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Study on a tetranuclear neodymium complex in polytrifluorochloroethylene oil for liquid lasers
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Optical Materials 31 (2008) 117–121 www.elsevier.com/locate/optmat
Study on a tetranuclear neodymium complex in polytrifluorochloroethylene oil for liquid lasers Xiaoming Qiu a, Kehan Yu a, Jiabao Lu a, Chao Gao b, Chaoqi Hou b, Junfang He b, Wei Wei a,*, Bo Peng a,b,* a
State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China b State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an Shaanxi 710119, PR China Received 26 September 2007; received in revised form 4 December 2007; accepted 23 January 2008 Available online 7 March 2008
Abstract A liquid solution on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 (HTTA: 2-thenoyltrifluoroacetone) in polytrifluorochloroethylene oil was presented. The molecular structure of the complex was characterized by single-crystal X-ray diffraction. The optical properties of the liquid medium were studied. Enhanced emission was observed at 1064 nm when excited at 340 nm due to efficient energy transfer from TTA to the Nd3+ ions and the mechanism for energy transfer process was discussed in detail. The obtained fluorescent lifetime was 1.7 ls and the quantum yield value was 0.63%, which compare well with some other organic liquid media reported previously. Besides, this liquid shows outstanding properties with no toxicity and causticity, nonvolatility and high stability. Ó 2008 Elsevier B.V. All rights reserved. PACS: 33.15; 78.20; 21.10; 42.55 Keywords: Crystal structure; Optical properties; Lifetime; Liquid laser
1. Introduction Since a laser system was first demonstrated by Maiman in 1960 [1], laser technologies had been widely applied in various fields and served for the humanity. As good gain media for lasers and amplifiers, the lanthanide were investigated in the past decades [2–7]. Recently, with the rapid development of modern science and technology, especially the electronics and photonics, solid state lasers containing Nd3+ have been largely investigated due to its important *
Corresponding authors. Address: State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China. Tel./fax: +86 21 55664170. E-mail addresses: [email protected] (W. Wei), bpeng@fudan. edu.cn (B. Peng). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.01.009
near infrared luminescence around 1060 nm and used in areas such as communication, microelectronics, national defenses, biotechnology and so on [8–10]. However, such lasing materials as crystals and glasses still have quite a few disadvantages that restrict their further applications [11], e.g., very costly and difficult to manufacture them in big size homogeneously. On the other hand, the solid materials cannot avoid defects in which thermal damaged spots will develop. The growing damaged spots are quite harmful for the performance and maintenances of high power laser system. What is more, difficulty in cooling the solid lasing materials as well as serious thermal shock by high thermal expansion can result in short running cycle period, low efficiency and working life. Such unsolved stubborn problems have become neck points to the further development of solid state neodymium lasers.
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X. Qiu et al. / Optical Materials 31 (2008) 117–121
Fortunately, liquid lasers can gracefully tackle such nuts. It is easy to get lasing liquid of almost unlimited volume in a uniform density. Even if some defects are developed while lasing, the damaged spots will get resolved by the flowing ambient branches. It is also quite convenient to realize the circumfluence cooling system to prolong the running cycle period and working life. To obtain the liquid laser media, Heller [12,13] dissolved Nd3+ in SeOCl2 and POCl3 for enhanced fluorescence in a liquid matrix in 1960 s. But such unconventional solvents are not appropriate for practical applications by their toxicity and causticity. Recently, developed Nd chelates in organic solvents were suffering from weak luminescence emission and poor lifetime [14,15] mainly due to the radiationless transition process via vibrational excitation of C–H and O–H bonds [11,16]. In this Letter, a liquid solution on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 in polytrifluorochloroethylene oil which has already been proved as an extreme low loss and stable liquid medium in high power laser conditions [17] was reported. Compared with some of other organic liquid materials investigated previously, this liquid represents good properties with stability, anti-temperature, no toxicity and causticity and easy to prepare. It shows potential application in the field of low loss neodymium liquid lasers in the future.
Table 1 Crystal data and structure refinement for Nd4(TTA)10O12H22 Empirical formula
C80H62F30Nd4O32S10
Formula weight (g/mol) Temperature (K) ˚) Wavelength (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (deg) b (deg) c (deg) ˚ 3), Z Volume (A Dcalc (Mg/m3) Absorption coefficient (mm1) F (000) Crystal size (mm) h range (deg) Limiting indices
3002.86 293(2) 0.71073 monoclinic C2/c 32.884(10) 12.968(4) 29.105(9) 90 117.651(4) 90 10,994(6), 4 1.814 2.171 5872 0.15 0.10 0.08 1.40–26.01 39 6 h 6 40 12 6 k 6 16 35 6 l 6 33 24,664/10,789 (Rint = 0.0621) 99.8% Semi-empirical from equivalents 0.8455 and 0.7366 Full-matrix least-squares on F2 10789/5/703 1.056 R1 = 0.0639, xR2 = 0.1400 R1 = 0.1085, xR2 = 0.1604 ˚ 3 1.284 and 0.760 e.A
Reflections collected/unique Completeness to h = 26.01 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest diff. peak and hole (e.A
2. Experimental 2.3. Synthesis of the tetranuclear complex 2.1. Instrument Elemental analysis was performed on a Vario ELIII elemental analyzer. Absorption spectrum of the liquid medium was recorded with a Shimadzu UV-3150 spectrophotometer at room temperature. Photoluminescence (PL) emission spectrum was measured using an Edinburgh Instruments FLS920 spectrophotometer. To characterize the fluorescence decay curve, the sample was pumped by a frequency-triplicated Nd:YAG laser with a pulse duration of 5 ns. The pump laser repetition is 10 Hz, and the energy of one pulse is 5 mJ. The fluorescence decay curve is recorded by a Si photodiode. 2.2. X-ray crystallography X-ray crystallography was performed using a Siemens P4 diffractometer with graphite-monochromated Mo Ka radiation. The intensity data of the single crystal for the complex were collected on the CCD-Bruker Smart APEX system. The data were collected at room temperature using the x scan technique. The structure was solved by direct methods, using Fourier techniques, and refined by a fullmatrix least-squares method. All the calculations were carried out with the Siemens SHELXTL-97 program. Crystallographic data are summarized in Table 1.
