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Photoredox reaction of (Pcts)FeIII(O22-)FeIII(Pcts) with PctsH2 = phthalocyaninetetrasulfonate induced by peroxide to Fe(III) charge transfer excitation
Inorganica Chimica Acta 358 (2005) 4086–4088 www.elsevier.com/locate/ica
Note
Photoredox reaction of ðPctsÞFeIII ðO22 ÞFeIIIðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate induced by peroxide to Fe(III) charge transfer excitation Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany Received 22 March 2005; accepted 1 April 2005 Available online 14 July 2005
Abstract The binuclear complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate is stable in aqueous solution for some time (1 h) before it is irreversibly converted to (Pcts)FeIII–O–FeIII(Pcts). The photolysis of the peroxo complex in argon-saturated water leads to the release of oxygen and the formation of FeII(Pcts) with / = 5 · 104 at kirr = 333 nm. It is suggested that this photoredox reaction originates from a peroxide to FeIII LMCT state. It is populated from Pcts IL states which are initially reached by light absorption. 2005 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Charge transfer; Iron complexes; Peroxide complexes; Photochemistry; Phthalocyanine
1. Introduction Generally, peroxo complexes of transition metals are light sensitive [1–6]. The photoreactivity is associated with the peroxide ligand and based on the redox properties of O2 2 . Since peroxide is reducing as well as oxidizing it can serve as CT donor and acceptor, respectively. When O2 2 is coordinated to a reducing metal center, low-energy MLCT (metal-to-ligand charge transfer) excited states are present and determine the photochemical properties of the complex. The photolysis leads to an oxidation of the metal and reduction of peroxide to oxide. In combination with an oxidizing metal, lowenergy LMCT (ligand-to-metal charge transfer) transitions occur and initiate the reduction of the metal and oxidation of peroxide. In particular, Co(III) peroxide
complexes have been shown to undergo this type of photoredox reaction [2,3]. In this context, it is remarkable that the excited state properties of isoelectronic iron (III) complexes have not yet been investigated. We explored this possibility and selected the complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate for the present study. SO3
SO3 O3S
O3S
N
N
N
III
Fe N
O N
Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler).
O
0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.04.045
Fe N
N
N
N N SO3
SO3 O3 S
III
N
N *
N
N
N
N
O3 S
H. Kunkely, A. Vogler / Inorganica Chimica Acta 358 (2005) 4086–4088
This subject is certainly of interest in its own right, but also of importance with regard to biological implications since Fe(III) peroxo complexes can occur in natural systems [7] where they might be exposed to ambient light.
2. Experimental The mono sodium salt of phthalocyanine-4,4 0 ,400 ,4000 tetrasulfonic acid iron (III) oxygen complex trihydrate was commercially available from Aldrich and used without further purification. All solvents used for spectroscopic measurements were of spectrograde quality (‘‘Uvasol’’) from Merck. The light source used for irradiation was an Osram HBO 200 W/2 or a Hanovia Xe/Hg 977 B-1 (1 kW) lamp. Monochromatic light was obtained using Schott PIL/IL interference filters or a Schoeffel GM/1 highintensity monochromator (band width 18 nm) with additional Schott cutoff filters to avoid short-wavelength and second order irradiation. In all cases the light beam was focused on a stirrable photolysis cell by a quartz lens. The photolyses were carried out in a mixture of water/ethanol (99:1) under argon in 1 cm spectrophotometer cells at room temperature under argon. Progress of the photolyses was monitored by UV–Vis spectrophotometry. For quantum yield determinations the complex concentrations were such as to have essentially complete light absorption. The total amount of photolysis was limited to less than 5% to avoid light absorption by the photoproduct. Absorbed light intensities were determined by a Polytec pyroelectric radiometer, which was calibrated by ferrioxalate actinometry and equipped with an RkP-345 detector. Absorption spectra were measured with a Varian Cary 50 or a Uvikon 860 spectrophotometer. Emission and excitation spectra were recorded on a Hitachi 850 fluorescence spectrometer equipped with a Hamamatsu 928 photomultiplier for measurements up to 900 nm. The luminescence spectra were corrected for monochromator and photomultiplier efficiency variations.
