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Low-vibrational luminescent polymers including tris(bis-perfluoromethane and ethanesulfonylaminate)neodymium(III) with 8 coordinated DMSO-d6
Journal of Luminescence 101 (2003) 235–242
Low-vibrational luminescent polymers including tris(bisperfluoromethane and ethanesulfonylaminate)neodymium(III) with 8 coordinated DMSO-d6 Yasuchika Hasegawaa, Kensaku Sogabeb, Yuji Wadaa, Shozo Yanagidaa,* a
Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan b New Development Division, New Japan Chemical Co., Ltd., Yoshijima 13, Yagura-cho, Fushimi-ku, Kyoto 612, Japan Received 31 May 2002; received in revised form 8 August 2002; accepted 8 August 2002
Abstract Novel Nd(III) complexes, tris(bis-perfluoromethane and ethanesulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide) were designed and synthesized to apply to luminescent materials in plastic optical fiber. The IR and the near-IR spectra analyses has been done to estimate the radiationless transition via vibrational excitation of the Nd(III) complexes. [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] in polyhexafluoroisopropylmethacrylate gave a emission quantum yield of 1.3% and 1.6%, which was the largest in luminescent Nd(III) polymers. A research field for telecommunication network using 1.3 mm light would be achieved by luminescent Nd(III) materials encapsulated lowvibrational coordination sites. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Neodymium; Optical fiber; Energy transfer
1. Introduction The Nd(III)-doped luminescent polymer systems are expected to open up some new optical fields, especially, telecommunication network and novel laser materials, because amplified spontaneous emission of Nd(III) ions using their ideal four level f–f transition leads to development of amplification of signal in plastic fiber and highpower laser materials [1–3]. Various researchers *Corresponding author. Tel.: +81-6-879-7924; fax: +81-6877-9067. E-mail address: [email protected] (S. Yanagida).
have recently succeeded in observing emission in certain organic Nd(III) systems [4–13]. However, the organic Nd(III) systems do not suit for plastic optical fiber (POF) in near-IR telecommunication, since general organic compounds have strong absorption in the near-IR area. The absorption of plastics is caused by the vibrational harmonics of C–H or O–H bonds. In order to overcome the intrinsic problem, the graded-index (GI)-type fiber using low-vibrational fluorinated polymer has been developed by Koike and co-workers [14]. Furthermore, they have succeeded in fabrication of the Nd(III)-doped GI plastic optical fiber (GI POF) [15]. The development of low-vibrational POF with Nd(III) is directly linked to the next
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 4 4 0 - 4
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generation optics. A key goal of POF system for near-IR telecommunication is to dope a Nd(III) complex with keeping efficient luminescence. Here, we designed and successfully synthesized the novel tris(bis-perfluoromethane and ethanesulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide), [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] (Fig. 1a and b). The IR and the near-IR spectra analyses has been done to estimate the radiationless transition via vibrational excitation of the Nd(III) complexes. The vibrational excitation of Nd(III) is discussed through the comparison of the calculated Frank–Condon (F–C) factors of [Nd(pms)3(DMSO-d6)8] with that of corresponding tris(bis-perfluoromethanesulfonylaminato)neodymium(III)octa(water), [Nd(pms)3(H2O)8] (Fig. 1c) [16,17]. In this paper, we also report about fabrication of luminescent polymers (PMMA: polymetylmethacrylate and P-FipMA: polyperfluoroisopropylmethacrylate)
combined with [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8]. The theoretical design from F–C factor calculation and their emission properties would lead to expansion of POF in near-IR field.
2. Experimental 2.1. Apparatus Thermal analysis was carried out using a SHIMADZU thermogravimetric analyzer TGA50 (rate: 51C/min). Infrared spectra for identification of synthesized materials were obtained with a Perkin-Elmer FT-IR 2000 spectrometer. Near-IR spectra were measured by use of JASCO V-570 UV/VIS/NIR spectrophotometer. Elemental analyses were performed by a Perkin-Elmer 240C. The 19F NMR data were obtained on a JEOL EX-270 spectrometer. The 19F NMR chemical shifts were determined using external hexafluorobenzene ðd ¼ 162:00Þ as an external standard. 2.2. Materials Neodymium acetate monohydrate (99.9%) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione were purchased from Wako Pure Chemical Industries Ltd. Bis-trifluoromethanesulfonimido (pms) was obtained from Fluka Chemika. Bis-perfluoroethanesulfonylamido lithium salt (pes-Li) was donated from Sumitomo 3 M. Methanol-d4 (CD3OD, 99.8%) and DMSO-d6 (CD3SOCD3, 99.8%) were obtained from Aldrich Chemical Company Inc. All other chemicals were reagent grade and were used as received. Tris(hexafluoroacetylacetonato)neodymium(III) dehydrates, [Nd(hfa)3(H2O)2] was prepared according to the procedure described in the literature [4,5].
Fig. 1. Chemical structures of the Nd(III) complexes and polymers investigated in this study: (a) n=1:Nd(pms)3(DMSO-d6)8; (b) n=2:Nd(pes)3(DMSO-d6)8 and (c) Nd(hfa-D)3(DMSO-d6)6.
