- Email: [email protected]
Influence of charge-transfer state on luminescence properties of Eu(III) complex with o-phenylenedioxydicarboxylic acid
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 496–499 www.elsevier.com/locate/jlumin
Influence of charge-transfer state on luminescence properties of Eu(III) complex with o-phenylenedioxydicarboxylic acid P. Gawryszewska, J. Legendziewicz Faculty of Chemistry, University of Wroc!aw, 14 F. Joliot-Curie Street, 50-383 Wroc!aw, Poland Available online 20 March 2006
Abstract The Na[Eu(C10H8O6)2(H2O)2].4H2O (1) crystals has been synthesized. Absorption, emission, and excitation spectra at 293, 77 and 4 K as well as the luminescence decay time measurements were applied to characterize the photophysical properties of the crystals. Strong temperature dependence of the luminescence decay time suggests that the ligand-to-metal charge-transfer state of Eu(III) deactivates the ligand excited state so that the energy transfer from ligand triplet state to metal ion levels does not occur. A correlation between spectroscopic properties of the complex in solid state and in solutions was made. The study for different pH values and various metal:ligand molar ratios showed the existence of more than one form of the complex where ML and ML2 forms dominated. r 2006 Elsevier B.V. All rights reserved. Keywords: Europium; Luminescence; Charge transfer state; o-phenylenedioxydicarboxylic acid
The design of efficient luminescent lanthanide-based probes for application to biological systems requires detailed knowledge concerning the efficiency of the ligand-to-metal energy transfer, solution structure and dynamics, and the influence that these have on the important photochemical and photophysical properties [1–6]. In recent years the great effort have been made to investigate structure and optical properties of chiral– lanthanide complexes because of their potential to be sensitive and selective probes in a wide variety of biological applications [7,8]. Looking for new chiral systems we have synthesized and investigated the Eu(III) compound with ophenylenedioxydiacetic acid.
obtained by crystallization from aqueous solution at pH ¼ 6 where metal:ligand molar ratio was kept 1:5. Preliminary examination and intensity data collections were carried out using a KUMA KM4CCD diffractometer using graphitemonochromated Mo–Ka radiation (0.71073 A˚). Absorption measurements at 293 and 4 K were performed using a Cary-Varian 500 spectrophotometer equipped with Oxford helium flow cryostat. The emission spectra at 293 and 77 K were recorded using a SPECTRAPRO-750 monochromator equipped with 450 W Xe lamp and liquid-N2-cooled cryostat. Excitation spectra were performed at 293 and 77 K using a SLM Aminco SPF500 spectrofluorometer equipped with a 300 W Xe lamp and a liquid-N2-cooled cryostat. IR spectrum at 293 K was measured using a Bruker IFS 66/s spectrometer.
2. Experimental
3. Results and discussion
o-phenylenedioxydicarboxylic acid (PDDA) purchased from Aldrich was used in the synthesis of complex 1. The Na[Eu(C10H8O6)2(H2O)2].4H2O (denoted 1) crystal of sufficient quality suitable for CCD X-ray analysis was
The compound Na[Eu(C10H8O6)2(H2O)2].4H2O (1) forms yellow crystals and is isostructural with Na[La(C10H8O6)2 (H2O)2].4H2O obtained by Choppin et al. [9]. The complex crystallizes in a triclinic system with P-1 space group. The Eu(III) ion is decacoordinate with the coordination sphere made up of two tetradentate PDDA ligands and two water molecules. The basic dimeric unit formed about
1. Introduction
Corresponding author. Tel.: +48 71 37 57 394; fax: +48 71 375 74 20.
