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Enhancement of luminescence of Nd3+ complexes with deuterated hexafluoroacetylacetonato ligands in organic solvent
5 January 1996
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 248 (1996) 8-12
Enhancement of luminescence of N d 3+ complexes with deuterated hexafluoroacetylacetonato ligands in organic solvent Yasuchika Hasegawa a, Kei Murakoshi a, Yuji Wada a, Shozo Yanagida a, Jeong-Ho Kim b, Nobuaki Nakashima b, Tatsuhiko Yamanaka b a Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan b Institute of Laser Engineering, Osaka University, Yaraada-oka, Suita, Osaka 565, Japan
Received 28 March 1995; in final form 3 November 1995
Abstract
Tris-(hexafluoroacetylacetonato)neodymium(III), [Nd(HFA-D)3], was prepared by chelation of Nd 3+ ion with deuterated hexafluoroacetylacetone in CD3OD.Luminescence of the Nd 3+ complex was observed for the first time in organic solvents and the quantum yield was estimated to be of the order of 10 -2 in deuterated acetone solution. The absorption spectrum of [Nd(HFA-D)3] dissolved in acetone was comparable with that of Nd 3+ ion in Y3A15015 matrix (Nd:YAG). Splitting of the 4F3/2 level was determined to be 82.3 cm- l in this system. These spectral characteristics suggest that the physical nature of Nd ~+ coordination environments should be uniform and well defined by coordination of HFA in solution.
1. Introduction
Applying a Nd 3 +-doped liquid medium to a laser system has been expected to solve the intrinsic problems of solid laser systems. Liquid media can cool the system by circulation at high power density operation and can replace thermally distorted components. A system with such features should achieve a high repetition rate of the emission of the order of kHz. The luminescence intensity of Nd 3÷ ion in solution, however, is extremely weak. The relatively low efficiency of Nd 3÷ emission in solution has been attributed to (1) the radiationless transition process via vibrational excitation of the surrounding medium [1], (2) dipole-dipole nonradiative energy transfer processes via cross-relaxation and excitation migration [2,3], and (3) characteristics of perturbation to Nd 3+ f orbitals by the surrounding environment. A Nd 3 +-doped liquid medium can be used as a
strong luminescent material if these contributions could be controlled by designing ligands of Nd 3+ complexes. The energy transfer process via vibrational excitation has been considered as a dominant process which quenches Nd 3+ excited states in solution. Higher frequency modes more effectively enhance the radiationless transitions, according to the energy gap theory [1,4,5]. The energy of the initial state has to be equal to the final state, which has a small number of the high frequency mode coupling with some low solvent modes, and the harmonic energy of vibrational quanta has to be equal to or greater than the energy gap of the radiative transition states. It has been pointed out that the energy gap of the radiative transition in Nd 3÷ ion (5400 c m - l : 4F3/2 "-')4Ii5/2) approximately matches the second harmonics of C - H and O - H bond vibrations (5900 and 6900 c m - l , respectively) of solvent molecules,
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01279-6
Y. Hasegawa et al./ Chemical Physics Letters 248 (1996) 8-12
and thus results in effective quenching of Nd 3+ excited states in solution. To improve the emission efficiency of a Nd 3+-doped liquid medium, the idea of introducing an organic molecule with low-energy vibrations into the system was proposed by Heller et al. [6,7] whose work led to the first successful utilization of an inorganic aprotic liquid, SeOCI 2 and POC13, for laser emission [8-10]. Although the emission efficiency was improved significantly by using the medium [11-13] those solvents were not appropriate to use for practical applications due to their instability and high toxicity. In this study, we successfully attempted to observe luminescence of Nd complexes in an organic solvent by using deuterated HFA as ligands of the complexes. The physical nature of the Nd 3+ coordination environment of the system was compared with that of Nd 3+ ion in strong luminescence materials such as Nd:YAG and LHG-8.