3. Results and discussion For the present work, we used a compound which is offered by Aldrich as mono sodium salt of a phthalocyanine-4,4 0 ,400 ,4000 -tetrasulfonic acid iron (III) oxygen complex. This material has been originally prepared by Weber and Busch [8]. The oxygen adduct with the analytical composition C32H15N8O14S4NaFe · 3H2O is formed when solid Na+[FeII(Pcts)] is exposed to air [8]. The identity of this complex is not quite clear, but the oxygen can be reversibly removed at least for 30 cycles. Upon dissolution of this oxygen complex or
4087
FeII(Pcts) in water in the presence of air a blue solution is formed which is characterized by a long-wavelength absorption at kmax = 634 nm with e = 75,000 M1 cm1 [9]. This is a typical Pc (phthalocyanine) IL (intraligand) pp* band [10]. It appears at this wavelength if Pcts is coordinated to Fe(III). The nature of this complex in aqueous solution has been controversially discussed [9,11,12]. A critical review of the literature and our own experience support the assumption that we are dealing with the complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ which is essentially stable for a short period (1 h). In aerated aqueous solution the equilibrium 2FeII ðPctsÞ þ O2 ¡ ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ
ð1Þ
is shifted to the right as indicated by the blue colour of the solution (kmax = 634 nm). However, upon addition of various organic solvents such as ethanol, acetonitrile, acetone and dimethylformamide the equilibrium is shifted to the left side. The formation of FeII(Pcts) is associated with a colour change from blue to green [9]. Simultaneously, the 634-nm band disappears and is replaced by a band at kmax = 670 nm which indicates the presence of FeII(Pcts) [9]. The concomitant spectral changes show sharp isosbestic points at 708, 555, 384 and 302 nm. When the aqueous solution of ðPctsÞFeIII ðO2 2 Þ III Fe ðPctsÞ is kept for longer times, e.g., several hours, only small further spectral changes take place but the reversibility is lost. Upon addition of organic solvents the blue colour is preserved. We suggest that ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ underwent an irreversible conversion to (Pcts)FeIII–O–FeIII(Pcts) which does not react with inert organic solvents. The spectra of the lperoxo and l-oxo complex are rather similar because the energy of the IL Pcts absorptions are essentially determined by the oxidation state of iron which are the same in both cases. The irreversible oxidation of Fe(II) by oxygen to FeIII–O–FeIII complexes is a general feature of many iron (II) compounds. In the absence of any further substrates the l-oxo complex in H2O is rather light stable (kirr = 333 nm) while the l-peroxo complex is photosensitive. Upon irradiation of aqueous ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ, spectral variations are observed (Fig. 1) which are the same as those which are seen when organic solvents are added in the dark. It follows that the photolysis takes place according to the equation hm
ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ ! 2FeII ðPctsÞ þ O2 II
ð2Þ
While under argon the photolysis product Fe (Pcts) is stable, exposure of the photolyzed solution to air leads to the regeneration of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ. The photolysis requires light with kirr < 380 nm. The quantum yield at kirr = 333 nm amounts to 5 · 104. The photolysis with white light largely leads to an irreversible decomposition of the Pcts ligand as indicated by a
4088
H. Kunkely, A. Vogler / Inorganica Chimica Acta 358 (2005) 4086–4088
that IL excitation is followed by the population of a reactive O2 2 ! FeIII LMCT state, but for ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ this does not happen with long-wavelength irradiation (>400 nm). However, it is known that the quantum yield for photoreduction of Fe(III) complexes decreases with increasing wavelength of irradiation independent of the energy of the longest-wavelength LMCT absorption [21–23]. Accordingly, only higher-energy IL states of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ are apparently deactivated to reactive LMCT states. Fig. 1. Spectral changes during the photolysis of 5.18 · 105 ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ in a mixture of water/ethanol (99:1) under argon at room temperature after 0 min (a), 10 and 25 min (b) irradiation times with kirr = 333 nm (Hanovia Xe/Hg 977 B-1 lamp), 1-cm cell.
