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Zn(II) coordination polymers and coordination supramolecules
Journal of Solid State Chemistry 197 (2013) 92–102
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Syntheses, structures and photoelectric properties of a series of Cd(II)/Zn(II) coordination polymers and coordination supramolecules Jing Jin, Xiao Han, Qin Meng, Dan Li, Yu-Xian Chi, Shu-Yun Niu n School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2012 Received in revised form 3 September 2012 Accepted 9 September 2012 Available online 19 September 2012
Five Cd(II)/Zn(II) complexes [Cd(1,2-bdc)(pz)2(H2O)]n (1), [Cd1Cd2(btec)(H2O)6]n (2), [Cd(3,4-pdc) (H2O)]n (3), [Zn(2,5-pdc)(H2O)4] 2H2O (4) and {[Zn(2,5-pdc)(H2O)2] H2O}n (5) (H2bdc¼1,2-benzenedicarboxylic acid, pz¼ pyrazole, H4btec¼ 1,2,4,5-benzenetetracarboxylic acid, H2pdc¼ pyridine-dicarboxylic acid) were hydrothermally synthesized and characterized by single-crystal X-ray diffraction, surface photovoltage spectroscopy, XRD, TG analysis, IR and UV–vis spectra and elemental analysis. Structural analyses show that complexes 1–3 are 1D, 2D and 3D Cd(II) coordination polymers, respectively. Complex 4 is a mononuclear Zn(II) complex. Complex 5 is a 3D Zn(II) coordination polymer. The surface photoelectric properties of complexes were investigated by SPS. The results indicate that all complexes exhibit photoelectric responses in the range of 300–600 nm, which reveals that they all possess certain photoelectric conversion properties. By the comparative analyses, it can be found that the species and coordination micro-environment of central metal ion, the species and property of ligands affect the intensity and scope of photoelectric response. & 2012 Elsevier Inc. All rights reserved.
Keywords: Cd (II)/Zn(II) complex Crystal structure Photoelectric property
1. Introduction It is well known that CdS and ZnS as excellent semiconductor materials have been widely used in the luminescence, photocatalysis, and solar cells field, etc. [1–4]. The coordination complexes containing Cd(II)/Zn(II) ions, due to their variable coordination numbers and different structural features, exhibit various special properties capable of promising applications in the luminescent materials, electrochemical, catalytic and molecular sieving fields etc. [5–11]. Therefore related reports gradually increase on an annual basis. For example, the [M(tza)(H2O)2]n polymers (M¼Cd and Zn) reported by Wu et al. [12] can emit strong blue fluorescence in the visible region. Niu [13–15] reported series of Cd(Zn)–Ln heterometallic complexes and found that the introduction of Cd(II)/Zn(II) ions can make changes to the emission bands of Ln(III) ions, such as red-shift, broadening and split. Mononuclear and binuclear Zn(II) complexes [ZnL(OAc)(H2O)2] and [Zn2L(OAc)2(H2O)2] ClO4 reported by Bharathi [16], their results of cyclic voltammetry indicate that different structures of complexes have important effects on the electrochemical properties. Kim et al. reported that complexes [Cd(O2CPh)2(bpa)1.5]n and [(Zn3(O2CPh)6) (m-bpe)(Zn2(O2CPh)4)]n can catalyze many transesterifications effectively [17–18]. In fact, these different properties of Cd(II) and Zn(II) complexes are usually related to electron-change behavior within themselves. For instance, the gain, loss and transfer of electrons between different components of complexes can give rise to the process of reduction and oxidation, which is one of the sources for
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catalysis; electron transitions under light-inducement as well as separation or transfer of photo-generated charges may bring about changes of voltage or current on the solid surface or interface, which is photoelectric conversion. Therefore, ascertaining the surface electron behaviors of complexes has an important significance for indepth investigation on various properties and exploration of their practical applications. Surface photovoltage spectroscopy (SPS) not only relates to the electron transitions under light-inducement, but also reflects the separation and transfer of photo-generated charges, so it is significant in research on electron diversions of surfaces and interfaces for solid, especially in semi-conductor materials [19–21]. In recent years, our group has explored on the surface electron behaviors and photoelectric conversion properties of transition metallic coordination compounds or coordination polymers, and obtained some preliminary results [22–26]. But, the photoelectric properties of Cd(II)/Zn(II) complexes have not yet been reported. In this paper, five Cd(II)/Zn(II) complexes were measured by SPS, their photoelectric conversion properties were analyzed, and associations between such properties and their coordination structures were studied. In addition, the SPS and UV–vis absorption spectra were compared, analyzed and correlated.
2. Experimental 2.1. Materials and instrumentation All chemicals used for the synthesis were of reagent grade and used as purchased without further purification. IR spectra were
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
recorded as KBr pellets with a JASCO FT-IR/480 spectrophotometer in the 4000–220 cm 1 region, and UV–vis diffuse reflectance spectra were recorded with a JASCO V-570 UV–vis–NIR spectrophotometer in the 200–2500 nm. The elemental analyses were detected on a PE-240C Analyzer. SPS were performed on a home-built surface photovoltage spectrophotometer in the range of 300–800 nm. The crystal structures were determined with Bruker Smart APEX-II CCD (complexes 1–3) and Rigaku R-AXIS RAPID (complexes 4–5) single-crystal X-ray diffractometers. The excitation and emission spectra in UV–vis region were measured with a JASCO FP-6500 fluorescence spectrometer. The X-ray powder diffraction (XRD) patterns were recorded on a Bruker ˚ at Advance-D8 diffractometer with CuKa radiation (l ¼1.5418 A) room temperature. Thermogravimetric (TG) analyses were performed under N2 atmosphere on a Perkin–Elmer Pyris Diamond TG/DTA instrument with a heating rate of 10 1C min 1. 2.2. Preparations of the complexes 2.2.1. [Cd(1,2-bdc)(pz)2(H2O)]n (1) CdCl2 2.5H2O (0.5 mmol, 0.11 g) was dissolved in deionized water (10 mL), which was added to a mixed solution (10 mL, V H2 O : VMeOH ¼3:1) containing potassium biphthalate (1 mmol, 0.20 g). Then, a colorless solution was obtained. Pyrazole (2 mmol, 0.14 g) was dissolved in a solution of methanol (5 mL) and water solution (5 mL), which was added to above the colorless solution with stirring. The pH value of the resulting solution was about 4. An aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 5. The final mixture was placed in a Teflon-lined stainless reactor and heated at 160 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Light-pink crystals of 1 suitable for X-ray diffraction analysis were obtained. The yield was 56% (0.12 g) based on Cd. Calc. for C14H14N4O5Cd 1: C 39.04, H 3.28, N 13.01%. Found: C 38.83, H 3.26, N 12.94%. IR(KBr pellet, nmax/cm 1): 3218 s, 3111 m, 3066 w, 1608 m, 1588 m, 1560 vs, 1488 w, 1476 w, 1450 w, 1410 vs, 2388 vs, 1131 m, 1057 m, 869 m, 761 s, 706 m, 605 w, 442 w, 405 w. 2.2.2. [Cd1Cd2(btec)(H2O)6]n (2) CdCl2 2.5H2O (1.0 mmol, 0.23 g) was dissolved in deionized water (5 mL), which was added to water and methanol solution (5 mL) containing H4btec acid (0.50 mmol, 0.13 g). An aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 7. The final mixture was placed in a Teflon-lined stainless reactor and heated at 105 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several weeks passed, light-pink crystals of 2 suitable for X-ray diffraction analysis were obtained. The yield was 48% (0.56 g) based on Cd. Calc. for C10H14O14Cd2 2: C 20.60, H 2.43%. Found: C 20.49, H 2.42%. IR(KBr pellet, nmax/cm 1): 3395 s, 1612w, 1556 s, 1493 w, 1434 w, 1385 vs, 1326 w, 1289 w, 1251 w, 1138 m, 876 m, 835 m, 813 w, 767 w, 621 w, 599 w, 509 w. 2.2.3. [Cd(3,4-pdc)(H2O)]n (3) An aqueous solution (10 mL) of containing 3,4-H2pdc acid (1 mmol, 0.10 g) was added into an aqueous solution(10 mL) of CdCl2 6H2O (0.5 mmol, 0.11 g), whose pH value was about 2. Then, an aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 6. The mixture was placed in a Teflon-lined stainless reactor and heated at 140 1C for 5 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several weeks later, light-pink crystals of 3 suitable for X-ray
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diffraction analysis were obtained. The yield was 54% (0.08 g) based on Cd. Calc. for C7H5NO5Cd 3: C 28.45, H 1.71, N 4.74%. Found: C 28.30, H 1.70, N 4.71%. IR(KBr pellet, nmax/cm 1): 3178 s, 3072 w, 1815 w, 1566 vs, 1488 w, 1409 vs, 1382 vs, 1283 m, 1199 m, 1170 m, 1064 m, 885 w, 860 w, 812 s, 783 s, 595 w, 525 w, 468 w. 2.2.4. Zn[(2,5-pdc)(H2O)4] 2H2O (4) An aqueous solution (10 mL) of NH2CH2COOH (1.0 mmol, 0.08 g) was added to an aqueous solution (10 mL) containing Zn(Ac)2 2H2O (1 mmol, 0.22 g), producing a colorless solution. After addition of an aqueous solution (10 mL) of 2,5-H2pdc acid (0.75 mmol, 0.12 g) to the above colorless solution, the precipitate occurred immediately. Then, the appropriate acetate acid and NaOH solutions (1 mol L 1) were added to adjust pH to about 5 respectively, when the precipitate vanished. The clear mixture was placed in a Teflon-lined stainless reactor and heated at 170 1C for 5 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several months later, light-yellow crystals of 4 suitable for X-ray diffraction analysis were obtained. The yield was 41% (0.14 g) based on Zn. Calc. for C7H15NO10Zn 4: C 24.83, H 4.47, N 4.14%. Found: C 24.70, H 4.45, N 4.12%. IR(KBr pellet, nmax/cm 1): 3471 s, 3096 w, 1697 s, 1642 s, 1604 w, 1575 s, 1433w, 1398 s, 1366 vs, 1282 vs, 1252 s, 1126 m, 1041 m, 894 w, 866 w, 802 w, 755 w, 688 w, 659 w, 531 w, 501 w, 413 w. 2.2.5. {[Zn(2,5-pdc)(H2O)2] H2O}n (5) 2,5-H2pdc acid (1.0 mmol, 0.15 g) and NaOH (2.0 mmol, 0.08 g) were dissolved in deionized water (15 mL), which was added to an aqueous solution (15 mL) containing Zn(Ac)2 2H2O (0.5 mmol, 0.11 g), producing a suspensible solution. The acetate acid was dropwise added to adjust pH value to about 5. The solution became clear, then it was placed in a Teflon-lined stainless reactor and heated at 150 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several months passed, lightyellow crystals of 5 for X-ray diffraction analysis were obtained. The yield was 42% (0.16 g) based on Zn. Calc. for C7H9NO7Zn 5: C 29.55, H 3.19, N 4.92%. Found: C 29.39, H 3.17, N 4.89%. IR(KBr pellet, nmax/cm 1): 3469 s, 3096 w, 1696 s, 1656 s, 1603 w, 1576 s, 1434 w, 1398 s, 1365 vs, 1288 vs, 1252 s, 1126 m, 1041 sm, 894 w, 866 w, 802 s, 755 w, 688 w, 659 w, 531 w, 502 w, 463 w, 413 w. 2.3. Single-crystal structural determinations X-ray intensity data of complexes 1–3 were collected on a Bruker Smart APEX-II CCD diffractometer at 293(2) K with MoKa ˚ by o scan mode. Single-crystal X-ray radiation (l ¼0.71073 A) diffraction data for complexes 4–5 were collected using the Rigaku R-AXIS RAPID IP area detector equipped with graphite˚ at 293(2) K. monochromated MoKa radiation (l ¼0.71073 A) Structures were solved by the direct methods and refined by full-matrix least squares on F2 using the SHELXTL version 5.1. All of the non-hydrogen atoms were refined anisotropically. The H atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. H atoms bound to water molecules were found via difference Fourier maps for 1, 4 and 5 and were included in the calculated position using the AFIX 93 parameters in SHELXL for 2 and 3. The crystallographic data and structural refinement parameters of complexes 1–5 are summarized in Table 1. For complex 2, the space group is P1¯. The check CIF report suggests that tests for missed symmetry should be done with ADDSYM. However, the result of ADDSYMCHECK is that space group is not changed and still is P1¯. The crystallographic data R values of complex 2 are some high, and
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J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
Table 1 Crystallographic data and structure refinement parameters for complexes 1–5. Complex
1
2
3
4
5
Empirical Formula Formula weight Crystal system Space group ˚ a (A)
C14H14N4O5Cd 430.69 Monoclinic C2/c 22.109(3)
C10H14O14Cd2 583.00 Triclinic P1¯
C7H15NO10Zn 338.57 Triclinic P1¯
5.5929(14)
C7H5NO5Cd 295.52 Monoclinic P2(1)/n 6.8713(8)
C7H9NO7Zn 284.52 Orthorhombic P212121 7.3333(15)
˚ b (A) ˚ c (A)
8.2852(13)
9.715(3)
14.8689(17)
8.3323(17)
9.4304(19)
17.887(3)
14.502(5)
7.9078(9)
11.449(2)
13.835(3)
a (1)
90 104.826(2) 90 3167.4(8)
101.628(3) 93.406(3) 104.912(2) 740.6(4)
90 102.9300(10) 90 787.44(16)
107.14(3) 99.42(3) 102.75(3) 610.7(2)
90 90 90 956.8(3)
8, 1.806 1712 9495 3792 (0.0325) 1.030 0.0366, 0.0828 0.0541, 0.0918 –
2, 2.605 480 4575 3398(0.0232) 1.018 0.0450, 0.1689 0.05591, 0.1780 –
4, 2.493 568 3691 1375(0.017) 1.114 0.0232, 0.0585 0.0255, 0.0595 –
2, 1.841 348 6085 2783(0.0175) 1.101 0.0229, 0.0551 0.0265, 0.0568 –
4, 1.975 576 9440 2189(0.0182) 1.098 0.0165, 0.0405 0.01707, 0.0406 0.01(3)
b (1)
g (1) V (A˚ 3) Z, Dcal (g cm 3) F(000) Reflns. collected Ind. reflns. (Rint) Goodness-of-fit on F2 R1, wR2 [I 42s(I)] R1, wR2 (all data) Absolute structure parameter
7.0857(14)
the ellipsoids of some carbon atoms have irregular shapes. These problems may be caused by the poor crystal quality and X-ray data of complex 2.
