Inorganica Chimica Acta 362 (2009) 2001–2005

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Inorganica Chimica Acta

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Optical properties of a novel neodymium pentafluoropropionate binuclear complex

Chao Gao a,b, Kai Cui a, Jiangbo She a, Chaoqi Hou a, Hongyan Guo b, Wei Zhao a, Wei Wei a,c,*, Bo Peng a,c

a 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 Chinab Photoelectronical Material Center, Xi’an Modern Chemistry Research Institute, Xi’an Shaanxi 710065, People’s Republic of Chinac State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 May 2008Received in revised form 30 August 2008Accepted 10 September 2008Available online 23 September 2008

Keywords:Neodymium complexCrystal structureJudd–Ofelt theoryOptical propertyEmission cross-section

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.09.033

* Corresponding author. Address: State Key LaboraPhotonics, Xi’an Institute of Optics and Precision MeScience (CAS), Xi’an Shaanxi 710119, People’s Rep88887553.

E-mail addresses: iamww@fudan.edu.cn (W. WPeng).

A novel neodymium pentafluoropropionate binuclear complex, Nd(C2F5COO)3Dipy (Dipy: 2,20-dipyridyl),was synthesized and characterized by single-crystal X-ray diffraction. At a concentration of 0.2 M inDMSO-d6, the Judd–Ofelt parameters (X2, X4, X6) were calculated from the UV–Vis spectrum. Accordingto the small value of X2 and the zero splitting energy of 4F/3/2 level, a symmetric ligand field of the com-plex was confirmed in DMSO-d6. Strong emission of the complex in DMSO-d6 at 1057 nm with a decaytime about 1.3 ls were detected when excited at 800 nm pumped by a laser diode. The stimulated emis-sion cross-section of 4F3/2 ?

4I11/2 fluorescence transition was 2.36 � 10�20 cm2 and comparable withsome laser glasses, which indicated good radiative properties of this neodymium pentafluoropropionatebinuclear complex in liquid matrix.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, much work has been devoted to lanthanidecomplexes for their unique physicochemical properties and variousapplications as functional materials, such as chem- and bio-sensors[1,2], electroluminescence [3–5], magnetic [6,7] and laser materi-als [8–12]. Among them, the neodymium (Nd) complex systemswith an output wavelength around 1060 nm are of great interestfor their potential application as liquid laser in high-power andhigh average-power outputs laser systems to overcome the inevi-table heat accumulation problems of solid laser [13–17].

The emission cross-section, one of the most important parame-ters for laser design, is the key factor of tuning emission propertiesof Nd(III) ions in the host matrixes. It depends considerably on thesurrounding host matrix and ligands. In this study, a novel neo-dymium pentafluoropropionate binuclear complex (Nd(C2F5COO)3-

Dipy) was synthesized. The solution of the complex with relativehigh stimulated emission cross-section in DMSO-d6 was reported.Optical properties of the complex have been characterized and dis-cussed using the theory developed by Judd [18] and Ofelt [19].

ll rights reserved.

tory of Transient Optics andchanics, Chinese Academy ofublic of China. Fax: +86 29

ei), bpeng@fudan.edu.cn (B.

2. Experimental

2.1. Instrument

The infrared spectrum was recorded use KBr single crystal on aNicolet 170SX FTIR spectrometer. 1H NMR spectrum was recordedon a Bruker Advance 500 MHz instrument with tetramethylsilane(TMS) as internal standard. Elemental analysis (C, H, O, and N)was determined with a German Vario EL III instrument. Absorptionspectrum was measured on a Unico UV-2000 scanning spectropho-tometer. Photoluminescence emission spectrum was measuredusing a Zolix Omini-k 300 fluorescence spectrophotometer pumpedby a laser diode at 800 nm. To characterize the fluorescence decaycurve, the measurements were performed by using a Ti:sapphire la-sers (Spectra-Physics Spitfire) with a frequency of 1 kHz and Si pho-todiode (time response < 1 ns). The sample was excited at 800 nm,and the energy of one pulse was 2.25 mJ. Luminescence quantumyields were determined by comparing the luminescence intensityof the complex solution with a solution of quinine bisulfate in1 M H2SO4. The quantum yield was calculated from the equationUsample ¼ ðn2

sampleIsampleAref=n2ref Iref AsampleÞUref with n the refractive in-

dex, A the absorption, and I the emission intensity and taking thequantum yield of quinine bisulfate as 54.4% [20].