All the reagents, including 2-thenoyltrifluoroacetone (HTTA) and Nd2O3 were reagent grade and used as received. NdCl3 6H2O was obtained from Nd2O3 by dissolution in hydrochloric acid and subsequent removal the solvent carefully. Firstly, HTTA (Aldrich) was dissolved in ethanol at 40 °C under intense stir. The pH of the solution was adjusted to about 8.0. Then, Neodymium chloride was added. The molar ratio of Nd3+/TTA was 1:2.5. The system was heated to 65 °C and refluxed for 2 h. After removing solvent, the obtained rough product was washed by deionized water and petroleum ether, respectively, and dried under vacuum. Finally, neodymium tetranuclear complex was achieved. Elemental analysis calculated for Nd4(TTA)10O12H22: C, 31.98; H, 2.07; S, 10.66%. Found: C, 32.21; H, 1.99; S, 11.02%.
2.4. Preparation of Nd4(TTA)10O12H22 complex liquid solution It was prepared by dissolving the tetranuclear neodymium complex in polytrifluorochloroethylene oil. After that, heat the solution to 80 °C for 15 min by stirring and then cool it to room temperature with stillness for half an hour.
X. Qiu et al. / Optical Materials 31 (2008) 117–121
Finally, a homogeneous liquid medium with high transparency and great stability was obtained. 3. Results and discussion Organic ligands such as b-diketones can photosensitize the luminescence of lanthanide ions and shield them from the surrounding environment. As it is well known, the great majority of lanthanide complexes studied are monometallic, such as Nd(TTA)3, Nd(POA-D)3, Nd(HFA-D)3, Nd(POM-D)3 [18–20], etc. There are few reports of ligand for recognition of two or more lanthanide ions [21,22]. Coordination control around the lanthanide by adjusting reaction conditions can lead to the formation of lanthanide supramolecular architectures. Here, a tetranuclear neodymium complex was synthesized. The crystal structure of the organic complex is displayed in Fig. 1, while selected bond lengths and angles are listed in Table 2. From Fig. 1, it can be seen clearly that the complex forms a tetranuclear structure. The four central Nd3+ ions are coordinated by 10 TTA ligands. Between the Nd3+ ions, there are several oxygen bridges to link them. The average bond length of Nd– ˚ . In the b-diketone O bonds with TTA ligands is 2.463 A rings, the average distances for the C–C and C–O bonds ˚ , respectively, which are between are 1.424 and 1.263 A the single- and double-bond distances. This can be explained by a conjugated structure between thiophene ring and the coordinated b-diketonate, which leads to the delocalization of electron density of the coordinated bdiketonate chelate ring [23]. Conventional Nd3+ complexes in organic matrices are usually short in luminescent lifetime, which is partly attributed to solvent molecule coordination. In traditional Nd(TTA)3, TTA ligands occupy six coordination sites, then the unoccupied sites would be coordinated by water or other molecules due to the reacting conditions. Thus, the emission efficiency would be decreased by the deactiva-
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Table 2 ˚ ) and angles (deg) for Nd4(TTA)10O12H22 Selected bond lengths (A Nd(1)–O(1) Nd(1)–O(3) Nd(1)–O(5) Nd(1)–O(9)#1 Nd(2)–O(10) Nd(2)–O(13) Nd(2)–O(7) Nd(2)–O(13)#1 O(1)–Nd(1)–O(3) O(2)–Nd(1)–O(6) O(2)–Nd(1)–O(4) O(13)#1–Nd(1)–O(5) O(3)–Nd(1)–O(4) O(5)–Nd(1)–O(4) O(3)–Nd(1)–O(6) O(5)–Nd(1)–O(6) O(1)–Nd(1)–O(11)#1 O(2)–Nd(1)–O(11)#1 O(10)–Nd(2)–O(8) O(8)–Nd(2)–O(13) O(8)–Nd(2)–O(13)#1 O(8)–Nd(2)–O(6) O(13)#1–Nd(2)–O(6) O(8)–Nd(2)–O(7) O(13)#1–Nd(2)–O(7) (10)–Nd(2)–O(9) O(13)–Nd(2)–O(9) O(6)–Nd(2)–O(9)