2.3. Preparation of tris(bisperfluoroalkylsulfonylaminato) neodymium(III) octahydrates ([Nd(pms)3(H2O)8]) Bis-trifluoromethanesulfonimide (0.95 g, 3.4 mmol) was dissolved in 20 ml of distilled water
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under stirring at 01C. Neodymium oxide (0.2 g, 0.8 mmol) was added to the above solution and the mixture was stirred for 24 h. The solution was evaporated and dissolved using methanol. The solution was filtered, and then concentrated. The resulting blue–purple solid were dried under vacuum (about 5 Torr) at 801C for 24 h, giving moisture-sensitive blue–purple solid: yield 98%, elimination point of water of crystallization: 130–2061C, weight decrease=15% (8 molecules of water), decomposition point: 3371C. 19 F-NMR (CD3OD) d ¼ 77:37 (s, CF3), IR (KBr): 3400, 1650, 1460, 1430, 1200, 1140 and 1050 cm1. Anal. calcd for C15H7O8F18Nd: C, 6.38; H, 1.43; N, 3.72%, found: C, 6.76; H, 1.49; N, 3.90%. Since [Nd(pms)3(H2O)8] has a characteristics of the moisture sensitive, the number of coordinated water is varied by the condition of the dry process. [Nd(pms)3(H2O)2] was obtained under high vacuum condition (103 Torr) at 1501C. [Nd(pes)3(H2O)n] were prepared from bis-perfluoroethanesulfonimide lithium salts by the same manner.
2.4. Preparation of tris(bisperfluoroalkylsulfonylaminato) neodymium(III) octa-deuterated dimethylsulfoxide ([Nd(pms)3(DMSO-d6)8]) [[Nd(pms)3(H2O)8] (0.5 g, 3.4 mmol) was dissolved in 3 ml of deuterated DMSO at room temperature. Then solvent (DMSO-d6) was evaporated. Solid was washed by chloroform several times. The resulting blue–purple powder were obtained and were dried under vacuum (about 5 Torr) at 801C for 24 h, giving blue–purple crystal; yield: 95%. The elimination point of DMSO-d6 of crystallization: 146–3651C, decomposition point: 3651C, 19F-NMR (CD3COCD3) d ¼ 77:28 (s, CF3), IR (KBr): 2250 (C–D), 1460, 1430, 1200, 1140, 1050, 1030, 990 cm1. Anal. calcd for C22O20S14F18D48F18Nd:C, 15.94; N, 2.53%, found: C, 15.62; N, 2.59%. [Nd(pes)3(DMSOd6)8] were obtained by the same manner.
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2.5. Preparation of polymers with Nd(III) complexes The [Nd(hfa-D)3(H2O)2] was prepared by exchange reaction via keto-enol tautomerism in CD3OD for 6 h under vacuum as described in our previous paper [5]. After evaporation of the solution under vacuum (B103 Torr) to dryness, the resulting [Nd(hfa-D)3(H2O)2] (0.05 M) was dissolved in a 1 ml mixture of purified anhydrous hexafluoroisopropylmethacrylate (FiPMA) or methylmethacrylate (MMA), AIBN and DMSOd6 in a Pyrex tube (Composition of polymerization; Nd(III) or Eu(III), 0.7 w%; AIBN, 0.05 w%; DMSO-d6, 6.6 w%.) In the fabrication of PFiPMA systems, MMA (10 w%) was added in FiPMA in order to solve AIBN and Nd complexes in FiPMA. The Pyrex tube was closed under 103 Torr, and thermostated at 601C for polymerization of FiPMA or MMA. For comparison, PMMA polymers incorporating DMSO-d6 and [Nd(hfa-D)3(H2O)2] or [Nd(hfa)3(H2O)2] were similarly prepared. P-FipMA and PMMA polymer with [Nd(pms)3(DMSO-d6)8] or [Nd(pes)3(DMSO-d6)8] were also prepared by polymerization of FiPMA and MMA with AIBN and [Nd(pms)3(DMSO-d6)8] or [Nd(pes)3(DMSOd6)8], respectively. A transparent PFiPMA polymer matrix was obtained by polymerization of FiPMA with 10% MMA. The resulting polymer films gave the transmittance of more than 70% at 4 mm thickness. Interestingly, the P-FiPMA matrices were found to give more transparent films with 90% transmittance.
2.6. Optical measurements Transmittance spectra were obtained using a monochromator (JASCO, PS-H150) and a photomultiplier (Hamamatsu Photonics, C659B). Scan rate and time constants are 180 nm/min and 100 mV/s, respectively. Emission spectra were measured by a Ge detector, and then were normalized so that the emission intensities of peak emission wavelength (lp : 4F3/2–4I11/2) were set for value of 100. The spectra presented here were corrected for detector sensitivity and lamp
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intensity. Quantum yields were measured as reported previously [5].
