- Email: [email protected]
Fluorescence of Kryptofix 5 metal complexes
Inorganic Chemistry Communications 51 (2015) 78–79
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Fluorescence of Kryptofix 5 metal complexes Arnd Vogler Institute of Inorganic Chemistry, University Regensburg, Germany
a r t i c l e
i n f o
Article history: Received 31 August 2014 Received in revised form 8 November 2014 Accepted 13 November 2014 Available online 15 November 2014 Keywords: Fluorescence Metal cations Kryptofix 5 Cryptands
a b s t r a c t Kryptofix 5 forms complexes with a variety of metal cations. The coordination with the crown ether moiety affects only slightly the emission wavelength of the free Kryptofix 5 ligand (λmax = 425 nm) in a buffer (pH = 9) solution, but the emission intensity of this blue fluorescence can increase strongly upon complex formation, probably owing to the rigidity of the complex. This applies e.g. to Ba2+, Zn2+, Cd2+, Hg2+, Ag+ and La3+. A second group of metal ions such as Al3+, Pb2+, Sb3+, Bi3+, Te4+, Gd3+ and Th4+ causes a large red shift of the fluorescence of free Kryptofix 5 to approximately λmax = 500 nm. This green fluorescence resembles that of Kryptofix 5 in acidic solution. It is suggested, that the second group of metal ions is also coordinated to the pyridine nitrogen atoms of the quinoline substituents. © 2014 Elsevier B.V. All rights reserved.
Cryptands represent an important family of multidendate ligands which forms a three-dimensional cavity for metal cations [1,2]. A rather intriguing ligand of this type is Kryptofix 5 (tradename of Merck), here abbreviated with K 5. It consists of a crown ether moiety and two quinoline substituents. A metal cation can be coordinated to the crown ether unit as well as to the quinoline chromophores. The electronic spectra will not be affected when only the crown ether is involved in coordination, but a strong effect is expected when the coordination includes also the pyridine ligand site of the quinoline substituents. This effect should influence the absorption as well as the emission spectra. We explored this possibility and selected the fluorescence as a suitable probe for the present study. This choice was based on the comparison with 8-quinolinol (Scheme 2) which is characterized by electronic features which should be similar to those of K 5. (See Scheme 1.) Kryptofix 5 is commercially available (Merck). Emission spectra were recorded with a Hitachi 850 spectrofluorometer equipped with a Hamamatsu 928 photomultiplier for measurements up to 900 nm. As solvent a buffer solution with a pH = 9 (Merck) was used. The fluorescence spectra of K 5 in a buffer solution at pH = 9 upon addition of ZnCl2 and KAl(SO4)2 are shown in Fig. 1. A buffer solution is required in order to avoid a decrease of the pH owing to the hydrolysis of several metal salts examined in this study. An acidification leads to a red shift of the longest-wavelength bands of the quinolinol substituent of K 5 in absorption and emission. The electronic spectra of free K 5 and its protonated form have been studied before [3]. It has been shown that the blue fluorescence (λmax = 410 nm) of K 5 in aqueous solution at pH = 11.5 changes to green (λmax = 495 nm) in acidic solution (pH = 2.5) as a consequence of the protonation of the quinolinol nitrogen. This observation resembles that of 8-quinolinol which shows a blue fluorescence at λ = 420 nm
http://dx.doi.org/10.1016/j.inoche.2014.11.011 1387-7003/© 2014 Elsevier B.V. All rights reserved.
