- Email: [email protected]
Excited state properties of diphenylmercury: photolysis and luminescence under ambient conditions
Inorganic Chemistry Communications 7 (2004) 741–743 www.elsevier.com/locate/inoche
Excited state properties of diphenylmercury: photolysis and luminescence under ambient conditions Horst Kunkely, Arnd Vogler
*
Institut f€ur Anorganische Chemie, Universit€at Regensburg, Universitaetsstrasse 31, D-93040, Regensburg, Germany Received 2 March 2004; accepted 8 March 2004 Available online 6 May 2004
Abstract In solution HgII Ph2 undergoes a photoredox decomposition (/ ¼ 0:006 at kirr ¼ 254 nm in CH3 CN) which is induced by ligandto-metal charge transfer excitation. In the solid state this decomposition is largely prevented but a phosphorescence appears which originates from the lowest-energy pp (phenyl) intraligand triplet. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Electronic Spectra; Photoluminescence; Photochemistry; Charge transfer; Mercury
Generally, simple mercury(II) compounds, in particular halide complexes, are not luminescent, but are frequently light sensitive [1]. In contrast, various Hg(II) complexes with cyclometalated phenyl ligands are known to show a luminescence [1–4]. These emissions have been assumed to originate from intraligand states and other excited states which are associated with metal–metal interactions. Moreover, many organometallic Hg(II) complexes are photoreactive [5–7]. Generally, the photolysis involves the homolytic cleavage of HgC r bonds. It is initiated by ligand-to-metal charge transfer (LMCT) excitation. Any relationship between photochemistry and photoluminescence has not yet been established. In this context, diphenylmercury seems to be of particular interest. HgPh2 can be considered to be the parent compound of all phenylated Hg(II) complexes. It is commercially available in high purity (Strem), thermally very stable and structurally well characterized [8]. Diphenylmercury has
Hg
* Corresponding author. Tel.: +49-941-9434485; fax: +49-9419434488. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.03.027
been shown to undergo a light-induced splitting of the HgPh bond [9–11]. Moreover, some preliminary reports on the photoluminescence of HgPh2 in low-temperature glasses have appeared many years ago [12,13] but the further significance of this emission has apparently not been evaluated. Accordingly, we decided to reexamine some of the excited state properties of HgPh2 and describe here our observations and conclusions. The electronic spectrum of HgPh2 in CH3 CN (Fig. 1) displays two absorptions at kmax ¼ 203 nm (e ¼ 18,000 M1 cm1 ) and 223 nm (13,700). At the long-wavelength side of the 223 nm band shoulders at 252, 257 and 264 are discernible. This spectrum agrees with that reported in the literature [14]. Solutions of HgPh2 are not luminescent. However, in accordance with previous observations [9–11] these solutions are light sensitive. The photolysis of HgPh2 in CH3 CN is accompanied by spectral changes (Fig. 2) which indicate the decomposition of HgPh2 . When the photolysis was carried out at higher concentrations of HgPh2 the formation of colloidal mercury became visible. It causes an absorption over the entire spectrum and its apparent extinction increases towards shorter wavelength [15,16]. Biphenyl was detected as another photoproduct by luminescence spectroscopy. It shows a diagnostic fluorescence which consists of a structured band [17] (kmax ¼ 325 nm in CH3 CN). The disappearance of HgPh2 was monitored
742
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 7 (2004) 741–743
transitions of the aryl ligands must occur. Indeed, the lower-energy tail of the longest-wavelength band of HgPh2 at kmax ¼ 223 nm (Fig. 1) shows inflections at 252, 257 and 264 nm which resemble the absorption maxima of many simple benzene derivatives [17]. These pp IL bands of HgPh2 are partially obscured by the more intense absorption at kmax ¼ 223 nm which is attributed to a (phenyl ! HgII ) LMCT transition. In analogy to the photochemical behavior of simple Hg(II) complexes [1] LMCT excitation of HgPh2 leads to a photolysis according to the following scheme: hm
Fig. 1. Electronic absorption, excitation and emission spectra of diphenylmercury under argon at room temperature. Absorption: 6.27 105 M in CH3 CN, 1-cm cell; excitation (solid, kem ¼ 540 nm) and emission (solid, kexc ¼ 300 nm), intensity in arbitrary units.
