Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectra, energy levels, and energy transition of lanthanide complexes with cinnamic acid and its derivatives Kaining Zhou, Zhongshan Feng, Jun Shen, Bing Wu, Xiaobing Luo, Sha Jiang, Li Li, Xianju Zhou ⁎ College of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
a r t i c l e
i n f o
Article history: Received 16 June 2015 Received in revised form 15 December 2015 Accepted 11 January 2016 Available online 12 January 2016 Keywords: Cinnamic acid Lanthanide complex Luminescence Energy level Triplet-state
a b s t r a c t High resolution spectra and luminescent lifetimes of 6 europium(III)–cinnamic acid complex {[Eu2L6(DMF) (H2O)]·nDMF·H2O}m (L = cinnamic acid I, 4-methyl-cinnamic acid II, 4-chloro-cinnamic acid III, 4-methoxycinnamic acid IV, 4-hydroxy-cinnamic acid V, 4-nitro-cinnamic acid VI; DMF = N, N-dimethylformamide, C3H7NO) were recorded from 8 K to room temperature. The energy levels of Eu3+ in these 6 complexes are obtained from the spectra analysis. It is found that the energy levels of the central Eu3+ ions are influenced by the nephelauxetic effect, while the triplet state of ligand is lowered by the p–π conjugation effect of the parasubstituted functional groups. The best energy matching between the ligand triplet state and the central ion excited state is found in complex I. While the other complexes show poorer matching because the gap of 5D0 and triplet state contracts. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The study of lanthanide complexes with organic ligands is constantly attracting research interest, due to the potential application of biosensor, electronic luminescent materials and anti-cancer drugs [1–10]. The choice of the ligands is very critical for the structure and properties of the complexes. And cinnamic acid has similar structure with benzoic acid except for one more C_C. Therefore, cinnamic acid with the halfrigid structure is feasible to act as ligands for metal–organic complex. These compounds are interesting in a wide range of applications [11–14]. Ga-Lai Law et al. have synthesized fifteen dipolar polymeric lanthanide complexes with trans-cinnamic acid as ligands. Strong second harmonic generation nonlinear optical activity has been observed in these compounds [15]. Cinnamic acid and its derivatives are also introduced to lanthanide complex polymer nanoparticles, and effective energy transfer is revealed from the ligand to the central metal ions. The fluorescent functional materials might be useful as bio-probe [16]. The synthesis, crystal structure of cinnamic acid europium complexes along with the rough optical properties at room temperature have been reported in our previous work [17,18]. The corresponding formulae and the coordination information of these complexes are summarized in Table 1. The coordination environment of the central Eu3+ ions which is published in ref. 17 could also be found in the Supporting information as Fig. S1. In this work, high resolution emission and excitation spectra of these complexes at various temperatures from 8–300 K have been recorded and analyzed carefully. Energy levels of Eu3+ ion ⁎ Corresponding author. E-mail address: [email protected] (X. Zhou).
http://dx.doi.org/10.1016/j.saa.2016.01.015 1386-1425/© 2016 Elsevier B.V. All rights reserved.
in these complexes were obtained based on the spectra. The factors which influence the energy levels are investigated as well. The relationship between ligand design and the transition energy for the central europium ion in the crystals was studied. Furthermore, energy transition from the ligand to central metal ions was discussed. Results show that the para-substituted group with different Hammett substituent constant σp could efficiently modulate the excitation band and triplet state of the complexes, and impact the energy transition within the complexes. 2. Experimental Synthesis of the complexes could be found in our previous work [17, 18]. The fine powders of these complexes were measured by powder Xray diffraction on an X-ray Diffractometer (XD-2, Beijing Purkinje General Instrument Co. Ltd.) with Cu-Kα radiation (λ = 1.5406 Å) at 36 kV and 20 mA. The scan rate was 2°/min and the scan range was between 10° and 80°. The results of the powder X-ray diffraction are shown in Fig. S2 in the Supporting information. The FT-IR spectra of the complexes were recorded in the region of 4000–400 cm−1 on an FT-IR spectrometer (Perkin Elmer spectrum 65) by the KBr pelleting technique. The FT-IR spectra and the corresponding assignments are shown in Fig. S3 and Table S1 respectively. The TG curves of the complexes (see Fig. S4 in the Supporting information) were recorded on a TA thermogravimetric system. Samples of about 7 mg were used in platinum crucibles, heated at a rate of 20 °C/min in N2 flowing at a rate of 20 mL/min at ambient pressure. Absorption spectra of the lanthanide complexes and the corresponding ligand in ethanol solution at a concentration of 10−4 M were measured on a double-beam spectrometer (TU 1901,
30
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Table 1 Structure information of the complexes. Complex
Para-substituent
Formula of complex
Space group
Za
Site sym.b
No. sitesc
C. N.d
II
CH3
{[Eu2(C10H9O2)6(DMF)(H2O)]∙2DMF∙H2O}n
P1(no. 2)
2
C1
2
9
III
Cl
{[Eu2(C9H6ClO2)6(DMF)(H2O)]∙2DMF∙H2O}n
P1(no. 2)
2
C1
2
9
VI
NO2
Eu2(C9H6NO4)6(DMF)2(H2O)2 ∙2DMF
2
C1
1
9
I IV V
H OCH3 OH
Eu2(C9H7O2)6(DMF)3(H2O)2 Eu2(C10H9O3)6(DMF)3(H2O)2 Eu2(C9H7O3)6(DMF)3(H2O)2
P1(no. 2) – – –
– – –
– – –
– – –
– – –
a b c d
Number of Eu3+ in the Bravais cell. Site symmetry of Eu3+. Number of distinct Eu3+ sites. Total coordination number of Eu3+.