3. Results and discussion 3.1. Structural descriptions 3.1.1. Complex [Cd(1,2-bdc)(pz)2(H2O)]n (1) Complex 1 is a coordination polymer bridged by 1,2-bdc2 anions with 1D stripline structure. The asymmetric unit consists of one Cd(II) ion, one 1,2-bdc2 anion, two pz molecules and one coordinated water molecule (Fig. 1). The Cd (II) ion is six-coordinate with O4N2 mode (Fig. S1), three O atoms from the carboxyl oxygen atoms (O1, O2 and O5A) of two 1,2-bdc2 anions, the other O atom (O3) is coordinated water molecule, and two N atoms (N1 and N3) are from two different pz molecules. The Cd(II) center exists in a slightly distorted octahedral geometry with the equatorial plane provided by three carboxylic O atoms (O1, O2 and O5A) and one N atom (N3), and the axial positions occupied by N1 and O3 atoms. The bond lengths of Cd–O are within 2.195(3)–2.411(2) A˚ and those of Cd–N ˚ The two carboxylic groups of 1,2-bdc2 are 2.263(4) and 2.381(3) A. anion bond to Cd(II) ions in the chelate bidentate and unidentate modes [27], respectively (Fig. S2), the distance of Cd CdB is ˚ In the crystal of 1, first, the 1,2-bdc2 anions bridge 5.738(1) A. [Cd(pz)2(H2O)]2 þ cations to form a 1D chain along the b-axis (Fig. 2). Then, the adjacent 1D chains are further linked by the intermolecular ˚ Table S1) formed hydrogen bonds (O(3)–H(3A)yO(4)#1, 2.748(4) A, between coordinated water molecule (O3) and uncoordinated carboxylic O atom (O4) along the c-axis, resulting in a hydrogen-bonded 2D layer in the bc plane (Fig. S3). 3.1.2. Complex [Cd1Cd2(btec)(H2O)6]n (2) The asymmetric unit of complex 2 comprises two crystallographic independent Cd(II) ions (Cd1 and Cd2), one btec4 anion and six coordinated water molecules (Fig. 3). The Cd1(II) ion is six-coordinate with O6 donors (Fig. S4a), three of which are the carboxyl oxygen atoms (O2, O4A and O8B) from three different btec4 anions, the other three O atoms (O9, O10 and O11) are coordinated water molecules. The bond lengths of Cd1–O are within 2.215(5)– ˚ The Cd2(II) ion is also six-coordinate by O6 donors 2.386(6) A.
Fig. 1. The asymmetric unit of complex 1. Thermal ellipsoid is of 30% probability.
(Fig. S4b), three of which are the carboxyl oxygen atoms (O4, O6B and O8C) from the three different btec4 groups, the other three oxygen atoms (O12, O13 and O14) are coordinated water molecules. ˚ The four The bond lengths of Cd2–O are within 2.224(5)–2.388(6) A. carboxylic groups of btec4 anion exhibit two kinds of coordination fashions (Fig. S5): (1) two carboxylic groups lying in 1- and 4-position all coordinate to one Cd(II) ion with unidentate mode [27], respectively; (2) two carboxylic groups lying in 2- and 5-position all bridge two Cd(II) ions with monoatomic bridging mode [27], respectively, ˚ In the crystal of 2, the two the distance of Cd1D Cd2 is 3.716(8) A. neighboring carboxyl groups lying in 4- and 5-positions of the btec4 anion bridge [Cd(H2O)2]2 þ cations to form a 1D chain along the a-axis (Fig. S6), and the other carboxyl groups of the btec4 anions in the chain are coordinated to the Cd (II) ions in the adjacent chains, resulting in a 2D layer in the ab plane (Fig. 4). The adjacent layers ˚ are further linked by the hydrogen bonds (O10–H O7, 2.726 A, ˚ ˚ O10–H O1, 2.807 A, O9–H O1, 2.793 A, O12–H O5, ˚ O13–H O5, 2.792 A, ˚ O12–H O3, 2.728 A) ˚ formed 2.817 A, between coordinated water molecules (O9, O10 and O12, O13) and uncoordinated carboxylic O-atoms (O1, O3 and O5) of btec4 anion along the c-axis, resulting in a 3D hydrogen-bonded network (Fig. S7).
3.1.3. Complex [Cd(3,4-pdc)(H2O)]n(3) Complex 3 is a coordination polymer with 2D structure. The asymmetric unit consists of one Cd(II) ion, one 3,4-pdc2 anion
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Fig. 2. 1D chain of complex 1 along b-axis.
carboxylic O atoms of 2,5-pdc2 group are contributed to formation of hydrogen bonds. First of all, the neighboring molecules are linked together by the bifurcation hydrogen bonds (O(6)–H(6A) O(2)#4, ˚ O(7)–H(7B) O(2)#6, 2.673(2) A) ˚ formed between coor2.789(2) A; dinated water molecule and uncoordinated carboxylic O atom of 2,5pdc2 anion along the a-axis, producing a 1D hydrogen-bonded chain (Fig. 8). Then, the adjacent 1D chains are further linked by hydrogen ˚ formed between coordibonds (O(7)–H(7A) O(4)#5, 2.664(2) A) nated water molecule and uncoordinated carboxylic O atom of 2,5pdc2 anion along the c-axis, resulting in formation of a hydrogenbonded 2D layer in the ac plane (Fig. 9). Finally, the adjacent 2D layers are further linked by hydrogen bonds (O(8)–H(8A) O(3)#2, ˚ between coordinated water molecule and uncoordinated 2.667(2) A) carboxylic O atom of 2,5-pdc2 anion along the b-axis, resulting in a 3D hydrogen-bonded network (Fig. S12). In addition, the lattice water molecules also take part in formation of hydrogen bonds (Table S1), which makes the structure more stable.
Fig. 3. The asymmetric unit of complex 2. Thermal ellipsoid is of 30% probability.
and one coordinated water molecule (Fig. S8). The Cd (II) ion is six-coordinate by O5N1 donors (Fig. 5), possessing a slightly distorted octahedral geometry. The bond lengths of Cd–O are ˚ The within 2.268(2)–2.379(3) A˚ and that of Cd–N is 2.301(3) A. four O atoms and one N atom of 3,4-pdc2 anion all participate in coordination. The two carboxylic groups of the 3,4-pdc2 anion all coordinate to Cd(II) ions as bridging bidentate groups in a syn–anti configuration [27] (Fig. S9). In the crystal of 3, the two carboxyl groups the 3,4-pdc2 anion alternately bridge [Cd(H2O)]2 þ cations to form a 1D chain along the c-axis (Fig. 6). Then, the N atoms of intrachain 3,4-pdc2 anions coordinate with the Cd(II) ions from the adjacent chains, linking the chains along the b-axis and resulting in a 2D coordination layer in the bc plane (Fig. S10). The adjacent layers are further linked by the intralayer remaining carboxylic O atoms coordinating to the Cd (II) ions from the neighboring layers, leading to formation of 3D network (Fig. S11).
3.1.4. Complex [Zn(2,5-pdc)(H2O)4] 2H2O(4) Structural analysis reveals that complex 4 is a mononuclear complex with the formula of [Zn(2,5-pdc)(H2O)4] 2H2O. The molecule contains one Zn(II) ion, one 2,5-pdc2 anion, four coordinated water molecules and two lattice molecules (Fig. 7). The Zn(II) ion is six-coordinate with O5N1 donors in distorted octahedral geometry. The bond lengths of Zn–O are within ˚ In the crystal 2.041(1)–2.162(2) A˚ and that of Zn–N is 2.128(2) A. of 4, a 3D coordination supramolecule is constructed by many O–H O hydrogen bonds (Table S1). The coordinated water molecules, the lattice water molecules and uncoordinated
3.1.5. Complex {[Zn(2,5-pdc)(H2O)2] H2O}n (5) Complex 5 is a coordination polymer with 2D infinite structure bridged by 2,5-pdc2 anion. The asymmetric unit consists of one Zn(II) ion, one 2,5-pdc2 anion, two coordinated water molecules and one lattice water molecule (Fig. S13). The Zn (II) ion is sixcoordinate by O5N1 donors with a distorted octahedral geometry (Fig. 10). The bond lengths of Zn–O are within 2.037(5)– ˚ Two carboxylic groups 2.270(5) A˚ and that of Zn–N is 2.133(6) A. of 2, 5-pdc2 anion coordinate to Zn(II) ions in unidentated and syn–anti bridging bidentated ways [27], respectively. The separation of ZnC ZnD is 4.940(9) A˚ (Fig. S14). In the crystal of 5, firstly, the N atom of 2,5-pdc2 anion together with its one neighboring carboxylic O atom chelate a Zn (II) ion, and simultaneously, one O atom from 5-position carboxylic group bridge another Zn (II) ion, resulting in a 1D chain along the a-axis (Fig. S15). Secondly, the remaining O atoms of 5-position carboxylic groups of intrachain 2,5-pdc2 anions coordinate to the Zn (II) ions from the adjacent chains, resulting in a 2D layer in the ab plane (Fig. 11). Finally, the adjacent layers are further linked by O–H O hydrogen bonds (Table S1) formed between coordinated water molecule (O6) and uncoordinated carboxylic O atom (O2) of 2, 5-pdc2 anion, between lattice water molecule (O7) and coordinated carboxylic O atom (O3) of 2, 5-pdc2 anion or coordinated water molecule (O5) along the b-axis, resulting in a 3D hydrogen-bonded network (Fig. S16). Although the structures of complexes 3 and 5 were same to those in the literatures [28–29], the synthesis methods of complexes, solvent system, the amount and nature of raw materials in this paper are different from those reported. In this paper, we focus on the surface photoelectric properties of the five complexes and the influence from structures of complexes on the surface photoelectric properties.
