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Enhanced photoluminescence of a tetranuclear neodymium complex: Fluorescent resonance energy transfer analysis
Chemical Physics Letters 457 (2008) 194–197
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Enhanced photoluminescence of a tetranuclear neodymium complex: Fluorescent resonance energy transfer analysis Xiaoming Qiu a, Kehan Yu a, Chao Gao b, Chaoqi Hou b, Junfang He b, Zhiwei Zhou c, Wei Wei a,*, Bo Peng a,b,* a
State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China 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, People’s Republic of China c Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 12 February 2008 In final form 31 March 2008 Available online 7 April 2008
a b s t r a c t A novel tetranuclear neodymium complex was synthesized and dissolved into N,N-dimethylformamide. The molecular structure of the complex was characterized by single-crystal X-ray diffraction. Fluorescent resonance energy transfer theory was applied to investigate the optical properties of the sample. It was found that the quenching of Nd3+ excited state via O–H vibrational excitation could be nearly neglected and the cross-relaxation or excitation migration rate of the tetranuclear complex was only 1/16 times of the traditional mononuclear one. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first laser system was invented by Maiman in 1960 [1], laser technology has significantly improved commercial and daily lives of human beings. As good media for laser amplifiers with high gains, the lanthanide inorganic materials such as crystals and glasses were investigated in the past decades [2–5]. However, the cost of these materials is often so high that their wide applications are restricted. In recent years, organic compounds employed in the area of organic lasers have been studied extensively due to their low cost and easy fabrication. Among these materials, neodymium-doped organic materials have attached more and more importance on account of their near infrared luminescence around 1060 nm, especially in the field of organic liquid lasers which can gracefully tackle thermal stress problems of solid lasers in high power laser systems [6–8]. Nd3+ chelates in organic solvents developed recently were suffering from weak luminescence emission and poor lifetime [9,10] due to some key problems, including: (1) the radiationless transition process via vibrational excitation of C–H and O–H; (2) dipole–dipole nonradiative energy transfer processes via cross-relaxation and excitation migration. In this study, a novel Nd3+ chelate in organic solvent with a fluorescent lifetime of 1.6 ls and a fluorescent quantum efficiency of 0.6% was prepared on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 (HTTA: 2-thenoyltrifluoroacetone) in N,N-Dimethylformamide (DMF). The schematic formula of the ligand TTA is shown as follows: * Corresponding authors. Address: State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China. Fax: +86 21 55664170 (B. Peng). E-mail address: [email protected] (B. Peng). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.093
S
O O F3C
Fluorescence resonance energy transfer (FRET) theory analysis revealed that the tetranuclear structure could effectively decrease the quenching via vibrational excitation of O–H in the solvent and weaken the concentration quenching compared with the mononuclear complex. 2. Experimental All the reagents, including 2-thenoyltrifluoroacetone (HTTA) and neodymium chloride were reagent grade and used as received. 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. X-ray crystallography was performed using a Siemens P4 diffractometer with graphite-monochromated Mo K 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
X. Qiu et al. / Chemical Physics Letters 457 (2008) 194–197
