Characteristic emission of β-diketonato Nd3+ complexes dressed with perfluoroalkyl groups in DMSO-d6

20 September 1996

ELSEVIER

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 260 (1996) 173- 177

Characteristic emission of 13-diketonato Nd 3÷ complexes dressed with perfluoroalkyl groups in DMSO-d 6 Yasuchika Hasegawa a, Kei Murakoshi a, Yuji Wada a, Jeong-Ho Kim h, Nobuaki Nakashima b, Tatsuhiko Yamanaka b, Shozo Yanagida a

a Materials and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan b Institute of Laser Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan Received 17 May 1996; in final form 28 June 1996

Abstract Tris(bis(perfluorooctanoyl)methanato)neodymium(III), Nd(POM-D) 3, was successfully synthesized to induce efficient emission in DMSO-d6. While the emission quantum yield of the tris(hexafluoroacetylacetonato)neodymium(III), Nd(HFAD)3, system decreased as the concentration increased, the Nd(POM-D) 3 system gave almost constant quantum yield (3.2%) in DMSO-d6 ranging in concentration from 0.01 to 0.07 M. The enhanced and characteristic emission of the Nd(POM-D) 3 system suggests that dipole-dipole energy transfer via cross-relaxation through collisions between Nd 3+ complex molecules should be suppressed in the fluid system by use of the bulky diketonato complex of Nd 3÷.

1. Introduction The development of a fluid Nd 3÷ laser medium is needed to obtain scale tunable high-power laser systems [1-8]. However, Nd 3÷ in fluid systems does not normally emit a photon because of their rapid deactivation of the excited states via vibrational excitation of the surrounding fluid molecules [9,10] or via an energy transfer between neighboring Nd 3+ complexes. The excitation migration process between neighboring Nd 3+ is also unavoidable, and enhances the cross-relaxation process through excitation migration although the process does not quench emission by itself. We recently revealed that the suppression of vibrational excitation by the use of deuterated hexafluoroacetylacetone as a ligand with low frequency vibrational mode ( C - D and C - F bonds) led to emission from Nd 3÷ in an organic solution [11]. Furthermore, we found that the use of

DMSO-d 6 as a solvent gave the most enhanced emission from tris(hexafluoroacetylacetonato) neodymium (III) (Nd(HFA-D)3; 1 in Scheme 1) because D20 with a high frequency vibrational mode ( O - D bond) should be replaced by DMSO-d 6 [12]. In order to induce more efficient emission of Nd 3+ in DMSO-d 6 solution, we have subsequently sought to suppress energy transfer via cross-relaxation. We have now succeeded in inducing the more enhanced emission from Nd 3+ in DMSO-d 6 by using deuterated bis-(perfluorooctanoylmethane) (POM) as a novel and bulky ligand (2 in Scheme 1). 2. Experimental 2.1. Materials

POM was synthesized by reacting perfluorooctanoyl chloride with t-butyl acetate and iso-pro-

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Y. Hasegawa et al.// Chemical Physics Letters 260 (1996) 173-177

174

for C17H2F3002: H, 0.25; C, 25.27; F, 70.6. Found: H, 0.88; C, 25.09; F, 71.5. Nd(POM)3 was prepared by reacting POM with neodymium nitrate hexahydrate in ethanol, and purified by recrystallization in a mixture of methanol and chloroform. Blue purple plates. 1H NMR (acetoned6//TMS): 3.66 (br) ppm; 19F NMR (methanol-d4/ hexafluorobenzene (-162.2 ppm)): -80.0 (t, 6F), - 114.77 (t, 2F), - 120.15 (m, 12F), - 121.45 (br, 4F), -125.0 (t, 4F) ppm; 13C NMR (acetone-d6/ TMS): 95.25 (S), 105-120 (m), 159.47 (t), 173.58 (s); IR (KBr): 1769, 1656 (C=O), 1230, 1204, 1146 (C-F) cm-~. Calcd for C51H3F9006NdI: H, 0.27; C, 23.54. Found: H, 0.21; C, 20.47.

1

~ C ~/CF2CF2CF2CF2CF2CF2CF3 \ \

CF2CF2CF2CF2CF2CF2CFa / 3 Scheme 1.

2.2. Optical measurements

pylmagnesium bromide, followed by decarboxylation catalyzed by p-toluenesulfonic acid. White crystals. IH NMR (acetone-d6/TMS): 4.41 (br) ppm; 19F NMR (methanol-dJhexafluorobenzene (-162.2 ppm)): -79.4 (t, 6F), -117.2 (t, 2F), -119.7 (t, 2F), 120.0, - 120.7, - 120.9 (m, 16F), - 124.3 (t, 4 F ) p p m ; 13C NMR (acetone-d6/ TMS): 95.25 (S), 105-120 (m), 159.47 (t), 173.58 (s); IR (KBr): 1769, 1656 (C=O), 1230, 1204, 1146 (C-F)cm -~. Calcd

I¸

2Pu2

•

The deuterated complex, Nd(POM-D)3, was obtained by treatment with excess methanol-d4 under vacuum (0.1 Pa). The previously reported complex Nd(HFA-D)3.2D20 [11] and commercially available Nd3+-doped phosphate laser glass (LHG-8) [13], were used for comparison. DMSO-d6 solutions were manipulated under vacuum (0.1 Pa). Absorption 400

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f

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700 80O

,0

20

0

'0

20

0

'0

Absorption coefficient / M'~.em "m

8 411S/2

m

m

41~

--

"-

4111/2

m

419/2

Is

4

~' ~

i

i

~f

0

0 Radiative decay

i d

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0

$0

0

SO

aoo~

Emission intensity / a.u.