Purkinje General Instrument Co. Ltd.) with quartz liquid sample cell. The emission and excitation spectra of the powder samples were recorded at different temperatures, by a spectrophotometer (FLSP 920, Edinburgh Instruments) equipped with a 450 W xenon lamp and PMT detector (R9287 PMT, Shimadzu). The luminescence lifetime experiments were carried out on the same instrument with pulsed xenon lamp. A closed cycle refrigerator system (CCS 150, Janis), and temperature controller (9700, Scientific Instruments) were utilized to control the sample temperature.
originating from two different coordination Eu3+ ions was not observed in refs. [17,18]. In this study, splitting of this transition, at 579.8 and 579.9 nm is recorded at a low temperature of 8 K in complex I, as shown in the inset of Fig. 3(a). But the splitting gap is very narrow, the energy difference between these two peaks is only 3 cm−1, which implies that the difference between these two coordination environments of europium ion is relatively small [19].
3.2. Energy levels of Eu3+ in the complexes 3. Results and discussion
In our previous work, the rough optical properties were studied by the spectra recorded at room temperature. But the energy levels and
3.1. Excitation and emission spectra From our previous work, the crystal structure of these complexes was found to be one dimensional zigzag chain or dimer. The coordination environment of europium(III) was alike, which could be described as distorted monocapped square antiprism [17,18]. Fig. 1 presents the excitation spectra of two representative complexes from 8 K to room temperature. It is shown that the complexes bear both indirect and direct excitations. Indirect excitation is the ligand to metal charge transfer band (LMCT) from 280–400 nm, overlapping with ligand absorption. Direct excitation is the characteristic f–f transition of europium(III). The detailed assignments of the excitation spectra are labeled in the figure. The shape and the position of the spectra profiles do not show dramatic change with temperature. The excitation intensity increases as temperature lowers; it illustrates that the thermal population is excluded in this case and the non-radiative relaxation decreases at low temperature. The ligand excitation band overlapped with LMCT band sees a blue shift as temperature decreases. In order to investigate the effect of the para-substituted functional group, excitation spectra of these six complexes at 8 K are compared as shown in Fig. 2. The characteristic f–f transitions of Eu3 + central ions do not show obvious change except for the intensity because of shielding effect of f-orbital. However, the shape and position of the broad LMCT and ligand excitation band show a sensitive response to the para-substituted groups. First, the intensities of the LMCT band in complexes II (x = CH3) and IV (x = OCH3) are much weaker than others, with integral area of this broad band being only 1/3 of complex I. Second, the broad band shifts with the electronic effect of the ligand. This phenomenon should be related to the electronic effect of substituted groups, which will be discussed later. In the emission spectrum shown in Fig. 3, europium intraconfigurational transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, 4) are observed. The strongest emission transition is the forced electric dipole allowed transition 5D0 → 7F2. The transition of 5D0 → 7F3 is very weak. From the single crystal X-ray diffraction data as reported in our previous work [17,18], there are two non-equal crystal Eu3+ ions due to different coordination environments. However, due to the low resolution of the spectrum at room temperature, splitting of 5D0 → 7F0 transition
Fig. 1. Representative excitation spectra at various temperatures of complex II (a) and complex III (b).
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Fig. 2. Comparison of the excitation spectra of these six complexes at 8 K.
