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Luminescence of divalent europium complexes with crown ethers, cryptands and polyethylene glycols
Journal
of the Less-Common
Metals,
93 (1983)81-87
81
LUMINESCENCE OF DIVALENT EUROPIUM COMPLEXES WITH CROWN ETHERS, CRYPTANDS AND POLYETHYLENE GLYCOLS*
GIN-YA ADACHI, JIRO SHIOKAWA
KENJI SORITA,
Department of Applied Osaka 565 (Japan)
Chemistry,
KENJI KAWATA,
Faculty of Engineering,
KATSUHIKO
TOMOKIYO
Osaka University,
and
Yamadaoka,
Suita,
(Received January 24,1983)
Summary The luminescence properties of the divalent europium complexes with crown ethers, cryptands and polyethylene glycols were studied. The observed intensity of luminescence is generally reported. The most intense luminescence is produced by a methanolic solution of the 15crown-5 complex with europium(II), the intensity of which is 600 times greater than that of a solution of EuCl, in methanol with the same europium(I1) concentration, The enhancement of the emission intensity as a result of the formation of complexes with crown ethers can be attributed to “insulation” of the europium(I1) ion from close approach of the solvent molecules which would produce radiationless energy losses.
1. Introduction The emission properties of crown complexes of the divalent lanthanides have received little attention until recently. Adachi et al. [l] and Donohue [2] have reported that the most intense fluorescence is produced by a methanolic solution of the l&crown-5 complex of europium(II), the intensity of which is 600 times greater than that of a methanolic solution of EuCl, with the same europium(I1) concentration Cl]. Sabbatini et al. [S] have reported the absorption and emission properties of europium(I1) cryptate in aqueous solution. The results obtained in our laboratory on the measurement of the fluorescence properties of the divalent europium complexes of crown ethers, cryptands and polyethylene glycols in methanolic solutions are reported in this paper. *Paper presented at the Sixteenth Rare Earth Research Conference, The Florida State University, Tallahassee, FL, U.S.A., April l&21,1983. 0022-5088/83/$3.00
(0 Elsevier Sequoia/Printed
in The Netherlands
82
2. Experimental
details
The solutions of the divalent europium polyether complexes used in this study were obtained by dissolving anhydrous EuCl, and polyethers into anhydrous degassed methanol. The crown ethers 12-crown-4 (l), Vi-crown-5 (2), 18-crown-6 (3), dicyclohexyl-18-crown-6 (DC-18-crown-6) (4), dicyclohexyl-24crown-8 (DC-24-crown-8) (5), monobenzo-15-crown-5 (6) and monobenzo-18crown-6 (7), the cryptands cryp.2.1., cryp.2.2., cryp.2.1.1., cryp.2.2.1. and cryp.2.2.2. (the nomenclature of the cryptands is defined in ref. 4), and the acyclic polyethers triethylene glycol (E03), tetraethylene glycol (E04), pentaethylene glycol (E05), hexaethylene glycol (E06) and heptaethylene glycol (E07) were used as ligands. EuCl, was prepared from Eu,O, (purity, 99.99%) which was mixed with an excess of NH,Cl and was then heated at 700 “C for 2 h in a hydrogen atmosphere (purity, 99.99%). The emission intensity was calculated from the area under the emission band envelope. The fluorescence quantum yields were determined by comparison with a solution of 1 x 10m5 M quinine bisulphate in 0.1 N H,SO, (@ = 0.55) [5]. The excitation wavelength for the standard quinine solution was 365 nm. The fluorescence lifetimes of the complexes were measured using a pulsed nitrogen laser (337 nm; pulse half-width, 28 ns). The decay of the intensity was recorded on a Sony-Tektronics 485 oscilloscope and the data were processed photographically. The proton magnetic resonance (PMR) spectra were obtained using a JEOL LMN-PS-100 spectrometer (100 MHz; sweep width, 1080Hz). All the measurements were performed on freshly prepared samples. 3. Results and discussion 3.1. Luminescence and absorption spectra of the complexes Complexes of europium(I1) with crown ethers, cryptands and polyethylene
200
300
400 Wdvelct-cJth
500
600
( nm )
Fig. 1. Emission and absorption spectra for EuCl, (---)
and E&2
complex (-)
in methanol
83
glycols are easily formed in methanol. Typical luminescence and absorption spectra of methanolic solutions of the Eu”-2 (1: 3) complex and EuCI, are shown in Fig. 1. The spectroscopic data for other complexes are summarized in Table 1. The absorption peaks in the near-UV region are attributable to the 4f + 5d transitions of europium(I1) ions [S]. The emission spectra consist of broad bands which are due to transitions between the ‘S,,, state and the crystal field components of the (4f)6(5d) configuration if the lowest (4f)6(5d) component is situated below the 6P, state of the (4f)’ configuration as is the case here. Complex formation usually shifts the emission peak towards a shorter wavelength with a concomitant increase in the intensity compared with the solvated species.