2.404(6) 2.405(6) 2.441(6) 2.688(6) 2.402(6) 2.429(5) 2.527(5) 2.448(5) 71.1(2) 137.1(2) 142.7(2) 115.0(2) 71.1(2) 137.9(2) 70.89(19) 67.6(2) 134.23(19) 68.58(19) 76.2(2) 119.48(19) 143.80(19) 79.96(19) 67.42(18) 68.01(19) 86.78(18) 69.40(19) 66.67(17) 150.54(19)
Nd(1)–O(2) Nd(1)–O(4) Nd(1)–O(6) Nd(1)–Nd(2)#1 Nd(2)–O(8) Nd(2)–O(6) Nd(2)–O(9) Nd(2)–Nd(1)#1 O(2)–Nd(1)–O(5) O(3)–Nd(1)–O(5) O(1)–Nd(1)–O(5) O(1)–Nd(1)–O(4) O(13)#1–Nd(1)–O(4) O(1)–Nd(1)–O(6) O(13)#1–Nd(1)–O(6) O(4)–Nd(1)–O(6) O(3)–Nd(1)–O(11)#1 O(13)#1–Nd(1)–O(11)#1 O(10)–Nd(2)–O(13) O(10)–Nd(2)–O(13)#1 O(10)–Nd(2)–O(6) O(13)–Nd(2)–O(6) O(10)–Nd(2)–O(7) O(13)–Nd(2)–O(7) O(6)–Nd(2)–O(7) O(8)–Nd(2)–O(9) O(13)#1–Nd(2)–O(9) O(7)–Nd(2)–O(9)
2.411(6) 2.459(6) 2.588(6) 3.9591(13) 2.427(6) 2.520(6) 2.546(5) 3.9591(13) 71.4(2) 73.8(2) 114.4(2) 74.8(2) 71.36(18) 139.14(19) 66.67(18) 79.4(2) 142.62(19) 63.09(17) 124.13(19) 131.21(19) 114.9(2) 120.43(18) 141.8(2) 67.08(19) 72.15(19) 72.8(2) 133.67(18) 87.23(19)
tion which caused by bond vibrations of C–H and O–H. In the crystal structure of Nd4(TTA)10O12H22 as shown in Fig. 1, the Nd3+ ions are shielded more efficiently to improve the luminescence. The absorption spectrum of the Nd3+ complex dissolved in polytrifluorochloroethylene oil is shown in Fig. 2. Two peaks were observed at 308 nm and 340 nm, respectively, which were due to the organic ligand TTA. Though the chelate concentration is low, about 105 M, It is found that the absorbance is still quite strong, the absorption coefficient of the liquid solution near 340 nm is 3.161 cm1, which illustrates that Nd3+ ion in chelate systems can be pumped more effectively than in the conventional inorganic
Abs.
4
2
0
400
600
800
1000
Wavelength (nm) Fig. 1. Crystal structure of the tetranuclear neodymium complex.
Fig. 2. Absorption spectrum of the complex in polytrifluorochloroethylene oil with the chelate concentration of 105 M.
X. Qiu et al. / Optical Materials 31 (2008) 117–121
Intensity (a.u.)
120
1000
1100
1200
1300
1400
Wavelength (nm) Fig. 3. Emission spectrum of the complex in polytrifluorochloroethylene oil pumped at 340 nm.
U ¼ s=s0
0.04
0.03
Intensity (a.u.)
not efficiently quench the excited state of Nd3+. The main reasons for this are: (a) there are few C–H bonds in close proximity to the metal ion; (b) the C–H aromatic vibration is usually slightly higher (3050–3100 cm1) than the aliphatic C–H vibration leading to a poorer match of the vibrational levels responsible for luminescence signal deactivation. In other reported Nd3+ complexes, the ligand design to maximize luminescence lifetime is based on either macrocylic [25] or fluorinated diketonate structures [19], which lead to complexes with similar lifetime values as in our case. An estimated quantum yield of Nd3+ luminescence of a certain complex may be calculated by comparison of the luminescence lifetime of the complex ion [26] with the natural lifetime of Nd, s0. By using Eq. (1) a value of 0.63% for the quantum yield of the complex is calculated, given a value, s0, for the natural lifetime of Nd(III) = 270 ls [27].
0.02
0.01
0.00 0
2
4
6
8
10
Decay time (μs) Fig. 4. Fluorescence decay curve of Nd3+ for the 4F3/2 ? 4I11/2 transition.