the relationship between the levels and the ligands are beyond discussion because of the limitation of the low resolution of the spectra. In this work, the low temperature excitation spectra of the complexes
31
clearly show the CTB of Eu–O, together with the characteristic Eu3+ f– f transitions of 7F0–5L6, 7F0–5D2, and 7F0–5D1, and the low temperature emission spectra have clearly presented the 5D1–7F1, 2 and 5D0–7FJ transitions with resolved structures. Since the site symmetry of Eu3+ ions in the complexes is C1, the electronic transitions occur following the Judd forced electric dipole mechanism with the selection rules Δs = 0; |ΔL ≤ 6| and if L = 0 or L′ = 0, |ΔL = 2,4,6|; |ΔJ ≤ 6| and if J = 0 or J′ = 0, |ΔJ = 2,4, 6|. The transitions from 5D0 are expected to be the strongest to terminal multiplets with even values of J. In addition, the 5D0 level is non-degenerate, so the number of zero-phonon lines in the lowtemperature emission spectra should be equal to 2J + 1 for transitions to J terminal multiplets [20] if there is only one Eu3+ site in the complex. But if there are two non-equal crystal Eu3 + ions, the maximum observed peaks should be doubled. In this study, even though two coordination different Eu3+ ions are found by single crystal X-ray diffraction, only 2 peaks for 5D0–7F0 in complex I and 4 peaks for 5D0–7F1 in complex VI were recorded. The spectrum splitting for other transitions in the complexes is not observed in the emission and excitation spectra. The number of peaks for 5D0–7FJ (J = 0, 1, 2, 4) transitions for all the complexes is listed in Table S2 (see in the Supporting information). The possible reason might be that the difference of these two coordination environment is relatively small and/or the resolution of the instrument is not high enough. The energy levels of europium(III) in 6 complexes are obtained from the spectra as shown in Table 2. The transition of 5 D0–7F3 is so weak, that only one or two energy levels were tentatively assigned. The values of 7F3 multiplet are not included in Table 2. 3.3. Nephelauxetic effect of the substituted group on central ion
Fig. 3. Representative emission spectra at various temperatures of complex I (a) and complex II (b). Inset: the splitting of 5D0 → 7F2 at 8 K in complex I.
Based on the findings of the energy levels of Eu3+ ion in these 6 complexes, it is seen that 5D0 and 5D2 excited states vary from each other. There has been constant debate concerning the explanation of Eu ion energy level variation, and different explanations that include nephelauxetic effects [21,22] and J-mixing [23] have been proposed. In this work, nephelauxetic effect is preferred to explain the spectral shift, and associate the variation of Eu 5D0 energy level with the decrease of inter-electronic repulsion, because the studied samples are lanthanide–organic ligand complexes with similar coordination environments and share the same site symmetry. Due to the nephelauxetic theory, short bond length and low coordination number can enhance the covalency between the metal ions and the ligands, and lower metal excitation level at the same time. A stronger Eu–O bond for europium(III) results in larger nephelauxetic effect and smaller value of the Slater integrals. It can cause the decrease of inter-electronic repulsion of europium(III), thus results in red-shift of 5D0 level. When the coordination number is the same, the key factor of nephelauxetic effect could be the electrostatic interaction of Eu–O which determines the bond length [24–26]. Single-crystal X-ray diffraction analysis shows that europium(III) bears the same coordination number of 9 in the complexes. The experimental results in this study found that 5D0 and 5D1 energy levels of Eu3+ central ions were increased by the replacement of para-position –H by substituted groups –CH3, –OCH3, –OH, –Cl, and – NO2, The similar trend was found in the 3P0 state of Pr3 + in the substituted benzoate complexes [27]. But in praseodymium complexes the electron-withdrawing substituent in benzoate moiety results in higher 3P0 energy than electron-donating groups. While in this work, the electron-withdrawing groups have done less work in lifting the energy of 5D0 of Eu3 + ions than electron-donating groups. The energy level of 5D2 has the same trend as that of 5D0. The relation between 5 D0 and 5D2 level and substituted groups are listed in Table 3. The effect of the substituted group is valued by the Hammet sigma factor for the para-substituent. The factors provide an estimate of the total electronic influence, including polarity, inductivity and resonance effect, in the absence of conjugation effects. The overall trend in Table 3 shows that the energy levels of central Eu3+ ions are lifted by the nephelauxetic effect of the para-substituted function groups.
