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Photolysis of aqueous europium(III) azide complexes: formation of europium(II) induced by ligand-to-metal charge transfer excitation
Inorganic Chemistry Communications 8 (2005) 117–118 www.elsevier.com/locate/inoche
Photolysis of aqueous europium(III) azide complexes: formation of europium(II) induced by ligand-to-metal charge transfer excitation Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universita¨tsstrasse 31, D-93040 Regensburg, Germany Received 16 September 2004; accepted 4 October 2004
Abstract Europium(III) azide complexes are formed when azide is added to aqueous EuCl3. A new absorption appears at kmax = 324 nm. It 3+ 2+ is assigned to a (N with / = 7 · 104 at kirr = 333 nm. 3 to Eu ) LMCT transition. LMCT excitation leads to the formation of Eu 2004 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Photochemistry; Europium; Azide complexes
It is well known that the reduction of Eu3+ to Eu2+ can be achieved photochemically [1–4]. This redox photolysis is induced by LMCT (ligand-to-metal charge transfer) excitation. In contrast to the simplicity of this process it is usually obscured by secondary thermal and photochemical reactions. Generally, an accumulation of Eu2+ has not been observed since it undergoes a facile thermal and/or photochemical reoxidation. While the quantum yield of the primary photogeneration of Eu2+ has been determined in a few cases, much less is known about the efficiency of the formation of Eu2+ as a permanent photoproduct. The present study was undertaken to identify such a simple LMCT-induced photoreduction of Eu3+ under conditions which limit an extensive reoxidation of Eu2+ in secondary processes. As a suitable ligand we selected azide for various reasons. Since N 3 has a rather low optical electronegativity (2.8) [5], N 3 to Eu3+ LMCT absorptions should occur at relatively long wavelengths. Secondary photooxidation of Eu2+ which requires short-wavelength irradiation could thus be * Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.10.024
diminished or avoided. Even more important, azide is well known to undergo a rapid irreversible oxidation to nitrogen [6]. Recombination of the radical pair Eu2+/N3 which is generated by LMCT excitation of europium(III) azide complexes is then less efficient. Finally, the choice of water as solvent offers several advantages. In particular, any interference by organic radicals which are formed in organic solvents [1–4] is excluded. Europium(III) complexes are well known to display LMCT bands in their electronic spectra [7–9]. With increasing reducing strength of the ligands these absorptions are shifted to longer wavelengths. For example, with Br as ligand the LMCT band appears at 320 nm [7]. Since Br and N 3 have the same optical electroneg3+ ativity (2.8) [5], the N LMCT absorption is ex3 to Eu pected to appear at nearly the same wavelength. Indeed, upon addition of azide to an aqueous solution of EuCl3 a new band shows up (Fig. 1) at kmax = 324 nm. With increasing N 3 concentration the optical density at 324 nm reaches an upper limit. At a concentration of 0.02 M for Eu3+ and 0.1 M for N 3 the 324 nm band obeys the Lambert–Beer law when this solution is diluted (1:10). The extinction coefficient at 324 nm amounts to 43.5 M1 cm1. The stoichiometry of europium(III) azide complexes which are formed under these
118
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 117–118
Fig. 1. Absorption spectrum (a) of an aqueous solution of 0.02 M EuCl3 · 6H2O and 0.1 M NaN3. Emission spectrum (e) of this solution after irradiation (kirr = 333 nm; Hanovia Xe/Hg 977 B-1 lamp; 30 min) and addition of a solution of 15-crown-5 in methanol (kexc = 320 nm, 1 cm cell, emission intensity in arbitrary units). All solutions were saturated with argon.
