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Synthesis, crystal structure and luminescent properties of a new samarium-fluorescein metal-organic framework
Journal of Molecular Structure 1098 (2015) 167e174
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, crystal structure and luminescent properties of a new samarium-fluorescein metal-organic framework Jesty Thomas*, K.S. Ambili Research Department of Chemistry, Kuriakose Elias College, Mannanam, Kottayam, Kerala 686561, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2015 Received in revised form 13 May 2015 Accepted 4 June 2015 Available online 9 June 2015
A new metal-organic framework with empirical formula C43H30NO12Sm was solvothermally synthesized using SmCl3, fluorescein and N, N-Dimethyl formamide (DMF) and characterized by single crystal X-ray diffraction, powder X-ray diffraction, infrared spectroscopy, UVeVisible spectroscopy, scanning electron microscopy, optical microscopy, photoluminescence spectroscopy, CHN elemental analysis and thermogravimetric analysis. Single crystal X-ray diffraction revealed that the crystal structure belongs to the triclinic system, P-1 space group with a ¼ 12.113 (6) Å, b ¼ 12.1734 (7) Å, c ¼ 13.2760(8) Å, a ¼ 67.930(3)⁰, b ¼ 87.779(3)⁰, g ¼ 77.603(3)⁰ and V ¼ 1769.71 (17) Å3. The photoluminescence spectrum showed emission peaks at 550 nm, 600 nm and 647 nm due to the characteristic transitions 4G5/2 to 6H5/2, 4G5/2 to 6 H7/2 and 4G5/2 to 6H9/2 respectively, when excited at 398 nm. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solvothermal synthesis Samarium-fluorescein metal-organic framework Photoluminescence C43H30NO12Sm
1. Introduction The design and synthesis of metal-organic frameworks (MOFs) containing lanthanide ions attracted much attention due to their versatile architecture and potential applications in catalysis, gas storage, magnetism, luminescence, sensors etc. [1e4]. The direct excitation of the 4f-4f transitions are parity forbidden and they consist mainly of magnetic dipole and electric dipole transitions. As a consequence of the forbidden nature, these transitions have relatively long excited state lifetime. This together with high colorimetric purity of the emitted light leads lanthanides as luminophores. In MOFs, the organic sensitizer (ligand) absorbs incident radiation and directly populates the f-excited state of lanthanides through energy transfer (antenna effect) and as a result they show high luminescence property [5]. The lanthanide centers have high and variable coordination numbers and flexible coordination environment. Hence it is possible to construct unusual topological frameworks and based on the selection of organic ligand, a variety of lanthanide complexes with various structures and properties can be obtained [6]. Also these complexes have excellent luminescent properties and give sharp and intense emission lines. Due to the high affinity of lanthanide ions to hard donor atoms,
ligands containing oxygen atoms have been extensively used in the synthesis of lanthanide complexes [7]. Fluorescein is an important ligand with oxygen atoms and shows high quantum yield of fluorescence and large absorption in visible region. It can give varied absorption spectra, fluorescence spectra, quantum yield and lifetime in different media as it exists in different structural forms [8]. Even though the synthesis and structural characterization of various fluorescein-metal complexes are reported [9], a metalorganic framework with samarium and fluorescein has not been explored to date. Here, we report the solvothermal synthesis, characterizations and luminescent property of a novel metalorganic framework with samarium and fluorescein. 2. Experimental section 2.1. Materials Fluorescein, N, N-Dimethyl formamide, and samarium chloride were purchased from M/S Merck Chemicals. All organic solvents were used without further purification. Distilled water was used throughout the experiment. 2.2. Synthesis of the compound
* Corresponding author. E-mail address: [email protected] (J. Thomas). http://dx.doi.org/10.1016/j.molstruc.2015.06.023 0022-2860/© 2015 Elsevier B.V. All rights reserved.
0.0664 g of fluorescein was dissolved in 6.0 mL of DMF/H2O (2:1) mixture. To this 0.4 mL of 0.5M aqueous SmCl3 solution was
J. Thomas, K.S. Ambili / Journal of Molecular Structure 1098 (2015) 167e174
added with stirring. After adjusting the pH to 7.0, the mixture was stirred for 1 h and transferred in to a stainless steel teflon lined autoclave. The solvothermal reaction was proceeded at 393 K for 48 h and then cooled to room temperature to get red needle shaped crystals arranged in a flower like manner. The crystals were filtered, washed with water and dried. The proposed reaction mechanism is given in Scheme 1. Elemental analyzes (%): calcd for C43H30NO12Sm (903.03): C 57.18, H 3.34, N 1.55. Found: C 57.13, H 3.33, N 1.54. 2.3. Measurements The single crystal X-Ray data collection for the crystal was performed using Bruker X8 KAPPA APEX II and powder X-ray diffraction patterns were obtained from Bruker D8 advanced diffractometer. Elemental analyses were carried out on Elementar Vario EL III instrument. FTIR spectra were taken within 400e4000 cm-1 region on PerkineElmer Spectrum 400FTIR/FTFIR spectrophotometer. Scanning electron microscopic image (SEM) was obtained from a JEOM JSM-6700 field-emission scanning electron microscope and optical microscopic image was taken by using Grandtec optical microscope. UVevisible absorption spectra were recorded from a UV-2400PC series spectrophotometer and fluorescence excitation and emission spectra were obtained on a SL174 spectrofluorometer using 150 W Xenon lamp as excitation source. Fluorescence lifetimes were measured using an IBH Picosecond single photon counting system. Shimadzu DTG-60 equipment was used for recording the thermogram of the synthesized crystals. 3. Results and discussion 3.1. Infrared spectra IR spectra of fluorescein and the MOF are shown in Fig. 1. The peak due to C]O stretching vibration (n(C]O)) at 1715 cm-1 in fluorescein disappears in the IR spectrum of MOF [10]. Also the difference between asymmetric (1589 cm-1) and symmetric (1457 cm-1 and 1385 cm-1) stretching of carboxylate group in the IR spectrum of fluorescein (nas - ns ¼ 132 and 204 cm-1) is decreased (nas - ns ¼ 112 and 184 cm-1) in the IR spectrum of complex indicating that the carboxylate groups are coordinated to the samarium ion in the complex [11]. Further the peak at 3068 cm-1 in
MOF
Transmittance (a.u)
168
3068 fluorescein
1715 1569 1457 1385 1589
500
1000
1500
2000
2500
3000
Fig. 1. Solid state Infrared spectra of fluorescein and synthesized MOF.