32
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Table 2 Energy levels of europium(III) in the 6 complexes. Group
7
7
H CH3 Cl OCH3 OH NO2
0 0 0 0 0 0
352, 372 321, 376, 420 297, 376, 455 338, 374, 411 294, 374, 453 306, 335, 378, 420
F0
F1
7
7
5
5
735, 971, 1007, 1090, 1130 918, 967, 1009, 1049, 1080, 1120 727, 945, 1005, 1130 732, 920, 972, 1000, 1034, 1100 747, 943, 1009, 1128 901, 968, 1007, 1034, 1060
2516, 2660, 2741, 2829, 3020, 3121 2730, 2782, 2818, 2897, 2925, 2968 2530, 2613, 2727, 273, 2818, 2885, 2914, 2959, 3082 2721, 2776, 2806, 2897, 2933, 2996, 3077 2530, 2612, 2715, 2770, 2816, 2912, 2958, 3082 2679, 2748, 2811, 2853, 2904, 2996
17,235 17,255 17,247 17,255 17,254 17,241
18,987, 19,008, 19,049(ex) 18,325 19,048(ex) 19,003, 19,012, 19,016 – 19,047(ex)
F2
F4
3.4. The p–π conjugation effect of the substituted group on the triplet state The absorption peak of benzoate moiety is generally believed to be the gap of the singlet state and the ground state in the metal–organic complexes [28]. In this study, the triplet states of complexes were measured by recording the emission peaks of the corresponding pure ligands in 0.1 mol/L DMF solution. The emission spectra and findings are shown in Fig. 4 and Table 4 respectively. According to the molecular orbital theory, the lone electron pair on p orbital of the substituted group conjugates with electrons on π orbital of benzene ring (p–π conjugation effect), and decrease the π– π′ transition energy [29]. The energy of the triplet state in all these six complexes has been lowered as expected. The energy of the singlet state, which is referred by the absorption spectra of the complexes (data not shown herein), has the same trend as that of the triplet state, possibly due to the electron effect of the substituted group as well. 3.5. Energy transition The emission spectra of 6 complexes show that the central Eu3+ ions could be excited by both direct and indirect excitations. However the luminescent intensity is greatly affected by the efficiency of indirect excitation. In indirect excitation process, the energy transfers from the triplet state to central ion and pumps it. The efficiency of central ion excitation is primarily based on the energy gap between the triplet state and central ion excited level. According to report, the best energy matching is between 3500–5000 cm−1 [30]. The triplet states of these 6 compounds are tentatively estimated by recording the triplet state of the corresponding ligand. The europium excited levels are obtained from the spectra, and therefore the gap is calculated from the difference. The best energy matching is found in complex I, followed by complexes II, III, IV, V and VI. This finding could be supported by the relative luminescent intensity, and the changing temperature emission spectrum.
D0
5
D1
D2
21,547(ex) 21,556(ex) 21,552(ex) 21,570(ex) 21,566(ex) 21,551(ex)
donating substituted group could enhance it more, while the triplet state of the ligands is lowered because of the p–π conjugation effect between the substituted group and the benzoic ring. The gap between the europium(III) excited state and the triplet state of the ligand primarily determines the energy transition efficiency. Complex I shows the best energy matching and leads to the strongest luminescent intensity upon the indirect excitation. The introduction of the para-substituted group in the other 5 complexes results in lower triplet state of the ligand, which causes poorer energy matching between the triplet state and the excited state of the central ion, and leading to a lower luminescent intensity. Acknowledgments The project was funded by the National Natural Science Foundation of China (11404047), Natural Science Foundation Project of Chongqing (CSTC 2012JJA90019, CSTC2015jcyjA50005), and Chongqing Innovation Program of Postgraduate Students (CYS14152). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2016.01.015.
4. Conclusion Europium(III) is well sensitized in direct and indirect excitations in 6 para-substituted cinnamic acid complexes. Strong red line emission is observed and high resolution excitation and emission spectra were recorded and analyzed. Energy levels of 5D0, 5D1, 5D2, 7F0, 7F1, 7F2 and 7F4 were obtained from the spectra. Due to the nephelauxetic effect of the para-substituted groups, the 5D0 energy level is lifted. The electronFig. 4. Emission spectra of these 6 ligands in 0.1 M solution in DMF.
Table 3 Relation between 5D0, 5D2 and substituted groups. σp is the Hammet constant of the parasubstituent. D0 (cm−1)
D2 (cm−1)
Table 4 Energy of the ligand triplet state and Hammet substituent constant.
σp
Complex
Group
5
5
Substituted group
σp
Triplet state (cm−1)
−0.37 −0.268 −0.17 0 +0.23 +0.78
V IV II I III VI
OH OCH3 CH3 H Cl NO2
17,254 17,255 17,255 17,235 17,247 17,241
21,566 21,570 21,556 21,547 21,552 21,551