decrease of the optical density over the entire UV–Vis spectral region. When the photolysis of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ is carried out in the presence of diphenylacetylene (solvent:methanol/H2O = 1:1) the formation of benzil (extracted with toluene) takes place as indicated by the appearance of its characteristic green fluorescence (kmax = 507 nm). This observation suggests that O2 is photochemically released as singlet oxygen which is known to add to diphenylacetylene with the formation of benzil [13]. The photochemical ejection of 1O2 from a variety of transition metal complexes which contain O2 as coordinated dioxygen, peroxide or superoxide has been frequently observed although the requirements for this photochemical reaction path are not well defined [14–16]. The photochemical release of O2 from ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ is certainly induced by peroxide to FeIII LMCT excitation. Product formation reflects directly the charge distribution in the excited state. Moreover, other peroxide complexes with oxidizing metal centers show an analogous behavior [1– 4,14,15]. Indeed, several FeIII ðO2 2 ÞFeIII complexes have been shown to display long-wavelength ðO2 2 ! FeIII Þ LMCT absorptions between 500 and 700 nm [17–20], but their photoreactivity has not been studied. These complexes do not show interfering IL bands at long wavelength. Unfortunately, phthalocyanine complexes exhibit intense IL absorptions which obscure bands of different origin including LMCT absorptions. IL as well as LMCT bands of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ may appear at comparable energies. It is thus conceivable
References [1] M.H. Gubelmann, A.F. Williams, Struct. Bonding 55 (1983) 1. [2] H.R. Ma¨cke, A.F. Williams, in: M.A. Fox, M. Chanon (Eds.), Photoinduced Electron Transfer, Part D, Elsevier, Amsterdam, 1988, p. 28. [3] N. Shinohara, Trends Inorg. Chem. 2 (1991) 49. [4] I.A. Guzei, A. Bakac, Inorg. Chem. 40 (2001) 2390, and references cited therein. [5] A. Vogler, H. Kunkely, Coord. Chem. Rev. 171 (1998) 399. [6] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 8 (2005) 467. [7] D.L. Wertz, J.S. Valentine, Struct. Bonding 97 (2000) 37. [8] J.H. Weber, D.H. Busch, Inorg. Chem. 4 (1965) 469. [9] D. Vonderschmitt, H.K. Bernauer, S. Fallab, Helv. Chim. Acta 48 (1965) 951. [10] M.J. Stillman, T. Nyokong, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines, VCH, New York, 1989, p. 133. [11] G. McLendon, A.E. Martell, Inorg. Chem. 16 (1977) 1812. [12] C. Ercolani, M. Gardini, F. Monacelli, G. Pennesi, G. Rossi, Inorg. Chem. 22 (1983) 2584. [13] H. Sugimoto, D.T. Sawyer, J. Am. Chem. Soc. 106 (1984) 4283. [14] M. Seip, H.-D. Brauer, J. Photochem. Photobiol. A: Chem. 76 (1993) 1. [15] I. Hatzopoulos, W.R. Thiel, H.-D. Brauer, J. Photochem. Photobiol. A: Chem. 102 (1997) 151. [16] W.-D. Wang, J.H. Espenson, Inorg. Chem. 36 (1997) 5069, and references cited therein. [17] Y. Dong, S. Me´nage, B.A. Brennan, T.E. Elgren, H.G. Jang, L.L. Pearce, L. Que Jr., J. Am. Chem. Soc. 115 (1993) 1851. [18] Y. Hayashi, T. Kayatani, H. Sugimoto, M. Suzuki, K. Inomata, A. Uehara, Y. Mizutani, T. Kitagawa, Y. Maeda, J. Am. Chem. Soc. 117 (1995) 11220. [19] T. Ookubo, H. Sugimoto, T. Nagayama, H. Masuda, T. Sato, K. Tanaka, Y. Maeda, H. Okawa, Y. Hayashi, A. Uehara, M. Suzuki, J. Am. Chem. Soc. 118 (1996) 701. [20] K. Kim, S.J. Lippard, J. Am. Chem. Soc. 118 (1996) 4914. [21] V. Balzani, V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, New York, 1970, p. 161. [22] O. Horva´th, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, New York, 1993, p. 218. [23] J. Sima, J. Makanova, Coord. Chem. Rev. 160 (1997) 161.