structure was solved by direct methods, using Fourier techniques, and refined by a full-matrix least-squares method. All the calculations were carried out with the Siemens SHELXTL-97 program. The liquid sample with high transparency and good stability was prepared by dissolving the complex in DMF with a chelate concentration of 0.1 M. The absorption spectrum of the sample was recorded with a UV-3150 spectrophotometer and the fluorescence spectrum was measured using an Edinburgh Instruments FLS920 spectrophotometer. To characterize the fluorescence decay curve, the measurements were performed by using a Ti: sapphire lasers (Spectra-Physics Spitfire) with a frequency of 1 kHz and Si photodiode (time response <1ns). The sample was excited at 800 nm, and the energy of one pulse was 2.25 mJ. 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 being well known, the great majority of lanthanide complexes studied are monometallic, such as Nd(TTA)3, Nd(HFA-D)3, Nd(POM-D)3 [11,12], etc. There are few reports of ligand for recognition of two or more lanthanide ions [13]. Here, a tetranuclear neodymium complex was synthesized. The crystal structure of the organic complex is displayed in Fig. 1. The crystal belongs to Monoclinic system, C2/c space group with a = 32.884(10), b = 12.968(4), c = 29.105(9) Å; b = 117.651(4)°, V = 10994(6) Å3, D = 1.814 g/cm3, Z = 4, F(0 0 0) = 5872, the final R = 0.0639 and wR = 0.1400. It can be seen clearly that the complex forms a tetranuclear structure. The four central Nd3+ ions are coordinated by ten TTA ligands. As for the central Nd3+ ions, they approximately forms a parallelogram with parameters of 3.9591 Å for Nd(2)–Nd(1A) distance, 4.0258 Å for Nd(2)–Nd(2A) distance and 62.217° for Nd(1A)–Nd(2)–Nd(2A) angle, respectively. Between the Nd3+ ions, there are oxygen bridges to link them. Among them, four ligands are bridging and the rest six are coordinated to single Nd centers. Besides, every pair of Nd atoms is bridged by one O-atom. The average bond length of Nd–O bonds with TTA ligands is 2.463 Å. In the b-diketone rings, the average distances for the C–C and C–O bonds are 1.424 and 1.263 Å, respectively, which are between 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
Fig. 1. Crystal structure of the tetranuclear neodymium complex.
195
delocalization of electron density of the coordinated b-diketonate chelate ring. 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 may be coordinated by water or other small molecules due to the reacting conditions. Thus, the emission efficiency would be decreased by the deactivation which caused by bond vibrations of C–H and O–H. Compared with the mononuclear complexes even with bulky ligands, the Nd3+ ions in the tetranuclear structure are wraped more close in the molecule and shielded more efficiently from the environment to improve the luminescence. To evaluate the potential of rare-earth-doped materials, Judd– Ofelt(JO) theory was applied to study the radiative properties of trivalent rare-earth-organic complex. According to JO theory, the oscillator strength of an electric dipole transition between an initial J manifold jðS; LÞJi and a terminal J0 manifold jðS0 ; L0 ÞJ 0 i can be determined by: 0
PðaJ; bJ Þ ¼
8p2 cmvm X 0 Xt jh4f N aJkU ðtÞ k4f N bJ ij2 3hð2J þ 1Þ t¼2;4;6
where h is the Plank constant and m is the frequency in inverse 2 þ2Þ2 centimeter, v ¼ ðn 9n is the local field correction factor. J is the total angular momentum quantum number of the initial level (for Nd3+ ion J = 9/2). ||U(t)|| are the doubly reduced matrix elements corresponding to the J–J0 transition. The JO parameters Xt( t = 2, 4, 6) can be obtained by a least-square method to correlate the oscillator strengths. The three intensity parameters exhibit the influence of the host on the radiative transition probabilities within the ground configuration, since they contain implicitly the effect of the odd symmetry crystal field terms, interconfigurational radial integrals and energy denominators. Following the Judd–Ofelt procedure [14,15], the JO parameter X2 is determined to be 1.08 1019 cm2 from the absorption spectra of the sample (Fig. 2). In particular, X2 is more sensitive to the symmetry and the degree of covalency in the lanthanide-first coordination shell interaction. In the sense of the dynamic coupling contribution to the total intensity, the polarization of the ligand field induces stronger lanthanide-ligand bonds and an increase in electric dipolar transitions for noncentrosymmetric ligand fields. The large value for X2, which is 1.08 1019 cm2 in this Letter, indicates the presence of covalent bonding between the Nd3+ ion and the surrounding ligands. This result suggests that the novel complex has no center of symmetry and the geometrical structure of the complex in DMF is asymmetric. Through analyzing the fluorescence spectrum and
Fig. 2. Absorption spectrum of the complex in DMF. The chelate concentration is 0.1 M.