fluorescein due to the OeH stretching is absent in the IR spectrum of MOF, which reveals the involvement of OeH group in bond formation [12]. 3.2. Crystal structure The structure of the synthesized crystal was characterized by single crystal X-ray diffraction analysis and the details of crystal data and data collection parameters are shown in Table 1. The compound crystallizes in the triclinic system, space group P-1with a ¼ 12.113 (6) A⁰, b ¼ 12.1734 (7) A⁰, c ¼ 13.2760(8) A⁰, a ¼ 67.930⁰, b ¼ 87.779⁰, g ¼ 77.603⁰. The analysis reveals that the compound has the empirical formula C43H30NO12Sm with formula weight 903.03. The compound shows a network structure in which the Sm3þ ion at the centre is coordinated to five different fluorescein molecules and a water molecule, giving distorted pentagonal bipyramidal geometry. The asymmetric unit of the MOF is shown in Fig. 2 and the coordination environment of the central Sm3þ ion is given in Fig. 3. One fluorescein molecule is coordinated to the Sm3þ ion through the oxygen atoms of the carboxylate group. Second and
HO
O O
O OH
+ SmCl3
DMF/H2O
OH
O
O
HO
O
O
O H2O
393 K for 48 hrs HO
3500
wavenumber (cm-1)
Sm
O 2
O
OH O O
O
Scheme 1. Proposed reaction mechanism.
O O
O
OH
O
O
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Table 1 Crystallographic data and structure refinements for the complex. Empirical formula Formula weight Temperature Wavelength Crystal system space group a (A ) b (A ) c (A ) Alpha ( ) Beta ( ) Gamma ( ) Volume Z Calculated density
C43 H30 N O12 Sm 903.03 293(2) K 0.71073 A Triclinic P-1 12.1113(6) 12.1734(7) 13.2760(8 67.930(3) 87.799(3) 77.603(3) 1769.71(17) A^3 2 1.695 Mg/m^3
Crystal size Q range for data collection Limiting indices Reflections collected Reflections unique R(int) Completeness to theta Max. and min. transmission Goodness-of-fit on F^2 Final R indices R indices (all data) Extinction coefficient Largest diff. peak and hole Absorption coefficient F(000)
third fluorescein molecules are coordinated to the Sm3þ ion through the oxygen atoms of keto groups. The fourth fluorescein molecule is coordinated through the OeH group of COOH and the fifth one is coordinated through its OH group. The seventh coordination site of the central Sm3þ is satisfied by a water molecule. The selected bond lengths and bond angles of the MOF are shown in Table 2 and is clear that SmeO bond lengths and O-Sm-O bond angles are in the range 2.1933 (15) - 2.4458 (17) and 73.55 164.57, respectively, which are comparable with the bond lengths and bond angles of Er3þ-fluorescein complex with similar structure reported by Huang et al. [9]. Table 3 shows the hydrogen bond distances and angles of the MOF and Fig. 4 shows the pack along Hbond.
0.25 0.20 0.15 mm 2.24e25.00 deg 14 h<¼14, 14 k<¼14, 15 l<¼15 31330 6232 0.0285 25.00 99.9% 0.7965 and 0.6532 1.119 [I > 2sigma(I)] R1 ¼ 0.0164, wR2 ¼ 0.0407 R1 ¼ 0.0185, wR2 ¼ 0.0420 0.00226(15) 0.353 and 0.290 e.A^-3 1.732 mm^-1 906
3.4. PXRD PXRD patterns (Fig. 7) indicate that the as synthesized MOF is highly crystalline and it matches with the simulated one except for some intensity differences. To study the effect of water molecule on the stability of the MOF, the synthesized MOF was heated at 180 C for 2 h based on the TG analysis. After heating the coordinated water molecules are lost and the color of the sample was changed from red to brown. The PXRD pattern of the dehydrated sample is identical to the simulated and as synthesized MOF which indicate that the frame work is stable after the removal of coordinated water molecule [14]. 3.5. UVeVis spectroscopy
3.3. Morphology The scanning electron micrograph (Fig. 5) and optical microscopic image (Fig. 6) of the synthesized metal-organic framework show uniform distribution of needle like crystals [13]. Also the needle like crystals arranged to get flower like appearance of the MOF (Fig. 6).
The UV visible absorption spectra of fluorescein and the synthesized MOF dissolved in DMF are shown in Fig. 8. The broad peak from 380 to 525 centered at 456 nm is due to the major p/ p* transition of the ligand (fluorescein). There is a red shift (from 456 to 513) for this transition in the spectrum of the MOF due to the formation of more extensive conjugated system between Sm3þ and
Fig. 2. Asymmetric unit of the MOF.
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Fig. 3. Coordination environment of Sm3þ ion in the MOF (hydrogen atoms are omitted for clarity).
fluorescein showing the expansion of p conjugated system [15]. Also the intensity of absorption peak is increased in MOF due to metal coordination [16].
