement of CO2 The adsorption data for CO2 collected at 273 K was shown in Fig. 9. The CO2 adsorption curve of 1 is a typical type-I shape adsorption isotherm and slowly rising, which indicates further high adsorption capacity under increasing pressure, as the case found in the literature [38]. The CO2 adsorption study reveals that
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Fig. 4. TG-DTA diagrams of 1.
2.0
400
427
300 Intensity(cps)
Absorbance (AU)
1.5
1.0
0.5
200
100
559 599
DMF 5 ¦ÌM
0.0
0
400
600
800
400
500
Wavelength (nm) Fig. 5. UV–Vis absorption spectrum measured at room temperature. Molar absorption coefficient (e/M 1 cm 1): e427 nm = 3.94 105.
700
800
Fig. 7. The excitation and emission spectra at room temperature (dashed line: excitation spectra; solid line: emission spectra).
from these MOFs (10 atm). At 273 K, no saturation behavior can be observed till the highest pressure obtainable by the apparatus. Such a CO2 adsorption ability is lower than most of MOFs, but higher than several [42]. In general, small gas molecules whose diameter is smaller than that of pore windows could go through the window and arrive anywhere of the pore of the open framework. However, the CO2 adsorption ability in compound 1 at 273 K is low, although the size of the pore window (5.6 Å 8.5 Å) is much larger than that of CO2 (3.3 Å). This phenomenon is uncommon for MOFs. The reason for that may be ascribed to that some CO2 molecules are strongly adsorbed by the window and block others to further enter the pores.
3
Eg = 1.18 eV
¦Á/S
2
1
3.5. Electrochemical studies
0 0.5
600 Wavelength(nm)
1.0
1.5
Eg (eV) Fig. 6. Diffuse reflectance spectrum of 1.
compound 1 adsorbed 0.8 mmol/g at the pressure up to 0.01 atm where the molar ratio of adsorbed CO2 to porphyrin moiety is 1. The CO2 uptake of 0.8 mmol/g in 1 seems lower than several famous MOFs, such as MIL-53 (ca. 8 mmol/g of CO2 at 10 atm) [39], MIL-96 (4.4 mmol/g of CO2 at 10 atm) [40], MIL-102 (3.1 mmol/g of CO2 at 10 atm) [41], but compound 1 was measured under very low pressure (only 0.01 atm), which is quite different
Kadish pointed out that the important factors mostly affecting the redox potential of a metalloporphyrin are the properties of the solvent, porphyrin itself and supporting electrolyte [43]. The redox potentials could be different up to 1.0 V or even much more, depending on the different substituents of the porphyrin. The measurements of the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) for compound 1 were conducted in benzonitrile and TBAPF6 (0.1 M) under room temperature. As shown in Fig. 10, slow sweep CV of 1 displays one quasi-reversible wave with E1/2 = 0.31 V that is close to the value of DPV (0.30 V). We also measured the CV and DPV for free-base H2TPPS under the same
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Fig. 8. Nanosecond transient spectra of 1 in benzonitrile.
3.6. Magnetic properties
CO2 absorbed (mmol/g)
0.8
0.6
0.4
0.2
0.0 0.000
0.002
0.004
0.006
0.008
0.010
P/P0 Fig. 9. CO2-adsorption isotherm of 1 at 273 K.
conditions as 1. The CV of free-base H2TPPS is characterized by one reversible redox wave with E1/2 = 0.84 V, which is similar to that of DPV ( 0.82 V) (as shown in the inset of Fig. 10).
The temperature dependent magnetic susceptibility of compound 1 was investigated from 300 to 2 K at 5000 Oe on the polycrystalline sample, as shown in Fig. 11a. The value of leff is 2.84 lB at 300 K, which is smaller than the expected value (3.40 lB) in the free-ion approximation for an isolated Eu3+ ion, and decreases with temperature to 0.35 lB at 2 K. When decreasing the temperature, the xM of compound 1 increases gradually from 3.23 10 3 emu/ mol at 300 K to reach 4.81 10 3 emu/mol at 25 K. On further cooling, the xM of compound 1 rises fast to about 9.24 10 3 emu/mol at 2 K. Such a behavior is typically observed for a paramagnetic system that displays a primarily antiferromagnetic interaction [44]. The origin of such an antiferromagnetic behavior of compound 1 could be ascribed to the progressive thermal depopulation of the Stark components of the europium ion, because the zinc ion is diamagnetic. The field dependence of the magnetization for compound 1 conducted at 2 K shows that the magnetization increases slowly with the increasing field. Clearly, the magnetization of compound 1 is unsaturated at 80 kOe with a value of 0.017 Nb, as shown in Fig. 11b.
Fig. 10. CV (green) and DPV (red) profiles for 1 under an argon atmosphere (Inset: free-base H2TPPS). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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1EZ, UK (Fax: +44-1223-336033; email: [email protected] or http://www.ccdc.cam.ac.uk). References
Fig. 11. (a) Thermal dependence of xM and effective magnetic moment of 1. (b) Magnetization vs H of 1.
In summary, by using a hydrothermal reaction, we have prepared a crystalline 4f–3d MPF, [EuZn(TPPS)H3O]n (1), which is characterized by a robust 3D porous open framework. This compound was characterized in detail by single-crystal X-ray diffraction, UV–Vis spectra, FT-IR, fluorescence, quantum yield, luminescence lifetime, CV/DPV, adsorption measurement, TG/DTA and magnetic measurements. Compound 1 displays both good thermal and aqueous stability, which is important for future practical applications. The magnetic measurements revealed that compound 1 possesses antiferromagnetic interactions. Future research in our laboratory will focus on preparing other crystalline 4f–3d MPFs, to gain deeper insights into the synthetic methodology, as well as the crystal structures and properties. Acknowledgments This work was supported by the NSF of China (21361013), the NSF of Jiangxi Province (20132BAB203010), and the open foundation (No. 20130014) of the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Appendix A. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 964911. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CBZ
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