3.2. Stoichiometry of the complexes Gradual addition of 2 to a methanolic solution of EuCI, resulted in an increase in emission intensity, which reached a maximum after approximately 3 equivalents of the ligand had been added. Job’s method of continuous variations was used to identify the stoichiometry of the complex and it was found that 1: 3 complexes were formed (Fig. 2). The ratio 1: 3 is unusual in the crown complexes which have been reported to date. Therefore a PMR measurement was undertaken to clarify the structure of the complexes. Crown complexes of strontium(I1) were used instead of the corresponding europium(I1) complexes because the latter would shift the signals. Non-coordinating 2 exhibits only one singlet signal at 3.64 ppm on its PMR spectrum. The downfield shifts of the crown ether’s protons by the complexation of strontium(I1) were measured in methanol-d,. A plot of the chemical shifts uersus the molar ratio (ratio of strontium(I1) concentration to crown ether concentration) displayed little detail and reached a limiting value at a metal-toligand ratio of 0.33, in good agreement with the value obtained from luminescence results. Low temperature measurements, however, revealed that the singlet signal split into two peaks. The areas under the peaks were roughly in the ratio l(low field): 2(high field). The low field signal became broader at temperatures lower than - 80 “C indicating an appreciable restriction ofthe proton exchange. These results suggest that the Eu”-2 complex or the corresponding strontium(I1) complex is a 1:3 complex capped with one crown and enclosed by two crowns located far from the central ion. The diameter of europium(I1) (or strontium(I1)) is slightly larger than that of the cavity of 2 [7]. Thus the capping crown is located away from the equatorial plane of the central cation. One of the possible structures is illustrated in Fig. 3. The stoichiometries of europium(I1) complexes with other ligands were determined by the same method and the results are summarized in Table 1. The Eu’i-cryp.2.1. complex also exhibited an unusual ratio of 1: 7. This ratio is unexpected for this type of complex because the ligand is fairly bulky. A more feasible ratio would be 1:l or 1: 2 and the rest of the ligands would cluster around the centre complex.
-6 1:3(1:3)’ 4:3 (1:l)’ 4:3 1:l _c l:l(l:l)’
_c 3:2 -c 3:l 1:l (l:l)C _c _c 1:l 2:l 2:l
-
E&l Eu”-2 Eu”-3 Eu”4 E&5 E&6 Eu’-7
Eu”-cryp.Z.l. Eu”-cryp.Z.Z. Eu”-cryp.Z.l.l. Eu”kryp.2.2.1. Eu’-cryp.2.2.2. Eu-E03 E&E04 Eu”-EO5 Eu”-EO6 E&E07
EuCl,
glycols
solution
93(1:10),148(1:20)d 620 150 81 180 95 9.2 129 6.8 =: 1.0 29 270 2.2 5.0 4.2 47 66 1.0
Relative intensity of emission b
from an EuCl,-methanol
458 489
316 328
of the emission
460 469 445 445 468 467 442 464 467
319 333 327 333 317 329 327 320 321
250 260 249 254 254 250 251 252 252 252
with the intensity
355 345 375 355 312 361 375 356 380 375 380 353 400 360 365 355 375 375
428,488 d 433 446 439 443 417 448
322 318 325 318 321 321 328
252 (228) d 248 251 251 252 267 268
by comparison
(nm)
(nm)
(nm)
248
Excitation wavelength
with crown ethers, cryptands and polyethylene Maximum emission band
of europium(I1) complexes
Maximum absorption band
solutions
“The europium(I1) concentration is 4.0 x 10e3 M. ‘The relative intensities of the emission were determined (europium(I1) concentration, 4.0 x 10m3 M). ‘The ratios could not be established. %ee text. ‘The ratio obtained by PMR.