Table 2 Selected bond distances (Å) and bond angles ( ) for the synthesized compound. O(2)-Sm(1) O(3)-Sm(1) O(4)-Sm(1) O(6)-H(6A) O(8)-Sm(1) O(10)-Sm(1) O(11)-Sm(1)#3 Sm(1)-O(8)#4 Sm(1)-O(1) Sm(1)-O(10)#3 Sm(1)-O(11)#3 Sm(1)-C(40)#3 O(1)-H(1B) O(1)-H(1A) O(10)-C(40)-Sm(1)#3 O(11)-C(40)-Sm(1)#3 C(39)-C(40)-Sm(1)#3 C(21)-O(2)-Sm(1) C(1)-O(3)-Sm(1) C(22)-O(4)-Sm(1) C(20)-O(8)-Sm(1)#4 C(40)-O(10)-Sm(1)#3 C(40)-O(11)-Sm(1)#3 O(2)-Sm(1)-O(8)#4 O(2)-Sm(1)-O(4) O(8)#4-Sm(1)-O(4)
2.1933(15) 2.2776(17) 2.2602(16) 0.8200 2.2564(16) 2.4242(16) 2.4458(17) 2.2563(16) 2.3752(17) 2.4243(16) 2.4458(17) 2.795(2) 0.915(17) 0.920(18) 59.95(12) 60.97(12) 173.78(15) 148.92(16) 137.25(17) 140.52(15) 131.68(15) 93.65(13) 92.40(13) 164.57(6) 91.45(6) 100.06(6)
O(2)-Sm(1)-O(3) O(8)#4-Sm(1)-O(3) O(4)-Sm(1)-O(3) O(2)-Sm(1)-O(1) O(8)#4-Sm(1)-O(1) O(4)-Sm(1)-O(1) O(3)-Sm(1)-O(1) O(2)-Sm(1)-O(10)#3 O(8)#4-Sm(1)-O(10)# O(4)-Sm(1)-O(10)#3 O(3)-Sm(1)-O(10)#3 O(1)-Sm(1)-O(10)#3 O(2)-Sm(1)-O(11)#3 O(8)#4-Sm(1)-(11)#3 O(4)-Sm(1)-O(11)#3 O(3)-Sm(1)-O(11)#3 O(1)-Sm(1)-O(11)#3 O(10)#3-Sm(1)-O(11)#3 O(2)-Sm(1)-C(40)#3 O(8)#4-Sm(1)-C(40)#3 O(4)-Sm(1)-C(40)#3 O(1)-Sm(1)-C(40)#3 O(10)#3-Sm(1)-C(40) O(11)#3-Sm(1)-C(40) Sm(1)-O(1)-H(1B) Sm(1)-O(1)-H(1A)
90.52(6) 81.33(6) 80.66(7) 82.92(6) 82.76(7) 159.61(7) 79.82(7) 108.04(6) 84.84(6) 76.27(6) 150.51(6) 124.10(6) 88.62(6) 92.79(6) 126.08(6) 153.26(6) 73.55(6) 52.90(5) 97.42(6) 90.59(6) 100.57(6) 99.58(6) 26.39(6) 26.64(6) 123.4(19) 130(2)
3.6. Photoluminescence properties The solid state excitation spectrum of the synthesized MOF recorded at 600 nm (the strongest emission wavelength of Sm3þ ion) is shown in Fig. 9. The peaks at 361 nm and 398 nm are due to the ligand excitation and the peak at 468 nm is due to the direct excitation of the Sm3þ ion [17]. The peaks due to the ligand excitation are stronger than the direct excitation of Sm3þ ion which
Fig. 4. Pack along H bond.
Table 3 Hydrogen bond lengths (Å) and bond angles ( ) of the MOF. D-H…A
d(D-H)
d(H…A)
d(D…A)
<(DHA)
C(2)-H(2)…O(6)#2 C(10)-H(10)…O(7)#5 C(27)-H(27)…O(10)#3 C(42)-H(42A)…O(7)#4 O(6)-H(6A)…O(7)#5 O(1)-H(1B)…O(12)#1 O(1)-H(1A)…O(11)#6
0.93 0.93 0.93 0.96 0.82 0.915(17) 0.920(18)
2.63 2.48 2.48 2.79 1.86 1.760(18) 1.87(2)
3.521(3) 3.110(3) 3.089(3) 3.420(4) 2.628(3) 2.671(3) 2.756(3)
159.6 124.7 123.4 124.1 154.7 173(3) 162(3)
Symmetry transformations used to generate equivalent atoms: #1 x-1,y,z #2 xþ1,y,z #3 -xþ2,-y,-zþ3 #4 -xþ1,-yþ1,-zþ2 #5 -x,-yþ1,-zþ2 #6 x-1,yþ1,z.
Fig. 5. SEM image of the synthesized MOF.
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Fig. 6. Photograph and Microscopic image of the synthesized MOF crystals.
513 MOF
Absorbance (a.u)
indicates that the luminescence sensitization through the excitation of ligand is much more efficient than the direct excitation of the Sm3þ ion. Further, the efficiency of intramolecular energy transfer from an organic ligand to rare earth ion is an important factor which influences the luminescence properties of rare earth complexes [18]. The solid state absorption spectrum of the ligand shows a broad peak (inset Fig. 9) which overlaps with the excitation spectrum of the MOF, indicating the energy transfer from ligand (fluorescein) to Sm3þ. Thus the coordination and sensitization of Sm3þ ions to fluorescein molecules are confirmed [19]. The emission spectra of the synthesized MOF at different excitation wavelengths are shown in Fig. 10. The emission spectrum shows three main lines at 550, 600 and 647 nm, due to the transitions, 4G5/2 / 6H5/2, 4G5/2 / 6H7/2 and 4G5/2 / 6H9/2, respectively, when excited at 398 nm. Here the magnetic dipole transition 4 G5/2 / 6H7/2 gives the strongest emission band as reported elsewhere [17,20]. Absence of emission bands from the ligand reveals that the ligand transfers the excitation energy efficiently to Sm3þ ion. The peaks due to 4G5/2 / 6H5/2 and 4G5/2 / 6H9/2 transitions were observed in the emission spectra when excited at 361 nm and 468 nm, respectively and all other Sm3þ based emissions are quenched which indicates that the direct excitation of the Sm3þ center is less efficient and the high luminescence property of the MOF is due to the antenna effect between the ligand (fluorescein) and Sm3þ ion. The results also showed that the excitation
fluorescein 456
400
450
500
550
600
650
Wavelength (nm) Fig. 8. UVeVisible absorption spectra of fluorescein and synthesized MOF in DMF.