3. Results and discussion 3.1. IR and near-IR spectra The Nd(pms)3(DMSO-d6)8 was obtained by replacement of coordinated waters with DMSOd6. Elemental analysis revealed the presence of Nd(pms)3 with eight coordinated DMSO-d6 molecules, [Nd(pms)3(DMSO-d6)8]. The IR spectra of [Nd(pms)3(H2O)8] and [Nd(pms)3(DMSO-d6)8] in a KBr pellet were in Fig. 2. Absorption intensity of spectra shown in Fig. 2 is normalized to the peak intensity of the C–F stretch (1145 cm1) in order to compare the relative intensity of the bands in wave number region among 3800–1000 cm1. In Fig. 2b, absorption at 3450 and 1600 cm1 were attributed to O–H stretch and bending mode of a coordinated water, respectively. In the IR spectrum of Nd(pms)3(DMSO-d6)8 (Fig. 2a), we can find the presence of a small amount of C–D bending (2250 cm1) and absence at 3450 and 1600 cm1. These results support that the Nd(pms)3(DMSOd6)8 has no coordinated waters, and agrees well with the data of elemental analysis. The band in the range 1000–1060 cm1 was assigned to the
absorption of SQO stretch mode of a coordinated DMSO-d6. The spectrum of Nd(pms)3 complexes has an absorption around 1450 cm1 owing to the coupled asymmetric stretch of the sulfonylaminate ligands. In fact, we can find that the shape of the band of OQSQO asymmetric stretch of pms ligands in Fig. 2a is a little different from the corresponding one in Fig. 2b (the band at 1430 cm1 in Fig. 2a decreased with increasing the band at 1460 cm1). The asymmetric stretch by the smaller OQSQO angle leads to a higher wave number [21]. The OQSQO angle of [Nd(pms)3(DMSO-d6)8] would be smaller than that of [Nd(pms)3(H2O)8] because of sterical hindrance by incorporated DMSO-d6 molecules. To sum up, IR analysis of [Nd(pms)3(DMSO-d6)8] indicates the absence of water molecules due to the presence of strongly coordinated DMSO-d6. Energy gap of Nd(III) was originated from the electron transition of 4F3/2-4I15/2. In order to determine the energy gap, we measured the nearIR absorption spectra of [Nd(pms)3(DMSO-d6)8] in DMSO-d6 (Fig. 3). Absorption bands of 870 nm (11494 cm1, fwhm=144 cm1) was assigned to the electron transition of 4I9/2-4F3/2. The absorption band of 4I9/2-4I11/2 splits upper level (1600 nm; 6250 cm1, fwhm=155 cm1) and lower level (1700 nm; 5882 cm1, fwhm=150 cm1) because of the field energy (stark splitting). The
Fig. 2. Normalized IR spectra of the Nd(III) complexes in KBr: (a) Nd(pms)3(H2O)8, (b) Nd(pms)3(DMSO-d6)8. The values of the vibrational quanta and Frank–Condon factor calculated by the energy gap of Nd(III) are shown on the IR spectra.
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energy level of 4I9/2-4I11/2 were estimated to be more than 1700 nm. The correlation between 4F3/2 and 4I9/2 originates from small off-set case because of narrow line of the electron transition [22]. Taking into consideration small off-set, the energy gap between 4F3/2 and 4I15/2 was estimated more than 5612 cm1 (Fig. 4). 3.2. Calculation of F–C factor The matching energies of the vibrational harmonics of each bond becomes larger than energy gap, 5612 cm1 (Fig. 2). The vibrational quanta (harmonic number) of O–H (3400 cm1), C–D (2250 cm1), OQSQO (1430–1460 cm1), C–F (1145 cm1) and SQO (1000–1060 cm1) were found to be 2 harmonics (6800 cm1), 3 harmonics (6750 cm1), 5 harmonics (7150–7300 cm1), 5 harmonics (5725 cm1) and 6 harmonics (6000– 6360 cm1), respectively. The vibrational transition probability is proportional to the F–C factor, i.e., overlap integrals between energy gap and vibrational harmonics. In terms of the energy gap theory, the rate constant for radiationless
Fig. 3. Near-IR spectra of the Nd(III): (a) LHG-8 (Nd(III)doped phosphite glass). (b) Differential spectra between Nd(pms)3(DMSO-d6)8 in DMSO-d6 and DMSO-d6 solvent. (c)The emission spectrum of the Nd(pms)3(DMSO-d6)8 in PFipMA. (880 nm: 4F3/2-4I9/2, 1054 nm: 4F3/2-4I11/2, 1325 nm: 4 F3/2-4I13/2). Excitation at 585 nm. Anti-symmetrical shape of 4 F3/2-4I13/2 transition is originated by stark splitting of the crystal field. Fig. 4. The postulated radiationless transition process via vibrational excitation. There is no storks shift between ground state and excited state because of small off-set of rare-earth ion.
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transition W RadiationlessTransition is given by 2
WRadiationlessTransition ¼ ð2pr=hÞJ F ;
ð1Þ
excitation much more than those of [Nd(hfaD)3(DMSO-d6)6] [5].
R
where r is the density of state, Jð¼ fi Qfj dtÞ is the coupling constant between the electronic wave functions due to nuclear motion, F is the F–C factor [23]. The factor F should be estimated quantitatively when approximation using undistorted oscillator model is adopted. In the model, F is given by F ðEÞ ¼ expðgÞgv =v!