2.2. Synthesis of Nd(C2F5COO)3Dipy complex

All the reagents, including Nd2O3, 2,20-dipyridyl, perfluropenta-noic acid were purchased from Alfa Aesar and used as received.

N

N

Nd

O

O

CC2F5

3

Fig. 1. The molecular structure of Nd(C2F5COO)3Dipy.

Table 2Selected bond lengths (Å) and bond angles (�) for Nd(C2F5COO)3Dipy crystal

Nd1–O2 2.406(7) Nd1–O1A 2.423(7)Nd1–O3 2.409(6) Nd1–O7 2.486(6)Nd1–O6A 2.418(6) Nd1–N2 2.610(7)Nd1–O5 2.442(6) Nd1–N1 2.615(7)O2–Nd1–O3 141.3(3) O1A–Nd1–N2 147.7(3)O2–Nd1–O1A 118.7(3) O5–Nd1—N2 134.2(2)O3–Nd1–O5 145.1(3) O7–Nd1–N2 114.2(2)O5–Nd1–O6A 120.3(2) O3–Nd1–N1 104.7(2)O2–Nd1–O7 140.2(3) O6–Nd1–N1 137.8(3)O6A–Nd1–O7 142.3(3) O1A–Nd1–N1 147.4(3)

Symmetry transformations used to generate equivalent atoms: A: 1 � x, 3/2 � y, z.

2002 C. Gao et al. / Inorganica Chimica Acta 362 (2009) 2001–2005

Nd(C2F5COO)3(H2O)2 was obtained from Nd2O3 by dissolution in amixture of C2F5COOH and water and subsequent volatilization ofthe water. The Nd(C2F5COO)3Dipy was prepared according to thefollowing process. Nd(C2F5COO)3(H2O)2 3.35 g (5 mmol) and 2,20-dipyridyl 0.078 g (5 mmol) were dissolved in ethanol, the reactionmixture was allowed to reflux for 24 h. The pink product wasachieved by dried at 120 �C for 12 h under reduced pressure. A sin-gle crystal for X-ray diffraction analysis was obtained in ethanolsolution after two weeks.

The molecular structure of the complex is displayed in Fig. 1.The central Nd(III) ion is bound to three perfluropentanoic acidand one 2,20-dipyridyl. Elemental Anal. Calc. for Nd(C2F5COO)3Di-py: C, 28.89; H, 1.01; O, 12.17; N, 3.55. Found: C, 28.97; H, 0.92;O, 12.24; N, 3.38%. FTIR (on KBr, cm�1): 1712(s), 1661(vs),1523(m), 1434(s), 1326(s), 1217(vs), 1173(vs), 1032(vs), 817(m),844(m), 730(vs). 1H NMR (DMSO-d6, ppm): d = 7.45(2H, m), 7.95(2H, t), 8.39(2H, d), 8.69(2H, d).

2.3. X-ray crystallography

X-ray crystallography was performed using a Bruker Smart-1000 CCD diffractometer with graphite monochromated Mo Karadiation (k = 0.71073 Å) using x–2h scan technique. The structurewas solved by direct method using SHELXS-97 [21] and refinedagainst F2 by full-matrix least-square using SHELXL-97 [22]. All

Table 1Crystal data and structure refinement for Nd(C2F5COO)3DiPY(C2H5OH)