crystal and glass systems because of efficient energy transfer in the chelate complexes. Fig. 3 is the luminescence spectrum of the Nd3+ complex liquid medium pumped at 340 nm. It is observed that a strong emission of the liquid sample occurred at 1064 nm due to the energy transfer from TTA to the central Nd3+ ions. This is the most characteristic peak of the neodymium by its 4F3/2 ? 4I11/2 transition. The fluorescent lifetime was demonstrated in Fig. 4. Analysis of the decay curve results in a lifetime of 1.7 ls. This value compares well with the reported lifetime of the tris(hexafluoroacetylacetonato) Nd(III) complex in DMF [24], indicating that the bond vibrations in the ligand do
ð1Þ
This method of quantum yield estimation does not take into account other factors such as intersystem crossing efficiency of absolute quantum yield methods using standards. The obtained quantum yield value compares well with some other Nd3+ complexes where all the protons of the ligands are either deuteurated or fluorinated, e.g., the Nd3+ complex of hexafluoroacetylacetonate [19] is reported to have a quantum yield of 0.3% and Nd2(1,3-bis(3-phenyl3-oxopropanoyl)benzene)3 in DMF-d7 is reported to 0.6% [28]. This result mainly attributed to the structure of the tetranuclear complex which efficiently protected the Nd3+ ions from C–H and O–H bond. On the other hand, the solution polytrifluorochloroethylene oil contains only C– F and C–Cl, which can also eliminate the influence of C– H and O–H bond vibration. A comparison of properties of our sample with some other organic neodymium liquid media reported is shown in Table 3. The energy process was described in Scheme 1 [29]. Firstly, the absorption of energy by TTA produces an excited singlet from the ground singlet state in the ligand. Secondly, intersystem crossing to a triplet state takes place by a radiationless process. Thirdly, radiationless transfer of energy occurs effectively from the triplet state to the lowlying 4f levels in the neodymium ion. Finally, enhanced emission of energy follows due to 4F3/2 ? 4IJ (J = 9/2, 11/2, 13/2) transition of Nd3+ ion.
Table 3 Comparison of the properties of our sample with some other organic neodymium liquid media reported [18–20,28] (4F3/2 ? 4I11/2) The liquid media
smed (s)
U (%)
Toxicity
Volatility
Boiling point (°C)
Nd(POA-D)3 in methanol-d4 Nd(HFA-D)3 in acetone-d6 Nd(HFA-D)3 in THF-d8 Nd(HFA-D)3 in DMSO-d6 Nd(POM-D)3 in methanol-d4 Nd2 (1,3-bis(3-phenyl-3-oxopropanoyl)benzene)3 in DMF-d7 This work
1.4 1.7 2.3 6.3 1.6 1.5 1.7
0.3 0.2 0.4 1.1 0.3 0.6 0.63
Low Low Low Low Low Low None
Volatile Volatile Volatile Nonvolatile Volatile Volatile Nonvolatile
64.6 56.2 65.4 189 64.6 153 >260
X. Qiu et al. / Optical Materials 31 (2008) 117–121
121
Technology Foundation of China (2004AA840003). The authors acknowledge the language processing by Johnson J. Penn. References [1] [2] [3] [4] [5] [6] [7] Scheme 1. Model of intramolecular energy relaxation processes of the tetranuclear complex.
4. Conclusions In conclusion, a novel Nd3+ liquid solution was prepared by dissolving a tetranuclear neodymium complex in polytrifluorochloroethylene oil. The structure of the complex was characterized by single-crystal X-ray diffraction. The optical properties of the liquid medium were investigated. Strong emission was observed at 1064 nm when excited at 340 nm due to efficient energy transfer from TTA to the Nd3+ ions. The fluorescent lifetime measured was 1.7 ls and a value of 0.63% for the quantum yield of the complex was calculated. Compared with some other organic Nd3+ liquid media reported previously, this liquid shows outstanding properties such as no toxicity and causticity, anti-temperature and so on. Based on these advantages, it is hoped to have applications in the field of rare earth low loss liquid lasers with further development.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Acknowledgements
[26]
This work was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 90401027, 20428304, 60578039) and National Science and
[27] [28] [29]
T. Maiman, Nature 187 (1960) 493. E. Snitzer, Phys. Rev. Lett. 7 (1961) 444. P. P Sorokin, M.J. Stevenson, IBM J. Res. Dev. 5 (1961) 6. B. Peng, Z.C. Fan, X.M. Qiu, L. Jiang, G.H. Tang, H.D. Ford, W. Huang, Adv. Mater. 17 (2005) 857. B. Peng, X.M. Qiu, L. Jiang, Z.C. Fan, W. Huang, Appl. Phys. Lett. 85 (2004) 1910. R.B. Yu et al., Adv. Mater. 19 (2007) 838. K.H. Yu, X.M. Qiu, X.X. Xu, W. Wei, B. Peng, Z.W. Zhou, Appl. Phys. Lett. 90 (2007) 091916. R.J. Mears, L. Reeki, S.B. Pools, D.N. Payne, Electron. Lett. 21 (1985) 738. H. Yajima, S. Kawase, Y. Sekimoto, Appl. Phys. Lett. 21 (1972) 407. T.G. Pavlopoulos, Prog. Quantum Electron. 26 (2002) 193. K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 102 (2002) 2347. A. Heller, Appl. Phys. Lett. 9 (1966) 106. A. Heller, J. Am. Chem. Soc. 88 (1966) 2058. Y. Hasegawa, T. Ohkubo, K. Sogabe, Y. Kawamura, Y. Wada, N. Nakasima, S. Yanagida, Chem. Int. Ed. 39 (2000) 357. S. Yanagida, Y. Hasegawa, Y. Wada, J. Lumin. 87 (2000) 995. Y. Wada, T. Okubo, M. Ryo, T. Nakazawa, Y. Hasegawa, S. Yanagida, J. Am. Chem. Soc. 122 (2000) 8583. Y. Yang, D.Y. Fan, Chin. J. Lasers 13 (1986) 49. S. Lin, R.J. Feuerstein, A.J. Mickelson, J. Appl. Phys. 79 (1996) 2868. S. Yanagida, Y. Hasegawa, K. Murakoshi, Y. Wada, N. Nakashima, T. Yamanaka, Coord. Chem. Rev. 171 (1998) 461. M. Iwamuro, Y. Hasegawa, Y. Wada, K. Murakoshi, N. Nakashima, T. Yamanaka, S. Yanagida, J. Lumin. 79 (1998) 29. I.A. Setyawati, S. Liu, S.J. Retting, C. Orvig, Inorg. Chem. 39 (2000) 496. R. Ziessel, M. Maestri, L. Prodi, V. Balzani, A. Van Dorsselaer, Inorg. Chem. 32 (1993) 1237. J.B. Yu et al., Inorg. Chem. Commun. 6 (2003) 852. Y. Hasegawa et al., J. Phys. Chem. 100 (1996) 10201. S. Faulkner, A. Beeby, M.C. Carrie, A. Dadabhoy, A.M. Kenwright, P.G. Sammes, Inorg. Chem. Commun. 4 (2001) 187. S.I. Klink, G.A. Hebbink, L. Grave, F.G.A. Peters, F.C.J.M. Van Veggel, D.N. Reinhoudt, J.W. Hofstraat, Eur. J. Org. Chem. 10 (2000) 1923. Weber, Phys. Rev. 171 (1968) 283. A.P. Bassett et al., J. Am. Chem. Soc. 126 (2004) 9413. R.H. Wu, Q.D. Su, J. Mol. Struct. 559 (2001) 195.