E-mail address: [email protected] (P. Gawryszewska). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.217
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499
inversion-related Na+ ions is polymerized along the crystallographic c-axis by the Eu–O–C–O–Na linkage [9]. The emission spectrum of solid 1 at 77 K is shown in Fig. 1. The spectral region displays overlaps with Eu(III) transitions from 5D0 state to 7FJ (J ¼ 0; 1; 2; 3; 4) terms in the ground state manifold. There are also related vibronic components marked. The superposition of the infrared spectrum of 1 onto the 0 phonon line shows the strong electron–phonon coupling mainly associated with vibrational modes of internal ligands and EuO10 moiety modes. The strongest coupling has been observed with the d OLnO vibrations of chelate ring. The vibronic components have been assigned on the basis of the IR spectrum and are listed in Table 1. Based on the emission spectrum at 77 K, the Eu(III) environment can be approximated by rather low C2v symmetry (see Table 2). The 5D0-7F0 transition consists of one peak with the half-width of 9 cm1 which is typical for polymeric structure of Eu(III) complexes [6,10,11]. The 5D0-7F1 transition exhibits splitting of the individual lines. There is a shoulder in the edge of the line marked 1.The energy difference between the maximum of the band and the shoulder is about 8 cm1. The splitting of the line marked 2 (16 cm –1) suggests a slight structure disorder of the respective europium centers in the dimeric unit. The splitting and the presence of the shoulder might also result from the resonant interaction of Stark components with n LnO vibrations. The resonance conditions are not fulfilled in the excitation energy range thus the 75000 5D
750000
7 0→ F0
497
vibronic coupling is not observed in the excitation spectrum at 77 K. Emission of the monocrystal of 1 is much weaker at 293 K than at 77 K. Very short decay time (t293 K ¼ 100 ms) of the 5D0 emitting level at 293 K and strong temperature dependence of the luminescence decay time (t77 K ¼ 610 ms) suggest that the ligand-to-metal charge transfer (LMCT) state of Eu(III) takes part in nonradiative deactivation of the excited Eu(III) levels. The most important mechanism for the temperature-independent nonradiative relaxation is Table 1 Vibronic components of the Na[Eu(C10H8O6)2(H2O)2].4H2O Transition D0-7F0
5
D0-7F2
5
Energy (cm1)
DE (cm1)
17,274 17,170 17,132 17,097 17,068 17,062 16,995 16,940 16,314
104d (OLnO) 142d (OLnO) 177d (OLnO) 206d (OLnO) 212d (OLnO) 279n (LnO) 334n (COO), NaO* chelate ring 960n (C–C)
16,334 16,251 16,234 16,188 16,001
83 lattice modes 100 lattice modes 146d (OLnO) 331n (COO), NaO* chelate ring
20000
1 5D
7 0→ F1
50000 500000 10000 2
Relative intensiry [arb.u.]
25000
250000
0
0
578
579
580
590
595
600
0
590
30000
1800000
5
D0→7F3
5 5D
D0→7F4
7 0→ F2
900000
15000
0
0
610
585
615
620
625
100000
0 648
651 λ [nm]
654
690
Fig. 1. The emission spectrum for the Na[Eu(C10H8O6)2(H2O)2].4H2O monocrystal at 77 K.
700
710
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499
498
Table 2 Splitting of the 7FJ levels of Eu(III) in non-centrosymmetric ligand field for C2v symmetry. J¼0
J¼1
J¼2
J¼3
J¼4
G
ED
EM
G
ED
EM
G
ED
EM
G
ED
EM
G
ED
EM
A1
+
A2 B1 B2
+ +
+ + +
2A1 A2 B1 B2
+ +
+ + +
A1 2A2 2B1 2B2
+ + +
+ + +
3A1 2A2 2B1 2B2
+ + +
+ + +
10
4
D
2 C
2
F0→5(G6,5,4)
F0→5D4 7
7
F0→5L6
7
F0→5D2
7
4
F0→5(H7,6,5,4,3)
6
7 5 F0→5D1 F0→ D0
7
Absorbance
8
0
B
7
Relative intensity [arb. uni]