Empirical formula C21H14F15N2NdO7

Formula weight 835.588Temperature (K) 291(2)Wavelength 0.71073Crystal system triclinicSpace group P�1Unit cell dimensions

a (Å) 10.4095(9)b (Å) 12.3912(11)c (Å) 13.0668(12)a (�) 68.5140(10)b (�) 73.0810(10)c (�) 74.9180(10)

V (Å3) 1477.9(2)Z 2Dcalc (Mg/m3) 1.878F(000) 810h Range for data collection 2.39–25.50Limiting indices �12 < h < 12,

�14 < k < 15�15 < l < 15

Reflections collected/unique (Rint) 11041/5458 (0.0175)Refinement method full-matrix least-squares on F2

Goodness-of-fit on F2 1.036Final R indices R = 0.0571, xR = 0.1628R indices (all data) R = 0.0644, xR = 0.1726Largest difference in peak and hole (e Å3) 1.650 and �1.348

non-hydrogen atoms were refined anisotropic ally. All hydrogenatoms were treated using a riding model. No decomposition wasshown in the crystal. Crystallographic data are summarized inTable 1.

3. Results and discussion

3.1. Crystal structure

The structure of Nd(C2F5COO)3Dipy crystal is crystallized in thetriclinic space group P�1 with Z = 2 formula units in the unit cell.Two Nd(III) ions, six pentafluoropropionate anions, two 2,20-dipyr-idyl and two ethanol are contained in a centrosymmetric unit. Theselected bond lengths and angles of the complex crystal are listedin Table 2. The molecular structure with the atom-labeling schemeand the detailed coordination sphere around the Nd(III) ion isshown in Fig. 2.

It can be seen that the Nd(III) ion is coordinated by six O atomsand two N atoms, which form strongly distorted square anti prismconfiguration. The eight oxygen atoms of the four pentafluoroprop-ionic acid anion group bridge two neodymium atoms together to

Fig. 2. The structure of Nd(C2F5COO)3Dipy crystal, showing 20% probabilitydisplacement ellipsoids. Hydrogen atoms are omitted for clarity. Symmetry codes:A = 1 � x, 3/2 � y, z.

Fig. 3. Packing diagram of the complex, showing hydrogen bonding scheme.

C. Gao et al. / Inorganica Chimica Acta 362 (2009) 2001–2005 2003

form a binuclear complex. The other two pentafluoropropionic acidanions act as unidentate ligand to neodymium ions by the hydroxyoxygen atom. The g2-chelate-l2-carboxylato-k1O:k1O0 bidentatepentafluoropropionic acid anions join two Nd(III) ions togetherwith the distance between Nd(III)–Nd(III) (4.640(7) Å) in dimerunit. The bipyridine as a second assistant ligands are bidentate tothe Nd(III) ions, at one time two solvent ethanol molecules arecoordinated with neodymium ions.

A packing diagram with the hydrogen-bonding scheme isshown in Fig. 3. The C atom of the ligand donate protons to theadjacent molecule to form C20–H20� � �F8 (�x, 3 � y, �z) hydrogenbonds. The distance of C20� � �F8 (�x, 3 � y, �z) is 3.287(16) Å andthe angle is 137.0(2)�. The hydrogen bonds connect binucleargroups together along b axis to a one dimension chain. In addition,

Fig. 4. The intermolecular p–p-stacking in the complex. Hydrogen atoms areomitted for clarity. Symmetry codes: B = 2 � x.

one kind of aromatic p–p-stacking interaction exists in this com-pound, as shown in Fig. 4. The bipyridyl ring (N1/N2/C12–C21)contacts with the invertible ring (N1B/N2B/C12B–C21B) in the dis-tance of C�g � C�I ¼ 3:851ð6Þ Å.