Optical Materials 31 (2008) 117–121 www.elsevier.com/locate/optmat
Study on a tetranuclear neodymium complex in polytrifluorochloroethylene oil for liquid lasers Xiaoming Qiu a, Kehan Yu a, Jiabao Lu a, Chao Gao b, Chaoqi Hou b, Junfang He b, Wei Wei a,*, Bo Peng a,b,* a
State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China b State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an Shaanxi 710119, PR China Received 26 September 2007; received in revised form 4 December 2007; accepted 23 January 2008 Available online 7 March 2008
Abstract A liquid solution on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 (HTTA: 2-thenoyltrifluoroacetone) in polytrifluorochloroethylene oil was presented. The molecular structure of the complex was characterized by single-crystal X-ray diffraction. The optical properties of the liquid medium were studied. Enhanced emission was observed at 1064 nm when excited at 340 nm due to efficient energy transfer from TTA to the Nd3+ ions and the mechanism for energy transfer process was discussed in detail. The obtained fluorescent lifetime was 1.7 ls and the quantum yield value was 0.63%, which compare well with some other organic liquid media reported previously. Besides, this liquid shows outstanding properties with no toxicity and causticity, nonvolatility and high stability. Ó 2008 Elsevier B.V. All rights reserved. PACS: 33.15; 78.20; 21.10; 42.55 Keywords: Crystal structure; Optical properties; Lifetime; Liquid laser
1. Introduction Since a laser system was first demonstrated by Maiman in 1960 [1], laser technologies had been widely applied in various fields and served for the humanity. As good gain media for lasers and amplifiers, the lanthanide were investigated in the past decades [2–7]. Recently, with the rapid development of modern science and technology, especially the electronics and photonics, solid state lasers containing Nd3+ have been largely investigated due to its important *
Corresponding authors. Address: State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China. Tel./fax: +86 21 55664170. E-mail addresses: [email protected] (W. Wei), bpeng@fudan. edu.cn (B. Peng). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.01.009
near infrared luminescence around 1060 nm and used in areas such as communication, microelectronics, national defenses, biotechnology and so on [8–10]. However, such lasing materials as crystals and glasses still have quite a few disadvantages that restrict their further applications [11], e.g., very costly and difficult to manufacture them in big size homogeneously. On the other hand, the solid materials cannot avoid defects in which thermal damaged spots will develop. The growing damaged spots are quite harmful for the performance and maintenances of high power laser system. What is more, difficulty in cooling the solid lasing materials as well as serious thermal shock by high thermal expansion can result in short running cycle period, low efficiency and working life. Such unsolved stubborn problems have become neck points to the further development of solid state neodymium lasers.
118
X. Qiu et al. / Optical Materials 31 (2008) 117–121
Fortunately, liquid lasers can gracefully tackle such nuts. It is easy to get lasing liquid of almost unlimited volume in a uniform density. Even if some defects are developed while lasing, the damaged spots will get resolved by the flowing ambient branches. It is also quite convenient to realize the circumfluence cooling system to prolong the running cycle period and working life. To obtain the liquid laser media, Heller [12,13] dissolved Nd3+ in SeOCl2 and POCl3 for enhanced fluorescence in a liquid matrix in 1960 s. But such unconventional solvents are not appropriate for practical applications by their toxicity and causticity. Recently, developed Nd chelates in organic solvents were suffering from weak luminescence emission and poor lifetime [14,15] mainly due to the radiationless transition process via vibrational excitation of C–H and O–H bonds [11,16]. In this Letter, a liquid solution on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 in polytrifluorochloroethylene oil which has already been proved as an extreme low loss and stable liquid medium in high power laser conditions [17] was reported. Compared with some of other organic liquid materials investigated previously, this liquid represents good properties with stability, anti-temperature, no toxicity and causticity and easy to prepare. It shows potential application in the field of low loss neodymium liquid lasers in the future.