that through OH vibrations. The ligand phosphorescence has been observed for crystals of Gd(III) complex with PDDA at the energy of 23,419 cm1. The Gd(III) complex with PDDA [Gd(C15H12O9)(H2O)2].2H2O [12] is not isostructural with the Eu(III) complex 1, but energy of the triplet state of the ligand should not change much. Both LMCT states and ligand 3pp* states control the energy transfer processes in Eu(III) complexes. It has been clearly showed in our earlier report [6,13,14] where, based on structural crystallographic data for Eu(III) complexes with cryptates and bipirydine molecules, the electronic structures have been calculated and theoretical models have been used to determine the intramolecular energy transfer rates. A comparison of the quantum yields and the decay times to the experimental data allowed to localize the energy of LMCT state and its role in the energy transfer process. There are five states that show appropriate resonance conditions with the ligand-excited states. The choice of 5D0, 5D1, 5D2, 5G6 and 5D4 results from the favorable resonance conditions and from the selection rules stemming from the theoretical models [15,16]. According to these selection rules, the direct energy transfer to the 5D0 level is not allowed. This rule is, however, relaxed due to the J-mixing effects and the thermal population of the 7F1 level. The barycenters of the 5D0, 5D1 and 5D2 of 1 are situated at 17,244, 19,058 and 21,533 cm1, respectively. Thus, the energy gap between the ligand 3pp* and the 5D2 state is 1886 cm1. The sensibilized emission of Eu(III) has not been observed inspite of quite good resonance conditions with respect to the Eu(III) levels where energy transfer (3pp*-5D2) can occurs by the dipole-2l pole mechanism. The excitation spectra at 77 K of the complex 1 in solid state and aqueous solution are presented in Fig. 2. The spectrum of monocrystal consists only of f–f transition bands from the 7F0 ground state to the excited states of the Eu(III) ion. Note the very low intensity of, usually strong, 7 F0-5GJ (J ¼ 4; 5; 6), 7F0-5D4 and 7F0-5HJ (J ¼ 3; 4; 5; 6; 7) transitions. Also intensity of 7F0-5L6 is lower then usual with respect to 7F0-5D2 and 7F0-5D1 transitions [17,18], because of low energy of the LMCT state which is seen in Fig. 2C, D. This state is in resonance with the 7F0-5D4, 7F0-5GJ and 7F0-5L6 transitions. This low energy and the offset of charge-transfer parabola in 1 are such that LMCT deactivates the ligand states and consequently the energy transfer from the ligand to the metal ion states does not exist. The LMCT state
A
0 250
300
350
400 450 λ [nm]
500
550
-2 600
Fig. 2. The excitation spectra at 77 K for the Na[Eu(C10H8O6)2(H2O)2].4H2O complex in solid state (A) and in aqueous solution (B) and the absorption spectra of this complex at 293 K in solid state (C) and in paraffin oil (D).
depopulates partially the Eu(III) excited states empties into the 7FJ ground state manifold resulting in strong reduction of luminescence. The oscillator strength values for the 7F0-5D2 and 7 F0-5D1 transitions are 7.05 108 and 2.0 108. They have similar values to dimeric complexes of Eu(III) with isoleucine and alanine [6,10,11]. When temperature is lowered to 4 K, the oscillator strength values increase to 10.7 108 and 4.1 108 for the 7F0-5D2 and 7F0-5D1, respectively. This effect clearly shows increasing population of the ground 7F0 level at low temperature and depopulation of the 7F1 and higher multiplets. Fig. 3 shows the emission spectra of solid compound and its aqueous solution of 1 at 77 K. The same shapes of the emission spectrum as shown in Fig. 3 has been observed for solutions at pH’s from 3 to 8, of course with different intensity. The solution of pH ¼ 4 and the M:L molar ratio1:5 has been used for this comparison. The 7F0-5D0 transition of solution spectrum consists of one peak with 30 cm1 half-width and at energy different (579.5 nm) than for solid state (578.9 nm). The large half-width suggests the existence of more than one form of the complex. The excitation spectrum for the aqueous solution of 1 differs from the respective spectrum of solid state, too, (see Fig. 2B) and exhibits a broad band centered at 295 nm,
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499 5
different for 1:1 molar ratio. The luminescence measurements suggest that both ML and ML2 complexes can exist in solution. This is in agreement with earlier stability constants calculations reported by Choppin and coworkers [19]. Both these forms are in equilibrium but the ML one dominates for 1:1 molar ratio.