196
X. Qiu et al. / Chemical Physics Letters 457 (2008) 194–197
lent to the rate of radiative decay (kDA = Ar). R0 is a function of the refractive index of the medium nD, Avogadro number NA, the donor PL quantum efficiency QD, the overlap integral I, and a parameter jp depending on the relative orientation of the donor and acceptor dipoles and is expressed as !1=6 9000ðln 10Þj2p Q D R0 ¼ ; ð3Þ N A 128p5 n4D I The spectral overlap integral I is defined as Z I¼ PLD ðkÞeA ðkÞk4 dk;
Fig. 3. Fluorescence spectrum of the complex in DMF pumped at 800 nm, and the inset is the fluorescence decay curves of Nd3+ for the 4F3/2 ? 4I11/2 transition.
fluorescence decay curve (Fig. 3), the fluorescence lifetime is determined to be 1.6 ls. The spontaneous emission cross-section of 4F3/ 4 20 cm2. 2 ? I11/2 (Gaussian lineshape) is 3.13 10 3+ An estimated quantum yield of Nd luminescence of a certain complex can be calculated by comparing the luminescence lifetime of the complex ion with the natural lifetime of Nd, s0 . By using Eq. (1), a value of 0.6% for the quantum yield of the complex is obtained, given a value s0 ¼ 270 ls for the natural lifetime of Nd(III) [16] U ¼ s=s0
ð1Þ
This method of quantum yield estimation does not take into account other factors such as intersystem crossing efficiency. The obtained quantum yield value compares well with some other Nd3+ complexes where all the protons of the ligands are either deuterated or fluorinated. For example, it is reported that the Nd3+ complex of hexafluoroacetylacetonate in Methanol-d4 [12] and the Nd2(1,3-bis(3-phenyl-3-oxopropanoyl)benzene)3 in DMF-d7 have quantum yields of about 0.3% and 0.6%, respectively [17]. This result was mainly attributed to the structure of the tetranuclear complex which efficiently protected the Nd3+ ions from C–H and O–H bonds. Fluorescence resonance energy transfer (FRET) was used to investigate the quenching mechanism of this material in the Letter. Generally, there are two types of quenching for emission from an active Nd3+ ion [18]. One is related to the environments of the ions and the other is connected to the interaction between the ions. Effective quenching for 4F3/2 of Nd3+ via vibrational excitation of C– H and O–H bonds is a function of the distance between C–H, O–H groups and Nd3+ ions. Concentration quenching is a function of the distance among Nd3+ ions. The vibrational energy of O–H group with vibrational quanta c = 2(6900 cm1) and 3 were well matched with two of the important radiative transition gaps of Nd3+ (4F3/2 ? 4I15/2 and 4F3/2 ? 4 I11/2), and vibrational excitation leads to effective quenching of the excited state of Nd3+. According to the fluorescence resonance energy transfer, it is very sensitive to the donor–acceptor separation distance and their relative dipole orientations [19]. FRET process is driven by dipole–dipole interactions and depends on the degree of spectral overlap between donor photoluminescence (PL) and acceptor absorption, and on the sixth power of the separation distance between the donor and acceptor pair r. Förster formula gives the rate of nonradiative energy transfer 6 R0 kDA ¼ Aed ; ð2Þ r where Aed is the excited state radiative rate of the donor and R0 is the Förster critical distance corresponding to a rate of FRET equiva-
ð4Þ
which quantitatively takes into account the donor–acceptor spectral overlap over all wavelengths k, where PLD and eA represent the donor emission (normalized dimensionless spectrum) and acceptor absorption extinction coefficient spectrum, respectively. According to Eq. (2), the interactions between donor and acceptor decay rapidly when the distance between them is beyond the critical range R0. The critical distance R0 of the sample in this work is calculated to be 0.132 nm using Eqs. (3) and (4). The PL quantum efficiency QD is 0.6% and jp is taken as (2/3)1/2 for fast Brownian rotation for both molecules. Through the JO analysis, the branching ratio of the 4F3/ 4 2 ? I15/2 transition is 0.00290, which is about 1/140 of the branching ratio of the 4F3/2 ? 4I11/2 transition (0.397). Therefore, the FRET calculation is carried out with the energy transfer from the Nd3+ 4 F3/2 ? 