ð2Þ
g ¼ 12 kðq% q% 0 Þ2 =_o;
ð3Þ
where q and q0 are equilibrium positions in vibrational initial and final states, respectively [24]. When g is assumed to be 1, F–C factors are obtained as 0.18, 0.061, 0.015, 0.0031and 0.0005 at n¼ 2; 3, 4, 5 and 6, respectively. In order to analyze the radiationless transition via vibrational excitation on IR spectra, we showed the energy lines which the energy gap (5612 cm1) divided by harmonics (2, 3, y, 6) equals (Fig. 4). The absorption of C–D bonds would not affect on the radiationless transition via vibrational excitation because of much smaller absorption coefficient. The F–C factor of OQSQO asymmetric vibration calculated to be 0.0031, which is smaller than that of corresponding CQO bonds (1650 cm1; F–C factor=0.015). This IR spectra analysis suggests that those coordination sites of [Nd(pms)3(DMSO-d6)8] suppress the vibrational
3.3. Emission quantum yields of [Nd(pms)3(DMSO-d6)8] in polymer matrices Since the geometrical factors of the Nd(III) complexes are hard to affect on the emission quantum yield, we can discuss the radiationless transition by the use of emission quantum yield without sterical factor. We carried out the measurements of the emission quantum yields of the Nd(III) complexes to analyze the suppression of radiationless transition via vibrational excitation. The emission quantum yields of Nd(III) complexes thus obtained in various polymer matrices are shown in Table 1. The typical emission spectra of the polymer systems containing Nd(III) were obtained by excitation at 585 nm (4I9/2-2G7/2) (Fig. 3). In particular, Nd(hfa-D)3 led to the observation of weak emission in PMMA matrix. The addition of DMSO-d6 enhances the emission to some extent. The coordinated DMSO-d6 molecules lead to suppression of radiationless transition via vibrational excitation of Nd(III) ion in organic media [18–20]. However, the quantum yield of the system of Nd(hfa-D)3/ DMSO-d6/PMMA was much smaller than that in liquid DMSO-d6 medium. This is explained as due to radiationless transition via vibrational
Table 1 Emission quantum yields of Nd(III) complexes in polymer matices Nd(III) complexa
Matrixb
Transmittance (%)c
Emission quantum yield (%)
Nd(hfa-H)3(H2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(pms)3(DMSO-d6)6 Nd(pms)3(DMSO-d6)6 Nd(pes)3(DMSO-d6)6 Nd(pes)3(DMSO-d6)6
PMMA PMMA PMMA/DMSO-d6 P-FipMA/DMSO-d6 DMSO-d6 PMMA/DMSO-d6 P-FipMA/DMSO-d6 PMMA/DMSO-d6 P-FipMA/DMSO-d6
79 79 83 90 90 90 80 90 80
o0.01 0.170.1 0.570.1 0.770.1 1.170.1 0.970.2 1.370.1 0.970.1 1.670.1
Nd(III) complex was excited at 585 nm (4I9/2-2G7/2). Composition of polymerization: Nd(III) or Eu(III), 0.7 w%; AIBN, 0.05 w%; DMSO-d6, 6.6 w%. c Transmittances of Nd(III) complexes were measured at 1060 nm. Optical lengths are 4 mm. a
b
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excitation of C–H bonds in the PMMA matrix. Generally, the quenching methanism follows an inverse power 6 law; however, we propose that Nd(III) complexes may be affected on the C–H bonds of the matrices. Because the vibrational energy transfer might be enhanced by the weak vibration of the fragments of polymer such as phonon quenching in solid state. Weight percentage of H atoms in the resulting polymer system is 8.0%. In order to minimize the vibrational excitation of C–H bond in the polymer matrix, we attempted to polymerize FiPMA, i.e., C–F bond containing methacrylate, in the presence of Nd(hfa-D)3 and DMSO-d6. On the other hand, the system of Nd(hfa-D)3/DMSO-d6/PFiPMA (the weight percentage of H atoms is 2.5%) was improved to give an effectively luminescent organic polymer of Nd(III). Near-IR spectrum of Nd(hfa-D)3(DMSO-d6)n in P-FiPMA was compared with that in PMMA (Fig. 1b). Absorption bands at 800 and 870 nm and those at 1170 and 1350–1500 nm were assigned to the f–f transitions of Nd(III) and vibrational harmonics of C–H bonds, respectively. The vibrational harmonic bands of Nd(hfa-D)3(DMSO-d6)n in P-FiPMA are smaller than that in the corresponding PMMA because of a smaller amount of H atoms. This result suggests that P-FiPMA matrix can suppress to appreciate the extent of the radiationless transition via vibrational excitation of the polymer matrix. In contrast, we observed higher efficient luminescence of [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] in polymer matrices. When compared with the emission of the [Nd(hfaD)3(DMSO-d6)n ]/PMMA system, the use of pms or pes ligands enhanced quantum yield of Nd(III) in PMMA. Especially, the system of [Nd(pes)3(DMSO-d6)8] in P-FiPMA gave a quantum yield of 1.6%, which was the largest in luminescent Nd(III) polymers. This can be explained as due to suppression of radiationless vibrational excitation on the coordination sites of Nd(III) by use of low-vibrational perfluoroalkylsulfonylaminate ligands. These results are agrees with the estimation of F–C factors using IR and near-IR spectra. The emission quantum yield of [Nd(pes)3(DMSO-d6)8] in PFipMA ðF ¼ 1:6%Þ
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was larger than that of [Nd(pms)3(DMSO-d6)8] ðF ¼ 1:3%Þ: In the prepared polymers, the weight percentage of H atoms of [Nd(pes)3(DMSO-d6)8] (molecular weight=1964) and [Nd(pes)3(DMSOd6)8] (molecular weight=1664) in PFipMA were caluculated to be 2.0 and 2.1 w%, respectively. The small deference would affect the suppression of vibrational excitation, effectively.
4. Conclusions we synthesized the tris(bis-perfluoroalkylsulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide), and discussed the comparison of the F–C factors of [Nd(pms)3(DMSO-d6)8] with [Nd(pms)3(H2O)8] and [Nd(hfa)3(H2O)2]. The F–C factor which is calculated by the use of IR and near-IR spectra can lead to the estimation of the emission quantum yield of Nd(III) complexes, significantly. The emission quantum yields of [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSOd6)8] in PFipMA were larger than those of corresponding [Nd(hfa)3(DMSO-d6)n ] in polymers and in DMSO-d6. The industrial combination of [Nd(pms)3(DMSO-d6)8] with the graded-indextype fiber using low-vibrational fluorinated polymer would achieve the next-generation POF. A research field for telecommunication network using 1.3 mm light would be achieved by luminescent Nd(III) materials encapsulated low-vibrational coordination sites.