OH OCH3 CH3 H Cl NO2
−0.37 −0.268 −0.17 0 0.227 0.778
21,392 ± 50 21,458 ± 50 21,514 ± 50 21,559 ± 50 21,491 ± 50 18,755 ± 50
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
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[16] D. Shi, X. Liu, F. Xiang, M. Chen, C. Yang, M. Akashi, Macromol. Chem. Phys. 210 (2009) 2063. [17] X.J. Zhou, K.N. Zhou, Z.L. Wang, S.G. Yang, Z. Anorg. Allg. Chem. 639 (2013) 121. [18] X.J. Zhou, K.N. Zhou, Z.L. Wang, T.J. Xiao, X.Q. Zhao, Q.X. Li, Chin. J. Inorg. Chem. 29 (2013) 1870. [19] K. Binnemans, K. Van Herck, C. Gorller-Walrand, Chem. Phys. Lett. 266 (1997) 297. [20] L. Ning, L. Lin, L. Li, C. Wu, C. Duan, Y. Zhang, L. Seijo, J. Mater. Chem. 22 (2012) 13723. [21] Y.R. Shen, W.B. Holzapfel, Phys. Rev. B 52 (1995) 12618. [22] C.K. JØrgensen, in: F.A. Cotton (Ed.), Process in Inorganic Chemisty, 4, Wiley, New York 1962, p. 73. [23] P.A. Tanner, Y.Y. Yeung, L. Ning, J. Phys. Chem. A 117 (2013) 2771. [24] Y.S. Liu, W.Q. Luo, R.F. Li, G.K. Liu, M.R. Antonio, X.Y. Chen, J. Phys. Chem. C 112 (2008) 686. [25] S.T. Frey, W.D.W. Horrocks, Inorg. Chim. Acta 229 (1995) 383. [26] Q.H. Zeng, Z.W. Pei, S.B. Wang, Q. Su, J. Alloys Compd. 275 (1998) 238. [27] X. Zhou, P.A. Tanner, J. Phys. Chem. A 119 (2015) 79. [28] B. Li, H.J. Zhang, J.F. Ma, Chin. J. Chem. Phys. 11 (1998) 152. [29] K. Driesen, R.V. Deun, C. Gorller-Walrand, K. Binnemans, Chem. Mater. 16 (2004) 1531. [30] F.J. Steemers, W. Verboom, D.N. Reinhoudt, E.B. van der Tol, J.W. Verhoeven, J. Am. Chem. Soc. 117 (1995) 9408.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectra, energy levels, and energy transition of lanthanide complexes with cinnamic acid and its derivatives Kaining Zhou, Zhongshan Feng, Jun Shen, Bing Wu, Xiaobing Luo, Sha Jiang, Li Li, Xianju Zhou ⁎ College of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
a r t i c l e
i n f o
Article history: Received 16 June 2015 Received in revised form 15 December 2015 Accepted 11 January 2016 Available online 12 January 2016 Keywords: Cinnamic acid Lanthanide complex Luminescence Energy level Triplet-state
a b s t r a c t High resolution spectra and luminescent lifetimes of 6 europium(III)–cinnamic acid complex {[Eu2L6(DMF) (H2O)]·nDMF·H2O}m (L = cinnamic acid I, 4-methyl-cinnamic acid II, 4-chloro-cinnamic acid III, 4-methoxycinnamic acid IV, 4-hydroxy-cinnamic acid V, 4-nitro-cinnamic acid VI; DMF = N, N-dimethylformamide, C3H7NO) were recorded from 8 K to room temperature. The energy levels of Eu3+ in these 6 complexes are obtained from the spectra analysis. It is found that the energy levels of the central Eu3+ ions are influenced by the nephelauxetic effect, while the triplet state of ligand is lowered by the p–π conjugation effect of the parasubstituted functional groups. The best energy matching between the ligand triplet state and the central ion excited state is found in complex I. While the other complexes show poorer matching because the gap of 5D0 and triplet state contracts. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The study of lanthanide complexes with organic ligands is constantly attracting research interest, due to the potential application of biosensor, electronic luminescent materials and anti-cancer drugs [1–10]. The choice of the ligands is very critical for the structure and properties of the complexes. And cinnamic acid has similar structure with benzoic acid except for one more C_C. Therefore, cinnamic acid with the halfrigid structure is feasible to act as ligands for metal–organic complex. These compounds are interesting in a wide range of applications [11–14]. Ga-Lai Law et al. have synthesized fifteen dipolar polymeric lanthanide complexes with trans-cinnamic acid as ligands. Strong second harmonic generation nonlinear optical activity has been observed in these compounds [15]. Cinnamic acid and its derivatives are also introduced to lanthanide complex polymer nanoparticles, and effective energy transfer is revealed from the ligand to the central metal ions. The fluorescent functional materials might be useful as bio-probe [16]. The synthesis, crystal structure of cinnamic acid europium complexes along with the rough optical properties at room temperature have been reported in our previous work [17,18]. The corresponding formulae and the coordination information of these complexes are summarized in Table 1. The coordination environment of the central Eu3+ ions which is published in ref. 17 could also be found in the Supporting information as Fig. S1. In this work, high resolution emission and excitation spectra of these complexes at various temperatures from 8–300 K have been recorded and analyzed carefully. Energy levels of Eu3+ ion ⁎ Corresponding author. E-mail address: [email protected] (X. Zhou).
http://dx.doi.org/10.1016/j.saa.2016.01.015 1386-1425/© 2016 Elsevier B.V. All rights reserved.