conditions has not been established but is without relevance for the present study. Upon irradiation (kirr > 300 nm) of an aqueous solution containing 0.01 M EuCl3 · 6H2O and 0.05 M NaN3 a photolysis takes place as indicated by an increase of the absorption above 260 nm. Simultaneously, gas bubbles evolve. There is little doubt that this gas is nitrogen which is also photochemically generated upon (N 3 to Mn+) LMCT excitation of numerous other azide complexes of reducing metal ions [6]. The photolysis of the Eu3+ azide complex is assumed to proceed according to the simple equation 2þ Eu3þ N þ 1:5N2 3 ! Eu
ð1Þ
The increase of the optical density during the photolysis is then attributed to the formation of Eu2+ which has a higher absorption (kmax = 320 nm; e = 602 and kmax = 250 nm; e = 1778) [10] than Eu3+ azide complexes in this spectral region. The photochemical formation of Eu2+ was confirmed by luminescence spectroscopy. While aqueous Eu2+ is not emissive it forms a fairly stable complex with the crown ether 15-crown-5 which shows an intense luminescence at kmax = 432 nm [11]. Indeed, when 15-crown-5 in methanol is added to the photolyzed solution this emission is nicely reproduced (Fig. 1). The progress of the photolysis is monitored by measuring the increase of the absorption at 320 nm taking into account the extinction coefficient of the Eu3+ azide complex (e = 42) and Eu2+ (e = 602) at this wavelength. At kirr = 333 nm the quantum yield of the Eu2+ formation is / = 7 · 104. Of course, all operations have to be carried out in the absence of oxygen since Eu2+ is instantaneously reoxidized by O2. When the photolysis of the Eu3+ azide complex is performed out in the presence of O2, spectral changes are not observed since the Eu3+ azide complexes are completely regenerated. In the absence of oxygen, Eu2+ ions are photochemically reoxidized by water [1,10,12]. This process prevents
an efficient accumulation of Eu2+ as a final product of the Eu(III) photoreduction. However, in the region of the LMCT band of Eu(III) azide complexes the extinction coefficient of Eu2+ is sufficiently small (e < 500) to restrict this interference. In contrast, previous studies have been carried out with EuCl3 or Eu(ClO4)3 in aqueous or alcoholic solution [1–4]. In these cases LMCT excitation requires much shorter-wavelength irradiation. Since in this spectral region Eu2+ is strongly absorbing and efficiently photooxidized (e.g., at 250 nm: e = 1778 and / = 0.2) [10], the secondary photolysis severely interferes with the primary photoreduction of Eu3+. Moreover, while the azide radicals as primary photooxidation products undergo a rapid irreversible decay [6], chloride atoms which are generated by LMCT excitation of EuCl3 are much more stable and accordingly favor a recombination (EuCl2 + Cl ! EuCl3). Even if a cage escape of chlorine atoms should be successful and a formation of Cl2 takes place, a subsequent reoxidation of Eu2+ by Cl2 would certainly occur. All these unfavorable conditions prevent the accumulation of Eu2+ as a permanent photoproduct of EuCl3. In conclusion, the addition of azide to an aqueous solution of Eu3+ leads to the formation of Eu3+ azide complexes which are characterized by a long-wavelength 3+ (N 3 to Eu ) LMCT band (kmax = 324 nm). Upon LMCT excitation azide irreversibly reduces Eu3+ to Eu2+ with a quantum yield of / = 7 · 104 at kirr = 333 nm. The present observation may be also of importance for the photoreduction of other lanthanide(III) ions such as Sm3+ [13].
References [1] O. Horva´th, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, New York, 1993, p. 89. [2] M. Kusaba, N. Nakashima, W. Kawamura, Chem. Phys. Lett. 197 (1992) 136. [3] M. Kusaba, N. Nakashima, W. Kawamura, Y. Izawa, C. Yamanaka, J. Alloys Comp. 192 (1993) 284. [4] M. Kusaba, N. Nakashima, Y. Izawa, C. Yamanaka, W. Kawamura, Chem. Phys. Lett. 221 (1994) 407. [5] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1984. [6] A. Vogler, H. Kunkely, in: K. Kalyanasundaram, M. Gra¨tzel (Eds.), Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, Kluwer Academic Publishers, Dordrecht, 1993, p. 71. [7] C.K. Jørgensen, Mol. Phys. 5 (1962) 271. [8] J.C. Barnes, H. Pincott, J. Chem. Soc. (A) (1966) 842. [9] N. Sabbatini, M. Guardigli, Coord. Chem. Rev. 123 (1993) 201. [10] D.D. Davis, K.L. Stevenson, G.K. King, Inorg. Chem. 16 (1977) 670. [11] J. Jiang, N. Higashiyama, K. Machida, G. Adachi, Coord. Chem. Rev. 170 (1998) 1. [12] M. Brandys, G. Stein, J. Phys. Chem. 82 (1978) 852. [13] M. Kusaba, Y. Tsunawaki, N. Nakashima, J. Photochem. Photobiol. A: Chem. 104 (1997) 35, and references cited therein.