which becomes green (λmax = 500 nm) in alkanes saturated with HCl [4]. The lowest-energy electronic transition, which involves a charge shift from the phenolic oxygen to the pyridine ring of the quinolinol, decreases in energy upon protonation of the pyridine nitrogen. The same effect is observed upon coordination. Numerous 8-quinolinolate (or oxinate) complexes are characterized by this behavior [5,6]. Accordingly, we suspected that such shift would be a rather useful probe also for the coordination of metal cations at the pyridine function of K 5. Indeed, the metal cations which were examined did either not change the blue fluorescence of K 5 or change the fluorescence color to green. At pH = 9 the longest-wavelength absorption of K 5 appears at λmax = 300 nm and the blue fluorescence at 425 nm. Upon addition of Ba(ClO4)2, ZnCl2, CdSO4, AgNO3, Hg(ClO4)2 and LaCl3 the absorption maximum stayed at 300 nm, and the blue fluorescence underwent a slight change to λmax = 420 nm (Ba2 +, Ag+ and La3 +) and 435 nm (Zn2 + Fig. 1, Cd2 + and Hg2 +). However, all six metal cations led to a considerable increase of the fluorescence intensity. In the case of Zn2+, it is,for example, more than 100 times more intense than the fluorescence of K 5 itself. This effect is probably caused by the increased rigidity of the complexed K 5 preventing a rapid radiationless deactivation by motions such as vibrations or rotations. Moreover, this observation is also an indication for complex formation at all, because the fact that the absorption and the fluorescence maximum is hardly shifted upon addition of these metal salts might otherwise be taken as evidence for the absence of complex formation. In conclusion, it is suggested that the complex formation of these metal cations is restricted to the crown ether moiety of K 5 (Scheme 3). It follows that the quinoline moieties of K 5 remain uncoordinated in these cases. In distinction, the second group of metal cations leads to a shift of the absorption from 300 nm to approximately 350 nm and of the blue fluorescence from 425 nm to a green fluorescence at almost 500 nm. This
A. Vogler / Inorganic Chemistry Communications 51 (2015) 78–79
79
Scheme 1.
Scheme 2.
takes place upon addition of GdCl 3 (485 nm),Pb(NO3 )2 , TeCl4 and Th(NO3 ) 4 (490 nm), KAl(SO 4 )2 (492 nm) Fig. 1, SbCl3 and BiCl3 (495 nm). Accordingly, it is concluded that in these cases the metal cations are also coordinated to the pyridine fragments of the quinoline chromophores. Typical transition metal ions such as Ni2+ lead, as it may be anticipated, to a complete or partial quenching of the K 5 fluorescence, probably owing to energy transfer to non-emitting ligand-field excited states. In other cases with kinetically stable complexes of transition metals such as Pt2+ (e.g. Pt(H2O)4+) a facile complex formation with K 5 may not occur. Unfortunately, it is not clear which correlation determines the luminescent behavior of both groups of metal cations as described above. It is apparently not the ion radius and not any specific property of the metal which is responsible for the type of fluorescence. Nevertheless, these fluorescence measurements which are rather simple to perform, represent a useful analytical probe for these, and possibly some other, metal cations. Moreover, our observations are quite interesting in their own right. Finally, it is an intriguing question if the conclusions, which are drawn here, can be confirmed by structural analysis. In one of the rare examples, X-ray structures have been determined for the K 5 complexes of HgI2 and Hg(SCN)2 [7]. In these cases complicated structures are formed with Hg2+ coordinated by the quinoline substituent while the crown ether forms an open chain. These results seem to be not consistent with our observation of the blue fluorescence of Hg2+ coordinated by K 5. However, in the solid state peculiar packing effects may determine the structure which could be quite different in solution. Another unresolved problem concerns the question why in the case of Hg2+, the heavy-atom effect does not lead to a fluorescence quenching and the appearance of a phosphorescence under ambient conditions as it is observed for CH3Hg(II)(8-quinolinolate) [8]. However, our observa-
Fig. 1. Fluorescence spectra of K 5 in a buffer solution (pH = 9) upon addition of ZnCl2 and KAl(SO4)2 (molar ratio of metal salt/K 5 = 100) in 1-cm quartz cell at r.t., λexc = 320 nm, intensity in arbitrary units.
Scheme 3.