in these complexes were obtained based on the spectra. The factors which influence the energy levels are investigated as well. The relationship between ligand design and the transition energy for the central europium ion in the crystals was studied. Furthermore, energy transition from the ligand to central metal ions was discussed. Results show that the para-substituted group with different Hammett substituent constant σp could efficiently modulate the excitation band and triplet state of the complexes, and impact the energy transition within the complexes. 2. Experimental Synthesis of the complexes could be found in our previous work [17, 18]. The fine powders of these complexes were measured by powder Xray diffraction on an X-ray Diffractometer (XD-2, Beijing Purkinje General Instrument Co. Ltd.) with Cu-Kα radiation (λ = 1.5406 Å) at 36 kV and 20 mA. The scan rate was 2°/min and the scan range was between 10° and 80°. The results of the powder X-ray diffraction are shown in Fig. S2 in the Supporting information. The FT-IR spectra of the complexes were recorded in the region of 4000–400 cm−1 on an FT-IR spectrometer (Perkin Elmer spectrum 65) by the KBr pelleting technique. The FT-IR spectra and the corresponding assignments are shown in Fig. S3 and Table S1 respectively. The TG curves of the complexes (see Fig. S4 in the Supporting information) were recorded on a TA thermogravimetric system. Samples of about 7 mg were used in platinum crucibles, heated at a rate of 20 °C/min in N2 flowing at a rate of 20 mL/min at ambient pressure. Absorption spectra of the lanthanide complexes and the corresponding ligand in ethanol solution at a concentration of 10−4 M were measured on a double-beam spectrometer (TU 1901,
30
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Table 1 Structure information of the complexes. Complex
Para-substituent
Formula of complex
Space group
Za
Site sym.b
No. sitesc
C. N.d
II
CH3
{[Eu2(C10H9O2)6(DMF)(H2O)]∙2DMF∙H2O}n
P1(no. 2)
2
C1
2
9
III
Cl
{[Eu2(C9H6ClO2)6(DMF)(H2O)]∙2DMF∙H2O}n
P1(no. 2)
2
C1
2
9
VI
NO2
Eu2(C9H6NO4)6(DMF)2(H2O)2 ∙2DMF
2
C1
1
9
I IV V
H OCH3 OH
Eu2(C9H7O2)6(DMF)3(H2O)2 Eu2(C10H9O3)6(DMF)3(H2O)2 Eu2(C9H7O3)6(DMF)3(H2O)2
P1(no. 2) – – –
– – –
– – –
– – –
– – –
a b c d
Number of Eu3+ in the Bravais cell. Site symmetry of Eu3+. Number of distinct Eu3+ sites. Total coordination number of Eu3+.
Purkinje General Instrument Co. Ltd.) with quartz liquid sample cell. The emission and excitation spectra of the powder samples were recorded at different temperatures, by a spectrophotometer (FLSP 920, Edinburgh Instruments) equipped with a 450 W xenon lamp and PMT detector (R9287 PMT, Shimadzu). The luminescence lifetime experiments were carried out on the same instrument with pulsed xenon lamp. A closed cycle refrigerator system (CCS 150, Janis), and temperature controller (9700, Scientific Instruments) were utilized to control the sample temperature.
originating from two different coordination Eu3+ ions was not observed in refs. [17,18]. In this study, splitting of this transition, at 579.8 and 579.9 nm is recorded at a low temperature of 8 K in complex I, as shown in the inset of Fig. 3(a). But the splitting gap is very narrow, the energy difference between these two peaks is only 3 cm−1, which implies that the difference between these two coordination environments of europium ion is relatively small [19].
3.2. Energy levels of Eu3+ in the complexes 3. Results and discussion
In our previous work, the rough optical properties were studied by the spectra recorded at room temperature. But the energy levels and
3.1. Excitation and emission spectra From our previous work, the crystal structure of these complexes was found to be one dimensional zigzag chain or dimer. The coordination environment of europium(III) was alike, which could be described as distorted monocapped square antiprism [17,18]. Fig. 1 presents the excitation spectra of two representative complexes from 8 K to room temperature. It is shown that the complexes bear both indirect and direct excitations. Indirect excitation is the ligand to metal charge transfer band (LMCT) from 280–400 nm, overlapping with ligand absorption. Direct excitation is the characteristic f–f transition of europium(III). The detailed assignments of the excitation spectra are labeled in the figure. The shape and the position of the spectra profiles do not show dramatic change with temperature. The excitation intensity increases as temperature lowers; it illustrates that the thermal population is excluded in this case and the non-radiative relaxation decreases at low temperature. The ligand excitation band overlapped with LMCT band sees a blue shift as temperature decreases. In order to investigate the effect of the para-substituted functional group, excitation spectra of these six complexes at 8 K are compared as shown in Fig. 2. The characteristic f–f transitions of Eu3 + central ions do not show obvious change except for the intensity because of shielding effect of f-orbital. However, the shape and position of the broad LMCT and ligand excitation band show a sensitive response to the para-substituted groups. First, the intensities of the LMCT band in complexes II (x = CH3) and IV (x = OCH3) are much weaker than others, with integral area of this broad band being only 1/3 of complex I. Second, the broad band shifts with the electronic effect of the ligand. This phenomenon should be related to the electronic effect of substituted groups, which will be discussed later. In the emission spectrum shown in Fig. 3, europium intraconfigurational transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, 4) are observed. The strongest emission transition is the forced electric dipole allowed transition 5D0 → 7F2. The transition of 5D0 → 7F3 is very weak. From the single crystal X-ray diffraction data as reported in our previous work [17,18], there are two non-equal crystal Eu3+ ions due to different coordination environments. However, due to the low resolution of the spectrum at room temperature, splitting of 5D0 → 7F0 transition
Fig. 1. Representative excitation spectra at various temperatures of complex II (a) and complex III (b).