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Low-vibrational luminescent polymers including tris(bisperfluoromethane and ethanesulfonylaminate)neodymium(III) with 8 coordinated DMSO-d6 Yasuchika Hasegawaa, Kensaku Sogabeb, Yuji Wadaa, Shozo Yanagidaa,* a
Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan b New Development Division, New Japan Chemical Co., Ltd., Yoshijima 13, Yagura-cho, Fushimi-ku, Kyoto 612, Japan Received 31 May 2002; received in revised form 8 August 2002; accepted 8 August 2002
Abstract Novel Nd(III) complexes, tris(bis-perfluoromethane and ethanesulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide) were designed and synthesized to apply to luminescent materials in plastic optical fiber. The IR and the near-IR spectra analyses has been done to estimate the radiationless transition via vibrational excitation of the Nd(III) complexes. [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] in polyhexafluoroisopropylmethacrylate gave a emission quantum yield of 1.3% and 1.6%, which was the largest in luminescent Nd(III) polymers. A research field for telecommunication network using 1.3 mm light would be achieved by luminescent Nd(III) materials encapsulated lowvibrational coordination sites. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Neodymium; Optical fiber; Energy transfer
1. Introduction The Nd(III)-doped luminescent polymer systems are expected to open up some new optical fields, especially, telecommunication network and novel laser materials, because amplified spontaneous emission of Nd(III) ions using their ideal four level f–f transition leads to development of amplification of signal in plastic fiber and highpower laser materials [1–3]. Various researchers *Corresponding author. Tel.: +81-6-879-7924; fax: +81-6877-9067. E-mail address: [email protected] (S. Yanagida).
have recently succeeded in observing emission in certain organic Nd(III) systems [4–13]. However, the organic Nd(III) systems do not suit for plastic optical fiber (POF) in near-IR telecommunication, since general organic compounds have strong absorption in the near-IR area. The absorption of plastics is caused by the vibrational harmonics of C–H or O–H bonds. In order to overcome the intrinsic problem, the graded-index (GI)-type fiber using low-vibrational fluorinated polymer has been developed by Koike and co-workers [14]. Furthermore, they have succeeded in fabrication of the Nd(III)-doped GI plastic optical fiber (GI POF) [15]. The development of low-vibrational POF with Nd(III) is directly linked to the next
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 4 4 0 - 4
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generation optics. A key goal of POF system for near-IR telecommunication is to dope a Nd(III) complex with keeping efficient luminescence. Here, we designed and successfully synthesized the novel tris(bis-perfluoromethane and ethanesulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide), [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] (Fig. 1a and b). The IR and the near-IR spectra analyses has been done to estimate the radiationless transition via vibrational excitation of the Nd(III) complexes. The vibrational excitation of Nd(III) is discussed through the comparison of the calculated Frank–Condon (F–C) factors of [Nd(pms)3(DMSO-d6)8] with that of corresponding tris(bis-perfluoromethanesulfonylaminato)neodymium(III)octa(water), [Nd(pms)3(H2O)8] (Fig. 1c) [16,17]. In this paper, we also report about fabrication of luminescent polymers (PMMA: polymetylmethacrylate and P-FipMA: polyperfluoroisopropylmethacrylate)
combined with [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8]. The theoretical design from F–C factor calculation and their emission properties would lead to expansion of POF in near-IR field.
2. Experimental 2.1. Apparatus Thermal analysis was carried out using a SHIMADZU thermogravimetric analyzer TGA50 (rate: 51C/min). Infrared spectra for identification of synthesized materials were obtained with a Perkin-Elmer FT-IR 2000 spectrometer. Near-IR spectra were measured by use of JASCO V-570 UV/VIS/NIR spectrophotometer. Elemental analyses were performed by a Perkin-Elmer 240C. The 19F NMR data were obtained on a JEOL EX-270 spectrometer. The 19F NMR chemical shifts were determined using external hexafluorobenzene ðd ¼ 162:00Þ as an external standard. 2.2. Materials Neodymium acetate monohydrate (99.9%) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione were purchased from Wako Pure Chemical Industries Ltd. Bis-trifluoromethanesulfonimido (pms) was obtained from Fluka Chemika. Bis-perfluoroethanesulfonylamido lithium salt (pes-Li) was donated from Sumitomo 3 M. Methanol-d4 (CD3OD, 99.8%) and DMSO-d6 (CD3SOCD3, 99.8%) were obtained from Aldrich Chemical Company Inc. All other chemicals were reagent grade and were used as received. Tris(hexafluoroacetylacetonato)neodymium(III) dehydrates, [Nd(hfa)3(H2O)2] was prepared according to the procedure described in the literature [4,5].
Fig. 1. Chemical structures of the Nd(III) complexes and polymers investigated in this study: (a) n=1:Nd(pms)3(DMSO-d6)8; (b) n=2:Nd(pes)3(DMSO-d6)8 and (c) Nd(hfa-D)3(DMSO-d6)6.