[Eu]: [ligand]
properties of methanolic
Complex a
Spectroscopic
TABLE 1
0
0.25
0.5
Fig. 2.. Job’s plots illustrating
1
0.75
the formation of the 1: 3 complex
Fig. 3. A model of the Eu”-2 complex.
3.3. Luminescence quantum yield Measurements of the luminescence quantum yields Qcomphave been carried out. The results are summarized in Table 2. The values of @ campwere markedly enhanced by complex formation, except for the cryp.2.1.1. complex. Figure 4 shows plots of Qcompas a function of the number of coordinating atoms around europium(I1). Possible quenching processes for the luminescence of the complexes are (1) an interaction of solvent TABLE 2 Luminescence
Complex
quantum yields @J_,,,,~, lifetimes 5 and rate constants
@camp
k ( :;06 s)
k * ( :;06 s)
(35) c (42) c (16)’ _d
0.12 1.7 0.59 0.67 0.099 0.13 (0.056) L (1.7)’ (0.29) c -d
3.2 4.5 5.7 9.3 3.4 7.0 (28)’ (21)’ (62)’ -d
(24)’ 200 _d _d _d
(0.58) c 0.47 -d -d -d
(41) c 4.5 _d -d -d
(43) c (59) c (20) c
(0.29) c (0.30) c (0.022) c
(23)’ (17)’ (50) E
T1
(4 E&l E&2 ELI”-3 Eu”4 ELI”-5 Eu”-6 E&7 Eu”Gryp.2.1. Eu”-cryp.Z.B. Eu”-cryp.Z.l.l. Eu”-cryp.2.2.1. Eu”kryp.2.2.2. E&E03 E&E04 ELI”-E05 Eu”~EO6 Eu”pE07 EuCl,
0.036 b 0.28 0.094 0.067 0.029 0.018 0.002 0.071 0.046 0.0005 0.014 0.093 0.00064 0.0014 0.0010 0.012 0.018 0.0004
a.Experimental errors within f 107,. bThe value for [Eu]: [ligand] = 1:lO. ‘Not very reliable. dNot measured.
300 160 160 100 290 140
86
molecules with the complexes, i.e. the collision of methanol molecules with the complexes, and (2) the intramolecular vibration of the complexes. Figure 4 demonstrates that the larger is the number of coordinating atoms around europium(I1) the higher are the quantum yields of the complexes, although there are some exceptions. This fact suggests that in higher complexes the central europium(I1) active sites are effectively protected by the ligands from collision with solvent molecules.
0.4 Q 0.3
0.2
QD
I
“L 0
5 10 15 No. of coordinating atoms
Fig. 4. Quantum yields and numbers of atoms coordinating to europium(I1) in the complexes: 11, Eu”-cryp.2.2.1.; 2, Eu”-2; 3, Eu”-3; 4, Eu”-4; 5, Eu”-5; 6, Eu”-6; 7, ELI”-7; 9, Eu”-cryp.2.2.; 12, Eu’-cryp.2.2.2.;
15, Eu”-E05;
16, Eu”-EOG; 17, Eu”-E07.
Some of the crown ethers with side chains or side rings, such as 5, exhibited a rather poor performance in view of their oxygen numbers. The ligand 5 has two cyclohexane rings in addition to an eight-membered oxygen ring. The side chains may cause intramolecular vibration which removes excitation energy from europium(I1) and thus wastes the energy non-radiatively. Polyethylene glycols did not give high quantum yields for europium(I1) luminescence. E05 forms a 1:l complex with europium(I1). However, its emission intensity is quite small compared with that of Eu”-EO6 or Eu”-E07. It appears that these ligands form floppy loose fitting complexes which are also apt to increase the intramolecular vibration. It is not surprising that cryp.2.1.1. does not enhance the emission because the cavity of the ligand is too small to accommodate a europium(I1) ion. The other ligand, cryp.2.1., also has a small ring although its europium(I1) complex shows a fairly high value of Qcomp(0.1) which suggests that the [Eu]: [ligand] ratio is 1:7. Therefore the bridging oxygen atom in cryp.2.1.1. must prevent complex formation with europium(I1). An aromatic ring attached to a crown ether also appears to lower acompfor a similar reason to that described above. Some sensitization of the complex as a result of the presence of the aromatic ring was expected, but such a phenomenon was not observed.