4I11/2 transition to the O–H third vibrational overtone. The spectral overlap integral I is calculated from Fig. 4. Concerning that the molecular radius of the complex is about 0.5 nm, the quenching rate of Nd3+ via O–H vibration excitation is 0.034% of the Aed at most according to Eq. (2). As a result, the quenching for 4F3/2 of Nd3+ via vibrational excitation of O–H is quite small. Meanwhile, the concentration quenching is a function of crossrelaxation and excitation migration processes in Nd3+ [18]. The cross-relaxation process is shown in Fig. 5a. There is a level (4I15/2) with energy between metastable state (4F3/2) and ground state (4I9/2) in the Nd3+ ion. Since Nd3+ ion has a 4I15/2 state, an excited Nd3+ ion shares its energy with neighbouring Nd3+ ions, the electrons are transferred to the 4I15/2 state rapidly and then decay to their respective ground states through the nonradiative decay. The transition probability of this process depends on Nd–Nd distance, therefore the concentration quenching is explained by the cross-relaxation. The excitation migration is shown in Fig. 5b. The excitation migration is a process by which excited state energy is transferred from excited ion to neighbouring unexcited ion. This process is also a function of Nd–Nd distance.
Fig. 4. Overlap spectrum of the O–H vibrational absorption (vibrational quanta c = 3) and the normalized PL intensity of Nd3+ 4F3/2 ? 4I11/2 transition.
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the complex was verified by single-crystal X-ray diffraction. Optical characterization and theoretical analysis using JO theory demonstrated that this novel material had good optical properties. FRET theory analysis found that the quenching of Nd3+ excited state via O–H vibrational excitation could be nearly neglected and the cross-relaxation or excitation migration rate of the tetranuclear complex was only 1/16 times of the traditional mononuclear one. With these advantages, it was believed to be a promising enhanced photoluminescence candidate in the field of neodymium organic materials. Acknowledgements
Fig. 5. Two relaxation processes in Nd doped materials. (a) Cross-relaxation: an excited Nd3+ ion shares its energy with a neighbouring Nd3+ ion. (b) Excitation migration: the excitation energy moves from one Nd3+ ion to another.
This work was financially supported by the National Natural Science Foundation of China (NNSFC, Nos. 90401027, 20428304, and 60578039) and National Science and Technology Foundation of China (2004AA840003). The authors acknowledge the language processing by Johnson J. Penn. References
In the case of the tetranuclear complex, the average Nd–Nd bond inside the molecule is 0.4 nm, which means the quenching inside the molecule among the Nd3+ ions is also small according to the critical distance R0. Besides, compared with the mononuclear complex, the FRET analysis reveals that the structure of tetranuclear complex can decrease the concentration quenching essentially. For the same Nd3+ ion concentration, the average distance between molecules of tetranuclear complex is about 1.583 times of the mononuclear one. Therefore, the cross-relaxation or excitation migration rate of the tetranuclear complex is only (1/1.583)6, which is 1/16 times of the mononuclear one. This is especially useful for increasing the concentration to obtain enough population inversion for ultimate lasing. 4. Conclusions In conclusion, a novel tetranuclear neodymium complex was synthesized and dissolved into DMF. The molecular structure of
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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) 56. 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. K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 102 (2002) 2347. 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. 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. 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. I.A. Setyawati, S. Liu, S.J. Retting, C. Orvig, Inorg. Chem. 39 (2000) 496. R. Judd, Phys. Rev. B 127 (1962) 750. G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. Weber, Phys. Rev. 171 (1968) 283. A.P. Bassett et al. , J. Am. Chem. Soc. 126 (2004) 9413. J.A. Caird, A.J. Ramponi, P.R. Staver, J. Opt. Soc. Am. B 8 (1991) 1391. T. Förster, Ann. Phys. 2 (1948) 55.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Enhanced photoluminescence of a tetranuclear neodymium complex: Fluorescent resonance energy transfer analysis Xiaoming Qiu a, Kehan Yu a, Chao Gao b, Chaoqi Hou b, Junfang He b, Zhiwei Zhou c, Wei Wei a,*, Bo Peng a,b,* a