Photolysis of aqueous europium(III) azide complexes: formation of europium(II) induced by ligand-to-metal charge transfer excitation Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universita¨tsstrasse 31, D-93040 Regensburg, Germany Received 16 September 2004; accepted 4 October 2004
Abstract Europium(III) azide complexes are formed when azide is added to aqueous EuCl3. A new absorption appears at kmax = 324 nm. It 3+ 2+ is assigned to a (N with / = 7 · 104 at kirr = 333 nm. 3 to Eu ) LMCT transition. LMCT excitation leads to the formation of Eu 2004 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Photochemistry; Europium; Azide complexes
It is well known that the reduction of Eu3+ to Eu2+ can be achieved photochemically [1–4]. This redox photolysis is induced by LMCT (ligand-to-metal charge transfer) excitation. In contrast to the simplicity of this process it is usually obscured by secondary thermal and photochemical reactions. Generally, an accumulation of Eu2+ has not been observed since it undergoes a facile thermal and/or photochemical reoxidation. While the quantum yield of the primary photogeneration of Eu2+ has been determined in a few cases, much less is known about the efficiency of the formation of Eu2+ as a permanent photoproduct. The present study was undertaken to identify such a simple LMCT-induced photoreduction of Eu3+ under conditions which limit an extensive reoxidation of Eu2+ in secondary processes. As a suitable ligand we selected azide for various reasons. Since N 3 has a rather low optical electronegativity (2.8) [5], N 3 to Eu3+ LMCT absorptions should occur at relatively long wavelengths. Secondary photooxidation of Eu2+ which requires short-wavelength irradiation could thus be * Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.10.024
diminished or avoided. Even more important, azide is well known to undergo a rapid irreversible oxidation to nitrogen [6]. Recombination of the radical pair Eu2+/N3 which is generated by LMCT excitation of europium(III) azide complexes is then less efficient. Finally, the choice of water as solvent offers several advantages. In particular, any interference by organic radicals which are formed in organic solvents [1–4] is excluded. Europium(III) complexes are well known to display LMCT bands in their electronic spectra [7–9]. With increasing reducing strength of the ligands these absorptions are shifted to longer wavelengths. For example, with Br as ligand the LMCT band appears at 320 nm [7]. Since Br and N 3 have the same optical electroneg3+ ativity (2.8) [5], the N LMCT absorption is ex3 to Eu pected to appear at nearly the same wavelength. Indeed, upon addition of azide to an aqueous solution of EuCl3 a new band shows up (Fig. 1) at kmax = 324 nm. With increasing N 3 concentration the optical density at 324 nm reaches an upper limit. At a concentration of 0.02 M for Eu3+ and 0.1 M for N 3 the 324 nm band obeys the Lambert–Beer law when this solution is diluted (1:10). The extinction coefficient at 324 nm amounts to 43.5 M1 cm1. The stoichiometry of europium(III) azide complexes which are formed under these
118
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 117–118
Fig. 1. Absorption spectrum (a) of an aqueous solution of 0.02 M EuCl3 · 6H2O and 0.1 M NaN3. Emission spectrum (e) of this solution after irradiation (kirr = 333 nm; Hanovia Xe/Hg 977 B-1 lamp; 30 min) and addition of a solution of 15-crown-5 in methanol (kexc = 320 nm, 1 cm cell, emission intensity in arbitrary units). All solutions were saturated with argon.