2. Experimental 2.1. Materials
Neodymium acetate monohydrates (99.9%) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (HFA) were purchased from Wako Pure Chemical Industry Ltd. All other chemicals were reagent grade and were used as received. Neodymium acetate monohydrate (5.0 g, 15 mmol) was dissolved in 20 ml of distilled water under stirring at 0°C. A solution of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (5 g, 24 mmol) in methanol (5 ml) was added dropwise, stirred for 3 h, giving a precipitation of tris-(hexafluoroacetylacetonato)neodymium(III) dihydrates ([Nd(HFA-H)3]-2H20). The reaction mixture was evaporated and filtered. The filtered blue-purple crystals were recrystallized from methanol and dried under vacuum (about 5 Torr) at 80°C for 24 h. IR: 661 (m), 743 (w), 806 (m), 955 (w), 1013 (w), 1104 (m), 1145 (s), 1217 (s), 1258 (s), 1351 (w), 1468 (s), 1535 (s), 1561 (m), 1612 (w), 1650 (s), 2985 (w), 3394 (m) cm -I. Analysis calculated for CI5H708FlaNd: C, 22.48; H, 0.88%; found: C, 22.12; H, 1.01%. Nd3+-doped phosphate glasses for solid lasers (LHG-8) were purchased from Hoya [14,15].
9
2.2. Deuterium exchange procedure
The preparation of liquid materials for optical measurements was as follows. Methanol-d4 (CD 3OD, 99.8%) and acetone-d 6 (CD3COCD3, > 99.95%) were obtained from Aldrich Chemical Company Inc. A solution containing 0.33 M of [ N d ( H F A - H ) 3 ] - 2 H 2 0 in deuterated-methanol (CH3OD) was introduced into an optical cell and dried to a powder under vacuum (about 10 -3 Torr). The powdered Nd 3+ complex was again deuterated using methanol-d 4 under vacuum. The dried deuterated Nd complex, [Nd(HFA-D)3].2D20, was obtained as a powder by evaporation of CDaOD and CD3OH. Deuterated-acetone dried with anhydrous CaSO 4 under vacuum was used as a solvent for optical measurement. 2.3. Optical measurements
The transmittance spectrum was obtained using a monochromator (JASCO, PS-H150) and a photomultiplier (Hamamatsu Photonics, C659B). A Ge detector was employed to detect emission induced in the near-infrared region. The quantum yield was determined by the standard procedure using an integral sphere [ 16].
3. Results and discussion As a promising ligand, HFA was chosen because of the small number of C - H bonds in the ligand. The Nd complex was prepared by chelation of HFA. The resulting complex was found to have water of crystallization based on IR and elemental analyses. In fact, DSC analysis of the Nd complex ([Nd(HFAH)3]" 2H20) gave two endothermic peaks at 132.5 and 163.2°C due to elimination of water molecules of crystallization with an endothermic peak at 243.5°C as melting point. These facts suggest that the structure of the Nd complex should have a square-anti prism with eight coordinated oxygens of three HFA and two water of crystallization [17-19]. To eliminate water of crystallization and to replace the C - H bond with C - D bonds, the Nd complex was treated with methanol-d I and methanoi-d 4 for deuteration using keto-enol equilibrium. The result-
10
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
~1600 cm"1 CF3
F3C 1¢/~
D ~'~ Q ~ O - ~
2100 era-!
F3C~O-~,~.O~C-,F
F2 ~7"1200 cm'l
O O A O,~jC , ~16$o cm" F3C ~ " , ~ ' ~ " C F 3
20
D
Fig. 1. Chemical structure of tris-(hexafluoroacetylacetonato)neodymium(IIl): [Nd(HFA-D) 3] and vibration wave numbers.
o 850
950
lO50
1150
1250
1350
145o
Wavelength / nm
Fig. 2. Emission spectrum of 2 mol dm -3 [Nd(HFA-D)3] in ing deuterated Nd complex [Nd(HFA-D) 3] could dissolve in various organic solvents such as methanol, pyridine, acetonitrile, DMSO, THF, DMF, ether, and acetic acid. In particular, [Nd(HFA-D) 3] could dissolve with an extremely high concentration of more than 2 mol dm -3 in acetone. In highly dehydrated solvents, the Nd complex should form a square-anti prism structure with two solvent molecules as ligands. With these facts in mind, acetone-d 6 was chosen as solvent for the optical measurement A solution of [Nd(HFA-D) 3] in acetone-d 6 showed emissions at 880, 1062 and 1345 nm when the system was excited at 585 nm (Fig. 2). The quantum yield of the emission was determined to be of the order of 10 -2 using an integral sphere [16]. No
Water contents of the system prepared by the present procedure should be less than 0.1301 wt%. Nd ~
0
emission 4F~
acetone-d6.