D0→7F2
600000
400000
5D →7F 0 0
relative intensity [ab.u.]
800000
5D →7F 0 1
5
200000
B
5 7 D0→7F3 D0→ F4
References
A
0 580
600
620
640 λ [nm]
700
720
Fig. 3. The emission spectra at 77 K for solid state (A) and aqueous solution (B) of the Na[Eu(C10H8O6)2(H2O)2].4H2O.
5D →7F 0 2
Relative intensity [arb. u.]
250000 200000
1:1 1:2 1:3 1:4 1:5
..... -.-. __ ---
150000 100000 50000 0 600
610
499
620 λ [nm]
630
640
Fig. 4. The 5D0-7F2 transition at 293 K for the aqueous solutions of Eu(ClO4)3 with PDDA with metal:ligand molar ratio ranging from 1:1 to 1:5.
arising from the ligand chromophore singlet state. Basing on the excitation spectra it is worth noting that the ligand to metal energy transfer occurs in aqueous solution of the complex but not in the solid. Fig. 4 presents the 5D0-7F2 transition for solutions of Eu(ClO4)3.8H2O with PDDA, of different molar ratios. Generally, the shapes and intensities of this transition are the same for solutions with molar ratios from 1:2 to 1:5 and
[1] N. Sabbatini, M. Guardigli, I. Manet antenna effect in encapsulation complexes of lanthanide ions, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier, Amsterdam, 1996. [2] S. Lis, M. Elbanowski, B. Ma˛ kowska, Z. Hnatejko, J. Photochem. Photobiol. A: Chem. 150 (2002) 233. [3] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.-C. Rodriguez-Ubis, J. Kankare, J. Luminesc. 75 (1997) 149. [4] H. Bazin, E. Trinquet, G. Mathis, Rev Mol Biotechnol 82 (2002) 233. [5] V.W.W. Yam, K.K.W. Lo, Coord. Chem. Rev. 184 (1999) 157. [6] P. Gawryszewska, J. Sokolnicki, J. Legendziewicz, Coord. Chem. Rev. 249 (2005) 2489. [7] J.P. Riehl, G. Muller, in: K.A. Gschneidner Jr., J.-C.G. Bu¨nzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 34, North-Holland, Amsterdam, 2005, pp. 289–357 (Chapter 220). [8] P. Gawryszewska, J. Sokolnicki, A. Dossing, J.P. Riehl, G. Muller, J. Legendziewicz, J. Phys. Chem. A 109 (2005) 3858. [9] H.B. Kerfoot, G.R. Choppin, T.J. Kistenmacher, Inorg. Chem. 18 (1979) 787. [10] J. Legendziewicz, Spectroscopy of dimeric and polymeric Ln(III) compounds single crystals, in: J. Legendziewicz, W. Strek (Eds.), Excited States of Transition Elements, World Scientific, Singapore, 1991, p. 149. [11] J. Legendziewicz, J. Alloys Compounds 300–301 (2000) 71. [12] P. Gawryszewska, Z. Ciunik, E. Huskowska, J. Legendziewicz, in preparation. [13] P. Gawryszewska, O.L. Malta, R. Longo, F.R.G.E. Silva, K. Mierzwicki, Z. Latajka, M. Pietraszkiewicz, J. Legendziewicz, Chem. Phys. Chem. 5 (2004) 1. [14] O.L. Malta, J. Legendziewicz, E. Huskowska, I. Turowska-Tyrk, R.Q. Albuquerque, C. de Mello Donega, F.R.G. e Silva, J. Alloys Compounds 323–324 (2001) 654. [15] O. Malta, J. Lumin 71 (1997) 229. [16] G.F. de Sa´, O. Malta, C. de Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva, Coord. Chem. Rev. 196 (2000) 165. [17] P.P. Gawryszewska, L. Jerzykiewicz, P. Sobota, J. Legendziewicz, J. Alloys Compounds 275 (2000) 300. [18] J. Legendziewicz, J. Alloys Compounds 341 (2002) 34. [19] Y. Hasegawa, G.R. Choppin, Inorg. Chem. 16 (1977) 2931.