lifetime Measurements of the lifetimes Tfor the complexes have been carried out at room temperature and the results are listed in Table 2. The radiative and nonand z. radiative rate constants 12,1 and k, 1* were derived from the values of (Pcomp It should be noted that t and k, 1* are of the same order except in a few cases. Of the complexes with lifetimes of the same order, k2 1 for the Eu”-2 complex is exceptionally high. As discussed above, the structure of the complex in solution appears to be responsible for this behaviour. It is concluded that the enhancement of the emission intensity as a result of the formation of complexes with crown ethers is attributable to “insulation” of the central europium(I1) ion from the close approach of the solvent molecules which would produce radiationless energy losses. 3.4. Luminescence
Acknowledgments This work was supported by a Scientific Research Grant-in-Aid (56 470 059) from the Ministry of Education of Japan. We are indebted to Dr. M. Okahara, Dr. I. Ikeda, Dr. S. Yanagida, Dr. T. Shono, Dr. K. Kimura, Dr. T. Maeda, Dr. H. Mikawa, Dr. Y. Shirota, Dr. T. Tanaka and Dr. K. Tanaka for their help in the preparation of materials and for measurements of the PMR spectra and the lifetimes. The authors appreciate gifts of samples of several crown ethers from Mr. Iwao Kato of the Nippon Soda Co.
References 1 G. Adachi, K. Tomokiyo, K. Sorita and J. Shiokawa, J. Chem. Sot., Chem. Commun., (1980) 914. 2 T. Donohue, in G. J. McCarthy, J. J. Rhyne and H. B. Silber (eds.), The Rare Earths in Modern Science and Technology, Vol. 2, Plenum, New York, 1980, pp. 105-110. 3 N. Sabbatini, M. Ciano, S. Dellonte, A. Bonazzi and V. Balzani, Chem. Phys. Lett., 90 (1982) 265. 4 0. A. Gansow, A. R. Kausar, K. M. Triplett, M. J. Weaver and E. L. Yee, J. Am. Chem. Sot., 99 (1977) 7087. 5 J. N. Demas and G. A. Crosby, J. Phys. Chem., 75 (1971) 991. 6 D. S. McClure and Z. J. Kiss, J. Chem. Phys., 39(1963) 3251. 7 C. J. Pedersen and H. K. Frensdorff, Angew. Chem., Znt. Edn. Engl., ll(l972) 16.
of the Less-Common
Metals,
93 (1983)81-87
81
LUMINESCENCE OF DIVALENT EUROPIUM COMPLEXES WITH CROWN ETHERS, CRYPTANDS AND POLYETHYLENE GLYCOLS*
GIN-YA ADACHI, JIRO SHIOKAWA
KENJI SORITA,
Department of Applied Osaka 565 (Japan)
Chemistry,
KENJI KAWATA,
Faculty of Engineering,
KATSUHIKO
TOMOKIYO
Osaka University,
and
Yamadaoka,
Suita,
(Received January 24,1983)
Summary The luminescence properties of the divalent europium complexes with crown ethers, cryptands and polyethylene glycols were studied. The observed intensity of luminescence is generally reported. The most intense luminescence is produced by a methanolic solution of the 15crown-5 complex with europium(II), the intensity of which is 600 times greater than that of a solution of EuCl, in methanol with the same europium(I1) concentration, The enhancement of the emission intensity as a result of the formation of complexes with crown ethers can be attributed to “insulation” of the europium(I1) ion from close approach of the solvent molecules which would produce radiationless energy losses.