Fig. 2. Spectral changes during the photolysis of 1.03 104 M diphenylmercury in CH3 CN under argon at room temperature after 0 min (a), 1, 2 and 4 min (b) irradiation times with kirr ¼ 254 nm (Hanovia Xe/Hg 977 B-1, 1 kW lamp), 1-cm cell.
at 223 nm. The quantum yield of decomposition was / ¼ 0:006 at kirr ¼ 254 nm. In the solid state HgPh2 is only slightly light sensitive but it shows a bright green photoluminescence (Fig. 1) at kmax ¼ 482 nm. This emission appears irrespective of the exciting wavelength. Interestingly, it can be also excited at wavelength above 290 nm. In this region where absorptions are not visible (Fig. 1) a structured excitation spectrum with maxima at 290, 325 and 357 nm is observed (Fig. 1). The slight photosensitivity of solid HgPh2 prevents reliable lifetime measurements. The cation Hg2þ is characterized by an empty valence shell with the electron configuration 6s0 6p0 . Accordingly, simple coordination compounds such as HgII X2 , II 2 HgII X 3 and Hg X4 with X ¼ halide or pseudohalide show only LMCT absorptions in their electronic spectra [1]. Such LMCT bands determine also the absorption spectra of many organometallic Hg(II) compounds [7]. In the case of Hg(II) aryl complexes additional pp
HgII Ph2 !HgI Ph þ Ph-radical
ð1Þ
HgI Ph ! Hg0 þ Ph-radical
ð2Þ
Elemental mercury is finally formed while the phenyl radicals participate in secondary processes, in particular hydrogen abstractions [9,10]. In addition, they can dimerize to biphenyl. In the solid state the photoredox decomposition of HgPh2 is largely suppressed owing to the rigid lattice structure. Instead, a luminescence appears (Fig. 1) which we attribute to an pp (phenyl) IL state. This emission is certainly a molecular property because there are no short intermolecular contacts in solid HgPh2 [8]. Moreover, it appears also in low-temperature glasses. The emission of HgPh2 is unlikely to be a fluorescence since luminescence and absorption spectra do not overlap. Accordingly, the luminescence of HgPh2 is assumed to be an IL phosphorescence. This conclusion is supported by the observation that a few other Hg(II) compounds display also an IL phosphorescence under ambient conditions [3,18] owing to the heavy-atom effect of mercury. However, the IL phosphorescence of HgPh2 does not seem to originate from the single phenyl ligands since the emission (Fig. 1) appears at much longer wavelength than the phosphorescence of benzene [19,20]. In addition, the excitation spectrum of HgPh2 (Fig. 1) shows bands at rather long wavelengths (kmax ¼ 290, 325 and 357 nm). These bands are apparently too weak to be observed in absorption. They are assigned to singlet–triplet IL transitions. Such spin-forbidden transitions have been detected in the phosphorescence excitation spectra of numerous aromatic compounds [20,21]. The phosphorescence spectrum of HgPh2 at room temperature (Fig. 1) roughly resembles that of biphenyl in a glassy matrix at 77 K (kmax ¼ 470 nm) [21]. In the ground state both phenyl groups of biphenyl interact only weakly and there is no net p-conjugation between the rings [22]. In contrast, the lowest-energy pp states of biphenyl are associated with a bonding interaction between both rings. As a consequence a contraction take place which causes the relatively large Stokes shift from absorption to emission. We suggest that this explanation applies also to HgPh2 . The electronic coupling of both phenyl ligands is mediated by Hg(II). While in the
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 7 (2004) 741–743
ground state the phenyl rings largely exist as separate ligands, pp IL excitation leads also to a bonding between both phenyl ligands since the HOMO is p-antibonding, while the LUMO is p-bonding with respect to the phenyl–phenyl interaction. This ligand–ligand coupling is facilitated by the coplanarity of both phenyl rings as it occurs in solid HgPh2 [8]. At this point it should be mentioned that an analogous picture applies to Mg(g-Cp)2 with Cp ¼ cyclopentadienyl. In this case both aromatic Cp rings which are aligned in a sandwich fashion undergo also a bonding interaction in the lowest-energy IL state [23]. In summary, solid HgPh2 shows a room temperature phosphorescence which originates from an IL pp triplet. This emission is based on an intramolecular intraligand interaction. In solution this luminescence is absent because the IL triplet is deactivated to a reactive LMCT state at higher energies. LMCT excitation leads to the reduction of Hg(II) to elemental mercury and oxidation of the anionic phenyl ligands to phenyl radicals which undergo secondary reactions including dimerization to biphenyl. References [1] H. Kunkely, O. Horvath, A. Vogler, Coord. Chem. Rev. 159 (1997) 85. [2] H. Kunkely, A. Vogler, Chem. Phys. Lett. 164 (1989) 621. [3] C.W. Chan, S.M. Peng, C.M. Che, Inorg. Chem. 33 (1994) 3656.