Note
Photoredox reaction of ðPctsÞFeIII ðO22 ÞFeIIIðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate induced by peroxide to Fe(III) charge transfer excitation Horst Kunkely, Arnd Vogler
*
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany Received 22 March 2005; accepted 1 April 2005 Available online 14 July 2005
Abstract The binuclear complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate is stable in aqueous solution for some time (1 h) before it is irreversibly converted to (Pcts)FeIII–O–FeIII(Pcts). The photolysis of the peroxo complex in argon-saturated water leads to the release of oxygen and the formation of FeII(Pcts) with / = 5 · 104 at kirr = 333 nm. It is suggested that this photoredox reaction originates from a peroxide to FeIII LMCT state. It is populated from Pcts IL states which are initially reached by light absorption. 2005 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Charge transfer; Iron complexes; Peroxide complexes; Photochemistry; Phthalocyanine
1. Introduction Generally, peroxo complexes of transition metals are light sensitive [1–6]. The photoreactivity is associated with the peroxide ligand and based on the redox properties of O2 2 . Since peroxide is reducing as well as oxidizing it can serve as CT donor and acceptor, respectively. When O2 2 is coordinated to a reducing metal center, low-energy MLCT (metal-to-ligand charge transfer) excited states are present and determine the photochemical properties of the complex. The photolysis leads to an oxidation of the metal and reduction of peroxide to oxide. In combination with an oxidizing metal, lowenergy LMCT (ligand-to-metal charge transfer) transitions occur and initiate the reduction of the metal and oxidation of peroxide. In particular, Co(III) peroxide
complexes have been shown to undergo this type of photoredox reaction [2,3]. In this context, it is remarkable that the excited state properties of isoelectronic iron (III) complexes have not yet been investigated. We explored this possibility and selected the complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ with PctsH2 = phthalocyaninetetrasulfonate for the present study. SO3
SO3 O3S
O3S
N
N
N
III
Fe N
O N
Corresponding author. Tel.: +49 941 9434485; fax: +49 941 9434488. E-mail address: [email protected] (A. Vogler).
O
0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.04.045
Fe N
N
N
N N SO3
SO3 O3 S
III
N
N *
N
N
N
N
O3 S
H. Kunkely, A. Vogler / Inorganica Chimica Acta 358 (2005) 4086–4088
This subject is certainly of interest in its own right, but also of importance with regard to biological implications since Fe(III) peroxo complexes can occur in natural systems [7] where they might be exposed to ambient light.
2. Experimental The mono sodium salt of phthalocyanine-4,4 0 ,400 ,4000 tetrasulfonic acid iron (III) oxygen complex trihydrate was commercially available from Aldrich and used without further purification. All solvents used for spectroscopic measurements were of spectrograde quality (‘‘Uvasol’’) from Merck. The light source used for irradiation was an Osram HBO 200 W/2 or a Hanovia Xe/Hg 977 B-1 (1 kW) lamp. Monochromatic light was obtained using Schott PIL/IL interference filters or a Schoeffel GM/1 highintensity monochromator (band width 18 nm) with additional Schott cutoff filters to avoid short-wavelength and second order irradiation. In all cases the light beam was focused on a stirrable photolysis cell by a quartz lens. The photolyses were carried out in a mixture of water/ethanol (99:1) under argon in 1 cm spectrophotometer cells at room temperature under argon. Progress of the photolyses was monitored by UV–Vis spectrophotometry. For quantum yield determinations the complex concentrations were such as to have essentially complete light absorption. The total amount of photolysis was limited to less than 5% to avoid light absorption by the photoproduct. Absorbed light intensities were determined by a Polytec pyroelectric radiometer, which was calibrated by ferrioxalate actinometry and equipped with an RkP-345 detector. Absorption spectra were measured with a Varian Cary 50 or a Uvikon 860 spectrophotometer. Emission and excitation spectra were recorded on a Hitachi 850 fluorescence spectrometer equipped with a Hamamatsu 928 photomultiplier for measurements up to 900 nm. The luminescence spectra were corrected for monochromator and photomultiplier efficiency variations.