emission was observed in either non-deuterated acetone or deuterated acetone without careful dehydration of the system. The quantum yield of the system containing C - H or O - H bond vibrations was estimated to be of the order of 10 -5 . Spectral peaks at 880, 1062, and 1345 nm were attributed to the f - f transitions 4[73/2 "->'119/2, 4F3/2 "->4Ill/2 , and 4F3/2 ---'4Ii3/2, respectively. The emission band at 1062 nm is important because it is applicable for laser emission. Interestingly, it had a symmetrical and narrow structure as is observed for the emission of Nd:YAG. It is said that the emission from Nd 3+ ion in glass matrices shows a relatively wide emission with multiple components due to the various circumstances of Nd 3 + ion in amorphous solid matrices. The band shape with highly symmetrical
O-H C.H O-D C-D C-O C-C C-F (~150) (2950) (2f~O) (2100) (1650) (IS$O) (1200)
-2000
E
Vffil
-4000 -6000 -8000 -10000
- I " 41l~Z
t
v
V=2
4111/2 Vffi3
-12000
-15000 Fig. 3. Schematic presentation of the vibrational energy levels in organic media and matching the electronic energy gap of Nd 3+ ion.
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
structure and relatively narrow width suggests that the environment of Nd 3+ should be uniform in the solution. The energy gap law indicates that the harmonic matching number of the vibrational quanta should be larger than that of O - H and C - H vibrations (3450 and 2950 cm-~). HFA ligands of [Nd(HFA-D) 3] molecules have vibrations of C - F at 1200 cm - t , C - D at 2100 cm - l , and O - D at 2500 cm -~. Fig. 3 shows the harmonic match number of C-D, C-O, C-C, and C - F vibrations. In the present system, the harmonic number was modified by deuteration from v = 2 ( C - H and O - H ) to v = 3 ( C - D and O-D); vibrational harmonics equal in energy to or greater than the gap in Nd 3÷ are capable of quenching the most effectively according to the energy gap theory [5]. As the vibrational quanta, v, increase, the Franck-Condon factor, F, decreases. The factor F should be estimated quantitatively and an approximation using the undistorted osillator model is adopted. In the model, F is given by F ( E ) = exp( - y ) y v / v ! ,
(1)
where y is the displacement factor defined by y = ½k(~-~°)2/hto (~ and T° are equilibrium positions in vibration states) [4]. If y is assumed to be 1, F is obtained as 0.18, 0.061, and 0.0031 at v = 2, 3, and 5, respectively. This estimation suggests that eliminating the proton from the ligand significantly decreases the probability of radiationless transitions. Absence of C - H and O - H vibrations should make it possible to observe the emission from Nd 3+ in an organic solvent. Luminescence of the [Nd(HFA-D) 3] complex in organic solvents was observed in the present system for the first time. Fig. 4 shows the absorption spectrum of 0.4 M [Nd(HFA-D) 3] in acetone-d6. Absorption bands were observed at 513, 524, 582, 680, 745, 800, and 865 nm and attributed to the Nd 3+ transitions 4 2 4 ....~ 2 of I9/2(ground state) "~ G l l / 2 , I9/2 G9/2, 41 __~ 2 [.~ 41 _...>417 ~9~/2 ~ 7 / 2 , , ~9/2 A ~ 9 / 2 ,
4'1 ...~4 F ~9/2 ~7/2,
41 "9/2
~ F s / 2 , and ~I9/2 ~ F 3 / 2 , respectively. The absorption spectrum is quite comparable with that of Nd:YAG. The broad absorption band at 320-350 nm is explained as due to HFA-D ligand molecules. One of the important characteristics of the spectrum is a splitting of the peak at 865 nm as shown in the inset of Fig. 4. The band peak at 865 nm is
11
100
b. 2O 87O 0
~
300
I
I
400
500
600
I
I
I
700
800
900
1~
Wavelength / nm