1. Introduction The emission properties of crown complexes of the divalent lanthanides have received little attention until recently. Adachi et al. [l] and Donohue [2] have reported that the most intense fluorescence is produced by a methanolic solution of the l&crown-5 complex of europium(II), the intensity of which is 600 times greater than that of a methanolic solution of EuCl, with the same europium(I1) concentration Cl]. Sabbatini et al. [S] have reported the absorption and emission properties of europium(I1) cryptate in aqueous solution. The results obtained in our laboratory on the measurement of the fluorescence properties of the divalent europium complexes of crown ethers, cryptands and polyethylene glycols in methanolic solutions are reported in this paper. *Paper presented at the Sixteenth Rare Earth Research Conference, The Florida State University, Tallahassee, FL, U.S.A., April l&21,1983. 0022-5088/83/$3.00
(0 Elsevier Sequoia/Printed
in The Netherlands
82
2. Experimental
details
The solutions of the divalent europium polyether complexes used in this study were obtained by dissolving anhydrous EuCl, and polyethers into anhydrous degassed methanol. The crown ethers 12-crown-4 (l), Vi-crown-5 (2), 18-crown-6 (3), dicyclohexyl-18-crown-6 (DC-18-crown-6) (4), dicyclohexyl-24crown-8 (DC-24-crown-8) (5), monobenzo-15-crown-5 (6) and monobenzo-18crown-6 (7), the cryptands cryp.2.1., cryp.2.2., cryp.2.1.1., cryp.2.2.1. and cryp.2.2.2. (the nomenclature of the cryptands is defined in ref. 4), and the acyclic polyethers triethylene glycol (E03), tetraethylene glycol (E04), pentaethylene glycol (E05), hexaethylene glycol (E06) and heptaethylene glycol (E07) were used as ligands. EuCl, was prepared from Eu,O, (purity, 99.99%) which was mixed with an excess of NH,Cl and was then heated at 700 “C for 2 h in a hydrogen atmosphere (purity, 99.99%). The emission intensity was calculated from the area under the emission band envelope. The fluorescence quantum yields were determined by comparison with a solution of 1 x 10m5 M quinine bisulphate in 0.1 N H,SO, (@ = 0.55) [5]. The excitation wavelength for the standard quinine solution was 365 nm. The fluorescence lifetimes of the complexes were measured using a pulsed nitrogen laser (337 nm; pulse half-width, 28 ns). The decay of the intensity was recorded on a Sony-Tektronics 485 oscilloscope and the data were processed photographically. The proton magnetic resonance (PMR) spectra were obtained using a JEOL LMN-PS-100 spectrometer (100 MHz; sweep width, 1080Hz). All the measurements were performed on freshly prepared samples. 3. Results and discussion 3.1. Luminescence and absorption spectra of the complexes Complexes of europium(I1) with crown ethers, cryptands and polyethylene
200
300
400 Wdvelct-cJth
500
600
( nm )
Fig. 1. Emission and absorption spectra for EuCl, (---)
and E&2
complex (-)
in methanol
83
glycols are easily formed in methanol. Typical luminescence and absorption spectra of methanolic solutions of the Eu”-2 (1: 3) complex and EuCI, are shown in Fig. 1. The spectroscopic data for other complexes are summarized in Table 1. The absorption peaks in the near-UV region are attributable to the 4f + 5d transitions of europium(I1) ions [S]. The emission spectra consist of broad bands which are due to transitions between the ‘S,,, state and the crystal field components of the (4f)6(5d) configuration if the lowest (4f)6(5d) component is situated below the 6P, state of the (4f)’ configuration as is the case here. Complex formation usually shifts the emission peak towards a shorter wavelength with a concomitant increase in the intensity compared with the solvated species.