Intensity (a.u)
Dehydrated
As-synthesized
Simulated
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 7. PXRD patterns of the simulated, as-synthesized and dehydrated samples.
Fig. 9. Solid-state excitation spectrum (lem ¼ 600 nm) of the synthesized MOF and solid state absorption spectrum of fluorescein (inset).
172
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wavelength affects the intensity of different transitions. The excitation and emission spectra of the dehydrated sample (Fig. 11) were also analyzed. It showed excitation peaks at 311 nm and 329 nm. The emission spectrum of the dehydrated sample at an excitation wavelength of 329 nm showed characteristic emission of Sm3þ ion confirming intramolecular energy transfer from ligand to metal center. The intensity of magnetic dipolar transition is decreased in the dehydrated sample due to the change in coordination environment of the sm3þ ion. Even though water molecule acts as a luminescence quencher the quenching effect is overcome in the synthesized MOF [21]. The absence of electric dipolar transition in the dehydrated sample reveals that the local environment of the Sm3þ ion is different in the as synthesized and dehydrated sample [22]. The photophysical property of the MOF in DMF was also studied and the excitation and emission spectra are shown in Fig. 12. The excitation spectrum shows peak at 361 nm due to the its absorption followed by excitation of the ligand in the UV region [23]. The emission spectrum showed a peak ranging from 400 to 490 nm with intensity maxima at 435 nm which corresponding to its blue emission band. The fluorescence decay profiles were deconvoluted using IBH datastation software V2.1, fitted with exponential decay and minimizing the c2 values of the fit to 1 ± 0.1 (1.03). The emission decay curve of the MOF in DMF was fitted with second exponential function and is shown in Fig. 13. Since the decay is second exponential, there are two different sites for luminescence and the lifetime values are t1 ¼ 0.87 ns (86.33%) and t2 ¼ 4.65 ns (13.67%) (the goodness-of-fit parameter c2 ¼ 1.03). The quantum yield of the MOF in DMF was determined by single point method using the equation;
Q ¼ Q R ½I=IR ½ODR =OD½n2 =n2R
i
where, ‘Q’ is the fluorescence quantum yield ‘I’ is the integrated fluorescence intensity, ‘n’ is the refractive index of the solvent and ‘OD’ is the optical density (absorption). The subscript ‘R’ refers to the reference fluorophore of known quantum yield. In this study, rhodamine B in ethanol is used as the standard reference [24,25]. The quantum yield of the synthesized MOF is high (16 ± 0.5%) compared to other samarium-organic complexes [26,27]. Also the quantum yield value is decreased to 5.99 ± 0.5% in the dehydrated sample. The low quantum yield value may be due to the absence of
Fig. 10. Solid state emission spectra of the synthesized MOF.
Fig. 11. Solid state excitation spectrum (lem ¼ 600 nm) and emission spectrum (lex ¼ 329 nm) of dehydrated sample.
electric dipolar transition in the dehydrated sample [22]. 3.7. Thermal analysis Fig. 14 shows the TG-DTA trace of the synthesized MOF conducted at a heating rate of 10⁰C/min. The MOF has lost 10% of its mass up to 318 C in two stages. This weight loss is due to the loss of coordinated water molecule [28] and the evaporation of the solvent DMF without the decomposition of chemical bonds [29]. Immediately after the removal of water and DMF, the third stage starts and it shows maximum endothermic decomposition peak at 532 C due to the decomposition of organic part. The graph shows 38.8% of weight loss and the decomposition was not completed up to 700 C [30]. 4. Conclusion A new samarium-fluorescein metal-organic framework with luminescence property has been successfully synthesized and fully characterized. The scanning electron micrograph and optical
Fig. 12. Excitation spectrum (lem ¼ 435 nm) and emission spectrum (lex ¼ 361 nm) of the MOF in DMF, Inset: Photograph of the MOF in DMF under UV light.
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References
Fig. 13. Decay profile for the MOF in DMF monitored around 435 nm and excited at 375 nm.
Fig. 14. TG-DTG curve of MOF.
microscopic image showed uniform needle like appearance of the synthesized MOF. The excitation spectrum of the MOF showed two major peaks due to the ligand excitation and the emission spectra taken at different excitation wavelengths showed the characteristic emission peaks of the central Sm3þ ion, which confirmed the energy transfer from the ligand to the metal center. The effect of coordinated H2O molecules on the luminescent properties of MOF was also studied. Photophysical property of the MOF in DMF was also studied. The strong luminescence property and thermal stability of the MOF reveals that it can be used as a good candidate for efficient luminescent materials. Acknowledgment This work is financially supported by Department of Science and Technology (DST), New Delhi, India under “Fast Track Young Scientist Scheme” (No: SR/FT/CS-33/2011). Authors thank Dr. Joshy Joseph, CSIR-NIIST, Thiruvananthapuram for lifetime measurement. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2015.06.023.