Fig. 4. Transmittancespeca'a of 0.4 tool dm-3 [Nd(HFA-D)3]in acetone-d6. Insert: 419/2-"~4F3/2(ground state to emitting level) transmission spectra. attributed to the t r a n s i t i o n 419/2 ---~4F3/2 . T h u s , the observed splitting indicates that the degenerate 4F3/2 level, i.e. the emitting level, splits slightly into two levels. The emitting levels are split under the influence of crystal or ligand fields. The Hamiltonian / t of Nd 3+ can be expressed as = H0 +/qso + 12e,,t,
(2)
where H0 is the Hamiltonian of the original state for LS quantization, ~so is the Hamiltonian for spinorbit interaction, Vext is the electron potential energy for crystal or figand fields. Generally, V~ is much smaller than H 0 and /~so, and l,~ext depends on the ligand distance, R, field anisotropy Rk(cos Oij) (Legendre polynomials), and field charge, Q, which increases under increasing influence of crystal or ligand fields. Slight splitting of the 4F3/2 degenerate levels should be induced by the crystal and ligand field in the system. Table 1 summarizes the splitting energy of Nd 3+ ion in [Nd(HFA-D)3] , Nd:YAG, and LHG-8. The splitting energy of [Nd(HFA-D) 3] 4F3/2 states of 82.3 cm- 1 is similar to that of Nd:YAG (84 cm- i ) [20]. On the other hand, LHG-8, Nd3+-doped glasses, have a splitting (I01.9 cm -1) larger than Table 1 Splitting energy of emitting level in Nd 3+ Compounds
4F3/ 2 (rim)
Splitting energy (cm- i )
Nd3 + YAG [20] LHG-8 [Nd(HFA-D3)]
868.1 865.5 864.8
84 101.9 82.3
874.5 873.2 871.2
12
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
that of [Nd(HFA-D) 3] and Nd:YAG. The splittings should reflect l~cxt in these matrices. LHG-8 is an amorphous glass, and does not have a symmetrical structure. Thus, the Vext parameters of LHG-8 should be larger. On the other hand, Nd:YAG is a pure crystal with the garnet structure. The splitting width observed for the absorption of Nd:YAG can be explained in terms o f the characteristics of the crystal structure of the matrix [20]. The comparable splitting width observed for the absorption of [Nd(HFA-D) 3] complex in liquid media with that of Nd:YAG suggests that the field due to the oxygens of the ligand molecules should be similar to that of Nd:YAG. Nd 3÷ ion in [Nd(HFA-D) 3] molecules should be coordinated to eight 0 2- atoms which are in three ligand molecules and two solvent molecules, and lead to strong crystal-field perturbation similar to that in Nd:YAG. The strong 4 f - 4 f transition intensity of Nd:YAG has been attributed to that kind of strong perturbation. Thus, an increase of the probability of 4 f - 4 f transitions is also expected for the present system after achievement of further optimization for the emission process. The extemal field also depends on the field charge Q as well as on R and Rk(cos Oij) which are determined by the molecular structure and charge site symmetry, as discussed above. The electron density on O atoms of [Nd(HFA-D) 3] should decrease and reduce the magnitude of the field charge Q. Vext should be affected by introducing trifluoromethyl groups into acetylacetonato ligands. The geometrical a n d / o r electronic structure of ligand molecules can also contribute to change the emission intensity. To elaborate the process, further investigation of the structures should be necessary. In conclusion, [Nd(HFA-D) 3] was prepared by chelation of Nd 3÷ ion with deuterated hexafluoroacetylacetone in acetone-d6. Effective emission of the Nd ion of [Nd(HFA-D) 3] in organic solution was observed for the first time. These characteristics of the emission and absorption spectra are similar to
that of Nd:YAG, suggesting that the electronic structure of the Nd 3÷ ion in the present system should be comparable with that of Nd 3+ ion in Nd:YAG. Higher quantum yield luminescence of Nd 3÷ complexes in liquid media should be achieved by designing molecules to control the Nd 3+ coordination structure.