743
[4] M.R. Haneline, M. Tsunoda, F.P. Gabbai, J. Am. Chem. Soc. 124 (2002) 3737. [5] K.C. Bass, Organomet. Chem. Rev. 1 (1966) 391. [6] V. Balzani, V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, New York, 1970, p. 281. [7] O. Horvath, A. Vogler, Inorg. Chem. Commun. 1 (1998) 270, and ref. cited therein. [8] D. Grdenic, B. Kamenar, A. Nagl, Acta Cryst. B 33 (1977) 587, and ref. cited therein. [9] W.A. Cramer, J. Phys. Chem. 71 (1967) 1171. [10] T. Wolff, J. Photochem. 18 (1982) 269. [11] A. Cerillos, M.A. Pradera Adrian, J.A. Navio, J. Photochem. Photobiol. A: Chemistry 84 (1994) 299. [12] M.S. Fadeeva, V.A. Kuznetsov, Y.A. Kaplin, J. Appl. Spectrosc. 10 (1969) 359; Zh. Prikl. Spectros. 10 (1969) 547. [13] A.V. Shabanov, V.A. Kuznetsov, A.N. Egorochkin, S.F. Zhiltsov, G.A. Razuvaev, Doklady Chemistry 239 (1978) 132; Doklady Akad. Nauk 239 (1978) 617. [14] B.G. Gowenlock, J. Trotman, J. Chem. Soc. (1955) 1454. [15] H. Kunkely, A. Vogler, Polyhedron 8 (1989) 2731. [16] O. Horvath, P.C. Ford, A. Vogler, Inorg. Chem. 32 (1993) 2614. [17] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971. [18] H. Kunkely, A. Vogler, J. Photochem. Photobiol. A: Chemistry 144 (2001) 69. [19] H. Sponer, Y. Kanda, L.A. Blackwell, Spectrochim. Acta 16 (1960) 1135. [20] J.B. Birks, The Photophysics of Aromatic Molecules, Wiley, New York, 1970. [21] A.P. Marchetti, D.R. Kearns, J. Am. Chem. Soc. 89 (1967) 768. [22] M. Rubio, M. Merchan, E. Orti, B.O. Roos, Chem. Phys. Lett. 234 (1995) 373. [23] H. Kunkely, A. Vogler (submitted).
Excited state properties of diphenylmercury: photolysis and luminescence under ambient conditions Horst Kunkely, Arnd Vogler
*
Institut f€ur Anorganische Chemie, Universit€at Regensburg, Universitaetsstrasse 31, D-93040, Regensburg, Germany Received 2 March 2004; accepted 8 March 2004 Available online 6 May 2004
Abstract In solution HgII Ph2 undergoes a photoredox decomposition (/ ¼ 0:006 at kirr ¼ 254 nm in CH3 CN) which is induced by ligandto-metal charge transfer excitation. In the solid state this decomposition is largely prevented but a phosphorescence appears which originates from the lowest-energy pp (phenyl) intraligand triplet. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Electronic Spectra; Photoluminescence; Photochemistry; Charge transfer; Mercury
Generally, simple mercury(II) compounds, in particular halide complexes, are not luminescent, but are frequently light sensitive [1]. In contrast, various Hg(II) complexes with cyclometalated phenyl ligands are known to show a luminescence [1–4]. These emissions have been assumed to originate from intraligand states and other excited states which are associated with metal–metal interactions. Moreover, many organometallic Hg(II) complexes are photoreactive [5–7]. Generally, the photolysis involves the homolytic cleavage of HgC r bonds. It is initiated by ligand-to-metal charge transfer (LMCT) excitation. Any relationship between photochemistry and photoluminescence has not yet been established. In this context, diphenylmercury seems to be of particular interest. HgPh2 can be considered to be the parent compound of all phenylated Hg(II) complexes. It is commercially available in high purity (Strem), thermally very stable and structurally well characterized [8]. Diphenylmercury has
Hg
* Corresponding author. Tel.: +49-941-9434485; fax: +49-9419434488. E-mail address: [email protected] (A. Vogler).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.03.027