Journal of Luminescence 122–123 (2007) 496–499 www.elsevier.com/locate/jlumin
Influence of charge-transfer state on luminescence properties of Eu(III) complex with o-phenylenedioxydicarboxylic acid P. Gawryszewska, J. Legendziewicz Faculty of Chemistry, University of Wroc!aw, 14 F. Joliot-Curie Street, 50-383 Wroc!aw, Poland Available online 20 March 2006
Abstract The Na[Eu(C10H8O6)2(H2O)2].4H2O (1) crystals has been synthesized. Absorption, emission, and excitation spectra at 293, 77 and 4 K as well as the luminescence decay time measurements were applied to characterize the photophysical properties of the crystals. Strong temperature dependence of the luminescence decay time suggests that the ligand-to-metal charge-transfer state of Eu(III) deactivates the ligand excited state so that the energy transfer from ligand triplet state to metal ion levels does not occur. A correlation between spectroscopic properties of the complex in solid state and in solutions was made. The study for different pH values and various metal:ligand molar ratios showed the existence of more than one form of the complex where ML and ML2 forms dominated. r 2006 Elsevier B.V. All rights reserved. Keywords: Europium; Luminescence; Charge transfer state; o-phenylenedioxydicarboxylic acid
The design of efficient luminescent lanthanide-based probes for application to biological systems requires detailed knowledge concerning the efficiency of the ligand-to-metal energy transfer, solution structure and dynamics, and the influence that these have on the important photochemical and photophysical properties [1–6]. In recent years the great effort have been made to investigate structure and optical properties of chiral– lanthanide complexes because of their potential to be sensitive and selective probes in a wide variety of biological applications [7,8]. Looking for new chiral systems we have synthesized and investigated the Eu(III) compound with ophenylenedioxydiacetic acid.
obtained by crystallization from aqueous solution at pH ¼ 6 where metal:ligand molar ratio was kept 1:5. Preliminary examination and intensity data collections were carried out using a KUMA KM4CCD diffractometer using graphitemonochromated Mo–Ka radiation (0.71073 A˚). Absorption measurements at 293 and 4 K were performed using a Cary-Varian 500 spectrophotometer equipped with Oxford helium flow cryostat. The emission spectra at 293 and 77 K were recorded using a SPECTRAPRO-750 monochromator equipped with 450 W Xe lamp and liquid-N2-cooled cryostat. Excitation spectra were performed at 293 and 77 K using a SLM Aminco SPF500 spectrofluorometer equipped with a 300 W Xe lamp and a liquid-N2-cooled cryostat. IR spectrum at 293 K was measured using a Bruker IFS 66/s spectrometer.
2. Experimental
3. Results and discussion
o-phenylenedioxydicarboxylic acid (PDDA) purchased from Aldrich was used in the synthesis of complex 1. The Na[Eu(C10H8O6)2(H2O)2].4H2O (denoted 1) crystal of sufficient quality suitable for CCD X-ray analysis was
The compound Na[Eu(C10H8O6)2(H2O)2].4H2O (1) forms yellow crystals and is isostructural with Na[La(C10H8O6)2 (H2O)2].4H2O obtained by Choppin et al. [9]. The complex crystallizes in a triclinic system with P-1 space group. The Eu(III) ion is decacoordinate with the coordination sphere made up of two tetradentate PDDA ligands and two water molecules. The basic dimeric unit formed about
1. Introduction
Corresponding author. Tel.: +48 71 37 57 394; fax: +48 71 375 74 20.