References [1] G. Stein and E. Wurzberg, J. Chem. Phys. 62 (1975) 208. [2] J.A. Caird, A.J. Ramponi and P.R. Staver, J. Opt. Soc. Am. B 87 (1991) 1391. [3] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [4] W. Siebrand, J. Chem. Phys. 46 (1967) 440. [5] Y. Haas, G. Stein and E. Wurzberg, J. Chem. Phys. 60 (1974) 258. [6] A. Heller, J. Am. Chem. Soc. 88 (1966) 2058. [7] A. Heller, Appl. Phys. Letters 9 (1966) 106. [8] E.J. Schimitscheck, J. Appl. Phys. 39 (1968) 6120. [9] M.N. Tolstoi, E.I. Lyubimov and I.M. Batyaev, Opt. Spectry. 28 (1970) 389. [10] N. Blumenthal, C.B. Ellis and D. Grafstein, J. Chem. Phys. 48 (1968) 5726. [11] C. Brecher and W. French, J. Phys. Chem. 77 (1973) 1370. [12] T. Sasaki, T. Yamanaka, G. Yamaguchi and C. Yamanaka, Japan J. Appl. Phys. 8 (1969) 1037. [13] M. Hongyo, T. Sasaki, Y. Nagao, K. Ueda and C. Yamanaka, IEEE J. Quant. Electron. QE-8 (1972) 192. [14] S.E. Stokowski, R.A. Saroyan and M.J. Weber, Nd-doped laser glass spectroscopic and physical properties, Lawrence Livermore National Laboratory, Livermore, CA, M-095 Rev. 2, 1981. [15] W.F. Krupke, M.D. Shinn, J.W. Marion, J.A. Caird and S.E. Stokowski, J. Opt. Soc. Am. B 3 (1986) 102. [16] K.L. Eckerle, W.H. Venable Jr. and V.R. Weidner, Appl. Opt. 153 (1976) 703. [17] M. Nakamura, R. Nakamura, K. Nagai, M. Shimoi, S. Tomoda, Y. Takeuchi and A. Ouchi, Bull. Chem. Soc. Japan 59 (1986). [18] R.C. Holz and L.C. Thompson, lnorg. Chem. 32 (1993) 5251. [19] M. Sayeed and N. Ahmad, J. Inrg. Nucl. Chem. 43 (1981) 3197. [20] G.W. Burdick, C.K. Jayasankar and F.S, Richardson, Phys. Rev. B 50 22 (1994) 16309.
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 248 (1996) 8-12
Enhancement of luminescence of N d 3+ complexes with deuterated hexafluoroacetylacetonato ligands in organic solvent Yasuchika Hasegawa a, Kei Murakoshi a, Yuji Wada a, Shozo Yanagida a, Jeong-Ho Kim b, Nobuaki Nakashima b, Tatsuhiko Yamanaka b a Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan b Institute of Laser Engineering, Osaka University, Yaraada-oka, Suita, Osaka 565, Japan
Received 28 March 1995; in final form 3 November 1995
Abstract
Tris-(hexafluoroacetylacetonato)neodymium(III), [Nd(HFA-D)3], was prepared by chelation of Nd 3+ ion with deuterated hexafluoroacetylacetone in CD3OD.Luminescence of the Nd 3+ complex was observed for the first time in organic solvents and the quantum yield was estimated to be of the order of 10 -2 in deuterated acetone solution. The absorption spectrum of [Nd(HFA-D)3] dissolved in acetone was comparable with that of Nd 3+ ion in Y3A15015 matrix (Nd:YAG). Splitting of the 4F3/2 level was determined to be 82.3 cm- l in this system. These spectral characteristics suggest that the physical nature of Nd ~+ coordination environments should be uniform and well defined by coordination of HFA in solution.
1. Introduction
Applying a Nd 3 +-doped liquid medium to a laser system has been expected to solve the intrinsic problems of solid laser systems. Liquid media can cool the system by circulation at high power density operation and can replace thermally distorted components. A system with such features should achieve a high repetition rate of the emission of the order of kHz. The luminescence intensity of Nd 3÷ ion in solution, however, is extremely weak. The relatively low efficiency of Nd 3÷ emission in solution has been attributed to (1) the radiationless transition process via vibrational excitation of the surrounding medium [1], (2) dipole-dipole nonradiative energy transfer processes via cross-relaxation and excitation migration [2,3], and (3) characteristics of perturbation to Nd 3+ f orbitals by the surrounding environment. A Nd 3 +-doped liquid medium can be used as a
strong luminescent material if these contributions could be controlled by designing ligands of Nd 3+ complexes. The energy transfer process via vibrational excitation has been considered as a dominant process which quenches Nd 3+ excited states in solution. Higher frequency modes more effectively enhance the radiationless transitions, according to the energy gap theory [1,4,5]. The energy of the initial state has to be equal to the final state, which has a small number of the high frequency mode coupling with some low solvent modes, and the harmonic energy of vibrational quanta has to be equal to or greater than the energy gap of the radiative transition states. It has been pointed out that the energy gap of the radiative transition in Nd 3÷ ion (5400 c m - l : 4F3/2 "-')4Ii5/2) approximately matches the second harmonics of C - H and O - H bond vibrations (5900 and 6900 c m - l , respectively) of solvent molecules,
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01279-6
Y. Hasegawa et al./ Chemical Physics Letters 248 (1996) 8-12
and thus results in effective quenching of Nd 3+ excited states in solution. To improve the emission efficiency of a Nd 3+-doped liquid medium, the idea of introducing an organic molecule with low-energy vibrations into the system was proposed by Heller et al. [6,7] whose work led to the first successful utilization of an inorganic aprotic liquid, SeOCI 2 and POC13, for laser emission [8-10]. Although the emission efficiency was improved significantly by using the medium [11-13] those solvents were not appropriate to use for practical applications due to their instability and high toxicity. In this study, we successfully attempted to observe luminescence of Nd complexes in an organic solvent by using deuterated HFA as ligands of the complexes. The physical nature of the Nd 3+ coordination environment of the system was compared with that of Nd 3+ ion in strong luminescence materials such as Nd:YAG and LHG-8.