been shown to undergo a light-induced splitting of the HgPh bond [9–11]. Moreover, some preliminary reports on the photoluminescence of HgPh2 in low-temperature glasses have appeared many years ago [12,13] but the further significance of this emission has apparently not been evaluated. Accordingly, we decided to reexamine some of the excited state properties of HgPh2 and describe here our observations and conclusions. The electronic spectrum of HgPh2 in CH3 CN (Fig. 1) displays two absorptions at kmax ¼ 203 nm (e ¼ 18,000 M1 cm1 ) and 223 nm (13,700). At the long-wavelength side of the 223 nm band shoulders at 252, 257 and 264 are discernible. This spectrum agrees with that reported in the literature [14]. Solutions of HgPh2 are not luminescent. However, in accordance with previous observations [9–11] these solutions are light sensitive. The photolysis of HgPh2 in CH3 CN is accompanied by spectral changes (Fig. 2) which indicate the decomposition of HgPh2 . When the photolysis was carried out at higher concentrations of HgPh2 the formation of colloidal mercury became visible. It causes an absorption over the entire spectrum and its apparent extinction increases towards shorter wavelength [15,16]. Biphenyl was detected as another photoproduct by luminescence spectroscopy. It shows a diagnostic fluorescence which consists of a structured band [17] (kmax ¼ 325 nm in CH3 CN). The disappearance of HgPh2 was monitored
742
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 7 (2004) 741–743
transitions of the aryl ligands must occur. Indeed, the lower-energy tail of the longest-wavelength band of HgPh2 at kmax ¼ 223 nm (Fig. 1) shows inflections at 252, 257 and 264 nm which resemble the absorption maxima of many simple benzene derivatives [17]. These pp IL bands of HgPh2 are partially obscured by the more intense absorption at kmax ¼ 223 nm which is attributed to a (phenyl ! HgII ) LMCT transition. In analogy to the photochemical behavior of simple Hg(II) complexes [1] LMCT excitation of HgPh2 leads to a photolysis according to the following scheme: hm
Fig. 1. Electronic absorption, excitation and emission spectra of diphenylmercury under argon at room temperature. Absorption: 6.27 105 M in CH3 CN, 1-cm cell; excitation (solid, kem ¼ 540 nm) and emission (solid, kexc ¼ 300 nm), intensity in arbitrary units.
Fig. 2. Spectral changes during the photolysis of 1.03 104 M diphenylmercury in CH3 CN under argon at room temperature after 0 min (a), 1, 2 and 4 min (b) irradiation times with kirr ¼ 254 nm (Hanovia Xe/Hg 977 B-1, 1 kW lamp), 1-cm cell.
at 223 nm. The quantum yield of decomposition was / ¼ 0:006 at kirr ¼ 254 nm. In the solid state HgPh2 is only slightly light sensitive but it shows a bright green photoluminescence (Fig. 1) at kmax ¼ 482 nm. This emission appears irrespective of the exciting wavelength. Interestingly, it can be also excited at wavelength above 290 nm. In this region where absorptions are not visible (Fig. 1) a structured excitation spectrum with maxima at 290, 325 and 357 nm is observed (Fig. 1). The slight photosensitivity of solid HgPh2 prevents reliable lifetime measurements. The cation Hg2þ is characterized by an empty valence shell with the electron configuration 6s0 6p0 . Accordingly, simple coordination compounds such as HgII X2 , II 2 HgII X 3 and Hg X4 with X ¼ halide or pseudohalide show only LMCT absorptions in their electronic spectra [1]. Such LMCT bands determine also the absorption spectra of many organometallic Hg(II) compounds [7]. In the case of Hg(II) aryl complexes additional pp
HgII Ph2 !HgI Ph þ Ph-radical
ð1Þ
HgI Ph ! Hg0 þ Ph-radical
ð2Þ
Elemental mercury is finally formed while the phenyl radicals participate in secondary processes, in particular hydrogen abstractions [9,10]. In addition, they can dimerize to biphenyl. In the solid state the photoredox decomposition of HgPh2 is largely suppressed owing to the rigid lattice structure. Instead, a luminescence appears (Fig. 1) which we attribute to an pp (phenyl) IL state. This emission is certainly a molecular property because there are no short intermolecular contacts in solid HgPh2 [8]. Moreover, it appears also in low-temperature glasses. The emission of HgPh2 is unlikely to be a fluorescence since luminescence and absorption spectra do not overlap. Accordingly, the luminescence of HgPh2 is assumed to be an IL phosphorescence. This conclusion is supported by the observation that a few other Hg(II) compounds display also an IL phosphorescence under ambient conditions [3,18] owing to the heavy-atom effect of mercury. However, the IL phosphorescence of HgPh2 does not seem to originate from the single phenyl