Fig. 1. Absorption and emission spectra of 0.1 M Nd(HFA-D) 3 in DMSO-d 6 (a, d), 0.1 M Nd(POM-D) 3 in DMSO-d 6 (b, e) and 0.6 w% LHG-8 (c, f).

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Y. Hasegawa et al./ Chemical Physics Letters 260 (1996) 173-177

spectra were obtained using a monochromator (JASCO, PS-HI50) and a photomultiplier (Hamamatsu Photonics, C659B). A G e detector was used to detect emission in the near-infrared region. The quantum yields were determined by standard procedures using an integral sphere (diameter 9 cm) [14] and a cell (optical path length 1 mm).

4

1

3. Results and discussion Fig. 1 shows the absorption and emission spectra of DMSO-d 6 solutions (0.05 M) of Nd(HFA-D) 3 • 2D20 and Nd(POM-D) 3 determined under the same conditions and those of LHG-8 glass. Absorption is attributed to excitation from the ground state, 419/2 , to various excited-state levels. These fluid solution systems gave a larger extinction coefficient than the glass system of LHG-8 (0.6 w%). Emission spectra were obtained by excitation at 585 nm (419/2-4G7/2). Emission peaks at 885, 1054 and 1325 nm were assigned to the f - f transitions of 4F3/2 ">419/2, 4F3/2 ~ 4111/2 and 4F3/2 "-~ 4113/2, respectively. The full width at half maximum (fwhm) of the 4F3/2 4111/2 transition was 25 nm, which is sharper than that of LHG-8 (35 nm) glass and that of Nd:YAG (30 nm). The shape of the emission band, which was more symmetrical and relatively narrower than that of LHG-8, suggests that the environment of Nd 3÷ in fluid solution systems is at least as uniform as that in solid Nd 3+ laser systems. To confirm the effect of ligands which should preve,it the collision between Nd 3+ and Nd 3÷, we measured the dependence of the emission efficiency on the concentration of Nd(POM-D) 3 in DMSO-d 6 and compared it with that in the Nd(HFA-D) 3 system (Fig. 2). The Nd(POM-D) 3 fluid system has a higher quantum efficiency than the Nd(HFA-D) 3 system over the entire range of concentrations tested. While the quantum yield of Nd(HFA-D) 3 decreased as the concentration increased, that of the Nd(POMD) 3 system was almost constant (3.2%) between 0.01 and 0.07 M. Interestingly, the quantum yield suddenly decreased to 1,5% at 0.1 M, and levelled off again at higher concentrations. Diffusion of Nd a+ complexes in a fluid system induces collisions between the molecules, leading to energy transfer via cross-relaxation and excitation

0

l

0.01

i

i

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I i il

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I

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i

0.1 Concentration / M

Fig. 2. Concentration dependence of quantum yields of Nd(HFA)3 (Q) and Nd(POM) 3 (C)).

migration. This quenching process is based on the dipole-dipole interaction of the molecules [15,16]. The critical distance Rox for nonradiative dipoledipole energy transfer is given by, 3c

R6x = 8,tr4n2arftr~m( A)o'~bS( A)

dA,

(1)

where n, A r and fo"D em(A)o"xabs ( X ) d a are the neodymium-ion density, total spontaneous radiative emission rate and resonance integral, respectively [17]. Typical values of the critical distances for the excitation migration (4F,~/2-419/2) and cross-relaxation (4F3/2-4115/2 and 4115/2--4~9/2) process in the system of NdS+-doped phosphate laser glass were estimated to be 11.14 and 4.07 A, respectively [17]. As reported in our previous paper, the emission decay ~- (-- WereI ) and quantum yield ~em of 0.1 M Nd(HFA-D) 3 were 6.3 /xs and 1.1%, respectively. The total spontaneous radiative emission rate, A r ( = r/emWem),of the present system is 1746 s -1 and is comparable with that of the Nd-doped glass (A r --2400 s-l). The absorption coefficient of Nd(HFAD) 3 (4.0 8 / M -1 cm-~) was also quite close to the value of Nd3+-doped glass (5.0 e / M -1 cm - I ) at 870 nm (419/2 --~ 41;:3/2 transition). According to Eq. (1), the critical distance of Nd(HFA-D) 3 was estimated as 11.7 A. In fluid systems, the diffusion processes dominate the energy transfer quenching. In fluid systems, the energy transfer quenching is concerned with collision by diffusion. Based on a pair probability method for the statistics of resonance