K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Fig. 2. Comparison of the excitation spectra of these six complexes at 8 K.
the relationship between the levels and the ligands are beyond discussion because of the limitation of the low resolution of the spectra. In this work, the low temperature excitation spectra of the complexes
31
clearly show the CTB of Eu–O, together with the characteristic Eu3+ f– f transitions of 7F0–5L6, 7F0–5D2, and 7F0–5D1, and the low temperature emission spectra have clearly presented the 5D1–7F1, 2 and 5D0–7FJ transitions with resolved structures. Since the site symmetry of Eu3+ ions in the complexes is C1, the electronic transitions occur following the Judd forced electric dipole mechanism with the selection rules Δs = 0; |ΔL ≤ 6| and if L = 0 or L′ = 0, |ΔL = 2,4,6|; |ΔJ ≤ 6| and if J = 0 or J′ = 0, |ΔJ = 2,4, 6|. The transitions from 5D0 are expected to be the strongest to terminal multiplets with even values of J. In addition, the 5D0 level is non-degenerate, so the number of zero-phonon lines in the lowtemperature emission spectra should be equal to 2J + 1 for transitions to J terminal multiplets [20] if there is only one Eu3+ site in the complex. But if there are two non-equal crystal Eu3 + ions, the maximum observed peaks should be doubled. In this study, even though two coordination different Eu3+ ions are found by single crystal X-ray diffraction, only 2 peaks for 5D0–7F0 in complex I and 4 peaks for 5D0–7F1 in complex VI were recorded. The spectrum splitting for other transitions in the complexes is not observed in the emission and excitation spectra. The number of peaks for 5D0–7FJ (J = 0, 1, 2, 4) transitions for all the complexes is listed in Table S2 (see in the Supporting information). The possible reason might be that the difference of these two coordination environment is relatively small and/or the resolution of the instrument is not high enough. The energy levels of europium(III) in 6 complexes are obtained from the spectra as shown in Table 2. The transition of 5 D0–7F3 is so weak, that only one or two energy levels were tentatively assigned. The values of 7F3 multiplet are not included in Table 2. 3.3. Nephelauxetic effect of the substituted group on central ion
Fig. 3. Representative emission spectra at various temperatures of complex I (a) and complex II (b). Inset: the splitting of 5D0 → 7F2 at 8 K in complex I.
Based on the findings of the energy levels of Eu3+ ion in these 6 complexes, it is seen that 5D0 and 5D2 excited states vary from each other. There has been constant debate concerning the explanation of Eu ion energy level variation, and different explanations that include nephelauxetic effects [21,22] and J-mixing [23] have been proposed. In this work, nephelauxetic effect is preferred to explain the spectral shift, and associate the variation of Eu 5D0 energy level with the decrease of inter-electronic repulsion, because the studied samples are lanthanide–organic ligand complexes with similar coordination environments and share the same site symmetry. Due to the nephelauxetic theory, short bond length and low coordination number can enhance the covalency between the metal ions and the ligands, and lower metal excitation level at the same time. A stronger Eu–O bond for europium(III) results in larger nephelauxetic effect and smaller value of the Slater integrals. It can cause the decrease of inter-electronic repulsion of europium(III), thus results in red-shift of 5D0 level. When the coordination number is the same, the key factor of nephelauxetic effect could be the electrostatic interaction of Eu–O which determines the bond length [24–26]. Single-crystal X-ray diffraction analysis shows that europium(III) bears the same coordination number of 9 in the complexes. The experimental results in this study found that 5D0 and 5D1 energy levels of Eu3+ central ions were increased by the replacement of para-position –H by substituted groups –CH3, –OCH3, –OH, –Cl, and – NO2, The similar trend was found in the 3P0 state of Pr3 + in the substituted benzoate complexes [27]. But in praseodymium complexes the electron-withdrawing substituent in benzoate moiety results in higher 3P0 energy than electron-donating groups. While in this work, the electron-withdrawing groups have done less work in lifting the energy of 5D0 of Eu3 + ions than electron-donating groups. The energy level of 5D2 has the same trend as that of 5D0. The relation between 5 D0 and 5D2 level and substituted groups are listed in Table 3. The effect of the substituted group is valued by the Hammet sigma factor for the para-substituent. The factors provide an estimate of the total electronic influence, including polarity, inductivity and resonance effect, in the absence of conjugation effects. The overall trend in Table 3 shows that the energy levels of central Eu3+ ions are lifted by the nephelauxetic effect of the para-substituted function groups.