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Fig. 4. 2D coordination layer of complex 2 in ab plane (coordinated water molecules are omitted for clarity).
Fig. 6. 1D chain of complex 3 along c-axis.
Fig. 5. Coordination environment around Cd(II) ion in complex 3. Thermal ellipsoid is of 30% probability Symmetry code A: x þ1/2, yþ 1/2, zþ 3/2; B: x, yþ 2, zþ 1; C: x þ1/2, yþ 1/2, z þ 1/2; D: xþ 1, yþ2, z þ 1.
3.2. XRD As shown in Fig. S17, the X-ray powder diffraction patterns for five complexes are consistent with the simulated ones on the basis of their single-crystal X-ray diffraction data, respectively. The positions of diffraction peaks on experimental and simulated patterns essentially correspond, indicating the phase purity of the samples. 3.3. Surface photoelectric properties of complexes 3.3.1. Analysis and assignment of SPS and UV–vis spectra of complexes Based on the UV–vis absorption spectra (Figs. 12b–16b), these five complexes all exhibit relatively strong absorption bands under inducement of UV and visible light. In addition, the energies of absorption bands are all in the range of band-gap energy of semiconductors, so they can be regarded as broad semiconductors. Thus, energy-band theory of semiconductor and crystal field theory of complex were combined to analyze and assign SPS and associate with UV–vis absorption spectra, i.e., 2s and 2p orbitals of direct coordination atoms (O or N) can form valence band, meanwhile
Fig. 7. The molecular structure of complex 4. Thermal ellipsoid is of 30% probability.
conjugated p orbitals of the ligands also belong to valence band. Empty 4s and 4p (or 5s and 5p) orbitals of central metal Zn(II) ion (or Cd(II) ion) together with pn orbitals of ligands can form conduction band. While the 3d or 4d orbitals of the central metal ions lie between valence band and conduction band, they are considered as impurity levels. Under common circumstance, the electron or hole transfer (or transition) from valance band to conduction band is called band-to-band transition (or sub-bandgap transition), while the electron transition between d orbitals of central metal ions is
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
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Fig. 8. 1D hydrogen-bonded chain of complex 4 along a-axis.
Fig. 9. 2D hydrogen-bonded layer of complex 4 in ac plane.
orbital probably form a new orbital with p orbital of ligands [32], and can also be categorized as valence band simultaneously. Thus we can assign this electron transition from the d orbital (which has been included in the valence band) to conduction band (MLCT) as also band-to-band transition of semiconductors.
Fig. 10. Coordination environment around Zn(II) ion in complex 5. Thermal ellipsoid is of 30% probability. Symmetry code A: x 1, y, z; B: xþ 1, y 1/2, zþ 1/2.
called impurity transition. However, 3d (or 4d) orbitals of transitional metal ions in coordination polymers often are affected by crystal field of coordination environment and split. The split situations of d orbitals of different metal ions in different coordination environments can be very different. Therefore, some split d orbitals may be close to valence band, the others may be close to conduction band instead, which makes the composition of valance and conduction bands more complicated [30–33]. For example, the split dxy
3.3.1.1. Analysis and assignment of SPS and UV–vis spectra of complexes 1–3. The SPS of complexes 1–3 are shown in Figs. 12a–14a. Complexes 1–3 are Cd(II) coordination polymers with aromatic carboxylic derivatives as bridging ligands, so their SPS exhibit similar characteristics. All of them show a wide, strong response band with many overlapped bands in 300–600 nm in SPS. Treated by Origin 7.0 program, the response band can be divided into several response bands with different intensities: complex 1, lmax ¼335, 361 and 446 nm (three bands); complex 2, lmax ¼350 and 531 nm (400– 700 nm) (two bands); complex 3, lmax ¼350, 403 and 465 nm (three bands). Obviously, these response bands belong to the band-to-band transition from valence band to conduction band. The response bands with relatively stronger intensity all lie in near ultraviolet region and they are assigned to band-to-band transition from the valence band (including p orbitals of direction coordination atoms with conjugated p orbitals) to conduction band, corresponding to the ligand-to-metal charge transfer (LMCT) transition. The response
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Fig. 11. 2D coordination layer of complex 5 in ab plane.
bands in visible region (lmax ¼446 nm, complex 1; 531 nm, complex 2; and 465 nm, complex 3) could be caused by the transition from the valence band (involving some post-split d orbitals of Cd(II) ion) to the conduction band, which should also belong to band-to-band transition of broad semiconductor; therefore, they lie in the visible region with lower energy and cover wide spectral region (430– 600 nm in 1; 400–700 nm in 2; 400–630 nm in 3). In addition, complexes 1 and 3 exhibit two response bands in the UV region, but complex 2 shows one broad response band, respectively. This may be due to that there are two species of direct coordination atoms (O and N atoms) in complexes 1 and 3, one species (O atom) in complex 2. The UV–vis absorption spectra of complexes 1–3 are shown in Figs. 12b–14b. By comparison, it can be found that the UV–vis absorption spectra are closely associated with SPS, and even consistent with one another. The UV–vis spectra of 1–3 show different numbers of absorption bands: lmax ¼252, 290, 330 and 544 nm (430–630 nm) for complex 1; lmax ¼ 252, 302 (290–440) and 510 nm (440–650 nm) for complex 2; lmax ¼258, 332 (290–450) and 550 nm (450–650 nm) for complex 3. The absorption bands at lmax ¼252 (1), 252 (2) and 258 nm (3) are assigned to intraligand p-pn. And the band in ultraviolet region for each complex is assigned to LMCT (O(N)-Cd). Because there are two species of direct coordinated atoms (O and N) with different electronegativity in complexes 1 and 3, the LMCT absorption bands in complexes 1 and 3 are split into two bands, i.e., O-Cd and N-Cd, respectively. In addition, the absorption bands in visible region for each complex (l ¼430–630 nm for 1; 440–650 nm for 2; 450–650 nm for 3) are relatively wide, which should be caused by metal-to-ligand charge transition (MLCT). Furthermore, the feature of SPS and UV–vis spectra in three Cd(II) complexes is that all of them show obvious bands in visible region, which should originate from MLCT in nature. This does not appear in other transitional metal complexes with d10 configuration, which in turn, is more favorable to photoelectric conversions.
3.3.1.2. Analysis and assignment of SPS and UV–visspectra of complexes 4 and 5. Complexes 4 and 5 are Zn(II) complexes with carboxylic derivatives as bridging ligands. As shown in Figs. 15a and 16a, the SPS of complexes 4 and 5 are similar to those of complexes 1–3, i.e., there is also a wide and strong response band with many overlapped bands for complex 4 as well as 5, respectively. Treated by Origin 7.0 program, the response band can be divided into three response bands: lmax ¼353, 386 and 442 nm for 4; lmax ¼341, 373 and 451 nm for 5. The response bands (lmax ¼353 and 386 nm for 4; 341 and 373 nm for 5) have higher energy and intensity and they are assigned to the band-to-band transition arising from valence band composed of ligands to conduction band, corresponding to LMCT. The bands in visible region (lmax ¼442 nm for 4; 451 nm for 5) are assigned to the transition from valence band formed by post-split d orbitals to conduction band, corresponding to MLCT. There are four absorption bands in the UV–vis spectra for complexes 4 and 5 (Figs. 15b and 16b), respectively: lmax ¼262, 336, 372 and 496 nm for 4; lmax ¼258, 308, 370 and 458 nm for 5. The bands at lmax ¼262 nm (for 4) and 258 nm (for 5) are assigned to intraligand p-pn. The bands at lmax ¼336, 372 nm (for 4) and 308, 370 nm (for 5) can be assigned to LMCT (O(or N) -Zn). In addition, the broad absorption bands in visible region (460–600 nm(for 4) and 400–600 nm(for 5)) are attributed to MLCT. By comparing SPS and UV–vis spectra, both of them are consistent with each other.
3.3.2. Comparison and analysis of SPS of complexes 1–5 By comparing and analyzing the SPS of complexes 1–5, the conclusions are summarized below: 1) There are obvious photovoltage response bands in the SPS of complexes 1–5 in the range of 300–600 nm, which indicates that they all possess certain photoelectric conversion properties.
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Fig. 12. SPS (a) and UV–vis (b) spectra of complex 1.
2)
3)
4)
5)
But the intensity and width of the SPS response bands are different. The SPS are closely associated with the UV–vis absorption spectra for each complex, which can be cross-verification for each other. There are different species of central metal ions in these five complexes: Cd(II) (4d10) is in complexes 1–3 and Zn(II) (3d10) is in complexes 4 and 5. Due to the difference of shell levels, the split energies of crystal filed are different, which in turn affect the results of SPS. For example, the response bands in visible region in complexes 1–3 are wider than those in complexes 4 and 5. The different direct coordination atoms with different electronegativity result in the splitting of the band-to-band responses based on LMCT. For example, there is one species of coordination atoms (i.e., O atom) in complex 2, so there is one LMCT response band. But in the complexes 1, 3, 4 and 5, there are two species of coordination atoms, respectively. So there are two LMCT response bands (O-Cd (or Zn) and N-Cd(or Zn)). The change of coordination micro-environment around central metal ions will affect the position and shape of response bands. For example, in complex 2, there are two crystallographically independent Cd(II) ions, thus, the SPS response band within 400–700 nm appear several splitting bands.
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Fig. 13. SPS (a) and UV–vis (b) spectra of complex 2.