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Y. Hasegawa et aL / Chemical Physics Letters 260 (1996) 173-177

energy transfer, the contribution of the diffusion process to the quenching was estimated by GSsele et al. [18]. In the estimation, the displacement factor Z 0 is given by Z 0 = 2(rAO) 2

,

(2)

where rAD, Ot and D are the collision distance, efficiency of resonance energy transfer and diffusion coefficient of the molecule, respectively. Since a is given by r 6 / r , Z o is rewritten as

Z° =

r0(r0/rAD) 2 2 DCD-~r '

(3)

where r 0 and r are the critical distance for resonance energy transfer and lifetime, respectively. In the present system, the values of r o, rAO, D and r were 11.73 ,~, 14 ~k (0.1 mol dm-3), 6 /zs and 1.82 × 10 - l ° m 2 s -~, respectively. When these values are substituted in Eq. (3), Z 0 is calculated to be 1.3 × 10 -2. The result of Z 0 << 1 supports that the diffusion dominates energy transfer quenching in the present system. The size of the Nd(HFA-D) 3 molecule is about 6 .~ which is smaller than the critical distance (11.7 ,~) of resonance energy transfer. The Nd(HFA-D) 3 complex should undergo collisional quenching with other Nd(HFA-D) 3 complexes even under 0.1 M (the average distance between molecules in liquid media is estimated to be 14 A [19]). In a fluid system, the yield of energy transfer quenching should increase as concentration becomes high. The observed concentration dependence of the emission quantum yields of Nd(HFA-D) 3 should be explained by an energy transfer process via collision. The effect of the concentration on the quenching in the system of Nd(HFA-D) 3 was more apparent than that in Nd 3+doped glass [17]. This fact supports the validity of our estimation of the critical distance in the present system. In the collision process, higher order multipole and exchange interactions could become important as the distance between Nd 3÷ ions becomes close to the ionic radii and chemical bond length [20]. In the present system, surrounding ligand molecules of the Nd 3+ complex should keep a certain distance (--~ 6 ,~) between the ions at the collisions. The rate

of excitation transfer via quadmpole interactions was estimated to be less than that of the dipole-dipole interaction by at least one order of magnitude [20]. Thus, we mainly consider the contribution of dipole-dipole interactions in this Letter. In the Nd(POM-D) 3 system, the absorption coefficient was comparable with that of Nd(HFA-D) 3, and the quantum yield of Nd(POM-D) 3 was about 3 times larger than that of Nd(HFA-D) 3. From these results, the critical distance is estimated to be about 6 ,~. The diameter of Nd(POM-D) 3 estimated by MM2 is 25 ,~ (CAChe ver. 6.10), suggesting that the collision distance is much greater than 6 A. Thus, the distance should be enough to prevent energy transfer. The observed concentration-independent behavior of the quantum yield proved that energy transfer has been suppressed in the fluid system by use of the bulky diketonato complex of Nd 3+. A sudden decrease in the emission quantum yield of Nd(POM) 3 is observed at concentrations greater than 0.07 M. Interestingly, a dynamic light-scattering spectrophotometer (DLS) measurement showed an increase in the average size (102 ,~) at a concentration of 0.4 M. The lipophobic perfluoroalkyl groups of Nd(POM) 3 should contribute to aggregation in DMSO, as is the case with surfactant molecules at CMC in water. We propose that Nd(POM) 3 aggregates at a high concentration and that this aggregation enhances energy transfer via an excitation hopping process, which leads to a decrease in the quantum yield of the emission. The high-order multipole and exchange interaction are also expected to contribute to the energy transfer process in such an aggregated system. These contributions of this process to the quenching are not clear at the present time. The ratio of the emission intensity at 885 nm to that at 1053 nm reflects the probability of excitation hopping between complexes (excitation migration) [17]. When the emission at 885 nm is absorbed by a neighboring Nd 3+ complex, the ratio changes because the re-absorbed light contributes to emission at 1054 nm. This ratio in the Nd(POM-D) 3 system was greater than that in the Nd(HFA-D) 3 system (Fig. 1). This indicates that excitation migration in a fluid solution is suppressed more in Nd(POM-D) 3 than in Nd(HFA-D) 3. Interestingly, this ratio in Nd(POMD) 3 became smaller at a high concentration of 0.3

Y. Hasegawa et al. / Chemical Physics Letters 260 (1996) 173-177

M, suggesting energy migration in the aggregation system. In conclusion, the quantum yield for Nd 3÷ emission can be increased by reducing the energy transfer via vibrational excitation and cross-relaxation processes in fluid systems. In other words, the dipoledipole energy transfer was successfully suppressed in the fluid system by use of the bulky [3-diketonato complex of Nd 3+.

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