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K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 158 (2016) 29–33
Table 2 Energy levels of europium(III) in the 6 complexes. Group
7
7
H CH3 Cl OCH3 OH NO2
0 0 0 0 0 0
352, 372 321, 376, 420 297, 376, 455 338, 374, 411 294, 374, 453 306, 335, 378, 420
F0
F1
7
7
5
5
735, 971, 1007, 1090, 1130 918, 967, 1009, 1049, 1080, 1120 727, 945, 1005, 1130 732, 920, 972, 1000, 1034, 1100 747, 943, 1009, 1128 901, 968, 1007, 1034, 1060
2516, 2660, 2741, 2829, 3020, 3121 2730, 2782, 2818, 2897, 2925, 2968 2530, 2613, 2727, 273, 2818, 2885, 2914, 2959, 3082 2721, 2776, 2806, 2897, 2933, 2996, 3077 2530, 2612, 2715, 2770, 2816, 2912, 2958, 3082 2679, 2748, 2811, 2853, 2904, 2996
17,235 17,255 17,247 17,255 17,254 17,241
18,987, 19,008, 19,049(ex) 18,325 19,048(ex) 19,003, 19,012, 19,016 – 19,047(ex)
F2
F4
3.4. The p–π conjugation effect of the substituted group on the triplet state The absorption peak of benzoate moiety is generally believed to be the gap of the singlet state and the ground state in the metal–organic complexes [28]. In this study, the triplet states of complexes were measured by recording the emission peaks of the corresponding pure ligands in 0.1 mol/L DMF solution. The emission spectra and findings are shown in Fig. 4 and Table 4 respectively. According to the molecular orbital theory, the lone electron pair on p orbital of the substituted group conjugates with electrons on π orbital of benzene ring (p–π conjugation effect), and decrease the π– π′ transition energy [29]. The energy of the triplet state in all these six complexes has been lowered as expected. The energy of the singlet state, which is referred by the absorption spectra of the complexes (data not shown herein), has the same trend as that of the triplet state, possibly due to the electron effect of the substituted group as well. 3.5. Energy transition The emission spectra of 6 complexes show that the central Eu3+ ions could be excited by both direct and indirect excitations. However the luminescent intensity is greatly affected by the efficiency of indirect excitation. In indirect excitation process, the energy transfers from the triplet state to central ion and pumps it. The efficiency of central ion excitation is primarily based on the energy gap between the triplet state and central ion excited level. According to report, the best energy matching is between 3500–5000 cm−1 [30]. The triplet states of these 6 compounds are tentatively estimated by recording the triplet state of the corresponding ligand. The europium excited levels are obtained from the spectra, and therefore the gap is calculated from the difference. The best energy matching is found in complex I, followed by complexes II, III, IV, V and VI. This finding could be supported by the relative luminescent intensity, and the changing temperature emission spectrum.
D0
5
D1
D2
21,547(ex) 21,556(ex) 21,552(ex) 21,570(ex) 21,566(ex) 21,551(ex)
donating substituted group could enhance it more, while the triplet state of the ligands is lowered because of the p–π conjugation effect between the substituted group and the benzoic ring. The gap between the europium(III) excited state and the triplet state of the ligand primarily determines the energy transition efficiency. Complex I shows the best energy matching and leads to the strongest luminescent intensity upon the indirect excitation. The introduction of the para-substituted group in the other 5 complexes results in lower triplet state of the ligand, which causes poorer energy matching between the triplet state and the excited state of the central ion, and leading to a lower luminescent intensity. Acknowledgments The project was funded by the National Natural Science Foundation of China (11404047), Natural Science Foundation Project of Chongqing (CSTC 2012JJA90019, CSTC2015jcyjA50005), and Chongqing Innovation Program of Postgraduate Students (CYS14152). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2016.01.015.
4. Conclusion Europium(III) is well sensitized in direct and indirect excitations in 6 para-substituted cinnamic acid complexes. Strong red line emission is observed and high resolution excitation and emission spectra were recorded and analyzed. Energy levels of 5D0, 5D1, 5D2, 7F0, 7F1, 7F2 and 7F4 were obtained from the spectra. Due to the nephelauxetic effect of the para-substituted groups, the 5D0 energy level is lifted. The electronFig. 4. Emission spectra of these 6 ligands in 0.1 M solution in DMF.
Table 3 Relation between 5D0, 5D2 and substituted groups. σp is the Hammet constant of the parasubstituent. D0 (cm−1)
D2 (cm−1)
Table 4 Energy of the ligand triplet state and Hammet substituent constant.
σp
Complex
Group
5
5
Substituted group
σp
Triplet state (cm−1)
−0.37 −0.268 −0.17 0 +0.23 +0.78
V IV II I III VI
OH OCH3 CH3 H Cl NO2
17,254 17,255 17,255 17,235 17,247 17,241
21,566 21,570 21,556 21,547 21,552 21,551
OH OCH3 CH3 H Cl NO2
−0.37 −0.268 −0.17 0 0.227 0.778
21,392 ± 50 21,458 ± 50 21,514 ± 50 21,559 ± 50 21,491 ± 50 18,755 ± 50
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