The average distance of Nd–N (2.613(7) Å) in this crystal isslight shorter than that of Nd–N (2.668(4) Å) in [Nd(phen)2Cl3-(H2O)]CH3OH [23]. The Nd–O distances between 2.406(7) Å (forO2) to 2.486(6) Å (for O7) are shorter by ca. 0.08–0.24 Å than thosereported for the neodymium eight-coordinated Nd(TTA)4Pyr(TTA = thenoyltrifluoroacetylacetonate) b-diketone complex [24].It is interesting that the C–O distances of carboxylatoes(–C1O1O2–) are same, 1.213(11) Å, which are all fallen into therange of C@O distance. At one time, the distances of C–O are aver-age, which indicates the electron-conjugate system is formed bythe atoms of carboxylato. But the C–O distances of carboxylatoes(–C7O5O6–) are 1.248(10), 1.217(11) Å, which behave a typicalcarboxylic bond distances.

3.2. Absorption spectrum and Judd–Ofelt analysis

The absorption spectrum of 0.2 M Nd(C2F5COO)3Dipy in DMSO-d6 is shown in Fig. 5. Absorption bands were observed at k =490–550, 560–610, 610–650, 650–710, 715–775, 775–845 and845–920 nm. They are attributed to the well-known Nd(III)absorption of 4I9/2 ?

4G7/2 + 4G9/2 + 2K13/2, 4I9/2 ?4G5/2 + 4G7/2,

4I9/2 ?2H11/2, 4I9/2 ?

4F9/2, 4I9/2 ?4F7/2 + 4S3/2, 4I9/2 ?

4F5/2 + 4H9/2

and 4I9/2 ?4F3/2, respectively.

The magnetic-dipole oscillator strengths (Pmd) do not dependon the matrixes and are tabulated in the literature [25]. The elec-tric-quadrupole oscillator strengths were estimated to be muchsmaller than the electric-dipole oscillator strengths (Ped) [26]. Sothe experimental oscillator strengths (Pexp) can be given by the fol-lowing equation:

Pexp ¼ Ped þ Pmd ð1Þ

The measured oscillator strength of an absorption band can bedetermined using the following equation [27]:

Pmd ¼ 4:32� 10�9 9n

ðn2 þ 2Þ2

" #ZaðkÞdk: ð2Þ

400 500 600 700 800 900 1000 1100

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce/

a.u

Wavelength/nm

Fig. 5. Absorption spectrum of Nd(C2F5COO)3Dipy with the Nd(III) concentration of0.2 M(1.22 � 1020/cm3).

Table 3The oscillator strengths and Judd–Ofelt parameters of Nd(C2F5COO)3Dipy in DMSO-d6

Excitedstates

Wavelengthrange (nm)

Integral absorptioncoefficient

Ra(k)dk

(nm/cm)

Oscillator strength,P � 106 (DMSO-d6

solution)

4G9/2, 4G7/2, 2K13/2 490–560 15.19 3.942G7/2, 2G5/2 560–610 33.27 6.992H11/2 610–650 1.66 0.304F9/2 650–710 3.33 0.514S3/2, 4F7/2 715–775 30.62 3.904F5/2, 2H9/2 775–845 34.23 3.804F3/2 845–920 15.67 1.49X2(�10�20 cm2) 0.62X4(�10�20 cm2) 4.96X6(�10�20 cm2) 3.56

2004 C. Gao et al. / Inorganica Chimica Acta 362 (2009) 2001–2005

where a(k) and n are absorption coefficient at wavelength k, indexof bulk refraction, respectively. The refractive index of 0.2 MNd(C2F5COO)3Dipy in DMSO-d6 is 1.474.

According to Judd–Ofelt theory, the oscillator strength of anelectric-dipole transition between an initial J manifold and|(S,L)Ji and a terminal J0 manifold |(S0,L0)J0i can be determined by[28]:

PðaJ; bJ0Þ ¼ 8p2cmvv3hð2J þ 1Þ

Xt¼2;4;6

Xtj 4f NaJ� ��jUðtÞj 4f NbJ0

�� �j2: ð3Þ

where h is the Plank constant and m is the frequency in inverse cen-timeter, v = (n2 + 2)2/9n is the local field correction factor. J is the to-tal angular momentum quantum number of the initial level (forNd(III) ion J = 9/2). |U(t)| are the doubly reduced matrix elementscorresponding to the J ? J0 transition.