E-mail address: [email protected] (P. Gawryszewska). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.217
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499
inversion-related Na+ ions is polymerized along the crystallographic c-axis by the Eu–O–C–O–Na linkage [9]. The emission spectrum of solid 1 at 77 K is shown in Fig. 1. The spectral region displays overlaps with Eu(III) transitions from 5D0 state to 7FJ (J ¼ 0; 1; 2; 3; 4) terms in the ground state manifold. There are also related vibronic components marked. The superposition of the infrared spectrum of 1 onto the 0 phonon line shows the strong electron–phonon coupling mainly associated with vibrational modes of internal ligands and EuO10 moiety modes. The strongest coupling has been observed with the d OLnO vibrations of chelate ring. The vibronic components have been assigned on the basis of the IR spectrum and are listed in Table 1. Based on the emission spectrum at 77 K, the Eu(III) environment can be approximated by rather low C2v symmetry (see Table 2). The 5D0-7F0 transition consists of one peak with the half-width of 9 cm1 which is typical for polymeric structure of Eu(III) complexes [6,10,11]. The 5D0-7F1 transition exhibits splitting of the individual lines. There is a shoulder in the edge of the line marked 1.The energy difference between the maximum of the band and the shoulder is about 8 cm1. The splitting of the line marked 2 (16 cm –1) suggests a slight structure disorder of the respective europium centers in the dimeric unit. The splitting and the presence of the shoulder might also result from the resonant interaction of Stark components with n LnO vibrations. The resonance conditions are not fulfilled in the excitation energy range thus the 75000 5D
750000
7 0→ F0
497
vibronic coupling is not observed in the excitation spectrum at 77 K. Emission of the monocrystal of 1 is much weaker at 293 K than at 77 K. Very short decay time (t293 K ¼ 100 ms) of the 5D0 emitting level at 293 K and strong temperature dependence of the luminescence decay time (t77 K ¼ 610 ms) suggest that the ligand-to-metal charge transfer (LMCT) state of Eu(III) takes part in nonradiative deactivation of the excited Eu(III) levels. The most important mechanism for the temperature-independent nonradiative relaxation is Table 1 Vibronic components of the Na[Eu(C10H8O6)2(H2O)2].4H2O Transition D0-7F0
5
D0-7F2
5
Energy (cm1)
DE (cm1)
17,274 17,170 17,132 17,097 17,068 17,062 16,995 16,940 16,314
104d (OLnO) 142d (OLnO) 177d (OLnO) 206d (OLnO) 212d (OLnO) 279n (LnO) 334n (COO), NaO* chelate ring 960n (C–C)
16,334 16,251 16,234 16,188 16,001
83 lattice modes 100 lattice modes 146d (OLnO) 331n (COO), NaO* chelate ring
20000
1 5D
7 0→ F1
50000 500000 10000 2
Relative intensiry [arb.u.]
25000
250000
0
0
578
579
580
590
595
600
0
590
30000
1800000
5
D0→7F3
5 5D
D0→7F4
7 0→ F2
900000
15000
0
0
610
585
615
620
625
100000
0 648
651 λ [nm]
654
690
Fig. 1. The emission spectrum for the Na[Eu(C10H8O6)2(H2O)2].4H2O monocrystal at 77 K.
700
710
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499
498
Table 2 Splitting of the 7FJ levels of Eu(III) in non-centrosymmetric ligand field for C2v symmetry. J¼0
J¼1
J¼2
J¼3
J¼4
G
ED
EM
G
ED
EM
G
ED
EM
G
ED
EM
G
ED
EM
A1
+
A2 B1 B2
+ +
+ + +
2A1 A2 B1 B2
+ +
+ + +
A1 2A2 2B1 2B2
+ + +
+ + +
3A1 2A2 2B1 2B2
+ + +
+ + +
10
4
D
2 C
2
F0→5(G6,5,4)
F0→5D4 7
7
F0→5L6
7
F0→5D2
7
4
F0→5(H7,6,5,4,3)
6
7 5 F0→5D1 F0→ D0
7
Absorbance
8
0
B
7
Relative intensity [arb. uni]