6) The increasing of conjugation degree of ligands maybe make response band widening and strengthening. For example, in complex 1, there are two types of cyclo-conjugated ligand, and in other four complexes, there is one type of cyclo-conjugated ligand, thus the response region of SPS in complex 1 is wider than that in other four complexes. In addition, the SPS of five complexes in this paper are compared with the SPS of Cd(Ac)2(H2O)4 and Zn(Ac)2(H2O)4 (Fig. S18), and the SPS of all five complexes are obviously strengthened and widened. It can be seen that the conjugated-organic-ligand can effectively increase the intensity of SPS. Because when the complex is irradiated by light, the conjugated ligand in the complex can absorb photos and give rise to the p-pn transition. Although these electrons in this transition process cannot directly flow and form current or voltage, they maybe inflow into the broad semiconductors and undertake the role of transferring energy, moreover, probably improving the photoelectric conversion efficiency. 3.4. TG analyses The thermal stability of complexes was investigated by TG analysis in the temperature 30–800 1C (Fig. S19). The TG curve of complex 1 shows that it is stable up to 150 1C, and then loses weight from 150 to 300 1C (observed: 40.12%; calculated: 41.33%),
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Fig. 15. SPS (a) and UV–vis (b) spectra of complex 4. Fig. 14. SPS (a) and UV–vis (b) spectra of complex 3.
corresponding to the removal of one coordinated water molecule, two organic pz molecules and one CO2 molecule from decomposition of 1,2-bdc2 ligand. The second weight loss of 42.4% (calculated: 45.69%) in the temperature range of 300–700 1C can be assigned to the release of 1,2-bdc2 ligand. The TG curve of complex 2 exhibits two continuous weight loss steps. The first weight loss starts at ca. 90–200 1C to give a weight loss of ca. 18.80%, corresponding to the loss of coordinated water molecules (18.52%). The second weight loss is 40.2% in the temperature range 350–500 1C, attributed to the decomposition of the btec ligand and the collapsing of the structure. The TG analysis indicates that complex 3 has good stability. The curve exhibits that there is no weight loss until 170 1C. It first loses the coordinated water molecules in the range of 170–250 1C (observed: 6.72%; calculated: 6.09%). Subsequently, a plateau region is observed from 250 to 320 1C. It keeps losing weight from 320 to 750 1C, ascribing to the decomposition of 3,4-pdc ligand and the collapsing of the structure. The whole weight loss (61.05%) is in good agreement with the calculated value (61.97%). For complex 4, it is thermally stable up to 100 1C. Above this temperature, the TG curve exhibits two weight loss stages. The first weight loss of 15.46% between 100 and 170 1C is attributable to loss of the three water molecules (calculated: 15.96%). The
second weight loss occurs from 170 to 800 1C, attributed to release of another three water molecules and the decomposition of organic ligand and the collapsing of the structure. The whole weight loss (79.16%) is in basic agreement with the calculated value (80.67%). The TG curve of complex 5 shows a weight loss (18.78%) from 130 to 250 1C corresponding to loss of the three water molecules (calculated: 18.98%). Then, a small plateau region is observed from 250 to 350 1C. The observed second weight loss (31.28%) in the range of 350–470 1C is attributed to loss of the two CO2 molecules from decomposition of 2,5-pdc2 ligand (calculated: 30.93%). 3.5. Luminescent properties of complexes At room temperature, the luminescent properties of the five complexes in the UV and visible region were investigated in the solid state (Bandwidth Ex.: 3 nm, Bandwidth Em.:3 nm). As shown in Fig. S20, five complexes all show weak emission bands: Em lEm max ¼400 nm (lEx ¼317 nm) for complex 1; lmax ¼301 nm (lEx ¼ Em 260 nm) for complex 2; lmax ¼408 nm (lEx ¼349 nm) for complex 3; Em lEm max ¼302 nm (lEx ¼260 nm) for complex 4 and lmax ¼469 nm (lEx ¼332 nm) for complex 5 in UV/visible regions, respectively. The observed emission bands are weak, which may be due to that the electrons are transited into an excited state under inducement of UV and visible light, then a portion of photo-generated electrons in
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can be classified the conduction band. Thus, this charge transition from any one orbital in the valence band to any one orbital in conduction band can lead to the band-to-band transition response. Obviously, this photovoltage response (caused by the charge transition from the d orbitals in the valence band to the conduction band) should be corresponded with MLCT, and this phenomenon is very rare. This can be verified by UV–vis absorption spectra of the complexes. SPS and UV–vis absorption spectra are basically consistent with one another. It is found that species and coordination micro-environment of central metal ion, the species and property of ligands all affect the intensity and scope of the photoelectric response.
Supporting Information The supporting information includes the portion of structure figures, Hydrogen-bond data, TG curves, XRD patterns, the SPS comparison and emission spectra.
Appendix A. Supplementary data CCDC: 861537, 861538, 861539, 861540 and 861541 contain the supplementary crystallographic data for complexes. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, or email: [email protected].
Acknowledgments The research was supported by National Natural Science Foundation of China (20571037) and the Educational Foundation of Liaoning Province in China (L2011193).
Appendix B. Supplementary material
Fig. 16. SPS (a) and UV–vis (b) spectra of complex 5.
excited state can transfer with help of the build-in electric field of samples and form current/voltage; another small portion of excited electrons lost by the non-radioactive process; so, the emission bands coming from electron recombination luminescence become weak.
4. Conclusions Five Cd(II)/Zn(II) complexes were hydrothermally synthesized. Their structures were determined by single-crystal X-ray diffraction. Surface electron behaviors of complexes were analyzed by SPS. The results of SPS indicate that all five complexes exhibit photoelectric responses in the range of 300–600 nm, which reveals that they all possess certain photoelectric conversion ability. So, the energy-band theory of semiconductor and the crystal field theory were combined to try to analyze and assign SPS and associate with UV–vis absorption spectra, i.e., 2s and 2p orbitals of direct coordination atoms (O or N), p orbitals of the conjugated ligand, and some post-split low-energy d orbitals of Cd(II)/Zn(II) ions caused by crystal field all can be classified the valence band. Empty 4s and 4p (or 5s and 5p) orbitals of central metal Zn(II) (or Cd(II)) ions, pn orbitals of the conjugated ligands, and some post-split high-energy d orbitals of Cd(II)/Zn(II) ions all
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.09.016.
References [1] S. Deka, A. Quarta, M.G. Lupo, A. Falqui, S. Boninelli, C. Giannini, G. Morello, M.D. Giorgi, G. Lanzani, C. Spinella, R. Cingolani, T. Pellegrino, L. Manna, J. Am. Chem. Soc. 131 (2009) 2948–2958. [2] A. Datta, S.K. Panda, S. Chaudhuri, J. Phys. Chem. C 111 (2007) 17260–17264. [3] H. Yin, Y. Wada, T. Kitamura, S. Yanagida, Environ. Sci. Technol. 35 (2001) 227–231. [4] H.J. Lee, J. Bang, J. Park, S. Kim, S.-M. Park, Chem. Mater. 22 (2010) 5636–5643. [5] R. Feng, F.-L. Jiang, L. Chen, C.-F. Yan, M.-Y. Wu, M.-C. Hong, Chem. Commun. 35 (2009) 5296–5298. [6] S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita, S. Kitagawa, J. Am. Chem. Soc. 129 (2007) 2607–2614. [7] P.K. Thallapally, C.A. Fernandez, R.K. Motkuri, S.K. Nune, J. Liu, C.H.F. Peden, Dalton Trans. 39 (2010) 1692–1694. [8] P.-X. Li, C.-L. Hu, Q.-P. Lin, N. Zhao, J.-G. Mao, Inorg. Chem. 48 (2009) 5454–5461. ˜ o, M. Esteban, Environ. Sci. [9] E. Chekmeneva, R. Prohens, J.M. Dı´az-Cruz, C. Arin Technol. 42 (2008) 2860–2866. [10] Y.M. Lee, Y.J. Song, J.I. Poong, S.H. Kim, H.G. Koo, J.A. Lee, C. Kim, S.-J. Kim, Y. Kim, Inorg. Chem. Commun. 13 (2010) 101–104. [11] Y.J. Song, H. Kwak, Y.M. Lee, S.H. Kim, S.H. Lee, B.K. Park, J.Y. Jun, S.M. Yu, C. Kim, S.-J. Kim, Y. Kim, Polyhedron 28 (2009) 1241–1252. [12] M.F. Wu, F.K. Zheng, G. Xu, A.Q. Wu, Y. Li, H.F. Chen, S.P. Guo, F. Chen, Z.F. Liu, G.C. Guo, J.S. Huang, Inorg. Chem. Commun. 13 (2010) 250–253. [13] Y.X. Chi, S.Y. Niu, J. Jin, Inorg. Chim. Acta 362 (2009) 3821–3828. [14] Y.-X. Chi, S.-Y. Niu, J. Jin, R. Wang, Y. Li, Dalton Trans. 37 (2009) 7653–7659. [15] Y.-X. Chi, S.-Y. Niu, R. Wang, J. Jin, G.-N. Zhang, J. Lumin. 131 (2011) 1707–1713.
102
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
[16] K.S. Bharathi, A.K. Rahiman, K. Rajesh, S. Sreedaran, P.G. Aravindan, D. Velmurugan, V. Narayanan, Polyhedron 25 (2006) 2859–2868. [17] S.H. Kim, B.K. Park, Y.J. Song, S.M. Yu, H.G. Koo, E.Y. Kim, J.I. Poong, J.H. Lee, C. Kim, S.-J. Kim, Y. Kim, Inorg. Chim. Acta 362 (2009) 4119–4126. [18] H. Kwak, S.H. Lee, S.H. Kim, Y.M. Lee, B.K. Park, Y.J. Lee, J.Y. Jun, C. Kim, S.-J. Kim, Y. Kim, Polyhedron 28 (2009) 553–561. [19] Y.F. Wang, J. Xu, W.Z. Zong, Y.F. Zhu, J. Solid State Chem. 184 (2011) 1433–1438. [20] T. Xie, D. Wang, L. Zhu, C. Wang, T. Li, X. Zhou, M. Wang, J. Phys. Chem. B 104 (2000) 8177–8181. [21] M.H. Qi, G.F. Liu, J. Phys. Chem. B 107 (2003) 7640–7646. [22] L.-P. Sun, S.-Y. Niu, J. Jin, G.-D. Yang, L. Ye, Eur. J. Inorg. Chem. 2006 (2006) 5130–5137. [23] L.-P. Sun, S.-Y. Niu, J. Jin, L. Zhang, Eur. J. Inorg. Chem. 2007 (2007) 3845–3852. [24] L. Zhang, S.-Y. Niu, J. Jin, L.-P. Sun, G.-D. Yang, L. Ye, Inorg. Chim. Acta 362 (2009) 1448–1454.
[25] L. Li, S.-Y. Niu, Z.-F. Shi, L.-C. Gong, J. Jin, Y.-X. Chi, Y.-H. Xing, Polyhedron 30 (2011) 976–982. [26] L. Li, S.-Y. Niu, J. Jin, Q. Meng, Y.-X. Chi, Y.-H. Xing, G.-N. Zhang, J. Solid State Chem. 184 (2011) 1279–1285. [27] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227–250. [28] X. Zhang, D. Huang, C. Chen, Q. Liu, D. Liao, L. Li, Inorg. Chem. Commun. 8 (2005) 22–26. [29] E.E. Sileo, O.E. Piro, G. Rigotti, M.A. Blesa, A.S. de Araujo, E.E. Castellano, Struct. Chem. 19 (2008) 651–657. [30] A.J. Sillanpaa, R. Aksela, K. Laasonen, Phys. Chem. Chem. Phys. 5 (2003) 3382–3393. [31] X. Zhou, H.-X. Zhang, Q.-J. Pan, B.-H. Xia, A.-C. Tang, J. Phys. Chem. A 109 (2005) 8809–8818. [32] L.T. Chen, T. Liu, J.Z. Chen, Acta Chim. Sin. 66 (2008) 1187–1195 (in Chinese). [33] T.M. McCormick, B.D. Calitree, A. Orchard, N.D. Kraut, F.V. Bright, M.R. Detty, R. Eisenberg, J. Am. Chem. Soc. 132 (2010) 15480–15483.