3. Results and discussion For the present work, we used a compound which is offered by Aldrich as mono sodium salt of a phthalocyanine-4,4 0 ,400 ,4000 -tetrasulfonic acid iron (III) oxygen complex. This material has been originally prepared by Weber and Busch [8]. The oxygen adduct with the analytical composition C32H15N8O14S4NaFe · 3H2O is formed when solid Na+[FeII(Pcts)] is exposed to air [8]. The identity of this complex is not quite clear, but the oxygen can be reversibly removed at least for 30 cycles. Upon dissolution of this oxygen complex or
4087
FeII(Pcts) in water in the presence of air a blue solution is formed which is characterized by a long-wavelength absorption at kmax = 634 nm with e = 75,000 M1 cm1 [9]. This is a typical Pc (phthalocyanine) IL (intraligand) pp* band [10]. It appears at this wavelength if Pcts is coordinated to Fe(III). The nature of this complex in aqueous solution has been controversially discussed [9,11,12]. A critical review of the literature and our own experience support the assumption that we are dealing with the complex ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ which is essentially stable for a short period (1 h). In aerated aqueous solution the equilibrium 2FeII ðPctsÞ þ O2 ¡ ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ
ð1Þ
is shifted to the right as indicated by the blue colour of the solution (kmax = 634 nm). However, upon addition of various organic solvents such as ethanol, acetonitrile, acetone and dimethylformamide the equilibrium is shifted to the left side. The formation of FeII(Pcts) is associated with a colour change from blue to green [9]. Simultaneously, the 634-nm band disappears and is replaced by a band at kmax = 670 nm which indicates the presence of FeII(Pcts) [9]. The concomitant spectral changes show sharp isosbestic points at 708, 555, 384 and 302 nm. When the aqueous solution of ðPctsÞFeIII ðO2 2 Þ III Fe ðPctsÞ is kept for longer times, e.g., several hours, only small further spectral changes take place but the reversibility is lost. Upon addition of organic solvents the blue colour is preserved. We suggest that ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ underwent an irreversible conversion to (Pcts)FeIII–O–FeIII(Pcts) which does not react with inert organic solvents. The spectra of the lperoxo and l-oxo complex are rather similar because the energy of the IL Pcts absorptions are essentially determined by the oxidation state of iron which are the same in both cases. The irreversible oxidation of Fe(II) by oxygen to FeIII–O–FeIII complexes is a general feature of many iron (II) compounds. In the absence of any further substrates the l-oxo complex in H2O is rather light stable (kirr = 333 nm) while the l-peroxo complex is photosensitive. Upon irradiation of aqueous ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ, spectral variations are observed (Fig. 1) which are the same as those which are seen when organic solvents are added in the dark. It follows that the photolysis takes place according to the equation hm
ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ ! 2FeII ðPctsÞ þ O2 II
ð2Þ
While under argon the photolysis product Fe (Pcts) is stable, exposure of the photolyzed solution to air leads to the regeneration of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ. The photolysis requires light with kirr < 380 nm. The quantum yield at kirr = 333 nm amounts to 5 · 104. The photolysis with white light largely leads to an irreversible decomposition of the Pcts ligand as indicated by a
4088
H. Kunkely, A. Vogler / Inorganica Chimica Acta 358 (2005) 4086–4088
that IL excitation is followed by the population of a reactive O2 2 ! FeIII LMCT state, but for ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ this does not happen with long-wavelength irradiation (>400 nm). However, it is known that the quantum yield for photoreduction of Fe(III) complexes decreases with increasing wavelength of irradiation independent of the energy of the longest-wavelength LMCT absorption [21–23]. Accordingly, only higher-energy IL states of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ are apparently deactivated to reactive LMCT states. Fig. 1. Spectral changes during the photolysis of 5.18 · 105 ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ in a mixture of water/ethanol (99:1) under argon at room temperature after 0 min (a), 10 and 25 min (b) irradiation times with kirr = 333 nm (Hanovia Xe/Hg 977 B-1 lamp), 1-cm cell.