that through OH vibrations. The ligand phosphorescence has been observed for crystals of Gd(III) complex with PDDA at the energy of 23,419 cm1. The Gd(III) complex with PDDA [Gd(C15H12O9)(H2O)2].2H2O [12] is not isostructural with the Eu(III) complex 1, but energy of the triplet state of the ligand should not change much. Both LMCT states and ligand 3pp* states control the energy transfer processes in Eu(III) complexes. It has been clearly showed in our earlier report [6,13,14] where, based on structural crystallographic data for Eu(III) complexes with cryptates and bipirydine molecules, the electronic structures have been calculated and theoretical models have been used to determine the intramolecular energy transfer rates. A comparison of the quantum yields and the decay times to the experimental data allowed to localize the energy of LMCT state and its role in the energy transfer process. There are five states that show appropriate resonance conditions with the ligand-excited states. The choice of 5D0, 5D1, 5D2, 5G6 and 5D4 results from the favorable resonance conditions and from the selection rules stemming from the theoretical models [15,16]. According to these selection rules, the direct energy transfer to the 5D0 level is not allowed. This rule is, however, relaxed due to the J-mixing effects and the thermal population of the 7F1 level. The barycenters of the 5D0, 5D1 and 5D2 of 1 are situated at 17,244, 19,058 and 21,533 cm1, respectively. Thus, the energy gap between the ligand 3pp* and the 5D2 state is 1886 cm1. The sensibilized emission of Eu(III) has not been observed inspite of quite good resonance conditions with respect to the Eu(III) levels where energy transfer (3pp*-5D2) can occurs by the dipole-2l pole mechanism. The excitation spectra at 77 K of the complex 1 in solid state and aqueous solution are presented in Fig. 2. The spectrum of monocrystal consists only of f–f transition bands from the 7F0 ground state to the excited states of the Eu(III) ion. Note the very low intensity of, usually strong, 7 F0-5GJ (J ¼ 4; 5; 6), 7F0-5D4 and 7F0-5HJ (J ¼ 3; 4; 5; 6; 7) transitions. Also intensity of 7F0-5L6 is lower then usual with respect to 7F0-5D2 and 7F0-5D1 transitions [17,18], because of low energy of the LMCT state which is seen in Fig. 2C, D. This state is in resonance with the 7F0-5D4, 7F0-5GJ and 7F0-5L6 transitions. This low energy and the offset of charge-transfer parabola in 1 are such that LMCT deactivates the ligand states and consequently the energy transfer from the ligand to the metal ion states does not exist. The LMCT state
A
0 250
300
350
400 450 λ [nm]
500
550
-2 600
Fig. 2. The excitation spectra at 77 K for the Na[Eu(C10H8O6)2(H2O)2].4H2O complex in solid state (A) and in aqueous solution (B) and the absorption spectra of this complex at 293 K in solid state (C) and in paraffin oil (D).
depopulates partially the Eu(III) excited states empties into the 7FJ ground state manifold resulting in strong reduction of luminescence. The oscillator strength values for the 7F0-5D2 and 7 F0-5D1 transitions are 7.05 108 and 2.0 108. They have similar values to dimeric complexes of Eu(III) with isoleucine and alanine [6,10,11]. When temperature is lowered to 4 K, the oscillator strength values increase to 10.7 108 and 4.1 108 for the 7F0-5D2 and 7F0-5D1, respectively. This effect clearly shows increasing population of the ground 7F0 level at low temperature and depopulation of the 7F1 and higher multiplets. Fig. 3 shows the emission spectra of solid compound and its aqueous solution of 1 at 77 K. The same shapes of the emission spectrum as shown in Fig. 3 has been observed for solutions at pH’s from 3 to 8, of course with different intensity. The solution of pH ¼ 4 and the M:L molar ratio1:5 has been used for this comparison. The 7F0-5D0 transition of solution spectrum consists of one peak with 30 cm1 half-width and at energy different (579.5 nm) than for solid state (578.9 nm). The large half-width suggests the existence of more than one form of the complex. The excitation spectrum for the aqueous solution of 1 differs from the respective spectrum of solid state, too, (see Fig. 2B) and exhibits a broad band centered at 295 nm,
ARTICLE IN PRESS P. Gawryszewska, J. Legendziewicz / Journal of Luminescence 122–123 (2007) 496–499 5
different for 1:1 molar ratio. The luminescence measurements suggest that both ML and ML2 complexes can exist in solution. This is in agreement with earlier stability constants calculations reported by Choppin and coworkers [19]. Both these forms are in equilibrium but the ML one dominates for 1:1 molar ratio.