2.3. Preparation of tris(bisperfluoroalkylsulfonylaminato) neodymium(III) octahydrates ([Nd(pms)3(H2O)8]) Bis-trifluoromethanesulfonimide (0.95 g, 3.4 mmol) was dissolved in 20 ml of distilled water
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under stirring at 01C. Neodymium oxide (0.2 g, 0.8 mmol) was added to the above solution and the mixture was stirred for 24 h. The solution was evaporated and dissolved using methanol. The solution was filtered, and then concentrated. The resulting blue–purple solid were dried under vacuum (about 5 Torr) at 801C for 24 h, giving moisture-sensitive blue–purple solid: yield 98%, elimination point of water of crystallization: 130–2061C, weight decrease=15% (8 molecules of water), decomposition point: 3371C. 19 F-NMR (CD3OD) d ¼ 77:37 (s, CF3), IR (KBr): 3400, 1650, 1460, 1430, 1200, 1140 and 1050 cm1. Anal. calcd for C15H7O8F18Nd: C, 6.38; H, 1.43; N, 3.72%, found: C, 6.76; H, 1.49; N, 3.90%. Since [Nd(pms)3(H2O)8] has a characteristics of the moisture sensitive, the number of coordinated water is varied by the condition of the dry process. [Nd(pms)3(H2O)2] was obtained under high vacuum condition (103 Torr) at 1501C. [Nd(pes)3(H2O)n] were prepared from bis-perfluoroethanesulfonimide lithium salts by the same manner.
2.4. Preparation of tris(bisperfluoroalkylsulfonylaminato) neodymium(III) octa-deuterated dimethylsulfoxide ([Nd(pms)3(DMSO-d6)8]) [[Nd(pms)3(H2O)8] (0.5 g, 3.4 mmol) was dissolved in 3 ml of deuterated DMSO at room temperature. Then solvent (DMSO-d6) was evaporated. Solid was washed by chloroform several times. The resulting blue–purple powder were obtained and were dried under vacuum (about 5 Torr) at 801C for 24 h, giving blue–purple crystal; yield: 95%. The elimination point of DMSO-d6 of crystallization: 146–3651C, decomposition point: 3651C, 19F-NMR (CD3COCD3) d ¼ 77:28 (s, CF3), IR (KBr): 2250 (C–D), 1460, 1430, 1200, 1140, 1050, 1030, 990 cm1. Anal. calcd for C22O20S14F18D48F18Nd:C, 15.94; N, 2.53%, found: C, 15.62; N, 2.59%. [Nd(pes)3(DMSOd6)8] were obtained by the same manner.
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2.5. Preparation of polymers with Nd(III) complexes The [Nd(hfa-D)3(H2O)2] was prepared by exchange reaction via keto-enol tautomerism in CD3OD for 6 h under vacuum as described in our previous paper [5]. After evaporation of the solution under vacuum (B103 Torr) to dryness, the resulting [Nd(hfa-D)3(H2O)2] (0.05 M) was dissolved in a 1 ml mixture of purified anhydrous hexafluoroisopropylmethacrylate (FiPMA) or methylmethacrylate (MMA), AIBN and DMSOd6 in a Pyrex tube (Composition of polymerization; Nd(III) or Eu(III), 0.7 w%; AIBN, 0.05 w%; DMSO-d6, 6.6 w%.) In the fabrication of PFiPMA systems, MMA (10 w%) was added in FiPMA in order to solve AIBN and Nd complexes in FiPMA. The Pyrex tube was closed under 103 Torr, and thermostated at 601C for polymerization of FiPMA or MMA. For comparison, PMMA polymers incorporating DMSO-d6 and [Nd(hfa-D)3(H2O)2] or [Nd(hfa)3(H2O)2] were similarly prepared. P-FipMA and PMMA polymer with [Nd(pms)3(DMSO-d6)8] or [Nd(pes)3(DMSO-d6)8] were also prepared by polymerization of FiPMA and MMA with AIBN and [Nd(pms)3(DMSO-d6)8] or [Nd(pes)3(DMSOd6)8], respectively. A transparent PFiPMA polymer matrix was obtained by polymerization of FiPMA with 10% MMA. The resulting polymer films gave the transmittance of more than 70% at 4 mm thickness. Interestingly, the P-FiPMA matrices were found to give more transparent films with 90% transmittance.
2.6. Optical measurements Transmittance spectra were obtained using a monochromator (JASCO, PS-H150) and a photomultiplier (Hamamatsu Photonics, C659B). Scan rate and time constants are 180 nm/min and 100 mV/s, respectively. Emission spectra were measured by a Ge detector, and then were normalized so that the emission intensities of peak emission wavelength (lp : 4F3/2–4I11/2) were set for value of 100. The spectra presented here were corrected for detector sensitivity and lamp
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intensity. Quantum yields were measured as reported previously [5].