State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China 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, People’s Republic of China c Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 12 February 2008 In final form 31 March 2008 Available online 7 April 2008
a b s t r a c t A novel tetranuclear neodymium complex was synthesized and dissolved into N,N-dimethylformamide. The molecular structure of the complex was characterized by single-crystal X-ray diffraction. Fluorescent resonance energy transfer theory was applied to investigate the optical properties of the sample. It was found that the quenching of Nd3+ excited state via O–H vibrational excitation could be nearly neglected and the cross-relaxation or excitation migration rate of the tetranuclear complex was only 1/16 times of the traditional mononuclear one. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first laser system was invented by Maiman in 1960 [1], laser technology has significantly improved commercial and daily lives of human beings. As good media for laser amplifiers with high gains, the lanthanide inorganic materials such as crystals and glasses were investigated in the past decades [2–5]. However, the cost of these materials is often so high that their wide applications are restricted. In recent years, organic compounds employed in the area of organic lasers have been studied extensively due to their low cost and easy fabrication. Among these materials, neodymium-doped organic materials have attached more and more importance on account of their near infrared luminescence around 1060 nm, especially in the field of organic liquid lasers which can gracefully tackle thermal stress problems of solid lasers in high power laser systems [6–8]. Nd3+ chelates in organic solvents developed recently were suffering from weak luminescence emission and poor lifetime [9,10] due to some key problems, including: (1) the radiationless transition process via vibrational excitation of C–H and O–H; (2) dipole–dipole nonradiative energy transfer processes via cross-relaxation and excitation migration. In this study, a novel Nd3+ chelate in organic solvent with a fluorescent lifetime of 1.6 ls and a fluorescent quantum efficiency of 0.6% was prepared on the basis of dissolving a novel tetranuclear neodymium complex Nd4(TTA)10O12H22 (HTTA: 2-thenoyltrifluoroacetone) in N,N-Dimethylformamide (DMF). The schematic formula of the ligand TTA is shown as follows: * Corresponding authors. Address: State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China. Fax: +86 21 55664170 (B. Peng). E-mail address: [email protected] (B. Peng). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.093
S
O O F3C
Fluorescence resonance energy transfer (FRET) theory analysis revealed that the tetranuclear structure could effectively decrease the quenching via vibrational excitation of O–H in the solvent and weaken the concentration quenching compared with the mononuclear complex. 2. Experimental All the reagents, including 2-thenoyltrifluoroacetone (HTTA) and neodymium chloride were reagent grade and used as received. 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. X-ray crystallography was performed using a Siemens P4 diffractometer with graphite-monochromated Mo K 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
X. Qiu et al. / Chemical Physics Letters 457 (2008) 194–197
structure was solved by direct methods, using Fourier techniques, and refined by a full-matrix least-squares method. All the calculations were carried out with the Siemens SHELXTL-97 program. The liquid sample with high transparency and good stability was prepared by dissolving the complex in DMF with a chelate concentration of 0.1 M. The absorption spectrum of the sample was recorded with a UV-3150 spectrophotometer and the fluorescence spectrum was measured using an Edinburgh Instruments FLS920 spectrophotometer. To characterize the fluorescence decay curve, the measurements were performed by using a Ti: sapphire lasers (Spectra-Physics Spitfire) with a frequency of 1 kHz and Si photodiode (time response <1ns). The sample was excited at 800 nm, and the energy of one pulse was 2.25 mJ. 