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Syntheses, structures and photoelectric properties of a series of Cd(II)/Zn(II) coordination polymers and coordination supramolecules Jing Jin, Xiao Han, Qin Meng, Dan Li, Yu-Xian Chi, Shu-Yun Niu n School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2012 Received in revised form 3 September 2012 Accepted 9 September 2012 Available online 19 September 2012
Five Cd(II)/Zn(II) complexes [Cd(1,2-bdc)(pz)2(H2O)]n (1), [Cd1Cd2(btec)(H2O)6]n (2), [Cd(3,4-pdc) (H2O)]n (3), [Zn(2,5-pdc)(H2O)4] 2H2O (4) and {[Zn(2,5-pdc)(H2O)2] H2O}n (5) (H2bdc¼1,2-benzenedicarboxylic acid, pz¼ pyrazole, H4btec¼ 1,2,4,5-benzenetetracarboxylic acid, H2pdc¼ pyridine-dicarboxylic acid) were hydrothermally synthesized and characterized by single-crystal X-ray diffraction, surface photovoltage spectroscopy, XRD, TG analysis, IR and UV–vis spectra and elemental analysis. Structural analyses show that complexes 1–3 are 1D, 2D and 3D Cd(II) coordination polymers, respectively. Complex 4 is a mononuclear Zn(II) complex. Complex 5 is a 3D Zn(II) coordination polymer. The surface photoelectric properties of complexes were investigated by SPS. The results indicate that all complexes exhibit photoelectric responses in the range of 300–600 nm, which reveals that they all possess certain photoelectric conversion properties. By the comparative analyses, it can be found that the species and coordination micro-environment of central metal ion, the species and property of ligands affect the intensity and scope of photoelectric response. & 2012 Elsevier Inc. All rights reserved.
Keywords: Cd (II)/Zn(II) complex Crystal structure Photoelectric property
1. Introduction It is well known that CdS and ZnS as excellent semiconductor materials have been widely used in the luminescence, photocatalysis, and solar cells field, etc. [1–4]. The coordination complexes containing Cd(II)/Zn(II) ions, due to their variable coordination numbers and different structural features, exhibit various special properties capable of promising applications in the luminescent materials, electrochemical, catalytic and molecular sieving fields etc. [5–11]. Therefore related reports gradually increase on an annual basis. For example, the [M(tza)(H2O)2]n polymers (M¼Cd and Zn) reported by Wu et al. [12] can emit strong blue fluorescence in the visible region. Niu [13–15] reported series of Cd(Zn)–Ln heterometallic complexes and found that the introduction of Cd(II)/Zn(II) ions can make changes to the emission bands of Ln(III) ions, such as red-shift, broadening and split. Mononuclear and binuclear Zn(II) complexes [ZnL(OAc)(H2O)2] and [Zn2L(OAc)2(H2O)2] ClO4 reported by Bharathi [16], their results of cyclic voltammetry indicate that different structures of complexes have important effects on the electrochemical properties. Kim et al. reported that complexes [Cd(O2CPh)2(bpa)1.5]n and [(Zn3(O2CPh)6) (m-bpe)(Zn2(O2CPh)4)]n can catalyze many transesterifications effectively [17–18]. In fact, these different properties of Cd(II) and Zn(II) complexes are usually related to electron-change behavior within themselves. For instance, the gain, loss and transfer of electrons between different components of complexes can give rise to the process of reduction and oxidation, which is one of the sources for
n
Corresponding author. Fax: þ86 411 82156832. E-mail address: [email protected] (S.-Y. Niu).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.09.016
catalysis; electron transitions under light-inducement as well as separation or transfer of photo-generated charges may bring about changes of voltage or current on the solid surface or interface, which is photoelectric conversion. Therefore, ascertaining the surface electron behaviors of complexes has an important significance for indepth investigation on various properties and exploration of their practical applications. Surface photovoltage spectroscopy (SPS) not only relates to the electron transitions under light-inducement, but also reflects the separation and transfer of photo-generated charges, so it is significant in research on electron diversions of surfaces and interfaces for solid, especially in semi-conductor materials [19–21]. In recent years, our group has explored on the surface electron behaviors and photoelectric conversion properties of transition metallic coordination compounds or coordination polymers, and obtained some preliminary results [22–26]. But, the photoelectric properties of Cd(II)/Zn(II) complexes have not yet been reported. In this paper, five Cd(II)/Zn(II) complexes were measured by SPS, their photoelectric conversion properties were analyzed, and associations between such properties and their coordination structures were studied. In addition, the SPS and UV–vis absorption spectra were compared, analyzed and correlated.
2. Experimental 2.1. Materials and instrumentation All chemicals used for the synthesis were of reagent grade and used as purchased without further purification. IR spectra were
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
recorded as KBr pellets with a JASCO FT-IR/480 spectrophotometer in the 4000–220 cm 1 region, and UV–vis diffuse reflectance spectra were recorded with a JASCO V-570 UV–vis–NIR spectrophotometer in the 200–2500 nm. The elemental analyses were detected on a PE-240C Analyzer. SPS were performed on a home-built surface photovoltage spectrophotometer in the range of 300–800 nm. The crystal structures were determined with Bruker Smart APEX-II CCD (complexes 1–3) and Rigaku R-AXIS RAPID (complexes 4–5) single-crystal X-ray diffractometers. The excitation and emission spectra in UV–vis region were measured with a JASCO FP-6500 fluorescence spectrometer. The X-ray powder diffraction (XRD) patterns were recorded on a Bruker ˚ at Advance-D8 diffractometer with CuKa radiation (l ¼1.5418 A) room temperature. Thermogravimetric (TG) analyses were performed under N2 atmosphere on a Perkin–Elmer Pyris Diamond TG/DTA instrument with a heating rate of 10 1C min 1. 2.2. Preparations of the complexes 2.2.1. [Cd(1,2-bdc)(pz)2(H2O)]n (1) CdCl2 2.5H2O (0.5 mmol, 0.11 g) was dissolved in deionized water (10 mL), which was added to a mixed solution (10 mL, V H2 O : VMeOH ¼3:1) containing potassium biphthalate (1 mmol, 0.20 g). Then, a colorless solution was obtained. Pyrazole (2 mmol, 0.14 g) was dissolved in a solution of methanol (5 mL) and water solution (5 mL), which was added to above the colorless solution with stirring. The pH value of the resulting solution was about 4. An aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 5. The final mixture was placed in a Teflon-lined stainless reactor and heated at 160 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Light-pink crystals of 1 suitable for X-ray diffraction analysis were obtained. The yield was 56% (0.12 g) based on Cd. Calc. for C14H14N4O5Cd 1: C 39.04, H 3.28, N 13.01%. Found: C 38.83, H 3.26, N 12.94%. IR(KBr pellet, nmax/cm 1): 3218 s, 3111 m, 3066 w, 1608 m, 1588 m, 1560 vs, 1488 w, 1476 w, 1450 w, 1410 vs, 2388 vs, 1131 m, 1057 m, 869 m, 761 s, 706 m, 605 w, 442 w, 405 w. 2.2.2. [Cd1Cd2(btec)(H2O)6]n (2) CdCl2 2.5H2O (1.0 mmol, 0.23 g) was dissolved in deionized water (5 mL), which was added to water and methanol solution (5 mL) containing H4btec acid (0.50 mmol, 0.13 g). An aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 7. The final mixture was placed in a Teflon-lined stainless reactor and heated at 105 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several weeks passed, light-pink crystals of 2 suitable for X-ray diffraction analysis were obtained. The yield was 48% (0.56 g) based on Cd. Calc. for C10H14O14Cd2 2: C 20.60, H 2.43%. Found: C 20.49, H 2.42%. IR(KBr pellet, nmax/cm 1): 3395 s, 1612w, 1556 s, 1493 w, 1434 w, 1385 vs, 1326 w, 1289 w, 1251 w, 1138 m, 876 m, 835 m, 813 w, 767 w, 621 w, 599 w, 509 w. 2.2.3. [Cd(3,4-pdc)(H2O)]n (3) An aqueous solution (10 mL) of containing 3,4-H2pdc acid (1 mmol, 0.10 g) was added into an aqueous solution(10 mL) of CdCl2 6H2O (0.5 mmol, 0.11 g), whose pH value was about 2. Then, an aqueous solution of NaOH (1 mol L 1) was dropwise added to adjust pH value to about 6. The mixture was placed in a Teflon-lined stainless reactor and heated at 140 1C for 5 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several weeks later, light-pink crystals of 3 suitable for X-ray
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diffraction analysis were obtained. The yield was 54% (0.08 g) based on Cd. Calc. for C7H5NO5Cd 3: C 28.45, H 1.71, N 4.74%. Found: C 28.30, H 1.70, N 4.71%. IR(KBr pellet, nmax/cm 1): 3178 s, 3072 w, 1815 w, 1566 vs, 1488 w, 1409 vs, 1382 vs, 1283 m, 1199 m, 1170 m, 1064 m, 885 w, 860 w, 812 s, 783 s, 595 w, 525 w, 468 w. 2.2.4. Zn[(2,5-pdc)(H2O)4] 2H2O (4) An aqueous solution (10 mL) of NH2CH2COOH (1.0 mmol, 0.08 g) was added to an aqueous solution (10 mL) containing Zn(Ac)2 2H2O (1 mmol, 0.22 g), producing a colorless solution. After addition of an aqueous solution (10 mL) of 2,5-H2pdc acid (0.75 mmol, 0.12 g) to the above colorless solution, the precipitate occurred immediately. Then, the appropriate acetate acid and NaOH solutions (1 mol L 1) were added to adjust pH to about 5 respectively, when the precipitate vanished. The clear mixture was placed in a Teflon-lined stainless reactor and heated at 170 1C for 5 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several months later, light-yellow crystals of 4 suitable for X-ray diffraction analysis were obtained. The yield was 41% (0.14 g) based on Zn. Calc. for C7H15NO10Zn 4: C 24.83, H 4.47, N 4.14%. Found: C 24.70, H 4.45, N 4.12%. IR(KBr pellet, nmax/cm 1): 3471 s, 3096 w, 1697 s, 1642 s, 1604 w, 1575 s, 1433w, 1398 s, 1366 vs, 1282 vs, 1252 s, 1126 m, 1041 m, 894 w, 866 w, 802 w, 755 w, 688 w, 659 w, 531 w, 501 w, 413 w. 2.2.5. {[Zn(2,5-pdc)(H2O)2] H2O}n (5) 2,5-H2pdc acid (1.0 mmol, 0.15 g) and NaOH (2.0 mmol, 0.08 g) were dissolved in deionized water (15 mL), which was added to an aqueous solution (15 mL) containing Zn(Ac)2 2H2O (0.5 mmol, 0.11 g), producing a suspensible solution. The acetate acid was dropwise added to adjust pH value to about 5. The solution became clear, then it was placed in a Teflon-lined stainless reactor and heated at 150 1C for 4 days. Cooled to room temperature, the reaction mixture was filtered and the filtrate was allowed to evaporate at room temperature. Several months passed, lightyellow crystals of 5 for X-ray diffraction analysis were obtained. The yield was 42% (0.16 g) based on Zn. Calc. for C7H9NO7Zn 5: C 29.55, H 3.19, N 4.92%. Found: C 29.39, H 3.17, N 4.89%. IR(KBr pellet, nmax/cm 1): 3469 s, 3096 w, 1696 s, 1656 s, 1603 w, 1576 s, 1434 w, 1398 s, 1365 vs, 1288 vs, 1252 s, 1126 m, 1041 sm, 894 w, 866 w, 802 s, 755 w, 688 w, 659 w, 531 w, 502 w, 463 w, 413 w. 2.3. Single-crystal structural determinations X-ray intensity data of complexes 1–3 were collected on a Bruker Smart APEX-II CCD diffractometer at 293(2) K with MoKa ˚ by o scan mode. Single-crystal X-ray radiation (l ¼0.71073 A) diffraction data for complexes 4–5 were collected using the Rigaku R-AXIS RAPID IP area detector equipped with graphite˚ at 293(2) K. monochromated MoKa radiation (l ¼0.71073 A) Structures were solved by the direct methods and refined by full-matrix least squares on F2 using the SHELXTL version 5.1. All of the non-hydrogen atoms were refined anisotropically. The H atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. H atoms bound to water molecules were found via difference Fourier maps for 1, 4 and 5 and were included in the calculated position using the AFIX 93 parameters in SHELXL for 2 and 3. The crystallographic data and structural refinement parameters of complexes 1–5 are summarized in Table 1. For complex 2, the space group is P1¯. The check CIF report suggests that tests for missed symmetry should be done with ADDSYM. However, the result of ADDSYMCHECK is that space group is not changed and still is P1¯. The crystallographic data R values of complex 2 are some high, and
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Table 1 Crystallographic data and structure refinement parameters for complexes 1–5. Complex
1
2
3
4
5
Empirical Formula Formula weight Crystal system Space group ˚ a (A)
C14H14N4O5Cd 430.69 Monoclinic C2/c 22.109(3)
C10H14O14Cd2 583.00 Triclinic P1¯
C7H15NO10Zn 338.57 Triclinic P1¯
5.5929(14)
C7H5NO5Cd 295.52 Monoclinic P2(1)/n 6.8713(8)
C7H9NO7Zn 284.52 Orthorhombic P212121 7.3333(15)
˚ b (A) ˚ c (A)
8.2852(13)
9.715(3)
14.8689(17)
8.3323(17)
9.4304(19)
17.887(3)
14.502(5)
7.9078(9)
11.449(2)
13.835(3)
a (1)
90 104.826(2) 90 3167.4(8)
101.628(3) 93.406(3) 104.912(2) 740.6(4)
90 102.9300(10) 90 787.44(16)
107.14(3) 99.42(3) 102.75(3) 610.7(2)
90 90 90 956.8(3)
8, 1.806 1712 9495 3792 (0.0325) 1.030 0.0366, 0.0828 0.0541, 0.0918 –
2, 2.605 480 4575 3398(0.0232) 1.018 0.0450, 0.1689 0.05591, 0.1780 –
4, 2.493 568 3691 1375(0.017) 1.114 0.0232, 0.0585 0.0255, 0.0595 –
2, 1.841 348 6085 2783(0.0175) 1.101 0.0229, 0.0551 0.0265, 0.0568 –
4, 1.975 576 9440 2189(0.0182) 1.098 0.0165, 0.0405 0.01707, 0.0406 0.01(3)
b (1)
g (1) V (A˚ 3) Z, Dcal (g cm 3) F(000) Reflns. collected Ind. reflns. (Rint) Goodness-of-fit on F2 R1, wR2 [I 42s(I)] R1, wR2 (all data) Absolute structure parameter
7.0857(14)
the ellipsoids of some carbon atoms have irregular shapes. These problems may be caused by the poor crystal quality and X-ray data of complex 2.