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Vasudevan, Molecular ladders of lanthanide-3-phenyl-4-benzoyl-5-isoxazolonate and bis(2-(diphenylphosphino) phenyl) ether oxide complexes: the role of ancillaryligand in the sensitization of Eu3þ and Tb3þ luminescence, Cryst. Growth Des. 9 (2009) 3562e3569. [22] S. Biju, Y.K. Eom, G.B. Jean-Claude, H.K. Kim, A new tetrakis b diketone ligand for NIR emitting LnIII ions: luminescent doped PMMA films and flexible resins for advanced photonic applications, J. Mater. Chem. C 1 (2013) 6935e6944. [23] R. Sjoback, J. Nygren, M. Kubista, Absorption and fluorescence properties of fluorescein, Spectrochim. Acta Part A. 51 (1995) L7eL21. [24] A.M. Brouwer, Standards for photoluminescence quantum yield measurements in solution (IUPAC Technical Report)*, Pure Appl. Chem. 83 (2011) 2213e2228. [25] Z. Yonghui, C. Mindong, G. Shengli, X. Jianqiang, G. Guizhi, K. Qinggang,
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, crystal structure and luminescent properties of a new samarium-fluorescein metal-organic framework Jesty Thomas*, K.S. Ambili Research Department of Chemistry, Kuriakose Elias College, Mannanam, Kottayam, Kerala 686561, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2015 Received in revised form 13 May 2015 Accepted 4 June 2015 Available online 9 June 2015
A new metal-organic framework with empirical formula C43H30NO12Sm was solvothermally synthesized using SmCl3, fluorescein and N, N-Dimethyl formamide (DMF) and characterized by single crystal X-ray diffraction, powder X-ray diffraction, infrared spectroscopy, UVeVisible spectroscopy, scanning electron microscopy, optical microscopy, photoluminescence spectroscopy, CHN elemental analysis and thermogravimetric analysis. Single crystal X-ray diffraction revealed that the crystal structure belongs to the triclinic system, P-1 space group with a ¼ 12.113 (6) Å, b ¼ 12.1734 (7) Å, c ¼ 13.2760(8) Å, a ¼ 67.930(3)⁰, b ¼ 87.779(3)⁰, g ¼ 77.603(3)⁰ and V ¼ 1769.71 (17) Å3. The photoluminescence spectrum showed emission peaks at 550 nm, 600 nm and 647 nm due to the characteristic transitions 4G5/2 to 6H5/2, 4G5/2 to 6 H7/2 and 4G5/2 to 6H9/2 respectively, when excited at 398 nm. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solvothermal synthesis Samarium-fluorescein metal-organic framework Photoluminescence C43H30NO12Sm
1. Introduction The design and synthesis of metal-organic frameworks (MOFs) containing lanthanide ions attracted much attention due to their versatile architecture and potential applications in catalysis, gas storage, magnetism, luminescence, sensors etc. [1e4]. The direct excitation of the 4f-4f transitions are parity forbidden and they consist mainly of magnetic dipole and electric dipole transitions. As a consequence of the forbidden nature, these transitions have relatively long excited state lifetime. This together with high colorimetric purity of the emitted light leads lanthanides as luminophores. In MOFs, the organic sensitizer (ligand) absorbs incident radiation and directly populates the f-excited state of lanthanides through energy transfer (antenna effect) and as a result they show high luminescence property [5]. The lanthanide centers have high and variable coordination numbers and flexible coordination environment. Hence it is possible to construct unusual topological frameworks and based on the selection of organic ligand, a variety of lanthanide complexes with various structures and properties can be obtained [6]. Also these complexes have excellent luminescent properties and give sharp and intense emission lines. Due to the high affinity of lanthanide ions to hard donor atoms,
ligands containing oxygen atoms have been extensively used in the synthesis of lanthanide complexes [7]. Fluorescein is an important ligand with oxygen atoms and shows high quantum yield of fluorescence and large absorption in visible region. It can give varied absorption spectra, fluorescence spectra, quantum yield and lifetime in different media as it exists in different structural forms [8]. Even though the synthesis and structural characterization of various fluorescein-metal complexes are reported [9], a metalorganic framework with samarium and fluorescein has not been explored to date. Here, we report the solvothermal synthesis, characterizations and luminescent property of a novel metalorganic framework with samarium and fluorescein. 2. Experimental section 2.1. Materials Fluorescein, N, N-Dimethyl formamide, and samarium chloride were purchased from M/S Merck Chemicals. All organic solvents were used without further purification. Distilled water was used throughout the experiment. 2.2. Synthesis of the compound
* Corresponding author. E-mail address: [email protected] (J. Thomas). http://dx.doi.org/10.1016/j.molstruc.2015.06.023 0022-2860/© 2015 Elsevier B.V. All rights reserved.
0.0664 g of fluorescein was dissolved in 6.0 mL of DMF/H2O (2:1) mixture. To this 0.4 mL of 0.5M aqueous SmCl3 solution was
J. Thomas, K.S. Ambili / Journal of Molecular Structure 1098 (2015) 167e174
added with stirring. After adjusting the pH to 7.0, the mixture was stirred for 1 h and transferred in to a stainless steel teflon lined autoclave. The solvothermal reaction was proceeded at 393 K for 48 h and then cooled to room temperature to get red needle shaped crystals arranged in a flower like manner. The crystals were filtered, washed with water and dried. The proposed reaction mechanism is given in Scheme 1. Elemental analyzes (%): calcd for C43H30NO12Sm (903.03): C 57.18, H 3.34, N 1.55. Found: C 57.13, H 3.33, N 1.54. 2.3. Measurements The single crystal X-Ray data collection for the crystal was performed using Bruker X8 KAPPA APEX II and powder X-ray diffraction patterns were obtained from Bruker D8 advanced diffractometer. Elemental analyses were carried out on Elementar Vario EL III instrument. FTIR spectra were taken within 400e4000 cm-1 region on PerkineElmer Spectrum 400FTIR/FTFIR spectrophotometer. Scanning electron microscopic image (SEM) was obtained from a JEOM JSM-6700 field-emission scanning electron microscope and optical microscopic image was taken by using Grandtec optical microscope. UVevisible absorption spectra were recorded from a UV-2400PC series spectrophotometer and fluorescence excitation and emission spectra were obtained on a SL174 spectrofluorometer using 150 W Xenon lamp as excitation source. Fluorescence lifetimes were measured using an IBH Picosecond single photon counting system. Shimadzu DTG-60 equipment was used for recording the thermogram of the synthesized crystals. 3. Results and discussion 3.1. Infrared spectra IR spectra of fluorescein and the MOF are shown in Fig. 1. The peak due to C]O stretching vibration (n(C]O)) at 1715 cm-1 in fluorescein disappears in the IR spectrum of MOF [10]. Also the difference between asymmetric (1589 cm-1) and symmetric (1457 cm-1 and 1385 cm-1) stretching of carboxylate group in the IR spectrum of fluorescein (nas - ns ¼ 132 and 204 cm-1) is decreased (nas - ns ¼ 112 and 184 cm-1) in the IR spectrum of complex indicating that the carboxylate groups are coordinated to the samarium ion in the complex [11]. Further the peak at 3068 cm-1 in
MOF
Transmittance (a.u)
168
3068 fluorescein
1715 1569 1457 1385 1589
500
1000
1500
2000
2500
3000
Fig. 1. Solid state Infrared spectra of fluorescein and synthesized MOF.