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 being well known, the great majority of lanthanide complexes studied are monometallic, such as Nd(TTA)3, Nd(HFA-D)3, Nd(POM-D)3 [11,12], etc. There are few reports of ligand for recognition of two or more lanthanide ions [13]. Here, a tetranuclear neodymium complex was synthesized. The crystal structure of the organic complex is displayed in Fig. 1. The crystal belongs to Monoclinic system, C2/c space group with a = 32.884(10), b = 12.968(4), c = 29.105(9) Å; b = 117.651(4)°, V = 10994(6) Å3, D = 1.814 g/cm3, Z = 4, F(0 0 0) = 5872, the final R = 0.0639 and wR = 0.1400. It can be seen clearly that the complex forms a tetranuclear structure. The four central Nd3+ ions are coordinated by ten TTA ligands. As for the central Nd3+ ions, they approximately forms a parallelogram with parameters of 3.9591 Å for Nd(2)–Nd(1A) distance, 4.0258 Å for Nd(2)–Nd(2A) distance and 62.217° for Nd(1A)–Nd(2)–Nd(2A) angle, respectively. Between the Nd3+ ions, there are oxygen bridges to link them. Among them, four ligands are bridging and the rest six are coordinated to single Nd centers. Besides, every pair of Nd atoms is bridged by one O-atom. The average bond length of Nd–O bonds with TTA ligands is 2.463 Å. In the b-diketone rings, the average distances for the C–C and C–O bonds are 1.424 and 1.263 Å, respectively, which are between 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
Fig. 1. Crystal structure of the tetranuclear neodymium complex.
195
delocalization of electron density of the coordinated b-diketonate chelate ring. 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 may be coordinated by water or other small molecules due to the reacting conditions. Thus, the emission efficiency would be decreased by the deactivation which caused by bond vibrations of C–H and O–H. Compared with the mononuclear complexes even with bulky ligands, the Nd3+ ions in the tetranuclear structure are wraped more close in the molecule and shielded more efficiently from the environment to improve the luminescence. To evaluate the potential of rare-earth-doped materials, Judd– Ofelt(JO) theory was applied to study the radiative properties of trivalent rare-earth-organic complex. According to JO theory, the oscillator strength of an electric dipole transition between an initial J manifold jðS; LÞJi and a terminal J0 manifold jðS0 ; L0 ÞJ 0 i can be determined by: 0
PðaJ; bJ Þ ¼
8p2 cmvm X 0 Xt jh4f N aJkU ðtÞ k4f N bJ ij2 3hð2J þ 1Þ t¼2;4;6
where h is the Plank constant and m is the frequency in inverse 2 þ2Þ2 centimeter, v ¼ ðn 9n is the local field correction factor. J is the total angular momentum quantum number of the initial level (for Nd3+ ion J = 9/2). ||U(t)|| are the doubly reduced matrix elements corresponding to the J–J0 transition. The JO parameters Xt( t = 2, 4, 6) can be obtained by a least-square method to correlate the oscillator strengths. The three intensity parameters exhibit the influence of the host on the radiative transition probabilities within the ground configuration, since they contain implicitly the effect of the odd symmetry crystal field terms, interconfigurational radial integrals and energy denominators. Following the Judd–Ofelt procedure [14,15], the JO parameter X2 is determined to be 1.08 1019 cm2 from the absorption spectra of the sample (Fig. 2). In particular, X2 is more sensitive to the symmetry and the degree of covalency in the lanthanide-first coordination shell interaction. In the sense of the dynamic coupling contribution to the total intensity, the polarization of the ligand field induces stronger lanthanide-ligand bonds and an increase in electric dipolar transitions for noncentrosymmetric ligand fields. The large value for X2, which is 1.08 1019 cm2 in this Letter, indicates the presence of covalent bonding between the Nd3+ ion and the surrounding ligands. This result suggests that the novel complex has no center of symmetry and the geometrical structure of the complex in DMF is asymmetric. Through analyzing the fluorescence spectrum and
Fig. 2. Absorption spectrum of the complex in DMF. The chelate concentration is 0.1 M.