3. Results and discussion 3.1. Structural descriptions 3.1.1. Complex [Cd(1,2-bdc)(pz)2(H2O)]n (1) Complex 1 is a coordination polymer bridged by 1,2-bdc2 anions with 1D stripline structure. The asymmetric unit consists of one Cd(II) ion, one 1,2-bdc2 anion, two pz molecules and one coordinated water molecule (Fig. 1). The Cd (II) ion is six-coordinate with O4N2 mode (Fig. S1), three O atoms from the carboxyl oxygen atoms (O1, O2 and O5A) of two 1,2-bdc2 anions, the other O atom (O3) is coordinated water molecule, and two N atoms (N1 and N3) are from two different pz molecules. The Cd(II) center exists in a slightly distorted octahedral geometry with the equatorial plane provided by three carboxylic O atoms (O1, O2 and O5A) and one N atom (N3), and the axial positions occupied by N1 and O3 atoms. The bond lengths of Cd–O are within 2.195(3)–2.411(2) A˚ and those of Cd–N ˚ The two carboxylic groups of 1,2-bdc2 are 2.263(4) and 2.381(3) A. anion bond to Cd(II) ions in the chelate bidentate and unidentate modes [27], respectively (Fig. S2), the distance of Cd CdB is ˚ In the crystal of 1, first, the 1,2-bdc2 anions bridge 5.738(1) A. [Cd(pz)2(H2O)]2 þ cations to form a 1D chain along the b-axis (Fig. 2). Then, the adjacent 1D chains are further linked by the intermolecular ˚ Table S1) formed hydrogen bonds (O(3)–H(3A)yO(4)#1, 2.748(4) A, between coordinated water molecule (O3) and uncoordinated carboxylic O atom (O4) along the c-axis, resulting in a hydrogen-bonded 2D layer in the bc plane (Fig. S3). 3.1.2. Complex [Cd1Cd2(btec)(H2O)6]n (2) The asymmetric unit of complex 2 comprises two crystallographic independent Cd(II) ions (Cd1 and Cd2), one btec4 anion and six coordinated water molecules (Fig. 3). The Cd1(II) ion is six-coordinate with O6 donors (Fig. S4a), three of which are the carboxyl oxygen atoms (O2, O4A and O8B) from three different btec4 anions, the other three O atoms (O9, O10 and O11) are coordinated water molecules. The bond lengths of Cd1–O are within 2.215(5)– ˚ The Cd2(II) ion is also six-coordinate by O6 donors 2.386(6) A.
Fig. 1. The asymmetric unit of complex 1. Thermal ellipsoid is of 30% probability.
(Fig. S4b), three of which are the carboxyl oxygen atoms (O4, O6B and O8C) from the three different btec4 groups, the other three oxygen atoms (O12, O13 and O14) are coordinated water molecules. ˚ The four The bond lengths of Cd2–O are within 2.224(5)–2.388(6) A. carboxylic groups of btec4 anion exhibit two kinds of coordination fashions (Fig. S5): (1) two carboxylic groups lying in 1- and 4-position all coordinate to one Cd(II) ion with unidentate mode [27], respectively; (2) two carboxylic groups lying in 2- and 5-position all bridge two Cd(II) ions with monoatomic bridging mode [27], respectively, ˚ In the crystal of 2, the two the distance of Cd1D Cd2 is 3.716(8) A. neighboring carboxyl groups lying in 4- and 5-positions of the btec4 anion bridge [Cd(H2O)2]2 þ cations to form a 1D chain along the a-axis (Fig. S6), and the other carboxyl groups of the btec4 anions in the chain are coordinated to the Cd (II) ions in the adjacent chains, resulting in a 2D layer in the ab plane (Fig. 4). The adjacent layers ˚ are further linked by the hydrogen bonds (O10–H O7, 2.726 A, ˚ ˚ O10–H O1, 2.807 A, O9–H O1, 2.793 A, O12–H O5, ˚ O13–H O5, 2.792 A, ˚ O12–H O3, 2.728 A) ˚ formed 2.817 A, between coordinated water molecules (O9, O10 and O12, O13) and uncoordinated carboxylic O-atoms (O1, O3 and O5) of btec4 anion along the c-axis, resulting in a 3D hydrogen-bonded network (Fig. S7).
3.1.3. Complex [Cd(3,4-pdc)(H2O)]n(3) Complex 3 is a coordination polymer with 2D structure. The asymmetric unit consists of one Cd(II) ion, one 3,4-pdc2 anion
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Fig. 2. 1D chain of complex 1 along b-axis.
carboxylic O atoms of 2,5-pdc2 group are contributed to formation of hydrogen bonds. First of all, the neighboring molecules are linked together by the bifurcation hydrogen bonds (O(6)–H(6A) O(2)#4, ˚ O(7)–H(7B) O(2)#6, 2.673(2) A) ˚ formed between coor2.789(2) A; dinated water molecule and uncoordinated carboxylic O atom of 2,5pdc2 anion along the a-axis, producing a 1D hydrogen-bonded chain (Fig. 8). Then, the adjacent 1D chains are further linked by hydrogen ˚ formed between coordibonds (O(7)–H(7A) O(4)#5, 2.664(2) A) nated water molecule and uncoordinated carboxylic O atom of 2,5pdc2 anion along the c-axis, resulting in formation of a hydrogenbonded 2D layer in the ac plane (Fig. 9). Finally, the adjacent 2D layers are further linked by hydrogen bonds (O(8)–H(8A) O(3)#2, ˚ between coordinated water molecule and uncoordinated 2.667(2) A) carboxylic O atom of 2,5-pdc2 anion along the b-axis, resulting in a 3D hydrogen-bonded network (Fig. S12). In addition, the lattice water molecules also take part in formation of hydrogen bonds (Table S1), which makes the structure more stable.
Fig. 3. The asymmetric unit of complex 2. Thermal ellipsoid is of 30% probability.
and one coordinated water molecule (Fig. S8). The Cd (II) ion is six-coordinate by O5N1 donors (Fig. 5), possessing a slightly distorted octahedral geometry. The bond lengths of Cd–O are ˚ The within 2.268(2)–2.379(3) A˚ and that of Cd–N is 2.301(3) A. four O atoms and one N atom of 3,4-pdc2 anion all participate in coordination. The two carboxylic groups of the 3,4-pdc2 anion all coordinate to Cd(II) ions as bridging bidentate groups in a syn–anti configuration [27] (Fig. S9). In the crystal of 3, the two carboxyl groups the 3,4-pdc2 anion alternately bridge [Cd(H2O)]2 þ cations to form a 1D chain along the c-axis (Fig. 6). Then, the N atoms of intrachain 3,4-pdc2 anions coordinate with the Cd(II) ions from the adjacent chains, linking the chains along the b-axis and resulting in a 2D coordination layer in the bc plane (Fig. S10). The adjacent layers are further linked by the intralayer remaining carboxylic O atoms coordinating to the Cd (II) ions from the neighboring layers, leading to formation of 3D network (Fig. S11).
3.1.4. Complex [Zn(2,5-pdc)(H2O)4] 2H2O(4) Structural analysis reveals that complex 4 is a mononuclear complex with the formula of [Zn(2,5-pdc)(H2O)4] 2H2O. The molecule contains one Zn(II) ion, one 2,5-pdc2 anion, four coordinated water molecules and two lattice molecules (Fig. 7). The Zn(II) ion is six-coordinate with O5N1 donors in distorted octahedral geometry. The bond lengths of Zn–O are within ˚ In the crystal 2.041(1)–2.162(2) A˚ and that of Zn–N is 2.128(2) A. of 4, a 3D coordination supramolecule is constructed by many O–H O hydrogen bonds (Table S1). The coordinated water molecules, the lattice water molecules and uncoordinated
3.1.5. Complex {[Zn(2,5-pdc)(H2O)2] H2O}n (5) Complex 5 is a coordination polymer with 2D infinite structure bridged by 2,5-pdc2 anion. The asymmetric unit consists of one Zn(II) ion, one 2,5-pdc2 anion, two coordinated water molecules and one lattice water molecule (Fig. S13). The Zn (II) ion is sixcoordinate by O5N1 donors with a distorted octahedral geometry (Fig. 10). The bond lengths of Zn–O are within 2.037(5)– ˚ Two carboxylic groups 2.270(5) A˚ and that of Zn–N is 2.133(6) A. of 2, 5-pdc2 anion coordinate to Zn(II) ions in unidentated and syn–anti bridging bidentated ways [27], respectively. The separation of ZnC ZnD is 4.940(9) A˚ (Fig. S14). In the crystal of 5, firstly, the N atom of 2,5-pdc2 anion together with its one neighboring carboxylic O atom chelate a Zn (II) ion, and simultaneously, one O atom from 5-position carboxylic group bridge another Zn (II) ion, resulting in a 1D chain along the a-axis (Fig. S15). Secondly, the remaining O atoms of 5-position carboxylic groups of intrachain 2,5-pdc2 anions coordinate to the Zn (II) ions from the adjacent chains, resulting in a 2D layer in the ab plane (Fig. 11). Finally, the adjacent layers are further linked by O–H O hydrogen bonds (Table S1) formed between coordinated water molecule (O6) and uncoordinated carboxylic O atom (O2) of 2, 5-pdc2 anion, between lattice water molecule (O7) and coordinated carboxylic O atom (O3) of 2, 5-pdc2 anion or coordinated water molecule (O5) along the b-axis, resulting in a 3D hydrogen-bonded network (Fig. S16). Although the structures of complexes 3 and 5 were same to those in the literatures [28–29], the synthesis methods of complexes, solvent system, the amount and nature of raw materials in this paper are different from those reported. In this paper, we focus on the surface photoelectric properties of the five complexes and the influence from structures of complexes on the surface photoelectric properties.