Table 1 Crystal data and structure refinement for Nd4(TTA)10O12H22 Empirical formula
C80H62F30Nd4O32S10
Formula weight (g/mol) Temperature (K) ˚) Wavelength (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (deg) b (deg) c (deg) ˚ 3), Z Volume (A Dcalc (Mg/m3) Absorption coefficient (mm1) F (000) Crystal size (mm) h range (deg) Limiting indices
3002.86 293(2) 0.71073 monoclinic C2/c 32.884(10) 12.968(4) 29.105(9) 90 117.651(4) 90 10,994(6), 4 1.814 2.171 5872 0.15 0.10 0.08 1.40–26.01 39 6 h 6 40 12 6 k 6 16 35 6 l 6 33 24,664/10,789 (Rint = 0.0621) 99.8% Semi-empirical from equivalents 0.8455 and 0.7366 Full-matrix least-squares on F2 10789/5/703 1.056 R1 = 0.0639, xR2 = 0.1400 R1 = 0.1085, xR2 = 0.1604 ˚ 3 1.284 and 0.760 e.A
Reflections collected/unique Completeness to h = 26.01 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest diff. peak and hole (e.A
2. Experimental 2.3. Synthesis of the tetranuclear complex 2.1. Instrument Elemental analysis was performed on a Vario ELIII elemental analyzer. Absorption spectrum of the liquid medium was recorded with a Shimadzu UV-3150 spectrophotometer at room temperature. Photoluminescence (PL) emission spectrum was measured using an Edinburgh Instruments FLS920 spectrophotometer. To characterize the fluorescence decay curve, the sample was pumped by a frequency-triplicated Nd:YAG laser with a pulse duration of 5 ns. The pump laser repetition is 10 Hz, and the energy of one pulse is 5 mJ. The fluorescence decay curve is recorded by a Si photodiode. 2.2. X-ray crystallography X-ray crystallography was performed using a Siemens P4 diffractometer with graphite-monochromated Mo Ka radiation. The intensity data of the single crystal for the complex were collected on the CCD-Bruker Smart APEX system. The data were collected at room temperature using the x scan technique. The structure was solved by direct methods, using Fourier techniques, and refined by a fullmatrix least-squares method. All the calculations were carried out with the Siemens SHELXTL-97 program. Crystallographic data are summarized in Table 1.
All the reagents, including 2-thenoyltrifluoroacetone (HTTA) and Nd2O3 were reagent grade and used as received. NdCl3 6H2O was obtained from Nd2O3 by dissolution in hydrochloric acid and subsequent removal the solvent carefully. Firstly, HTTA (Aldrich) was dissolved in ethanol at 40 °C under intense stir. The pH of the solution was adjusted to about 8.0. Then, Neodymium chloride was added. The molar ratio of Nd3+/TTA was 1:2.5. The system was heated to 65 °C and refluxed for 2 h. After removing solvent, the obtained rough product was washed by deionized water and petroleum ether, respectively, and dried under vacuum. Finally, neodymium tetranuclear complex was achieved. Elemental analysis calculated for Nd4(TTA)10O12H22: C, 31.98; H, 2.07; S, 10.66%. Found: C, 32.21; H, 1.99; S, 11.02%.
2.4. Preparation of Nd4(TTA)10O12H22 complex liquid solution It was prepared by dissolving the tetranuclear neodymium complex in polytrifluorochloroethylene oil. After that, heat the solution to 80 °C for 15 min by stirring and then cool it to room temperature with stillness for half an hour.