fluorescein due to the OeH stretching is absent in the IR spectrum of MOF, which reveals the involvement of OeH group in bond formation [12]. 3.2. Crystal structure The structure of the synthesized crystal was characterized by single crystal X-ray diffraction analysis and the details of crystal data and data collection parameters are shown in Table 1. The compound crystallizes in the triclinic system, space group P-1with a ¼ 12.113 (6) A⁰, b ¼ 12.1734 (7) A⁰, c ¼ 13.2760(8) A⁰, a ¼ 67.930⁰, b ¼ 87.779⁰, g ¼ 77.603⁰. The analysis reveals that the compound has the empirical formula C43H30NO12Sm with formula weight 903.03. The compound shows a network structure in which the Sm3þ ion at the centre is coordinated to five different fluorescein molecules and a water molecule, giving distorted pentagonal bipyramidal geometry. The asymmetric unit of the MOF is shown in Fig. 2 and the coordination environment of the central Sm3þ ion is given in Fig. 3. One fluorescein molecule is coordinated to the Sm3þ ion through the oxygen atoms of the carboxylate group. Second and
HO
O O
O OH
+ SmCl3
DMF/H2O
OH
O
O
HO
O
O
O H2O
393 K for 48 hrs HO
3500
wavenumber (cm-1)
Sm
O 2
O
OH O O
O
Scheme 1. Proposed reaction mechanism.
O O
O
OH
O
O
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Table 1 Crystallographic data and structure refinements for the complex. Empirical formula Formula weight Temperature Wavelength Crystal system space group a (A ) b (A ) c (A ) Alpha ( ) Beta ( ) Gamma ( ) Volume Z Calculated density
C43 H30 N O12 Sm 903.03 293(2) K 0.71073 A Triclinic P-1 12.1113(6) 12.1734(7) 13.2760(8 67.930(3) 87.799(3) 77.603(3) 1769.71(17) A^3 2 1.695 Mg/m^3
Crystal size Q range for data collection Limiting indices Reflections collected Reflections unique R(int) Completeness to theta Max. and min. transmission Goodness-of-fit on F^2 Final R indices R indices (all data) Extinction coefficient Largest diff. peak and hole Absorption coefficient F(000)
third fluorescein molecules are coordinated to the Sm3þ ion through the oxygen atoms of keto groups. The fourth fluorescein molecule is coordinated through the OeH group of COOH and the fifth one is coordinated through its OH group. The seventh coordination site of the central Sm3þ is satisfied by a water molecule. The selected bond lengths and bond angles of the MOF are shown in Table 2 and is clear that SmeO bond lengths and O-Sm-O bond angles are in the range 2.1933 (15) - 2.4458 (17) and 73.55 164.57, respectively, which are comparable with the bond lengths and bond angles of Er3þ-fluorescein complex with similar structure reported by Huang et al. [9]. Table 3 shows the hydrogen bond distances and angles of the MOF and Fig. 4 shows the pack along Hbond.
0.25 0.20 0.15 mm 2.24e25.00 deg 14 h<¼14, 14 k<¼14, 15 l<¼15 31330 6232 0.0285 25.00 99.9% 0.7965 and 0.6532 1.119 [I > 2sigma(I)] R1 ¼ 0.0164, wR2 ¼ 0.0407 R1 ¼ 0.0185, wR2 ¼ 0.0420 0.00226(15) 0.353 and 0.290 e.A^-3 1.732 mm^-1 906
3.4. PXRD PXRD patterns (Fig. 7) indicate that the as synthesized MOF is highly crystalline and it matches with the simulated one except for some intensity differences. To study the effect of water molecule on the stability of the MOF, the synthesized MOF was heated at 180 C for 2 h based on the TG analysis. After heating the coordinated water molecules are lost and the color of the sample was changed from red to brown. The PXRD pattern of the dehydrated sample is identical to the simulated and as synthesized MOF which indicate that the frame work is stable after the removal of coordinated water molecule [14]. 3.5. UVeVis spectroscopy
3.3. Morphology The scanning electron micrograph (Fig. 5) and optical microscopic image (Fig. 6) of the synthesized metal-organic framework show uniform distribution of needle like crystals [13]. Also the needle like crystals arranged to get flower like appearance of the MOF (Fig. 6).
The UV visible absorption spectra of fluorescein and the synthesized MOF dissolved in DMF are shown in Fig. 8. The broad peak from 380 to 525 centered at 456 nm is due to the major p/ p* transition of the ligand (fluorescein). There is a red shift (from 456 to 513) for this transition in the spectrum of the MOF due to the formation of more extensive conjugated system between Sm3þ and
Fig. 2. Asymmetric unit of the MOF.