3.2. Stoichiometry of the complexes Gradual addition of 2 to a methanolic solution of EuCI, resulted in an increase in emission intensity, which reached a maximum after approximately 3 equivalents of the ligand had been added. Job’s method of continuous variations was used to identify the stoichiometry of the complex and it was found that 1: 3 complexes were formed (Fig. 2). The ratio 1: 3 is unusual in the crown complexes which have been reported to date. Therefore a PMR measurement was undertaken to clarify the structure of the complexes. Crown complexes of strontium(I1) were used instead of the corresponding europium(I1) complexes because the latter would shift the signals. Non-coordinating 2 exhibits only one singlet signal at 3.64 ppm on its PMR spectrum. The downfield shifts of the crown ether’s protons by the complexation of strontium(I1) were measured in methanol-d,. A plot of the chemical shifts uersus the molar ratio (ratio of strontium(I1) concentration to crown ether concentration) displayed little detail and reached a limiting value at a metal-toligand ratio of 0.33, in good agreement with the value obtained from luminescence results. Low temperature measurements, however, revealed that the singlet signal split into two peaks. The areas under the peaks were roughly in the ratio l(low field): 2(high field). The low field signal became broader at temperatures lower than - 80 “C indicating an appreciable restriction ofthe proton exchange. These results suggest that the Eu”-2 complex or the corresponding strontium(I1) complex is a 1:3 complex capped with one crown and enclosed by two crowns located far from the central ion. The diameter of europium(I1) (or strontium(I1)) is slightly larger than that of the cavity of 2 [7]. Thus the capping crown is located away from the equatorial plane of the central cation. One of the possible structures is illustrated in Fig. 3. The stoichiometries of europium(I1) complexes with other ligands were determined by the same method and the results are summarized in Table 1. The Eu’i-cryp.2.1. complex also exhibited an unusual ratio of 1: 7. This ratio is unexpected for this type of complex because the ligand is fairly bulky. A more feasible ratio would be 1:l or 1: 2 and the rest of the ligands would cluster around the centre complex.
-6 1:3(1:3)’ 4:3 (1:l)’ 4:3 1:l _c l:l(l:l)’
_c 3:2 -c 3:l 1:l (l:l)C _c _c 1:l 2:l 2:l
-
E&l Eu”-2 Eu”-3 Eu”4 E&5 E&6 Eu’-7
Eu”-cryp.Z.l. Eu”-cryp.Z.Z. Eu”-cryp.Z.l.l. Eu”kryp.2.2.1. Eu’-cryp.2.2.2. Eu-E03 E&E04 Eu”-EO5 Eu”-EO6 E&E07
EuCl,
glycols
solution
93(1:10),148(1:20)d 620 150 81 180 95 9.2 129 6.8 =: 1.0 29 270 2.2 5.0 4.2 47 66 1.0
Relative intensity of emission b
from an EuCl,-methanol
458 489
316 328
of the emission
460 469 445 445 468 467 442 464 467
319 333 327 333 317 329 327 320 321
250 260 249 254 254 250 251 252 252 252
with the intensity
355 345 375 355 312 361 375 356 380 375 380 353 400 360 365 355 375 375
428,488 d 433 446 439 443 417 448
322 318 325 318 321 321 328
252 (228) d 248 251 251 252 267 268
by comparison
(nm)
(nm)
(nm)
248
Excitation wavelength
with crown ethers, cryptands and polyethylene Maximum emission band
of europium(I1) complexes
Maximum absorption band
solutions
“The europium(I1) concentration is 4.0 x 10e3 M. ‘The relative intensities of the emission were determined (europium(I1) concentration, 4.0 x 10m3 M). ‘The ratios could not be established. %ee text. ‘The ratio obtained by PMR.
[Eu]: [ligand]
properties of methanolic
Complex a
Spectroscopic
TABLE 1
0
0.25
0.5
Fig. 2.. Job’s plots illustrating
1
0.75
the formation of the 1: 3 complex
Fig. 3. A model of the Eu”-2 complex.
3.3. Luminescence quantum yield Measurements of the luminescence quantum yields Qcomphave been carried out. The results are summarized in Table 2. The values of @ campwere markedly enhanced by complex formation, except for the cryp.2.1.1. complex. Figure 4 shows plots of Qcompas a function of the number of coordinating atoms around europium(I1). Possible quenching processes for the luminescence of the complexes are (1) an interaction of solvent TABLE 2 Luminescence
Complex
quantum yields @J_,,,,~, lifetimes 5 and rate constants
@camp
k ( :;06 s)
k * ( :;06 s)
(35) c (42) c (16)’ _d
0.12 1.7 0.59 0.67 0.099 0.13 (0.056) L (1.7)’ (0.29) c -d
3.2 4.5 5.7 9.3 3.4 7.0 (28)’ (21)’ (62)’ -d
(24)’ 200 _d _d _d
(0.58) c 0.47 -d -d -d
(41) c 4.5 _d -d -d
(43) c (59) c (20) c
(0.29) c (0.30) c (0.022) c
(23)’ (17)’ (50) E
T1
(4 E&l E&2 ELI”-3 Eu”4 ELI”-5 Eu”-6 E&7 Eu”Gryp.2.1. Eu”-cryp.Z.B. Eu”-cryp.Z.l.l. Eu”-cryp.2.2.1. Eu”kryp.2.2.2. E&E03 E&E04 ELI”-E05 Eu”~EO6 Eu”pE07 EuCl,
0.036 b 0.28 0.094 0.067 0.029 0.018 0.002 0.071 0.046 0.0005 0.014 0.093 0.00064 0.0014 0.0010 0.012 0.018 0.0004
a.Experimental errors within f 107,. bThe value for [Eu]: [ligand] = 1:lO. ‘Not very reliable. dNot measured.