tion of fluorescence enhancement in the case of the Hg2+–K5 complex is not without precedent [9,10]. This may occur when the metal cation increases the rigidity of the complex but a strong metal–ligand bonding, which facilitates an efficient electronic interaction, is absent. References [1] J.M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, Weinheim, VCH, 1995. [2] J.M. Lehn, Acc. Chem. Res. 11 (1978) 49–57. [3] O.S. Wolfbeis, H. Offenbacher, Monatsh. Chem. 115 (1984) 647–654. [4] M. Goldman, E.l. Wehry, Anal. Chem. 42 (1970) 1178–1185. [5] A. Vogler, Coord. Chem. Rev. 251 (2007) 577–583. [6] H. Kunkely, A. Vogler, Inorg. Chim. Acta 362 (2009) 196–198. [7] J. Pickardt, S. Wiese, Z. Naturforsch. 55 b (2000) 971–974. [8] H. Kunkely, A. Vogler, J. Photochem. Photobiol. A Chem. 144 (2001) 69–72. [9] K. Rurack, Spectrochim. Acta A 57 (2001) 2161–2195. [10] B. San Vicente de la Riva, J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, Anal. Chim. Acta. 419 (2000) 33–40.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Fluorescence of Kryptofix 5 metal complexes Arnd Vogler Institute of Inorganic Chemistry, University Regensburg, Germany
a r t i c l e
i n f o
Article history: Received 31 August 2014 Received in revised form 8 November 2014 Accepted 13 November 2014 Available online 15 November 2014 Keywords: Fluorescence Metal cations Kryptofix 5 Cryptands
a b s t r a c t Kryptofix 5 forms complexes with a variety of metal cations. The coordination with the crown ether moiety affects only slightly the emission wavelength of the free Kryptofix 5 ligand (λmax = 425 nm) in a buffer (pH = 9) solution, but the emission intensity of this blue fluorescence can increase strongly upon complex formation, probably owing to the rigidity of the complex. This applies e.g. to Ba2+, Zn2+, Cd2+, Hg2+, Ag+ and La3+. A second group of metal ions such as Al3+, Pb2+, Sb3+, Bi3+, Te4+, Gd3+ and Th4+ causes a large red shift of the fluorescence of free Kryptofix 5 to approximately λmax = 500 nm. This green fluorescence resembles that of Kryptofix 5 in acidic solution. It is suggested, that the second group of metal ions is also coordinated to the pyridine nitrogen atoms of the quinoline substituents. © 2014 Elsevier B.V. All rights reserved.
Cryptands represent an important family of multidendate ligands which forms a three-dimensional cavity for metal cations [1,2]. A rather intriguing ligand of this type is Kryptofix 5 (tradename of Merck), here abbreviated with K 5. It consists of a crown ether moiety and two quinoline substituents. A metal cation can be coordinated to the crown ether unit as well as to the quinoline chromophores. The electronic spectra will not be affected when only the crown ether is involved in coordination, but a strong effect is expected when the coordination includes also the pyridine ligand site of the quinoline substituents. This effect should influence the absorption as well as the emission spectra. We explored this possibility and selected the fluorescence as a suitable probe for the present study. This choice was based on the comparison with 8-quinolinol (Scheme 2) which is characterized by electronic features which should be similar to those of K 5. (See Scheme 1.) Kryptofix 5 is commercially available (Merck). Emission spectra were recorded with a Hitachi 850 spectrofluorometer equipped with a Hamamatsu 928 photomultiplier for measurements up to 900 nm. As solvent a buffer solution with a pH = 9 (Merck) was used. The fluorescence spectra of K 5 in a buffer solution at pH = 9 upon addition of ZnCl2 and KAl(SO4)2 are shown in Fig. 1. A buffer solution is required in order to avoid a decrease of the pH owing to the hydrolysis of several metal salts examined in this study. An acidification leads to a red shift of the longest-wavelength bands of the quinolinol substituent of K 5 in absorption and emission. The electronic spectra of free K 5 and its protonated form have been studied before [3]. It has been shown that the blue fluorescence (λmax = 410 nm) of K 5 in aqueous solution at pH = 11.5 changes to green (λmax = 495 nm) in acidic solution (pH = 2.5) as a consequence of the protonation of the quinolinol nitrogen. This observation resembles that of 8-quinolinol which shows a blue fluorescence at λ = 420 nm
http://dx.doi.org/10.1016/j.inoche.2014.11.011 1387-7003/© 2014 Elsevier B.V. All rights reserved.
which becomes green (λmax = 500 nm) in alkanes saturated with HCl [4]. The lowest-energy electronic transition, which involves a charge shift from the phenolic oxygen to the pyridine ring of the quinolinol, decreases in energy upon protonation of the pyridine nitrogen. The same effect is observed upon coordination. Numerous 8-quinolinolate (or oxinate) complexes are characterized by this behavior [5,6]. Accordingly, we suspected that such shift would be a rather useful probe also for the coordination of metal cations at the pyridine function of K 5. Indeed, the metal cations which were examined did either not change the blue fluorescence of K 5 or change the fluorescence color to green. At pH = 9 the longest-wavelength absorption of K 5 appears at λmax = 300 nm and the blue fluorescence at 425 nm. Upon addition of Ba(ClO4)2, ZnCl2, CdSO4, AgNO3, Hg(ClO4)2 and LaCl3 the absorption maximum stayed at 300 nm, and the blue fluorescence underwent a slight change to λmax = 420 nm (Ba2 +, Ag+ and La3 +) and 435 nm (Zn2 + Fig. 1, Cd2 + and Hg2 +). However, all six metal cations led to a considerable increase of the fluorescence intensity. In the case of Zn2+, it is,for example, more than 100 times more intense than the fluorescence of K 5 itself. This effect is probably caused by the increased rigidity of the complexed K 5 preventing a rapid radiationless deactivation by motions such as vibrations or rotations. Moreover, this observation is also an indication for complex formation at all, because the fact that the absorption and the fluorescence maximum is hardly shifted upon addition of these metal salts might otherwise be taken as evidence for the absence of complex formation. In conclusion, it is suggested that the complex formation of these metal cations is restricted to the crown ether moiety of K 5 (Scheme 3). It follows that the quinoline moieties of K 5 remain uncoordinated in these cases. In distinction, the second group of metal cations leads to a shift of the absorption from 300 nm to approximately 350 nm and of the blue fluorescence from 425 nm to a green fluorescence at almost 500 nm. This
A. Vogler / Inorganic Chemistry Communications 51 (2015) 78–79
79
Scheme 1.