2. Experimental 2.1. Materials
Neodymium acetate monohydrates (99.9%) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (HFA) were purchased from Wako Pure Chemical Industry Ltd. All other chemicals were reagent grade and were used as received. Neodymium acetate monohydrate (5.0 g, 15 mmol) was dissolved in 20 ml of distilled water under stirring at 0°C. A solution of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (5 g, 24 mmol) in methanol (5 ml) was added dropwise, stirred for 3 h, giving a precipitation of tris-(hexafluoroacetylacetonato)neodymium(III) dihydrates ([Nd(HFA-H)3]-2H20). The reaction mixture was evaporated and filtered. The filtered blue-purple crystals were recrystallized from methanol and dried under vacuum (about 5 Torr) at 80°C for 24 h. IR: 661 (m), 743 (w), 806 (m), 955 (w), 1013 (w), 1104 (m), 1145 (s), 1217 (s), 1258 (s), 1351 (w), 1468 (s), 1535 (s), 1561 (m), 1612 (w), 1650 (s), 2985 (w), 3394 (m) cm -I. Analysis calculated for CI5H708FlaNd: C, 22.48; H, 0.88%; found: C, 22.12; H, 1.01%. Nd3+-doped phosphate glasses for solid lasers (LHG-8) were purchased from Hoya [14,15].
9
2.2. Deuterium exchange procedure
The preparation of liquid materials for optical measurements was as follows. Methanol-d4 (CD 3OD, 99.8%) and acetone-d 6 (CD3COCD3, > 99.95%) were obtained from Aldrich Chemical Company Inc. A solution containing 0.33 M of [ N d ( H F A - H ) 3 ] - 2 H 2 0 in deuterated-methanol (CH3OD) was introduced into an optical cell and dried to a powder under vacuum (about 10 -3 Torr). The powdered Nd 3+ complex was again deuterated using methanol-d 4 under vacuum. The dried deuterated Nd complex, [Nd(HFA-D)3].2D20, was obtained as a powder by evaporation of CDaOD and CD3OH. Deuterated-acetone dried with anhydrous CaSO 4 under vacuum was used as a solvent for optical measurement. 2.3. Optical measurements
The transmittance spectrum was obtained using a monochromator (JASCO, PS-H150) and a photomultiplier (Hamamatsu Photonics, C659B). A Ge detector was employed to detect emission induced in the near-infrared region. The quantum yield was determined by the standard procedure using an integral sphere [ 16].
3. Results and discussion As a promising ligand, HFA was chosen because of the small number of C - H bonds in the ligand. The Nd complex was prepared by chelation of HFA. The resulting complex was found to have water of crystallization based on IR and elemental analyses. In fact, DSC analysis of the Nd complex ([Nd(HFAH)3]" 2H20) gave two endothermic peaks at 132.5 and 163.2°C due to elimination of water molecules of crystallization with an endothermic peak at 243.5°C as melting point. These facts suggest that the structure of the Nd complex should have a square-anti prism with eight coordinated oxygens of three HFA and two water of crystallization [17-19]. To eliminate water of crystallization and to replace the C - H bond with C - D bonds, the Nd complex was treated with methanol-d I and methanoi-d 4 for deuteration using keto-enol equilibrium. The result-
10
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
~1600 cm"1 CF3
F3C 1¢/~
D ~'~ Q ~ O - ~
2100 era-!