300 160 160 100 290 140
86
molecules with the complexes, i.e. the collision of methanol molecules with the complexes, and (2) the intramolecular vibration of the complexes. Figure 4 demonstrates that the larger is the number of coordinating atoms around europium(I1) the higher are the quantum yields of the complexes, although there are some exceptions. This fact suggests that in higher complexes the central europium(I1) active sites are effectively protected by the ligands from collision with solvent molecules.
0.4 Q 0.3
0.2
QD
I
“L 0
5 10 15 No. of coordinating atoms
Fig. 4. Quantum yields and numbers of atoms coordinating to europium(I1) in the complexes: 11, Eu”-cryp.2.2.1.; 2, Eu”-2; 3, Eu”-3; 4, Eu”-4; 5, Eu”-5; 6, Eu”-6; 7, ELI”-7; 9, Eu”-cryp.2.2.; 12, Eu’-cryp.2.2.2.;
15, Eu”-E05;
16, Eu”-EOG; 17, Eu”-E07.
Some of the crown ethers with side chains or side rings, such as 5, exhibited a rather poor performance in view of their oxygen numbers. The ligand 5 has two cyclohexane rings in addition to an eight-membered oxygen ring. The side chains may cause intramolecular vibration which removes excitation energy from europium(I1) and thus wastes the energy non-radiatively. Polyethylene glycols did not give high quantum yields for europium(I1) luminescence. E05 forms a 1:l complex with europium(I1). However, its emission intensity is quite small compared with that of Eu”-EO6 or Eu”-E07. It appears that these ligands form floppy loose fitting complexes which are also apt to increase the intramolecular vibration. It is not surprising that cryp.2.1.1. does not enhance the emission because the cavity of the ligand is too small to accommodate a europium(I1) ion. The other ligand, cryp.2.1., also has a small ring although its europium(I1) complex shows a fairly high value of Qcomp(0.1) which suggests that the [Eu]: [ligand] ratio is 1:7. Therefore the bridging oxygen atom in cryp.2.1.1. must prevent complex formation with europium(I1). An aromatic ring attached to a crown ether also appears to lower acompfor a similar reason to that described above. Some sensitization of the complex as a result of the presence of the aromatic ring was expected, but such a phenomenon was not observed.
lifetime Measurements of the lifetimes Tfor the complexes have been carried out at room temperature and the results are listed in Table 2. The radiative and nonand z. radiative rate constants 12,1 and k, 1* were derived from the values of (Pcomp It should be noted that t and k, 1* are of the same order except in a few cases. Of the complexes with lifetimes of the same order, k2 1 for the Eu”-2 complex is exceptionally high. As discussed above, the structure of the complex in solution appears to be responsible for this behaviour. It is concluded that the enhancement of the emission intensity as a result of the formation of complexes with crown ethers is attributable to “insulation” of the central europium(I1) ion from the close approach of the solvent molecules which would produce radiationless energy losses. 3.4. Luminescence
Acknowledgments This work was supported by a Scientific Research Grant-in-Aid (56 470 059) from the Ministry of Education of Japan. We are indebted to Dr. M. Okahara, Dr. I. Ikeda, Dr. S. Yanagida, Dr. T. Shono, Dr. K. Kimura, Dr. T. Maeda, Dr. H. Mikawa, Dr. Y. Shirota, Dr. T. Tanaka and Dr. K. Tanaka for their help in the preparation of materials and for measurements of the PMR spectra and the lifetimes. The authors appreciate gifts of samples of several crown ethers from Mr. Iwao Kato of the Nippon Soda Co.
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