decrease of the optical density over the entire UV–Vis spectral region. When the photolysis of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ is carried out in the presence of diphenylacetylene (solvent:methanol/H2O = 1:1) the formation of benzil (extracted with toluene) takes place as indicated by the appearance of its characteristic green fluorescence (kmax = 507 nm). This observation suggests that O2 is photochemically released as singlet oxygen which is known to add to diphenylacetylene with the formation of benzil [13]. The photochemical ejection of 1O2 from a variety of transition metal complexes which contain O2 as coordinated dioxygen, peroxide or superoxide has been frequently observed although the requirements for this photochemical reaction path are not well defined [14–16]. The photochemical release of O2 from ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ is certainly induced by peroxide to FeIII LMCT excitation. Product formation reflects directly the charge distribution in the excited state. Moreover, other peroxide complexes with oxidizing metal centers show an analogous behavior [1– 4,14,15]. Indeed, several FeIII ðO2 2 ÞFeIII complexes have been shown to display long-wavelength ðO2 2 ! FeIII Þ LMCT absorptions between 500 and 700 nm [17–20], but their photoreactivity has not been studied. These complexes do not show interfering IL bands at long wavelength. Unfortunately, phthalocyanine complexes exhibit intense IL absorptions which obscure bands of different origin including LMCT absorptions. IL as well as LMCT bands of ðPctsÞFeIII ðO2 2 ÞFeIII ðPctsÞ may appear at comparable energies. It is thus conceivable
References [1] M.H. Gubelmann, A.F. Williams, Struct. Bonding 55 (1983) 1. [2] H.R. Ma¨cke, A.F. Williams, in: M.A. Fox, M. Chanon (Eds.), Photoinduced Electron Transfer, Part D, Elsevier, Amsterdam, 1988, p. 28. [3] N. Shinohara, Trends Inorg. Chem. 2 (1991) 49. [4] I.A. Guzei, A. Bakac, Inorg. Chem. 40 (2001) 2390, and references cited therein. [5] A. Vogler, H. Kunkely, Coord. Chem. Rev. 171 (1998) 399. [6] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 8 (2005) 467. [7] D.L. Wertz, J.S. Valentine, Struct. Bonding 97 (2000) 37. [8] J.H. Weber, D.H. Busch, Inorg. Chem. 4 (1965) 469. [9] D. Vonderschmitt, H.K. Bernauer, S. Fallab, Helv. Chim. Acta 48 (1965) 951. [10] M.J. Stillman, T. Nyokong, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines, VCH, New York, 1989, p. 133. [11] G. McLendon, A.E. Martell, Inorg. Chem. 16 (1977) 1812. [12] C. Ercolani, M. Gardini, F. Monacelli, G. Pennesi, G. Rossi, Inorg. Chem. 22 (1983) 2584. [13] H. Sugimoto, D.T. Sawyer, J. Am. Chem. Soc. 106 (1984) 4283. [14] M. Seip, H.-D. Brauer, J. Photochem. Photobiol. A: Chem. 76 (1993) 1. [15] I. Hatzopoulos, W.R. Thiel, H.-D. Brauer, J. Photochem. Photobiol. A: Chem. 102 (1997) 151. [16] W.-D. Wang, J.H. Espenson, Inorg. Chem. 36 (1997) 5069, and references cited therein. [17] Y. Dong, S. Me´nage, B.A. Brennan, T.E. Elgren, H.G. Jang, L.L. Pearce, L. Que Jr., J. Am. Chem. Soc. 115 (1993) 1851. [18] Y. Hayashi, T. Kayatani, H. Sugimoto, M. Suzuki, K. Inomata, A. Uehara, Y. Mizutani, T. Kitagawa, Y. Maeda, J. Am. Chem. Soc. 117 (1995) 11220. [19] T. Ookubo, H. Sugimoto, T. Nagayama, H. Masuda, T. Sato, K. Tanaka, Y. Maeda, H. Okawa, Y. Hayashi, A. Uehara, M. Suzuki, J. Am. Chem. Soc. 118 (1996) 701. [20] K. Kim, S.J. Lippard, J. Am. Chem. Soc. 118 (1996) 4914. [21] V. Balzani, V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, New York, 1970, p. 161. [22] O. Horva´th, K.L. Stevenson, Charge Transfer Photochemistry of Coordination Compounds, VCH, New York, 1993, p. 218. [23] J. Sima, J. Makanova, Coord. Chem. Rev. 160 (1997) 161.