ligands since the emission (Fig. 1) appears at much longer wavelength than the phosphorescence of benzene [19,20]. In addition, the excitation spectrum of HgPh2 (Fig. 1) shows bands at rather long wavelengths (kmax ¼ 290, 325 and 357 nm). These bands are apparently too weak to be observed in absorption. They are assigned to singlet–triplet IL transitions. Such spin-forbidden transitions have been detected in the phosphorescence excitation spectra of numerous aromatic compounds [20,21]. The phosphorescence spectrum of HgPh2 at room temperature (Fig. 1) roughly resembles that of biphenyl in a glassy matrix at 77 K (kmax ¼ 470 nm) [21]. In the ground state both phenyl groups of biphenyl interact only weakly and there is no net p-conjugation between the rings [22]. In contrast, the lowest-energy pp states of biphenyl are associated with a bonding interaction between both rings. As a consequence a contraction take place which causes the relatively large Stokes shift from absorption to emission. We suggest that this explanation applies also to HgPh2 . The electronic coupling of both phenyl ligands is mediated by Hg(II). While in the
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 7 (2004) 741–743
ground state the phenyl rings largely exist as separate ligands, pp IL excitation leads also to a bonding between both phenyl ligands since the HOMO is p-antibonding, while the LUMO is p-bonding with respect to the phenyl–phenyl interaction. This ligand–ligand coupling is facilitated by the coplanarity of both phenyl rings as it occurs in solid HgPh2 [8]. At this point it should be mentioned that an analogous picture applies to Mg(g-Cp)2 with Cp ¼ cyclopentadienyl. In this case both aromatic Cp rings which are aligned in a sandwich fashion undergo also a bonding interaction in the lowest-energy IL state [23]. In summary, solid HgPh2 shows a room temperature phosphorescence which originates from an IL pp triplet. This emission is based on an intramolecular intraligand interaction. In solution this luminescence is absent because the IL triplet is deactivated to a reactive LMCT state at higher energies. LMCT excitation leads to the reduction of Hg(II) to elemental mercury and oxidation of the anionic phenyl ligands to phenyl radicals which undergo secondary reactions including dimerization to biphenyl. References [1] H. Kunkely, O. Horvath, A. Vogler, Coord. Chem. Rev. 159 (1997) 85. [2] H. Kunkely, A. Vogler, Chem. Phys. Lett. 164 (1989) 621. [3] C.W. Chan, S.M. Peng, C.M. Che, Inorg. Chem. 33 (1994) 3656.
743
[4] M.R. Haneline, M. Tsunoda, F.P. Gabbai, J. Am. Chem. Soc. 124 (2002) 3737. [5] K.C. Bass, Organomet. Chem. Rev. 1 (1966) 391. [6] V. Balzani, V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, New York, 1970, p. 281. [7] O. Horvath, A. Vogler, Inorg. Chem. Commun. 1 (1998) 270, and ref. cited therein. [8] D. Grdenic, B. Kamenar, A. Nagl, Acta Cryst. B 33 (1977) 587, and ref. cited therein. [9] W.A. Cramer, J. Phys. Chem. 71 (1967) 1171. [10] T. Wolff, J. Photochem. 18 (1982) 269. [11] A. Cerillos, M.A. Pradera Adrian, J.A. Navio, J. Photochem. Photobiol. A: Chemistry 84 (1994) 299. [12] M.S. Fadeeva, V.A. Kuznetsov, Y.A. Kaplin, J. Appl. Spectrosc. 10 (1969) 359; Zh. Prikl. Spectros. 10 (1969) 547. [13] A.V. Shabanov, V.A. Kuznetsov, A.N. Egorochkin, S.F. Zhiltsov, G.A. Razuvaev, Doklady Chemistry 239 (1978) 132; Doklady Akad. Nauk 239 (1978) 617. [14] B.G. Gowenlock, J. Trotman, J. Chem. Soc. (1955) 1454. [15] H. Kunkely, A. Vogler, Polyhedron 8 (1989) 2731. [16] O. Horvath, P.C. Ford, A. Vogler, Inorg. Chem. 32 (1993) 2614. [17] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971. [18] H. Kunkely, A. Vogler, J. Photochem. Photobiol. A: Chemistry 144 (2001) 69. [19] H. Sponer, Y. Kanda, L.A. Blackwell, Spectrochim. Acta 16 (1960) 1135. [20] J.B. Birks, The Photophysics of Aromatic Molecules, Wiley, New York, 1970. [21] A.P. Marchetti, D.R. Kearns, J. Am. Chem. Soc. 89 (1967) 768. [22] M. Rubio, M. Merchan, E. Orti, B.O. Roos, Chem. Phys. Lett. 234 (1995) 373. [23] H. Kunkely, A. Vogler (submitted).