X. Qiu et al. / Optical Materials 31 (2008) 117–121
Finally, a homogeneous liquid medium with high transparency and great stability was obtained. 3. Results and discussion Organic ligands such as b-diketones can photosensitize the luminescence of lanthanide ions and shield them from the surrounding environment. As it is well known, the great majority of lanthanide complexes studied are monometallic, such as Nd(TTA)3, Nd(POA-D)3, Nd(HFA-D)3, Nd(POM-D)3 [18–20], etc. There are few reports of ligand for recognition of two or more lanthanide ions [21,22]. Coordination control around the lanthanide by adjusting reaction conditions can lead to the formation of lanthanide supramolecular architectures. Here, a tetranuclear neodymium complex was synthesized. The crystal structure of the organic complex is displayed in Fig. 1, while selected bond lengths and angles are listed in Table 2. From Fig. 1, it can be seen clearly that the complex forms a tetranuclear structure. The four central Nd3+ ions are coordinated by 10 TTA ligands. Between the Nd3+ ions, there are several oxygen bridges to link them. The average bond length of Nd– ˚ . In the b-diketone O bonds with TTA ligands is 2.463 A rings, the average distances for the C–C and C–O bonds ˚ , respectively, which are between are 1.424 and 1.263 A the single- and double-bond distances. This can be explained by a conjugated structure between thiophene ring and the coordinated b-diketonate, which leads to the delocalization of electron density of the coordinated bdiketonate chelate ring [23]. Conventional Nd3+ complexes in organic matrices are usually short in luminescent lifetime, which is partly attributed to solvent molecule coordination. In traditional Nd(TTA)3, TTA ligands occupy six coordination sites, then the unoccupied sites would be coordinated by water or other molecules due to the reacting conditions. Thus, the emission efficiency would be decreased by the deactiva-
119
Table 2 ˚ ) and angles (deg) for Nd4(TTA)10O12H22 Selected bond lengths (A Nd(1)–O(1) Nd(1)–O(3) Nd(1)–O(5) Nd(1)–O(9)#1 Nd(2)–O(10) Nd(2)–O(13) Nd(2)–O(7) Nd(2)–O(13)#1 O(1)–Nd(1)–O(3) O(2)–Nd(1)–O(6) O(2)–Nd(1)–O(4) O(13)#1–Nd(1)–O(5) O(3)–Nd(1)–O(4) O(5)–Nd(1)–O(4) O(3)–Nd(1)–O(6) O(5)–Nd(1)–O(6) O(1)–Nd(1)–O(11)#1 O(2)–Nd(1)–O(11)#1 O(10)–Nd(2)–O(8) O(8)–Nd(2)–O(13) O(8)–Nd(2)–O(13)#1 O(8)–Nd(2)–O(6) O(13)#1–Nd(2)–O(6) O(8)–Nd(2)–O(7) O(13)#1–Nd(2)–O(7) (10)–Nd(2)–O(9) O(13)–Nd(2)–O(9) O(6)–Nd(2)–O(9)
2.404(6) 2.405(6) 2.441(6) 2.688(6) 2.402(6) 2.429(5) 2.527(5) 2.448(5) 71.1(2) 137.1(2) 142.7(2) 115.0(2) 71.1(2) 137.9(2) 70.89(19) 67.6(2) 134.23(19) 68.58(19) 76.2(2) 119.48(19) 143.80(19) 79.96(19) 67.42(18) 68.01(19) 86.78(18) 69.40(19) 66.67(17) 150.54(19)
Nd(1)–O(2) Nd(1)–O(4) Nd(1)–O(6) Nd(1)–Nd(2)#1 Nd(2)–O(8) Nd(2)–O(6) Nd(2)–O(9) Nd(2)–Nd(1)#1 O(2)–Nd(1)–O(5) O(3)–Nd(1)–O(5) O(1)–Nd(1)–O(5) O(1)–Nd(1)–O(4) O(13)#1–Nd(1)–O(4) O(1)–Nd(1)–O(6) O(13)#1–Nd(1)–O(6) O(4)–Nd(1)–O(6) O(3)–Nd(1)–O(11)#1 O(13)#1–Nd(1)–O(11)#1 O(10)–Nd(2)–O(13) O(10)–Nd(2)–O(13)#1 O(10)–Nd(2)–O(6) O(13)–Nd(2)–O(6) O(10)–Nd(2)–O(7) O(13)–Nd(2)–O(7) O(6)–Nd(2)–O(7) O(8)–Nd(2)–O(9) O(13)#1–Nd(2)–O(9) O(7)–Nd(2)–O(9)
2.411(6) 2.459(6) 2.588(6) 3.9591(13) 2.427(6) 2.520(6) 2.546(5) 3.9591(13) 71.4(2) 73.8(2) 114.4(2) 74.8(2) 71.36(18) 139.14(19) 66.67(18) 79.4(2) 142.62(19) 63.09(17) 124.13(19) 131.21(19) 114.9(2) 120.43(18) 141.8(2) 67.08(19) 72.15(19) 72.8(2) 133.67(18) 87.23(19)
tion which caused by bond vibrations of C–H and O–H. In the crystal structure of Nd4(TTA)10O12H22 as shown in Fig. 1, the Nd3+ ions are shielded more efficiently to improve the luminescence. The absorption spectrum of the Nd3+ complex dissolved in polytrifluorochloroethylene oil is shown in Fig. 2. Two peaks were observed at 308 nm and 340 nm, respectively, which were due to the organic ligand TTA. Though the chelate concentration is low, about 105 M, It is found that the absorbance is still quite strong, the absorption coefficient of the liquid solution near 340 nm is 3.161 cm1, which illustrates that Nd3+ ion in chelate systems can be pumped more effectively than in the conventional inorganic
Abs.
4
2
0
400
600
800
1000
Wavelength (nm) Fig. 1. Crystal structure of the tetranuclear neodymium complex.
Fig. 2. Absorption spectrum of the complex in polytrifluorochloroethylene oil with the chelate concentration of 105 M.
X. Qiu et al. / Optical Materials 31 (2008) 117–121
Intensity (a.u.)
120
1000
1100
1200
1300
1400
Wavelength (nm) Fig. 3. Emission spectrum of the complex in polytrifluorochloroethylene oil pumped at 340 nm.