D0→7F2
600000
400000
5D →7F 0 0
relative intensity [ab.u.]
800000
5D →7F 0 1
5
200000
B
5 7 D0→7F3 D0→ F4
References
A
0 580
600
620
640 λ [nm]
700
720
Fig. 3. The emission spectra at 77 K for solid state (A) and aqueous solution (B) of the Na[Eu(C10H8O6)2(H2O)2].4H2O.
5D →7F 0 2
Relative intensity [arb. u.]
250000 200000
1:1 1:2 1:3 1:4 1:5
..... -.-. __ ---
150000 100000 50000 0 600
610
499
620 λ [nm]
630
640
Fig. 4. The 5D0-7F2 transition at 293 K for the aqueous solutions of Eu(ClO4)3 with PDDA with metal:ligand molar ratio ranging from 1:1 to 1:5.
arising from the ligand chromophore singlet state. Basing on the excitation spectra it is worth noting that the ligand to metal energy transfer occurs in aqueous solution of the complex but not in the solid. Fig. 4 presents the 5D0-7F2 transition for solutions of Eu(ClO4)3.8H2O with PDDA, of different molar ratios. Generally, the shapes and intensities of this transition are the same for solutions with molar ratios from 1:2 to 1:5 and
[1] N. Sabbatini, M. Guardigli, I. Manet antenna effect in encapsulation complexes of lanthanide ions, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier, Amsterdam, 1996. [2] S. Lis, M. Elbanowski, B. Ma˛ kowska, Z. Hnatejko, J. Photochem. Photobiol. A: Chem. 150 (2002) 233. [3] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.-C. Rodriguez-Ubis, J. Kankare, J. Luminesc. 75 (1997) 149. [4] H. Bazin, E. Trinquet, G. Mathis, Rev Mol Biotechnol 82 (2002) 233. [5] V.W.W. Yam, K.K.W. Lo, Coord. Chem. Rev. 184 (1999) 157. [6] P. Gawryszewska, J. Sokolnicki, J. Legendziewicz, Coord. Chem. Rev. 249 (2005) 2489. [7] J.P. Riehl, G. Muller, in: K.A. Gschneidner Jr., J.-C.G. Bu¨nzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 34, North-Holland, Amsterdam, 2005, pp. 289–357 (Chapter 220). [8] P. Gawryszewska, J. Sokolnicki, A. Dossing, J.P. Riehl, G. Muller, J. Legendziewicz, J. Phys. Chem. A 109 (2005) 3858. [9] H.B. Kerfoot, G.R. Choppin, T.J. Kistenmacher, Inorg. Chem. 18 (1979) 787. [10] J. Legendziewicz, Spectroscopy of dimeric and polymeric Ln(III) compounds single crystals, in: J. Legendziewicz, W. Strek (Eds.), Excited States of Transition Elements, World Scientific, Singapore, 1991, p. 149. [11] J. Legendziewicz, J. Alloys Compounds 300–301 (2000) 71. [12] P. Gawryszewska, Z. Ciunik, E. Huskowska, J. Legendziewicz, in preparation. [13] P. Gawryszewska, O.L. Malta, R. Longo, F.R.G.E. Silva, K. Mierzwicki, Z. Latajka, M. Pietraszkiewicz, J. Legendziewicz, Chem. Phys. Chem. 5 (2004) 1. [14] O.L. Malta, J. Legendziewicz, E. Huskowska, I. Turowska-Tyrk, R.Q. Albuquerque, C. de Mello Donega, F.R.G. e Silva, J. Alloys Compounds 323–324 (2001) 654. [15] O. Malta, J. Lumin 71 (1997) 229. [16] G.F. de Sa´, O. Malta, C. de Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva, Coord. Chem. Rev. 196 (2000) 165. [17] P.P. Gawryszewska, L. Jerzykiewicz, P. Sobota, J. Legendziewicz, J. Alloys Compounds 275 (2000) 300. [18] J. Legendziewicz, J. Alloys Compounds 341 (2002) 34. [19] Y. Hasegawa, G.R. Choppin, Inorg. Chem. 16 (1977) 2931.