F3C~O-~,~.O~C-,F
F2 ~7"1200 cm'l
O O A O,~jC , ~16$o cm" F3C ~ " , ~ ' ~ " C F 3
20
D
Fig. 1. Chemical structure of tris-(hexafluoroacetylacetonato)neodymium(IIl): [Nd(HFA-D) 3] and vibration wave numbers.
o 850
950
lO50
1150
1250
1350
145o
Wavelength / nm
Fig. 2. Emission spectrum of 2 mol dm -3 [Nd(HFA-D)3] in ing deuterated Nd complex [Nd(HFA-D) 3] could dissolve in various organic solvents such as methanol, pyridine, acetonitrile, DMSO, THF, DMF, ether, and acetic acid. In particular, [Nd(HFA-D) 3] could dissolve with an extremely high concentration of more than 2 mol dm -3 in acetone. In highly dehydrated solvents, the Nd complex should form a square-anti prism structure with two solvent molecules as ligands. With these facts in mind, acetone-d 6 was chosen as solvent for the optical measurement A solution of [Nd(HFA-D) 3] in acetone-d 6 showed emissions at 880, 1062 and 1345 nm when the system was excited at 585 nm (Fig. 2). The quantum yield of the emission was determined to be of the order of 10 -2 using an integral sphere [16]. No
Water contents of the system prepared by the present procedure should be less than 0.1301 wt%. Nd ~
0
emission 4F~
acetone-d6.
emission was observed in either non-deuterated acetone or deuterated acetone without careful dehydration of the system. The quantum yield of the system containing C - H or O - H bond vibrations was estimated to be of the order of 10 -5 . Spectral peaks at 880, 1062, and 1345 nm were attributed to the f - f transitions 4[73/2 "->'119/2, 4F3/2 "->4Ill/2 , and 4F3/2 ---'4Ii3/2, respectively. The emission band at 1062 nm is important because it is applicable for laser emission. Interestingly, it had a symmetrical and narrow structure as is observed for the emission of Nd:YAG. It is said that the emission from Nd 3+ ion in glass matrices shows a relatively wide emission with multiple components due to the various circumstances of Nd 3 + ion in amorphous solid matrices. The band shape with highly symmetrical
O-H C.H O-D C-D C-O C-C C-F (~150) (2950) (2f~O) (2100) (1650) (IS$O) (1200)
-2000
E
Vffil
-4000 -6000 -8000 -10000
- I " 41l~Z
t
v
V=2
4111/2 Vffi3
-12000
-15000 Fig. 3. Schematic presentation of the vibrational energy levels in organic media and matching the electronic energy gap of Nd 3+ ion.
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
structure and relatively narrow width suggests that the environment of Nd 3+ should be uniform in the solution. The energy gap law indicates that the harmonic matching number of the vibrational quanta should be larger than that of O - H and C - H vibrations (3450 and 2950 cm-~). HFA ligands of [Nd(HFA-D) 3] molecules have vibrations of C - F at 1200 cm - t , C - D at 2100 cm - l , and O - D at 2500 cm -~. Fig. 3 shows the harmonic match number of C-D, C-O, C-C, and C - F vibrations. In the present system, the harmonic number was modified by deuteration from v = 2 ( C - H and O - H ) to v = 3 ( C - D and O-D); vibrational harmonics equal in energy to or greater than the gap in Nd 3÷ are capable of quenching the most effectively according to the energy gap theory [5]. As the vibrational quanta, v, increase, the Franck-Condon factor, F, decreases. The factor F should be estimated quantitatively and an approximation using the undistorted osillator model is adopted. In the model, F is given by F ( E ) = exp( - y ) y v / v ! ,
(1)
where y is the displacement factor defined by y = ½k(~-~°)2/hto (~ and T° are equilibrium positions in vibration states) [4]. If y is assumed to be 1, F is obtained as 0.18, 0.061, and 0.0031 at v = 2, 3, and 5, respectively. This estimation suggests that eliminating the proton from the ligand significantly decreases the probability of radiationless transitions. Absence of C - H and O - H vibrations should make it possible to observe the emission from Nd 3+ in an organic solvent. Luminescence of the [Nd(HFA-D) 3] complex in organic solvents was observed in the present system for the first time. Fig. 4 shows the absorption spectrum of 0.4 M [Nd(HFA-D) 3] in acetone-d6. Absorption bands were observed at 513, 524, 582, 680, 745, 800, and 865 nm and attributed to the Nd 3+ transitions 4 2 4 ....~ 2 of I9/2(ground state) "~ G l l / 2 , I9/2 G9/2, 41 __~ 2 [.~ 41 _...>417 ~9~/2 ~ 7 / 2 , , ~9/2 A ~ 9 / 2 ,
4'1 ...~4 F ~9/2 ~7/2,
41 "9/2