U ¼ s=s0
0.04
0.03
Intensity (a.u.)
not efficiently quench the excited state of Nd3+. The main reasons for this are: (a) there are few C–H bonds in close proximity to the metal ion; (b) the C–H aromatic vibration is usually slightly higher (3050–3100 cm1) than the aliphatic C–H vibration leading to a poorer match of the vibrational levels responsible for luminescence signal deactivation. In other reported Nd3+ complexes, the ligand design to maximize luminescence lifetime is based on either macrocylic [25] or fluorinated diketonate structures [19], which lead to complexes with similar lifetime values as in our case. An estimated quantum yield of Nd3+ luminescence of a certain complex may be calculated by comparison of the luminescence lifetime of the complex ion [26] with the natural lifetime of Nd, s0. By using Eq. (1) a value of 0.63% for the quantum yield of the complex is calculated, given a value, s0, for the natural lifetime of Nd(III) = 270 ls [27].
0.02
0.01
0.00 0
2
4
6
8
10
Decay time (μs) Fig. 4. Fluorescence decay curve of Nd3+ for the 4F3/2 ? 4I11/2 transition.
crystal and glass systems because of efficient energy transfer in the chelate complexes. Fig. 3 is the luminescence spectrum of the Nd3+ complex liquid medium pumped at 340 nm. It is observed that a strong emission of the liquid sample occurred at 1064 nm due to the energy transfer from TTA to the central Nd3+ ions. This is the most characteristic peak of the neodymium by its 4F3/2 ? 4I11/2 transition. The fluorescent lifetime was demonstrated in Fig. 4. Analysis of the decay curve results in a lifetime of 1.7 ls. This value compares well with the reported lifetime of the tris(hexafluoroacetylacetonato) Nd(III) complex in DMF [24], indicating that the bond vibrations in the ligand do
ð1Þ
This method of quantum yield estimation does not take into account other factors such as intersystem crossing efficiency of absolute quantum yield methods using standards. The obtained quantum yield value compares well with some other Nd3+ complexes where all the protons of the ligands are either deuteurated or fluorinated, e.g., the Nd3+ complex of hexafluoroacetylacetonate [19] is reported to have a quantum yield of 0.3% and Nd2(1,3-bis(3-phenyl3-oxopropanoyl)benzene)3 in DMF-d7 is reported to 0.6% [28]. This result mainly attributed to the structure of the tetranuclear complex which efficiently protected the Nd3+ ions from C–H and O–H bond. On the other hand, the solution polytrifluorochloroethylene oil contains only C– F and C–Cl, which can also eliminate the influence of C– H and O–H bond vibration. A comparison of properties of our sample with some other organic neodymium liquid media reported is shown in Table 3. The energy process was described in Scheme 1 [29]. Firstly, the absorption of energy by TTA produces an excited singlet from the ground singlet state in the ligand. Secondly, intersystem crossing to a triplet state takes place by a radiationless process. Thirdly, radiationless transfer of energy occurs effectively from the triplet state to the lowlying 4f levels in the neodymium ion. Finally, enhanced emission of energy follows due to 4F3/2 ? 4IJ (J = 9/2, 11/2, 13/2) transition of Nd3+ ion.
Table 3 Comparison of the properties of our sample with some other organic neodymium liquid media reported [18–20,28] (4F3/2 ? 4I11/2) The liquid media
smed (s)
U (%)
Toxicity
Volatility
Boiling point (°C)
Nd(POA-D)3 in methanol-d4 Nd(HFA-D)3 in acetone-d6 Nd(HFA-D)3 in THF-d8 Nd(HFA-D)3 in DMSO-d6 Nd(POM-D)3 in methanol-d4 Nd2 (1,3-bis(3-phenyl-3-oxopropanoyl)benzene)3 in DMF-d7 This work
1.4 1.7 2.3 6.3 1.6 1.5 1.7
0.3 0.2 0.4 1.1 0.3 0.6 0.63
Low Low Low Low Low Low None
Volatile Volatile Volatile Nonvolatile Volatile Volatile Nonvolatile
64.6 56.2 65.4 189 64.6 153 >260
X. Qiu et al. / Optical Materials 31 (2008) 117–121
121
Technology Foundation of China (2004AA840003). The authors acknowledge the language processing by Johnson J. Penn. References [1] [2] [3] [4] [5] [6] [7] Scheme 1. Model of intramolecular energy relaxation processes of the tetranuclear complex.
4. Conclusions In conclusion, a novel Nd3+ liquid solution was prepared by dissolving a tetranuclear neodymium complex in polytrifluorochloroethylene oil. The structure of the complex was characterized by single-crystal X-ray diffraction. The optical properties of the liquid medium were investigated. Strong emission was observed at 1064 nm when excited at 340 nm due to efficient energy transfer from TTA to the Nd3+ ions. The fluorescent lifetime measured was 1.7 ls and a value of 0.63% for the quantum yield of the complex was calculated. Compared with some other organic Nd3+ liquid media reported previously, this liquid shows outstanding properties such as no toxicity and causticity, anti-temperature and so on. Based on these advantages, it is hoped to have applications in the field of rare earth low loss liquid lasers with further development.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Acknowledgements
[26]
This work was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 90401027, 20428304, 60578039) and National Science and
[27] [28] [29]
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