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Fig. 4. 2D coordination layer of complex 2 in ab plane (coordinated water molecules are omitted for clarity).
Fig. 6. 1D chain of complex 3 along c-axis.
Fig. 5. Coordination environment around Cd(II) ion in complex 3. Thermal ellipsoid is of 30% probability Symmetry code A: x þ1/2, yþ 1/2, zþ 3/2; B: x, yþ 2, zþ 1; C: x þ1/2, yþ 1/2, z þ 1/2; D: xþ 1, yþ2, z þ 1.
3.2. XRD As shown in Fig. S17, the X-ray powder diffraction patterns for five complexes are consistent with the simulated ones on the basis of their single-crystal X-ray diffraction data, respectively. The positions of diffraction peaks on experimental and simulated patterns essentially correspond, indicating the phase purity of the samples. 3.3. Surface photoelectric properties of complexes 3.3.1. Analysis and assignment of SPS and UV–vis spectra of complexes Based on the UV–vis absorption spectra (Figs. 12b–16b), these five complexes all exhibit relatively strong absorption bands under inducement of UV and visible light. In addition, the energies of absorption bands are all in the range of band-gap energy of semiconductors, so they can be regarded as broad semiconductors. Thus, energy-band theory of semiconductor and crystal field theory of complex were combined to analyze and assign SPS and associate with UV–vis absorption spectra, i.e., 2s and 2p orbitals of direct coordination atoms (O or N) can form valence band, meanwhile
Fig. 7. The molecular structure of complex 4. Thermal ellipsoid is of 30% probability.
conjugated p orbitals of the ligands also belong to valence band. Empty 4s and 4p (or 5s and 5p) orbitals of central metal Zn(II) ion (or Cd(II) ion) together with pn orbitals of ligands can form conduction band. While the 3d or 4d orbitals of the central metal ions lie between valence band and conduction band, they are considered as impurity levels. Under common circumstance, the electron or hole transfer (or transition) from valance band to conduction band is called band-to-band transition (or sub-bandgap transition), while the electron transition between d orbitals of central metal ions is
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
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Fig. 8. 1D hydrogen-bonded chain of complex 4 along a-axis.
Fig. 9. 2D hydrogen-bonded layer of complex 4 in ac plane.
orbital probably form a new orbital with p orbital of ligands [32], and can also be categorized as valence band simultaneously. Thus we can assign this electron transition from the d orbital (which has been included in the valence band) to conduction band (MLCT) as also band-to-band transition of semiconductors.
Fig. 10. Coordination environment around Zn(II) ion in complex 5. Thermal ellipsoid is of 30% probability. Symmetry code A: x 1, y, z; B: xþ 1, y 1/2, zþ 1/2.
called impurity transition. However, 3d (or 4d) orbitals of transitional metal ions in coordination polymers often are affected by crystal field of coordination environment and split. The split situations of d orbitals of different metal ions in different coordination environments can be very different. Therefore, some split d orbitals may be close to valence band, the others may be close to conduction band instead, which makes the composition of valance and conduction bands more complicated [30–33]. For example, the split dxy
3.3.1.1. Analysis and assignment of SPS and UV–vis spectra of complexes 1–3. The SPS of complexes 1–3 are shown in Figs. 12a–14a. Complexes 1–3 are Cd(II) coordination polymers with aromatic carboxylic derivatives as bridging ligands, so their SPS exhibit similar characteristics. All of them show a wide, strong response band with many overlapped bands in 300–600 nm in SPS. Treated by Origin 7.0 program, the response band can be divided into several response bands with different intensities: complex 1, lmax ¼335, 361 and 446 nm (three bands); complex 2, lmax ¼350 and 531 nm (400– 700 nm) (two bands); complex 3, lmax ¼350, 403 and 465 nm (three bands). Obviously, these response bands belong to the band-to-band transition from valence band to conduction band. The response bands with relatively stronger intensity all lie in near ultraviolet region and they are assigned to band-to-band transition from the valence band (including p orbitals of direction coordination atoms with conjugated p orbitals) to conduction band, corresponding to the ligand-to-metal charge transfer (LMCT) transition. The response
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Fig. 11. 2D coordination layer of complex 5 in ab plane.
bands in visible region (lmax ¼446 nm, complex 1; 531 nm, complex 2; and 465 nm, complex 3) could be caused by the transition from the valence band (involving some post-split d orbitals of Cd(II) ion) to the conduction band, which should also belong to band-to-band transition of broad semiconductor; therefore, they lie in the visible region with lower energy and cover wide spectral region (430– 600 nm in 1; 400–700 nm in 2; 400–630 nm in 3). In addition, complexes 1 and 3 exhibit two response bands in the UV region, but complex 2 shows one broad response band, respectively. This may be due to that there are two species of direct coordination atoms (O and N atoms) in complexes 1 and 3, one species (O atom) in complex 2. The UV–vis absorption spectra of complexes 1–3 are shown in Figs. 12b–14b. By comparison, it can be found that the UV–vis absorption spectra are closely associated with SPS, and even consistent with one another. The UV–vis spectra of 1–3 show different numbers of absorption bands: lmax ¼252, 290, 330 and 544 nm (430–630 nm) for complex 1; lmax ¼ 252, 302 (290–440) and 510 nm (440–650 nm) for complex 2; lmax ¼258, 332 (290–450) and 550 nm (450–650 nm) for complex 3. The absorption bands at lmax ¼252 (1), 252 (2) and 258 nm (3) are assigned to intraligand p-pn. And the band in ultraviolet region for each complex is assigned to LMCT (O(N)-Cd). Because there are two species of direct coordinated atoms (O and N) with different electronegativity in complexes 1 and 3, the LMCT absorption bands in complexes 1 and 3 are split into two bands, i.e., O-Cd and N-Cd, respectively. In addition, the absorption bands in visible region for each complex (l ¼430–630 nm for 1; 440–650 nm for 2; 450–650 nm for 3) are relatively wide, which should be caused by metal-to-ligand charge transition (MLCT). Furthermore, the feature of SPS and UV–vis spectra in three Cd(II) complexes is that all of them show obvious bands in visible region, which should originate from MLCT in nature. This does not appear in other transitional metal complexes with d10 configuration, which in turn, is more favorable to photoelectric conversions.
3.3.1.2. Analysis and assignment of SPS and UV–visspectra of complexes 4 and 5. Complexes 4 and 5 are Zn(II) complexes with carboxylic derivatives as bridging ligands. As shown in Figs. 15a and 16a, the SPS of complexes 4 and 5 are similar to those of complexes 1–3, i.e., there is also a wide and strong response band with many overlapped bands for complex 4 as well as 5, respectively. Treated by Origin 7.0 program, the response band can be divided into three response bands: lmax ¼353, 386 and 442 nm for 4; lmax ¼341, 373 and 451 nm for 5. The response bands (lmax ¼353 and 386 nm for 4; 341 and 373 nm for 5) have higher energy and intensity and they are assigned to the band-to-band transition arising from valence band composed of ligands to conduction band, corresponding to LMCT. The bands in visible region (lmax ¼442 nm for 4; 451 nm for 5) are assigned to the transition from valence band formed by post-split d orbitals to conduction band, corresponding to MLCT. There are four absorption bands in the UV–vis spectra for complexes 4 and 5 (Figs. 15b and 16b), respectively: lmax ¼262, 336, 372 and 496 nm for 4; lmax ¼258, 308, 370 and 458 nm for 5. The bands at lmax ¼262 nm (for 4) and 258 nm (for 5) are assigned to intraligand p-pn. The bands at lmax ¼336, 372 nm (for 4) and 308, 370 nm (for 5) can be assigned to LMCT (O(or N) -Zn). In addition, the broad absorption bands in visible region (460–600 nm(for 4) and 400–600 nm(for 5)) are attributed to MLCT. By comparing SPS and UV–vis spectra, both of them are consistent with each other.
3.3.2. Comparison and analysis of SPS of complexes 1–5 By comparing and analyzing the SPS of complexes 1–5, the conclusions are summarized below: 1) There are obvious photovoltage response bands in the SPS of complexes 1–5 in the range of 300–600 nm, which indicates that they all possess certain photoelectric conversion properties.
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
Fig. 12. SPS (a) and UV–vis (b) spectra of complex 1.
2)
3)
4)
5)
But the intensity and width of the SPS response bands are different. The SPS are closely associated with the UV–vis absorption spectra for each complex, which can be cross-verification for each other. There are different species of central metal ions in these five complexes: Cd(II) (4d10) is in complexes 1–3 and Zn(II) (3d10) is in complexes 4 and 5. Due to the difference of shell levels, the split energies of crystal filed are different, which in turn affect the results of SPS. For example, the response bands in visible region in complexes 1–3 are wider than those in complexes 4 and 5. The different direct coordination atoms with different electronegativity result in the splitting of the band-to-band responses based on LMCT. For example, there is one species of coordination atoms (i.e., O atom) in complex 2, so there is one LMCT response band. But in the complexes 1, 3, 4 and 5, there are two species of coordination atoms, respectively. So there are two LMCT response bands (O-Cd (or Zn) and N-Cd(or Zn)). The change of coordination micro-environment around central metal ions will affect the position and shape of response bands. For example, in complex 2, there are two crystallographically independent Cd(II) ions, thus, the SPS response band within 400–700 nm appear several splitting bands.
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Fig. 13. SPS (a) and UV–vis (b) spectra of complex 2.