3. Results and discussion 3.1. IR and near-IR spectra The Nd(pms)3(DMSO-d6)8 was obtained by replacement of coordinated waters with DMSOd6. Elemental analysis revealed the presence of Nd(pms)3 with eight coordinated DMSO-d6 molecules, [Nd(pms)3(DMSO-d6)8]. The IR spectra of [Nd(pms)3(H2O)8] and [Nd(pms)3(DMSO-d6)8] in a KBr pellet were in Fig. 2. Absorption intensity of spectra shown in Fig. 2 is normalized to the peak intensity of the C–F stretch (1145 cm1) in order to compare the relative intensity of the bands in wave number region among 3800–1000 cm1. In Fig. 2b, absorption at 3450 and 1600 cm1 were attributed to O–H stretch and bending mode of a coordinated water, respectively. In the IR spectrum of Nd(pms)3(DMSO-d6)8 (Fig. 2a), we can find the presence of a small amount of C–D bending (2250 cm1) and absence at 3450 and 1600 cm1. These results support that the Nd(pms)3(DMSOd6)8 has no coordinated waters, and agrees well with the data of elemental analysis. The band in the range 1000–1060 cm1 was assigned to the
absorption of SQO stretch mode of a coordinated DMSO-d6. The spectrum of Nd(pms)3 complexes has an absorption around 1450 cm1 owing to the coupled asymmetric stretch of the sulfonylaminate ligands. In fact, we can find that the shape of the band of OQSQO asymmetric stretch of pms ligands in Fig. 2a is a little different from the corresponding one in Fig. 2b (the band at 1430 cm1 in Fig. 2a decreased with increasing the band at 1460 cm1). The asymmetric stretch by the smaller OQSQO angle leads to a higher wave number [21]. The OQSQO angle of [Nd(pms)3(DMSO-d6)8] would be smaller than that of [Nd(pms)3(H2O)8] because of sterical hindrance by incorporated DMSO-d6 molecules. To sum up, IR analysis of [Nd(pms)3(DMSO-d6)8] indicates the absence of water molecules due to the presence of strongly coordinated DMSO-d6. Energy gap of Nd(III) was originated from the electron transition of 4F3/2-4I15/2. In order to determine the energy gap, we measured the nearIR absorption spectra of [Nd(pms)3(DMSO-d6)8] in DMSO-d6 (Fig. 3). Absorption bands of 870 nm (11494 cm1, fwhm=144 cm1) was assigned to the electron transition of 4I9/2-4F3/2. The absorption band of 4I9/2-4I11/2 splits upper level (1600 nm; 6250 cm1, fwhm=155 cm1) and lower level (1700 nm; 5882 cm1, fwhm=150 cm1) because of the field energy (stark splitting). The
Fig. 2. Normalized IR spectra of the Nd(III) complexes in KBr: (a) Nd(pms)3(H2O)8, (b) Nd(pms)3(DMSO-d6)8. The values of the vibrational quanta and Frank–Condon factor calculated by the energy gap of Nd(III) are shown on the IR spectra.
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energy level of 4I9/2-4I11/2 were estimated to be more than 1700 nm. The correlation between 4F3/2 and 4I9/2 originates from small off-set case because of narrow line of the electron transition [22]. Taking into consideration small off-set, the energy gap between 4F3/2 and 4I15/2 was estimated more than 5612 cm1 (Fig. 4). 3.2. Calculation of F–C factor The matching energies of the vibrational harmonics of each bond becomes larger than energy gap, 5612 cm1 (Fig. 2). The vibrational quanta (harmonic number) of O–H (3400 cm1), C–D (2250 cm1), OQSQO (1430–1460 cm1), C–F (1145 cm1) and SQO (1000–1060 cm1) were found to be 2 harmonics (6800 cm1), 3 harmonics (6750 cm1), 5 harmonics (7150–7300 cm1), 5 harmonics (5725 cm1) and 6 harmonics (6000– 6360 cm1), respectively. The vibrational transition probability is proportional to the F–C factor, i.e., overlap integrals between energy gap and vibrational harmonics. In terms of the energy gap theory, the rate constant for radiationless
Fig. 3. Near-IR spectra of the Nd(III): (a) LHG-8 (Nd(III)doped phosphite glass). (b) Differential spectra between Nd(pms)3(DMSO-d6)8 in DMSO-d6 and DMSO-d6 solvent. (c)The emission spectrum of the Nd(pms)3(DMSO-d6)8 in PFipMA. (880 nm: 4F3/2-4I9/2, 1054 nm: 4F3/2-4I11/2, 1325 nm: 4 F3/2-4I13/2). Excitation at 585 nm. Anti-symmetrical shape of 4 F3/2-4I13/2 transition is originated by stark splitting of the crystal field. Fig. 4. The postulated radiationless transition process via vibrational excitation. There is no storks shift between ground state and excited state because of small off-set of rare-earth ion.
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transition W RadiationlessTransition is given by 2
WRadiationlessTransition ¼ ð2pr=hÞJ F ;
ð1Þ
excitation much more than those of [Nd(hfaD)3(DMSO-d6)6] [5].
R
where r is the density of state, Jð¼ fi Qfj dtÞ is the coupling constant between the electronic wave functions due to nuclear motion, F is the F–C factor [23]. The factor F should be estimated quantitatively when approximation using undistorted oscillator model is adopted. In the model, F is given by F ðEÞ ¼ expðgÞgv =v!