The experimental oscillator strength P for the electric-dipoletransition can be obtained by using Eq. (1). The Judd–Ofelt param-eters Xt (t = 2,4,6) can be obtained by a least-square method tocorrelate the oscillator strengths. Substituting Xt into Eq. (3), theoscillator strengths for all transitions can be calculated. Changesof the Xt parameters obtained in Judd–Ofelt analysis arise fromthe influence of the ligand field and chemical bonding propertiesaround the Nd(III) center. The X2 is particularly sensitive to achange of ligand field among them, and the more asymmetric li-gand field leads to more significant increase of X2.

The oscillator strengths and Judd–Ofelt parameters Xt values ofthe neodymium complex are listed in Table 3. A weak oscillatorstrength as 1.49 � 10�6 at the hypersensitive transition and a small

Table 4Oscillator strength of the hypersensitive transition P, Splitting energy and X2 of some neo

Complexesa Matrix P (10�6) Splitting

4F/3/2 lev

Nd(DBM)3Phen MMA 102.1 868 877Nd(POA-D)3 Methanol-d4 49.54 866 875Nd(HFA-D)3(D2O)2 Methanol-d4 52.52 866 871Nd(fod)3 Methanol-d4 52.49 866 876Nd(thd)3 Hexane 26.36 865Nd(POM–D)3 Methanol-d4 19.66 868Nd(C2F5COO)3Dipy DMSO-d6 1.49 868

a DBM: dibenzoylmethane; POA: perfluorooctanoylacetic acid, POM: bisperfluoroocdimethyl-4,6-octanedione, thd:2,2,6,6-tetramethyl-3,5-heptanedione.

X2 value of 0.62 � 10�20 cm2 were obtained for Nd(C2F5COO)3Dipyin DMSO-d6. The splitting energies [29] of the 4F3/2 degenerate lev-els of some neodymium complexes are listed in Table 4. The split-ting energy of 4F3/2 as zero is observed for Nd(C2F5COO)3Dipy. Theoscillator strength, Judd–Ofelt parameter X2 and splitting energyindicated a symmetric coordination environment of Nd(C2F5COO)3-

Dipy in DMSO-d6. Like some other symmetric Nd(thd)3 andNd(POM–D)3 in hexane and methanol-d4, the oscillator strengthsof the hypersensitive transition and the Judd–Ofelt parameter X2

are much small than those of asymmetric complex, as shown inTable 4.

3.3. Radiative properties

It has been considered to be impossible to observe the lumines-cence of Nd(III), if the bonds with higher frequency mode such asO–H or C–H are present in ligands or solvent molecules aroundNd(III), due to the radiationless transition through vibrational exci-tation. The complex Nd(C2F5COO)3Dipy without O–H bond showedstrong emission in DMSO-d6 by excitation at 800 nm is describedin Fig. 6. There are three main peaks and the wavelength centeredat 886 (4F3/2 ?

4I9/2), 1057 (4F3/2 ?4I11/2) and 1328 nm (4F3/

2 ?4I13/2), respectively. The intensity of the 4F3/2 ?

4I11/2 is thestrongest. The inset of Fig. 6 is the fluorescence decay time forthe 4F3/2 ? 4I11/2 transition. The measured fluorescence lifetime(sm) then obtained as 1.3 ls, which is comparable with some neo-dymium complex in solution [30]. According to the measured life-time (sm) and emission quantum yield (U = 0.12%), the radiativetransition rates of 4F3/2 ?