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Fig. 3. Coordination environment of Sm3þ ion in the MOF (hydrogen atoms are omitted for clarity).
fluorescein showing the expansion of p conjugated system [15]. Also the intensity of absorption peak is increased in MOF due to metal coordination [16].
Table 2 Selected bond distances (Å) and bond angles ( ) for the synthesized compound. O(2)-Sm(1) O(3)-Sm(1) O(4)-Sm(1) O(6)-H(6A) O(8)-Sm(1) O(10)-Sm(1) O(11)-Sm(1)#3 Sm(1)-O(8)#4 Sm(1)-O(1) Sm(1)-O(10)#3 Sm(1)-O(11)#3 Sm(1)-C(40)#3 O(1)-H(1B) O(1)-H(1A) O(10)-C(40)-Sm(1)#3 O(11)-C(40)-Sm(1)#3 C(39)-C(40)-Sm(1)#3 C(21)-O(2)-Sm(1) C(1)-O(3)-Sm(1) C(22)-O(4)-Sm(1) C(20)-O(8)-Sm(1)#4 C(40)-O(10)-Sm(1)#3 C(40)-O(11)-Sm(1)#3 O(2)-Sm(1)-O(8)#4 O(2)-Sm(1)-O(4) O(8)#4-Sm(1)-O(4)
2.1933(15) 2.2776(17) 2.2602(16) 0.8200 2.2564(16) 2.4242(16) 2.4458(17) 2.2563(16) 2.3752(17) 2.4243(16) 2.4458(17) 2.795(2) 0.915(17) 0.920(18) 59.95(12) 60.97(12) 173.78(15) 148.92(16) 137.25(17) 140.52(15) 131.68(15) 93.65(13) 92.40(13) 164.57(6) 91.45(6) 100.06(6)
O(2)-Sm(1)-O(3) O(8)#4-Sm(1)-O(3) O(4)-Sm(1)-O(3) O(2)-Sm(1)-O(1) O(8)#4-Sm(1)-O(1) O(4)-Sm(1)-O(1) O(3)-Sm(1)-O(1) O(2)-Sm(1)-O(10)#3 O(8)#4-Sm(1)-O(10)# O(4)-Sm(1)-O(10)#3 O(3)-Sm(1)-O(10)#3 O(1)-Sm(1)-O(10)#3 O(2)-Sm(1)-O(11)#3 O(8)#4-Sm(1)-(11)#3 O(4)-Sm(1)-O(11)#3 O(3)-Sm(1)-O(11)#3 O(1)-Sm(1)-O(11)#3 O(10)#3-Sm(1)-O(11)#3 O(2)-Sm(1)-C(40)#3 O(8)#4-Sm(1)-C(40)#3 O(4)-Sm(1)-C(40)#3 O(1)-Sm(1)-C(40)#3 O(10)#3-Sm(1)-C(40) O(11)#3-Sm(1)-C(40) Sm(1)-O(1)-H(1B) Sm(1)-O(1)-H(1A)
90.52(6) 81.33(6) 80.66(7) 82.92(6) 82.76(7) 159.61(7) 79.82(7) 108.04(6) 84.84(6) 76.27(6) 150.51(6) 124.10(6) 88.62(6) 92.79(6) 126.08(6) 153.26(6) 73.55(6) 52.90(5) 97.42(6) 90.59(6) 100.57(6) 99.58(6) 26.39(6) 26.64(6) 123.4(19) 130(2)
3.6. Photoluminescence properties The solid state excitation spectrum of the synthesized MOF recorded at 600 nm (the strongest emission wavelength of Sm3þ ion) is shown in Fig. 9. The peaks at 361 nm and 398 nm are due to the ligand excitation and the peak at 468 nm is due to the direct excitation of the Sm3þ ion [17]. The peaks due to the ligand excitation are stronger than the direct excitation of Sm3þ ion which
Fig. 4. Pack along H bond.
Table 3 Hydrogen bond lengths (Å) and bond angles ( ) of the MOF. D-H…A
d(D-H)
d(H…A)
d(D…A)
<(DHA)
C(2)-H(2)…O(6)#2 C(10)-H(10)…O(7)#5 C(27)-H(27)…O(10)#3 C(42)-H(42A)…O(7)#4 O(6)-H(6A)…O(7)#5 O(1)-H(1B)…O(12)#1 O(1)-H(1A)…O(11)#6
0.93 0.93 0.93 0.96 0.82 0.915(17) 0.920(18)
2.63 2.48 2.48 2.79 1.86 1.760(18) 1.87(2)
3.521(3) 3.110(3) 3.089(3) 3.420(4) 2.628(3) 2.671(3) 2.756(3)
159.6 124.7 123.4 124.1 154.7 173(3) 162(3)
Symmetry transformations used to generate equivalent atoms: #1 x-1,y,z #2 xþ1,y,z #3 -xþ2,-y,-zþ3 #4 -xþ1,-yþ1,-zþ2 #5 -x,-yþ1,-zþ2 #6 x-1,yþ1,z.
Fig. 5. SEM image of the synthesized MOF.
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Fig. 6. Photograph and Microscopic image of the synthesized MOF crystals.