ð2Þ
g ¼ 12 kðq% q% 0 Þ2 =_o;
ð3Þ
where q and q0 are equilibrium positions in vibrational initial and final states, respectively [24]. When g is assumed to be 1, F–C factors are obtained as 0.18, 0.061, 0.015, 0.0031and 0.0005 at n¼ 2; 3, 4, 5 and 6, respectively. In order to analyze the radiationless transition via vibrational excitation on IR spectra, we showed the energy lines which the energy gap (5612 cm1) divided by harmonics (2, 3, y, 6) equals (Fig. 4). The absorption of C–D bonds would not affect on the radiationless transition via vibrational excitation because of much smaller absorption coefficient. The F–C factor of OQSQO asymmetric vibration calculated to be 0.0031, which is smaller than that of corresponding CQO bonds (1650 cm1; F–C factor=0.015). This IR spectra analysis suggests that those coordination sites of [Nd(pms)3(DMSO-d6)8] suppress the vibrational
3.3. Emission quantum yields of [Nd(pms)3(DMSO-d6)8] in polymer matrices Since the geometrical factors of the Nd(III) complexes are hard to affect on the emission quantum yield, we can discuss the radiationless transition by the use of emission quantum yield without sterical factor. We carried out the measurements of the emission quantum yields of the Nd(III) complexes to analyze the suppression of radiationless transition via vibrational excitation. The emission quantum yields of Nd(III) complexes thus obtained in various polymer matrices are shown in Table 1. The typical emission spectra of the polymer systems containing Nd(III) were obtained by excitation at 585 nm (4I9/2-2G7/2) (Fig. 3). In particular, Nd(hfa-D)3 led to the observation of weak emission in PMMA matrix. The addition of DMSO-d6 enhances the emission to some extent. The coordinated DMSO-d6 molecules lead to suppression of radiationless transition via vibrational excitation of Nd(III) ion in organic media [18–20]. However, the quantum yield of the system of Nd(hfa-D)3/ DMSO-d6/PMMA was much smaller than that in liquid DMSO-d6 medium. This is explained as due to radiationless transition via vibrational
Table 1 Emission quantum yields of Nd(III) complexes in polymer matices Nd(III) complexa
Matrixb
Transmittance (%)c
Emission quantum yield (%)
Nd(hfa-H)3(H2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(hfa-D)3(D2O)2 Nd(pms)3(DMSO-d6)6 Nd(pms)3(DMSO-d6)6 Nd(pes)3(DMSO-d6)6 Nd(pes)3(DMSO-d6)6
PMMA PMMA PMMA/DMSO-d6 P-FipMA/DMSO-d6 DMSO-d6 PMMA/DMSO-d6 P-FipMA/DMSO-d6 PMMA/DMSO-d6 P-FipMA/DMSO-d6
79 79 83 90 90 90 80 90 80
o0.01 0.170.1 0.570.1 0.770.1 1.170.1 0.970.2 1.370.1 0.970.1 1.670.1
Nd(III) complex was excited at 585 nm (4I9/2-2G7/2). Composition of polymerization: Nd(III) or Eu(III), 0.7 w%; AIBN, 0.05 w%; DMSO-d6, 6.6 w%. c Transmittances of Nd(III) complexes were measured at 1060 nm. Optical lengths are 4 mm. a
b
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excitation of C–H bonds in the PMMA matrix. Generally, the quenching methanism follows an inverse power 6 law; however, we propose that Nd(III) complexes may be affected on the C–H bonds of the matrices. Because the vibrational energy transfer might be enhanced by the weak vibration of the fragments of polymer such as phonon quenching in solid state. Weight percentage of H atoms in the resulting polymer system is 8.0%. In order to minimize the vibrational excitation of C–H bond in the polymer matrix, we attempted to polymerize FiPMA, i.e., C–F bond containing methacrylate, in the presence of Nd(hfa-D)3 and DMSO-d6. On the other hand, the system of Nd(hfa-D)3/DMSO-d6/PFiPMA (the weight percentage of H atoms is 2.5%) was improved to give an effectively luminescent organic polymer of Nd(III). Near-IR spectrum of Nd(hfa-D)3(DMSO-d6)n in P-FiPMA was compared with that in PMMA (Fig. 1b). Absorption bands at 800 and 870 nm and those at 1170 and 1350–1500 nm were assigned to the f–f transitions of Nd(III) and vibrational harmonics of C–H bonds, respectively. The vibrational harmonic bands of Nd(hfa-D)3(DMSO-d6)n in P-FiPMA are smaller than that in the corresponding PMMA because of a smaller amount of H atoms. This result suggests that P-FiPMA matrix can suppress to appreciate the extent of the radiationless transition via vibrational excitation of the polymer matrix. In contrast, we observed higher efficient luminescence of [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSO-d6)8] in polymer matrices. When compared with the emission of the [Nd(hfaD)3(DMSO-d6)n ]/PMMA system, the use of pms or pes ligands enhanced quantum yield of Nd(III) in PMMA. Especially, the system of [Nd(pes)3(DMSO-d6)8] in P-FiPMA gave a quantum yield of 1.6%, which was the largest in luminescent Nd(III) polymers. This can be explained as due to suppression of radiationless vibrational excitation on the coordination sites of Nd(III) by use of low-vibrational perfluoroalkylsulfonylaminate ligands. These results are agrees with the estimation of F–C factors using IR and near-IR spectra. The emission quantum yield of [Nd(pes)3(DMSO-d6)8] in PFipMA ðF ¼ 1:6%Þ
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was larger than that of [Nd(pms)3(DMSO-d6)8] ðF ¼ 1:3%Þ: In the prepared polymers, the weight percentage of H atoms of [Nd(pes)3(DMSO-d6)8] (molecular weight=1964) and [Nd(pes)3(DMSOd6)8] (molecular weight=1664) in PFipMA were caluculated to be 2.0 and 2.1 w%, respectively. The small deference would affect the suppression of vibrational excitation, effectively.
4. Conclusions we synthesized the tris(bis-perfluoroalkylsulfonylaminato)neodymium(III)octa(deuterated dimethylsulfoxide), and discussed the comparison of the F–C factors of [Nd(pms)3(DMSO-d6)8] with [Nd(pms)3(H2O)8] and [Nd(hfa)3(H2O)2]. The F–C factor which is calculated by the use of IR and near-IR spectra can lead to the estimation of the emission quantum yield of Nd(III) complexes, significantly. The emission quantum yields of [Nd(pms)3(DMSO-d6)8] and [Nd(pes)3(DMSOd6)8] in PFipMA were larger than those of corresponding [Nd(hfa)3(DMSO-d6)n ] in polymers and in DMSO-d6. The industrial combination of [Nd(pms)3(DMSO-d6)8] with the graded-indextype fiber using low-vibrational fluorinated polymer would achieve the next-generation POF. A research field for telecommunication network using 1.3 mm light would be achieved by luminescent Nd(III) materials encapsulated low-vibrational coordination sites.
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