6) The increasing of conjugation degree of ligands maybe make response band widening and strengthening. For example, in complex 1, there are two types of cyclo-conjugated ligand, and in other four complexes, there is one type of cyclo-conjugated ligand, thus the response region of SPS in complex 1 is wider than that in other four complexes. In addition, the SPS of five complexes in this paper are compared with the SPS of Cd(Ac)2(H2O)4 and Zn(Ac)2(H2O)4 (Fig. S18), and the SPS of all five complexes are obviously strengthened and widened. It can be seen that the conjugated-organic-ligand can effectively increase the intensity of SPS. Because when the complex is irradiated by light, the conjugated ligand in the complex can absorb photos and give rise to the p-pn transition. Although these electrons in this transition process cannot directly flow and form current or voltage, they maybe inflow into the broad semiconductors and undertake the role of transferring energy, moreover, probably improving the photoelectric conversion efficiency. 3.4. TG analyses The thermal stability of complexes was investigated by TG analysis in the temperature 30–800 1C (Fig. S19). The TG curve of complex 1 shows that it is stable up to 150 1C, and then loses weight from 150 to 300 1C (observed: 40.12%; calculated: 41.33%),
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Fig. 15. SPS (a) and UV–vis (b) spectra of complex 4. Fig. 14. SPS (a) and UV–vis (b) spectra of complex 3.
corresponding to the removal of one coordinated water molecule, two organic pz molecules and one CO2 molecule from decomposition of 1,2-bdc2 ligand. The second weight loss of 42.4% (calculated: 45.69%) in the temperature range of 300–700 1C can be assigned to the release of 1,2-bdc2 ligand. The TG curve of complex 2 exhibits two continuous weight loss steps. The first weight loss starts at ca. 90–200 1C to give a weight loss of ca. 18.80%, corresponding to the loss of coordinated water molecules (18.52%). The second weight loss is 40.2% in the temperature range 350–500 1C, attributed to the decomposition of the btec ligand and the collapsing of the structure. The TG analysis indicates that complex 3 has good stability. The curve exhibits that there is no weight loss until 170 1C. It first loses the coordinated water molecules in the range of 170–250 1C (observed: 6.72%; calculated: 6.09%). Subsequently, a plateau region is observed from 250 to 320 1C. It keeps losing weight from 320 to 750 1C, ascribing to the decomposition of 3,4-pdc ligand and the collapsing of the structure. The whole weight loss (61.05%) is in good agreement with the calculated value (61.97%). For complex 4, it is thermally stable up to 100 1C. Above this temperature, the TG curve exhibits two weight loss stages. The first weight loss of 15.46% between 100 and 170 1C is attributable to loss of the three water molecules (calculated: 15.96%). The
second weight loss occurs from 170 to 800 1C, attributed to release of another three water molecules and the decomposition of organic ligand and the collapsing of the structure. The whole weight loss (79.16%) is in basic agreement with the calculated value (80.67%). The TG curve of complex 5 shows a weight loss (18.78%) from 130 to 250 1C corresponding to loss of the three water molecules (calculated: 18.98%). Then, a small plateau region is observed from 250 to 350 1C. The observed second weight loss (31.28%) in the range of 350–470 1C is attributed to loss of the two CO2 molecules from decomposition of 2,5-pdc2 ligand (calculated: 30.93%). 3.5. Luminescent properties of complexes At room temperature, the luminescent properties of the five complexes in the UV and visible region were investigated in the solid state (Bandwidth Ex.: 3 nm, Bandwidth Em.:3 nm). As shown in Fig. S20, five complexes all show weak emission bands: Em lEm max ¼400 nm (lEx ¼317 nm) for complex 1; lmax ¼301 nm (lEx ¼ Em 260 nm) for complex 2; lmax ¼408 nm (lEx ¼349 nm) for complex 3; Em lEm max ¼302 nm (lEx ¼260 nm) for complex 4 and lmax ¼469 nm (lEx ¼332 nm) for complex 5 in UV/visible regions, respectively. The observed emission bands are weak, which may be due to that the electrons are transited into an excited state under inducement of UV and visible light, then a portion of photo-generated electrons in
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can be classified the conduction band. Thus, this charge transition from any one orbital in the valence band to any one orbital in conduction band can lead to the band-to-band transition response. Obviously, this photovoltage response (caused by the charge transition from the d orbitals in the valence band to the conduction band) should be corresponded with MLCT, and this phenomenon is very rare. This can be verified by UV–vis absorption spectra of the complexes. SPS and UV–vis absorption spectra are basically consistent with one another. It is found that species and coordination micro-environment of central metal ion, the species and property of ligands all affect the intensity and scope of the photoelectric response.
Supporting Information The supporting information includes the portion of structure figures, Hydrogen-bond data, TG curves, XRD patterns, the SPS comparison and emission spectra.
Appendix A. Supplementary data CCDC: 861537, 861538, 861539, 861540 and 861541 contain the supplementary crystallographic data for complexes. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, or email: [email protected].
Acknowledgments The research was supported by National Natural Science Foundation of China (20571037) and the Educational Foundation of Liaoning Province in China (L2011193).
Appendix B. Supplementary material
Fig. 16. SPS (a) and UV–vis (b) spectra of complex 5.
excited state can transfer with help of the build-in electric field of samples and form current/voltage; another small portion of excited electrons lost by the non-radioactive process; so, the emission bands coming from electron recombination luminescence become weak.
4. Conclusions Five Cd(II)/Zn(II) complexes were hydrothermally synthesized. Their structures were determined by single-crystal X-ray diffraction. Surface electron behaviors of complexes were analyzed by SPS. The results of SPS indicate that all five complexes exhibit photoelectric responses in the range of 300–600 nm, which reveals that they all possess certain photoelectric conversion ability. So, the energy-band theory of semiconductor and the crystal field theory were combined to try to analyze and assign SPS and associate with UV–vis absorption spectra, i.e., 2s and 2p orbitals of direct coordination atoms (O or N), p orbitals of the conjugated ligand, and some post-split low-energy d orbitals of Cd(II)/Zn(II) ions caused by crystal field all can be classified the valence band. Empty 4s and 4p (or 5s and 5p) orbitals of central metal Zn(II) (or Cd(II)) ions, pn orbitals of the conjugated ligands, and some post-split high-energy d orbitals of Cd(II)/Zn(II) ions all
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.09.016.
References [1] S. Deka, A. Quarta, M.G. Lupo, A. Falqui, S. Boninelli, C. Giannini, G. Morello, M.D. Giorgi, G. Lanzani, C. Spinella, R. Cingolani, T. Pellegrino, L. Manna, J. Am. Chem. Soc. 131 (2009) 2948–2958. [2] A. Datta, S.K. Panda, S. Chaudhuri, J. Phys. Chem. C 111 (2007) 17260–17264. [3] H. Yin, Y. Wada, T. Kitamura, S. Yanagida, Environ. Sci. Technol. 35 (2001) 227–231. [4] H.J. Lee, J. Bang, J. Park, S. Kim, S.-M. Park, Chem. Mater. 22 (2010) 5636–5643. [5] R. Feng, F.-L. Jiang, L. Chen, C.-F. Yan, M.-Y. Wu, M.-C. Hong, Chem. Commun. 35 (2009) 5296–5298. [6] S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita, S. Kitagawa, J. Am. Chem. Soc. 129 (2007) 2607–2614. [7] P.K. Thallapally, C.A. Fernandez, R.K. Motkuri, S.K. Nune, J. Liu, C.H.F. Peden, Dalton Trans. 39 (2010) 1692–1694. [8] P.-X. Li, C.-L. Hu, Q.-P. Lin, N. Zhao, J.-G. Mao, Inorg. Chem. 48 (2009) 5454–5461. ˜ o, M. Esteban, Environ. Sci. [9] E. Chekmeneva, R. Prohens, J.M. Dı´az-Cruz, C. Arin Technol. 42 (2008) 2860–2866. [10] Y.M. Lee, Y.J. Song, J.I. Poong, S.H. Kim, H.G. Koo, J.A. Lee, C. Kim, S.-J. Kim, Y. Kim, Inorg. Chem. Commun. 13 (2010) 101–104. [11] Y.J. Song, H. Kwak, Y.M. Lee, S.H. Kim, S.H. Lee, B.K. Park, J.Y. Jun, S.M. Yu, C. Kim, S.-J. Kim, Y. Kim, Polyhedron 28 (2009) 1241–1252. [12] M.F. Wu, F.K. Zheng, G. Xu, A.Q. Wu, Y. Li, H.F. Chen, S.P. Guo, F. Chen, Z.F. Liu, G.C. Guo, J.S. Huang, Inorg. Chem. Commun. 13 (2010) 250–253. [13] Y.X. Chi, S.Y. Niu, J. Jin, Inorg. Chim. Acta 362 (2009) 3821–3828. [14] Y.-X. Chi, S.-Y. Niu, J. Jin, R. Wang, Y. Li, Dalton Trans. 37 (2009) 7653–7659. [15] Y.-X. Chi, S.-Y. Niu, R. Wang, J. Jin, G.-N. Zhang, J. Lumin. 131 (2011) 1707–1713.
102
J. Jin et al. / Journal of Solid State Chemistry 197 (2013) 92–102
[16] K.S. Bharathi, A.K. Rahiman, K. Rajesh, S. Sreedaran, P.G. Aravindan, D. Velmurugan, V. Narayanan, Polyhedron 25 (2006) 2859–2868. [17] S.H. Kim, B.K. Park, Y.J. Song, S.M. Yu, H.G. Koo, E.Y. Kim, J.I. Poong, J.H. Lee, C. Kim, S.-J. Kim, Y. Kim, Inorg. Chim. Acta 362 (2009) 4119–4126. [18] H. Kwak, S.H. Lee, S.H. Kim, Y.M. Lee, B.K. Park, Y.J. Lee, J.Y. Jun, C. Kim, S.-J. Kim, Y. Kim, Polyhedron 28 (2009) 553–561. [19] Y.F. Wang, J. Xu, W.Z. Zong, Y.F. Zhu, J. Solid State Chem. 184 (2011) 1433–1438. [20] T. Xie, D. Wang, L. Zhu, C. Wang, T. Li, X. Zhou, M. Wang, J. Phys. Chem. B 104 (2000) 8177–8181. [21] M.H. Qi, G.F. Liu, J. Phys. Chem. B 107 (2003) 7640–7646. [22] L.-P. Sun, S.-Y. Niu, J. Jin, G.-D. Yang, L. Ye, Eur. J. Inorg. Chem. 2006 (2006) 5130–5137. [23] L.-P. Sun, S.-Y. Niu, J. Jin, L. Zhang, Eur. J. Inorg. Chem. 2007 (2007) 3845–3852. [24] L. Zhang, S.-Y. Niu, J. Jin, L.-P. Sun, G.-D. Yang, L. Ye, Inorg. Chim. Acta 362 (2009) 1448–1454.
[25] L. Li, S.-Y. Niu, Z.-F. Shi, L.-C. Gong, J. Jin, Y.-X. Chi, Y.-H. Xing, Polyhedron 30 (2011) 976–982. [26] L. Li, S.-Y. Niu, J. Jin, Q. Meng, Y.-X. Chi, Y.-H. Xing, G.-N. Zhang, J. Solid State Chem. 184 (2011) 1279–1285. [27] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227–250. [28] X. Zhang, D. Huang, C. Chen, Q. Liu, D. Liao, L. Li, Inorg. Chem. Commun. 8 (2005) 22–26. [29] E.E. Sileo, O.E. Piro, G. Rigotti, M.A. Blesa, A.S. de Araujo, E.E. Castellano, Struct. Chem. 19 (2008) 651–657. [30] A.J. Sillanpaa, R. Aksela, K. Laasonen, Phys. Chem. Chem. Phys. 5 (2003) 3382–3393. [31] X. Zhou, H.-X. Zhang, Q.-J. Pan, B.-H. Xia, A.-C. Tang, J. Phys. Chem. A 109 (2005) 8809–8818. [32] L.T. Chen, T. Liu, J.Z. Chen, Acta Chim. Sin. 66 (2008) 1187–1195 (in Chinese). [33] T.M. McCormick, B.D. Calitree, A. Orchard, N.D. Kraut, F.V. Bright, M.R. Detty, R. Eisenberg, J. Am. Chem. Soc. 132 (2010) 15480–15483.