Scheme 2.
takes place upon addition of GdCl 3 (485 nm),Pb(NO3 )2 , TeCl4 and Th(NO3 ) 4 (490 nm), KAl(SO 4 )2 (492 nm) Fig. 1, SbCl3 and BiCl3 (495 nm). Accordingly, it is concluded that in these cases the metal cations are also coordinated to the pyridine fragments of the quinoline chromophores. Typical transition metal ions such as Ni2+ lead, as it may be anticipated, to a complete or partial quenching of the K 5 fluorescence, probably owing to energy transfer to non-emitting ligand-field excited states. In other cases with kinetically stable complexes of transition metals such as Pt2+ (e.g. Pt(H2O)4+) a facile complex formation with K 5 may not occur. Unfortunately, it is not clear which correlation determines the luminescent behavior of both groups of metal cations as described above. It is apparently not the ion radius and not any specific property of the metal which is responsible for the type of fluorescence. Nevertheless, these fluorescence measurements which are rather simple to perform, represent a useful analytical probe for these, and possibly some other, metal cations. Moreover, our observations are quite interesting in their own right. Finally, it is an intriguing question if the conclusions, which are drawn here, can be confirmed by structural analysis. In one of the rare examples, X-ray structures have been determined for the K 5 complexes of HgI2 and Hg(SCN)2 [7]. In these cases complicated structures are formed with Hg2+ coordinated by the quinoline substituent while the crown ether forms an open chain. These results seem to be not consistent with our observation of the blue fluorescence of Hg2+ coordinated by K 5. However, in the solid state peculiar packing effects may determine the structure which could be quite different in solution. Another unresolved problem concerns the question why in the case of Hg2+, the heavy-atom effect does not lead to a fluorescence quenching and the appearance of a phosphorescence under ambient conditions as it is observed for CH3Hg(II)(8-quinolinolate) [8]. However, our observa-
Fig. 1. Fluorescence spectra of K 5 in a buffer solution (pH = 9) upon addition of ZnCl2 and KAl(SO4)2 (molar ratio of metal salt/K 5 = 100) in 1-cm quartz cell at r.t., λexc = 320 nm, intensity in arbitrary units.
Scheme 3.
tion of fluorescence enhancement in the case of the Hg2+–K5 complex is not without precedent [9,10]. This may occur when the metal cation increases the rigidity of the complex but a strong metal–ligand bonding, which facilitates an efficient electronic interaction, is absent. References [1] J.M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, Weinheim, VCH, 1995. [2] J.M. Lehn, Acc. Chem. Res. 11 (1978) 49–57. [3] O.S. Wolfbeis, H. Offenbacher, Monatsh. Chem. 115 (1984) 647–654. [4] M. Goldman, E.l. Wehry, Anal. Chem. 42 (1970) 1178–1185. [5] A. Vogler, Coord. Chem. Rev. 251 (2007) 577–583. [6] H. Kunkely, A. Vogler, Inorg. Chim. Acta 362 (2009) 196–198. [7] J. Pickardt, S. Wiese, Z. Naturforsch. 55 b (2000) 971–974. [8] H. Kunkely, A. Vogler, J. Photochem. Photobiol. A Chem. 144 (2001) 69–72. [9] K. Rurack, Spectrochim. Acta A 57 (2001) 2161–2195. [10] B. San Vicente de la Riva, J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, Anal. Chim. Acta. 419 (2000) 33–40.