4I11/2 transition (A1057) of the complexcan be calculated by the following equation [31]:

A1057 ¼Usm

ð4Þ

with the value of the radiative transition rate calculated above andthe corresponding emission spectrum, the stimulated emissioncross-section r is calculated by [31]:

dymium complexes in different matrix

energy X2 (10�20 cm2) Reference

el (nm) DE (cm�1)

120 37.54 [11]120 22.26 [30]

70 21.13 [30]130 23.52 [30]

0 8.70 [30]0 2.50 [30]0 0.62 This work

tanoylmethane, HFA: hexafluoroacetylacetone, fod: 1,1,1,2,2,3,3-heptafluoro- 7,7-

Table 5The emission cross-section r of 4F3/2 ? 4I11/2 fluorescence transition of Nd(III) indifferent matrixes

Materials r (�10�20 cm2) References

Nd(DBM)3Phen doped silica gel 1.41 [33]Nd(III) doped silicate glasses 1.5 [32]Nd(DBM)3Phen in PMMA 1.87 [11]Glasses doped with Nd2O3 2.6 [34]Nd(III) doped borate and phosphate glasses 3.2 [35]Nd(TTA)3(TPPO)2 in PMMA 2.75 [14]Nd(C2F5COO)3Dipy in DMSO-d6 2.36 This work

900 1000 1100 1200 1300 14000

100

200

300

400

500

0 2 4 6 8 100.00

0.05

0.10

0.15

0.20

0.25

Inte

nsity

(a.

u)

Decay time ( s)

Inte

nsity

(a.

u)

Wavelength (nm)

μ

Fig. 6. Fluorescence spectrum of the complex in DMSO-d6 pumped at 800 nm, insetis the fluorescence decay curve of Nd(III) for the 4F3/2 ? 4I11/2 transition.

C. Gao et al. / Inorganica Chimica Acta 362 (2009) 2001–2005 2005

r ¼ A1057k40

8pn2cDk: ð5Þ

where the k0 is the peak wavelength of fluorescence as 1057 nm,and Dk is the line width of fluorescence peak, determined as thefull-width at half-maximum as 30 nm. The calculated emissioncross-section r of 4F3/2 ? 4I11/2 fluorescence transition can bedecided as 2.36 � 10�20 cm2.

It is well known that the emission cross-section r of4F3/2 ?

4I11/2 fluorescence transition of Nd(III) is one of the mostimportant parameters for laser design and it is dependent onlyon X4 and X6. For a large emission cross-section, the magnitudeof X4 and X6 should be as high as possible. In this case, the sym-metric ligand filed of the complex in DMSO-d6 was confirmed bythe X-ray diffraction as well as the Judd–Ofelt parameter X2 as illu-minated above. However, the emission cross-section r of4F3/2 ?

4I11/2 fluorescence transition is 2.36 � 10�20 cm2, which isa little smaller than some asymmetric neodymium complexes[14] but comparable with laser glasses [32,34–36] as shown inTable 5. Therefore, with the high r value and other radiative prop-erties discussed above, Nd(C2F5COO)3Dipy complex has good pho-toluminescence emission in solution.

4. Conclusion

In conclusion, a novel binuclear neodymium complex was syn-thesized and dissolved into DMSO-d6. The crystal structure of thecomplex was verified by single-crystal X-ray diffraction. TheJudd–Ofelt parameters (X2, X4, X6) were obtained from theabsorption spectrum of Nd(C2F5COO)3Dipy in DMSO-d6 with a con-centration of 0.2 M. Small value of the Judd–Ofelt parameter X2 aswell as zero splitting energy of 4F/3/2 level were obtained, which

indicated a symmetric ligand field of the complex in DMSO-d6. Inspite of symmetric ligand field, strong emission was detected at1057 nm when excited at 800 nm. The emission cross-section of4F3/2 ?

4I11/2 fluorescence transition was 2.36 � 10�20 cm2 andcomparable with some laser glasses, which indicated good radia-tive properties of this neodymium pentafluoropropionate binu-clear complex in liquid matrix.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (NSFC, No. 60578039), The Programfor Tackling Key Industrial Problems of Shaanxi Province(2007K06-06) and the Postdoctoral Science Foundation of China(No. 20070421134).

Appendix A. Supplementary material

CCDC 666540 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2008.09.033.

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