513 MOF
Absorbance (a.u)
indicates that the luminescence sensitization through the excitation of ligand is much more efficient than the direct excitation of the Sm3þ ion. Further, the efficiency of intramolecular energy transfer from an organic ligand to rare earth ion is an important factor which influences the luminescence properties of rare earth complexes [18]. The solid state absorption spectrum of the ligand shows a broad peak (inset Fig. 9) which overlaps with the excitation spectrum of the MOF, indicating the energy transfer from ligand (fluorescein) to Sm3þ. Thus the coordination and sensitization of Sm3þ ions to fluorescein molecules are confirmed [19]. The emission spectra of the synthesized MOF at different excitation wavelengths are shown in Fig. 10. The emission spectrum shows three main lines at 550, 600 and 647 nm, due to the transitions, 4G5/2 / 6H5/2, 4G5/2 / 6H7/2 and 4G5/2 / 6H9/2, respectively, when excited at 398 nm. Here the magnetic dipole transition 4 G5/2 / 6H7/2 gives the strongest emission band as reported elsewhere [17,20]. Absence of emission bands from the ligand reveals that the ligand transfers the excitation energy efficiently to Sm3þ ion. The peaks due to 4G5/2 / 6H5/2 and 4G5/2 / 6H9/2 transitions were observed in the emission spectra when excited at 361 nm and 468 nm, respectively and all other Sm3þ based emissions are quenched which indicates that the direct excitation of the Sm3þ center is less efficient and the high luminescence property of the MOF is due to the antenna effect between the ligand (fluorescein) and Sm3þ ion. The results also showed that the excitation
fluorescein 456
400
450
500
550
600
650
Wavelength (nm) Fig. 8. UVeVisible absorption spectra of fluorescein and synthesized MOF in DMF.
Intensity (a.u)
Dehydrated
As-synthesized
Simulated
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 7. PXRD patterns of the simulated, as-synthesized and dehydrated samples.
Fig. 9. Solid-state excitation spectrum (lem ¼ 600 nm) of the synthesized MOF and solid state absorption spectrum of fluorescein (inset).
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wavelength affects the intensity of different transitions. The excitation and emission spectra of the dehydrated sample (Fig. 11) were also analyzed. It showed excitation peaks at 311 nm and 329 nm. The emission spectrum of the dehydrated sample at an excitation wavelength of 329 nm showed characteristic emission of Sm3þ ion confirming intramolecular energy transfer from ligand to metal center. The intensity of magnetic dipolar transition is decreased in the dehydrated sample due to the change in coordination environment of the sm3þ ion. Even though water molecule acts as a luminescence quencher the quenching effect is overcome in the synthesized MOF [21]. The absence of electric dipolar transition in the dehydrated sample reveals that the local environment of the Sm3þ ion is different in the as synthesized and dehydrated sample [22]. The photophysical property of the MOF in DMF was also studied and the excitation and emission spectra are shown in Fig. 12. The excitation spectrum shows peak at 361 nm due to the its absorption followed by excitation of the ligand in the UV region [23]. The emission spectrum showed a peak ranging from 400 to 490 nm with intensity maxima at 435 nm which corresponding to its blue emission band. The fluorescence decay profiles were deconvoluted using IBH datastation software V2.1, fitted with exponential decay and minimizing the c2 values of the fit to 1 ± 0.1 (1.03). The emission decay curve of the MOF in DMF was fitted with second exponential function and is shown in Fig. 13. Since the decay is second exponential, there are two different sites for luminescence and the lifetime values are t1 ¼ 0.87 ns (86.33%) and t2 ¼ 4.65 ns (13.67%) (the goodness-of-fit parameter c2 ¼ 1.03). The quantum yield of the MOF in DMF was determined by single point method using the equation;
Q ¼ Q R ½I=IR ½ODR =OD½n2 =n2R
i
where, ‘Q’ is the fluorescence quantum yield ‘I’ is the integrated fluorescence intensity, ‘n’ is the refractive index of the solvent and ‘OD’ is the optical density (absorption). The subscript ‘R’ refers to the reference fluorophore of known quantum yield. In this study, rhodamine B in ethanol is used as the standard reference [24,25]. The quantum yield of the synthesized MOF is high (16 ± 0.5%) compared to other samarium-organic complexes [26,27]. Also the quantum yield value is decreased to 5.99 ± 0.5% in the dehydrated sample. The low quantum yield value may be due to the absence of
Fig. 10. Solid state emission spectra of the synthesized MOF.
Fig. 11. Solid state excitation spectrum (lem ¼ 600 nm) and emission spectrum (lex ¼ 329 nm) of dehydrated sample.
electric dipolar transition in the dehydrated sample [22]. 3.7. Thermal analysis Fig. 14 shows the TG-DTA trace of the synthesized MOF conducted at a heating rate of 10⁰C/min. The MOF has lost 10% of its mass up to 318 C in two stages. This weight loss is due to the loss of coordinated water molecule [28] and the evaporation of the solvent DMF without the decomposition of chemical bonds [29]. Immediately after the removal of water and DMF, the third stage starts and it shows maximum endothermic decomposition peak at 532 C due to the decomposition of organic part. The graph shows 38.8% of weight loss and the decomposition was not completed up to 700 C [30]. 4. Conclusion A new samarium-fluorescein metal-organic framework with luminescence property has been successfully synthesized and fully characterized. The scanning electron micrograph and optical
Fig. 12. Excitation spectrum (lem ¼ 435 nm) and emission spectrum (lex ¼ 361 nm) of the MOF in DMF, Inset: Photograph of the MOF in DMF under UV light.
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References
Fig. 13. Decay profile for the MOF in DMF monitored around 435 nm and excited at 375 nm.
Fig. 14. TG-DTG curve of MOF.
microscopic image showed uniform needle like appearance of the synthesized MOF. The excitation spectrum of the MOF showed two major peaks due to the ligand excitation and the emission spectra taken at different excitation wavelengths showed the characteristic emission peaks of the central Sm3þ ion, which confirmed the energy transfer from the ligand to the metal center. The effect of coordinated H2O molecules on the luminescent properties of MOF was also studied. Photophysical property of the MOF in DMF was also studied. The strong luminescence property and thermal stability of the MOF reveals that it can be used as a good candidate for efficient luminescent materials. Acknowledgment This work is financially supported by Department of Science and Technology (DST), New Delhi, India under “Fast Track Young Scientist Scheme” (No: SR/FT/CS-33/2011). Authors thank Dr. Joshy Joseph, CSIR-NIIST, Thiruvananthapuram for lifetime measurement. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2015.06.023.
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