conditions has not been established but is without relevance for the present study. Upon irradiation (kirr > 300 nm) of an aqueous solution containing 0.01 M EuCl3 · 6H2O and 0.05 M NaN3 a photolysis takes place as indicated by an increase of the absorption above 260 nm. Simultaneously, gas bubbles evolve. There is little doubt that this gas is nitrogen which is also photochemically generated upon (N 3 to Mn+) LMCT excitation of numerous other azide complexes of reducing metal ions [6]. The photolysis of the Eu3+ azide complex is assumed to proceed according to the simple equation 2þ Eu3þ N þ 1:5N2 3 ! Eu
ð1Þ
The increase of the optical density during the photolysis is then attributed to the formation of Eu2+ which has a higher absorption (kmax = 320 nm; e = 602 and kmax = 250 nm; e = 1778) [10] than Eu3+ azide complexes in this spectral region. The photochemical formation of Eu2+ was confirmed by luminescence spectroscopy. While aqueous Eu2+ is not emissive it forms a fairly stable complex with the crown ether 15-crown-5 which shows an intense luminescence at kmax = 432 nm [11]. Indeed, when 15-crown-5 in methanol is added to the photolyzed solution this emission is nicely reproduced (Fig. 1). The progress of the photolysis is monitored by measuring the increase of the absorption at 320 nm taking into account the extinction coefficient of the Eu3+ azide complex (e = 42) and Eu2+ (e = 602) at this wavelength. At kirr = 333 nm the quantum yield of the Eu2+ formation is / = 7 · 104. Of course, all operations have to be carried out in the absence of oxygen since Eu2+ is instantaneously reoxidized by O2. When the photolysis of the Eu3+ azide complex is performed out in the presence of O2, spectral changes are not observed since the Eu3+ azide complexes are completely regenerated. In the absence of oxygen, Eu2+ ions are photochemically reoxidized by water [1,10,12]. This process prevents
an efficient accumulation of Eu2+ as a final product of the Eu(III) photoreduction. However, in the region of the LMCT band of Eu(III) azide complexes the extinction coefficient of Eu2+ is sufficiently small (e < 500) to restrict this interference. In contrast, previous studies have been carried out with EuCl3 or Eu(ClO4)3 in aqueous or alcoholic solution [1–4]. In these cases LMCT excitation requires much shorter-wavelength irradiation. Since in this spectral region Eu2+ is strongly absorbing and efficiently photooxidized (e.g., at 250 nm: e = 1778 and / = 0.2) [10], the secondary photolysis severely interferes with the primary photoreduction of Eu3+. Moreover, while the azide radicals as primary photooxidation products undergo a rapid irreversible decay [6], chloride atoms which are generated by LMCT excitation of EuCl3 are much more stable and accordingly favor a recombination (EuCl2 + Cl ! EuCl3). Even if a cage escape of chlorine atoms should be successful and a formation of Cl2 takes place, a subsequent reoxidation of Eu2+ by Cl2 would certainly occur. All these unfavorable conditions prevent the accumulation of Eu2+ as a permanent photoproduct of EuCl3. In conclusion, the addition of azide to an aqueous solution of Eu3+ leads to the formation of Eu3+ azide complexes which are characterized by a long-wavelength 3+ (N 3 to Eu ) LMCT band (kmax = 324 nm). Upon LMCT excitation azide irreversibly reduces Eu3+ to Eu2+ with a quantum yield of / = 7 · 104 at kirr = 333 nm. The present observation may be also of importance for the photoreduction of other lanthanide(III) ions such as Sm3+ [13].
References [1] O. Horva´th, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, New York, 1993, p. 89. [2] M. Kusaba, N. Nakashima, W. Kawamura, Chem. Phys. Lett. 197 (1992) 136. [3] M. Kusaba, N. Nakashima, W. Kawamura, Y. Izawa, C. Yamanaka, J. Alloys Comp. 192 (1993) 284. [4] M. Kusaba, N. Nakashima, Y. Izawa, C. Yamanaka, W. Kawamura, Chem. Phys. Lett. 221 (1994) 407. [5] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1984. [6] A. Vogler, H. Kunkely, in: K. Kalyanasundaram, M. Gra¨tzel (Eds.), Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, Kluwer Academic Publishers, Dordrecht, 1993, p. 71. [7] C.K. Jørgensen, Mol. Phys. 5 (1962) 271. [8] J.C. Barnes, H. Pincott, J. Chem. Soc. (A) (1966) 842. [9] N. Sabbatini, M. Guardigli, Coord. Chem. Rev. 123 (1993) 201. [10] D.D. Davis, K.L. Stevenson, G.K. King, Inorg. Chem. 16 (1977) 670. [11] J. Jiang, N. Higashiyama, K. Machida, G. Adachi, Coord. Chem. Rev. 170 (1998) 1. [12] M. Brandys, G. Stein, J. Phys. Chem. 82 (1978) 852. [13] M. Kusaba, Y. Tsunawaki, N. Nakashima, J. Photochem. Photobiol. A: Chem. 104 (1997) 35, and references cited therein.