~ F s / 2 , and ~I9/2 ~ F 3 / 2 , respectively. The absorption spectrum is quite comparable with that of Nd:YAG. The broad absorption band at 320-350 nm is explained as due to HFA-D ligand molecules. One of the important characteristics of the spectrum is a splitting of the peak at 865 nm as shown in the inset of Fig. 4. The band peak at 865 nm is
11
100
b. 2O 87O 0
~
300
I
I
400
500
600
I
I
I
700
800
900
1~
Wavelength / nm
Fig. 4. Transmittancespeca'a of 0.4 tool dm-3 [Nd(HFA-D)3]in acetone-d6. Insert: 419/2-"~4F3/2(ground state to emitting level) transmission spectra. attributed to the t r a n s i t i o n 419/2 ---~4F3/2 . T h u s , the observed splitting indicates that the degenerate 4F3/2 level, i.e. the emitting level, splits slightly into two levels. The emitting levels are split under the influence of crystal or ligand fields. The Hamiltonian / t of Nd 3+ can be expressed as = H0 +/qso + 12e,,t,
(2)
where H0 is the Hamiltonian of the original state for LS quantization, ~so is the Hamiltonian for spinorbit interaction, Vext is the electron potential energy for crystal or figand fields. Generally, V~ is much smaller than H 0 and /~so, and l,~ext depends on the ligand distance, R, field anisotropy Rk(cos Oij) (Legendre polynomials), and field charge, Q, which increases under increasing influence of crystal or ligand fields. Slight splitting of the 4F3/2 degenerate levels should be induced by the crystal and ligand field in the system. Table 1 summarizes the splitting energy of Nd 3+ ion in [Nd(HFA-D)3] , Nd:YAG, and LHG-8. The splitting energy of [Nd(HFA-D) 3] 4F3/2 states of 82.3 cm- 1 is similar to that of Nd:YAG (84 cm- i ) [20]. On the other hand, LHG-8, Nd3+-doped glasses, have a splitting (I01.9 cm -1) larger than Table 1 Splitting energy of emitting level in Nd 3+ Compounds
4F3/ 2 (rim)
Splitting energy (cm- i )
Nd3 + YAG [20] LHG-8 [Nd(HFA-D3)]
868.1 865.5 864.8
84 101.9 82.3
874.5 873.2 871.2
12
Y. Hasegawa et al. / Chemical Physics Letters 248 (1996) 8-12
that of [Nd(HFA-D) 3] and Nd:YAG. The splittings should reflect l~cxt in these matrices. LHG-8 is an amorphous glass, and does not have a symmetrical structure. Thus, the Vext parameters of LHG-8 should be larger. On the other hand, Nd:YAG is a pure crystal with the garnet structure. The splitting width observed for the absorption of Nd:YAG can be explained in terms o f the characteristics of the crystal structure of the matrix [20]. The comparable splitting width observed for the absorption of [Nd(HFA-D) 3] complex in liquid media with that of Nd:YAG suggests that the field due to the oxygens of the ligand molecules should be similar to that of Nd:YAG. Nd 3÷ ion in [Nd(HFA-D) 3] molecules should be coordinated to eight 0 2- atoms which are in three ligand molecules and two solvent molecules, and lead to strong crystal-field perturbation similar to that in Nd:YAG. The strong 4 f - 4 f transition intensity of Nd:YAG has been attributed to that kind of strong perturbation. Thus, an increase of the probability of 4 f - 4 f transitions is also expected for the present system after achievement of further optimization for the emission process. The extemal field also depends on the field charge Q as well as on R and Rk(cos Oij) which are determined by the molecular structure and charge site symmetry, as discussed above. The electron density on O atoms of [Nd(HFA-D) 3] should decrease and reduce the magnitude of the field charge Q. Vext should be affected by introducing trifluoromethyl groups into acetylacetonato ligands. The geometrical a n d / o r electronic structure of ligand molecules can also contribute to change the emission intensity. To elaborate the process, further investigation of the structures should be necessary. In conclusion, [Nd(HFA-D) 3] was prepared by chelation of Nd 3÷ ion with deuterated hexafluoroacetylacetone in acetone-d6. Effective emission of the Nd ion of [Nd(HFA-D) 3] in organic solution was observed for the first time. These characteristics of the emission and absorption spectra are similar to
that of Nd:YAG, suggesting that the electronic structure of the Nd 3÷ ion in the present system should be comparable with that of Nd 3+ ion in Nd:YAG. Higher quantum yield luminescence of Nd 3÷ complexes in liquid media should be achieved by designing molecules to control the Nd 3+ coordination structure.
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