196
X. Qiu et al. / Chemical Physics Letters 457 (2008) 194–197
lent to the rate of radiative decay (kDA = Ar). R0 is a function of the refractive index of the medium nD, Avogadro number NA, the donor PL quantum efficiency QD, the overlap integral I, and a parameter jp depending on the relative orientation of the donor and acceptor dipoles and is expressed as !1=6 9000ðln 10Þj2p Q D R0 ¼ ; ð3Þ N A 128p5 n4D I The spectral overlap integral I is defined as Z I¼ PLD ðkÞeA ðkÞk4 dk;
Fig. 3. Fluorescence spectrum of the complex in DMF pumped at 800 nm, and the inset is the fluorescence decay curves of Nd3+ for the 4F3/2 ? 4I11/2 transition.
fluorescence decay curve (Fig. 3), the fluorescence lifetime is determined to be 1.6 ls. The spontaneous emission cross-section of 4F3/ 4 20 cm2. 2 ? I11/2 (Gaussian lineshape) is 3.13 10 3+ An estimated quantum yield of Nd luminescence of a certain complex can be calculated by comparing the luminescence lifetime of the complex ion with the natural lifetime of Nd, s0 . By using Eq. (1), a value of 0.6% for the quantum yield of the complex is obtained, given a value s0 ¼ 270 ls for the natural lifetime of Nd(III) [16] U ¼ s=s0
ð1Þ
This method of quantum yield estimation does not take into account other factors such as intersystem crossing efficiency. The obtained quantum yield value compares well with some other Nd3+ complexes where all the protons of the ligands are either deuterated or fluorinated. For example, it is reported that the Nd3+ complex of hexafluoroacetylacetonate in Methanol-d4 [12] and the Nd2(1,3-bis(3-phenyl-3-oxopropanoyl)benzene)3 in DMF-d7 have quantum yields of about 0.3% and 0.6%, respectively [17]. This result was mainly attributed to the structure of the tetranuclear complex which efficiently protected the Nd3+ ions from C–H and O–H bonds. Fluorescence resonance energy transfer (FRET) was used to investigate the quenching mechanism of this material in the Letter. Generally, there are two types of quenching for emission from an active Nd3+ ion [18]. One is related to the environments of the ions and the other is connected to the interaction between the ions. Effective quenching for 4F3/2 of Nd3+ via vibrational excitation of C– H and O–H bonds is a function of the distance between C–H, O–H groups and Nd3+ ions. Concentration quenching is a function of the distance among Nd3+ ions. The vibrational energy of O–H group with vibrational quanta c = 2(6900 cm1) and 3 were well matched with two of the important radiative transition gaps of Nd3+ (4F3/2 ? 4I15/2 and 4F3/2 ? 4 I11/2), and vibrational excitation leads to effective quenching of the excited state of Nd3+. According to the fluorescence resonance energy transfer, it is very sensitive to the donor–acceptor separation distance and their relative dipole orientations [19]. FRET process is driven by dipole–dipole interactions and depends on the degree of spectral overlap between donor photoluminescence (PL) and acceptor absorption, and on the sixth power of the separation distance between the donor and acceptor pair r. Förster formula gives the rate of nonradiative energy transfer 6 R0 kDA ¼ Aed ; ð2Þ r where Aed is the excited state radiative rate of the donor and R0 is the Förster critical distance corresponding to a rate of FRET equiva-
ð4Þ
which quantitatively takes into account the donor–acceptor spectral overlap over all wavelengths k, where PLD and eA represent the donor emission (normalized dimensionless spectrum) and acceptor absorption extinction coefficient spectrum, respectively. According to Eq. (2), the interactions between donor and acceptor decay rapidly when the distance between them is beyond the critical range R0. The critical distance R0 of the sample in this work is calculated to be 0.132 nm using Eqs. (3) and (4). The PL quantum efficiency QD is 0.6% and jp is taken as (2/3)1/2 for fast Brownian rotation for both molecules. Through the JO analysis, the branching ratio of the 4F3/ 4 2 ? I15/2 transition is 0.00290, which is about 1/140 of the branching ratio of the 4F3/2 ? 4I11/2 transition (0.397). Therefore, the FRET calculation is carried out with the energy transfer from the Nd3+ 4 F3/2 ? 4I11/2 transition to the O–H third vibrational overtone. The spectral overlap integral I is calculated from Fig. 4. Concerning that the molecular radius of the complex is about 0.5 nm, the quenching rate of Nd3+ via O–H vibration excitation is 0.034% of the Aed at most according to Eq. (2). As a result, the quenching for 4F3/2 of Nd3+ via vibrational excitation of O–H is quite small. Meanwhile, the concentration quenching is a function of crossrelaxation and excitation migration processes in Nd3+ [18]. The cross-relaxation process is shown in Fig. 5a. There is a level (4I15/2) with energy between metastable state (4F3/2) and ground state (4I9/2) in the Nd3+ ion. Since Nd3+ ion has a 4I15/2 state, an excited Nd3+ ion shares its energy with neighbouring Nd3+ ions, the electrons are transferred to the 4I15/2 state rapidly and then decay to their respective ground states through the nonradiative decay. The transition probability of this process depends on Nd–Nd distance, therefore the concentration quenching is explained by the cross-relaxation. The excitation migration is shown in Fig. 5b. The excitation migration is a process by which excited state energy is transferred from excited ion to neighbouring unexcited ion. This process is also a function of Nd–Nd distance.
Fig. 4. Overlap spectrum of the O–H vibrational absorption (vibrational quanta c = 3) and the normalized PL intensity of Nd3+ 4F3/2 ? 4I11/2 transition.
X. Qiu et al. / Chemical Physics Letters 457 (2008) 194–197
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the complex was verified by single-crystal X-ray diffraction. Optical characterization and theoretical analysis using JO theory demonstrated that this novel material had good optical properties. FRET theory analysis found that the quenching of Nd3+ excited state via O–H vibrational excitation could be nearly neglected and the cross-relaxation or excitation migration rate of the tetranuclear complex was only 1/16 times of the traditional mononuclear one. With these advantages, it was believed to be a promising enhanced photoluminescence candidate in the field of neodymium organic materials. Acknowledgements
Fig. 5. Two relaxation processes in Nd doped materials. (a) Cross-relaxation: an excited Nd3+ ion shares its energy with a neighbouring Nd3+ ion. (b) Excitation migration: the excitation energy moves from one Nd3+ ion to another.
This work was financially supported by the National Natural Science Foundation of China (NNSFC, Nos. 90401027, 20428304, and 60578039) and National Science and Technology Foundation of China (2004AA840003). The authors acknowledge the language processing by Johnson J. Penn. References
In the case of the tetranuclear complex, the average Nd–Nd bond inside the molecule is 0.4 nm, which means the quenching inside the molecule among the Nd3+ ions is also small according to the critical distance R0. Besides, compared with the mononuclear complex, the FRET analysis reveals that the structure of tetranuclear complex can decrease the concentration quenching essentially. For the same Nd3+ ion concentration, the average distance between molecules of tetranuclear complex is about 1.583 times of the mononuclear one. Therefore, the cross-relaxation or excitation migration rate of the tetranuclear complex is only (1/1.583)6, which is 1/16 times of the mononuclear one. This is especially useful for increasing the concentration to obtain enough population inversion for ultimate lasing. 4. Conclusions In conclusion, a novel tetranuclear neodymium complex was synthesized and dissolved into DMF. The molecular structure of
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