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Comment on “Analysis of mixed solvent effects on the properties of singlet oxygen (1Δg)” [Chem. Phys. 300 (2004) 33–39]
Chemical Physics 304 (2004) 315–316 www.elsevier.com/locate/chemphys
Comment on ‘‘Analysis of mixed solvent effects on the properties of singlet oxygen (1Dg)’’ [Chem. Phys. 300 (2004) 33–39] Reinhard Schmidt
*
Institut fu¨r Physikalische und Theoretische Chemie, J.W. Goethe-Universita¨t, Marie-Curie-Str.11, D60439 Frankfurt am Main, Germany Received 17 May 2004; accepted 23 June 2004 Available online 28 July 2004
Recently [1], literature data of mixed solvent effects on the rate constant of the charge transfer (CT) induced quenching and on the rate constant kp of the a1 Dg ! X3 R g radiative deactivation of singlet oxygen O2(1Dg) have been used to demonstrate the benefits of linear complexation energy relationships. Analysis was done in linear multi-parameter fits using quantities as independent variables, which characterize the solvent with respect to dipolarity/polarizability (SPP), basicity (SB), and acidity (SA). This analysis may provide some physical insight for the CT induced quenching of O2(1Dg). But this is not the case for the solvent dependence of the radiative rate constant kp. Different explanations have been presented for the strong solvent effect on kp [2], as is mentioned in [1]. But in the last years a consistent and quantitative understanding of the perturbation effects on the radiative transition of singlet oxygen has been gained by contributions of different groups [4–11]. This development is concealed in [1]. Instead, it is claimed to contribute by the investigation of [1] to the open question how solvent affects the a1 Dg ! X3 R g radiative transition of singlet oxygen. Using values of kp measured by Bilski et al. [3] for the binary solvent mixtures propylene carbonate/1,4-dioxane, water/1,4-dioxane, and water/acetonitrile and the corresponding SPP, SB and SA data it was found that the variation of kp in these three mixtures is well described by Eq. (1) log k p ¼ ð0:703 0:11ÞSPP þ ð1:10 0:07ÞSB ð0:60 0:02ÞSA ð1:38 0:11Þ
ð1Þ
*
Tel.: +49-69-798-29448; fax: +49-69-798-29445. E-mail address: [email protected] (R. Schmidt).
0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.06.043
This correlation, resting on the rather small data basis of only three solvent mixtures, led to the assertion that the variables SPP, SA, and SB can be used to rationalize the solvatochromism and spectroscopic properties of singlet oxygen in different types of molecular environment [1]. It was furthermore concluded that not only polarizability governs the changes of kp but also more specific interactions such as acidity and basicity have an important role on the kinetic of phosphorescence. The proof of these assertions is missing. The extremely weak solvatochromism of the a1 Dg ! X3 R g phosphorescence, that has been explained by Ogilby and coworkers [4,5] in experimental and theoretical studies which consider the importance of equilibrium and nonequilibrium solvation, is not at all treated in [1]. Furthermore, not even one of the three coefficients 0.70, 1.10, and 0.60 of the variables SSP, SB and SA of Eq. (1) is discussed. Thus, their possible physical meaning remains unclear. The independent term 1.38 of Eq. (1) is interpreted as value of log kp for the gas phase (kp = 4 · 102), which is claimed to be in good agreement with that calculated for the perturbed O2 molecule [1]. But that is not true. The kp data of [1] are relative numbers related to kp = 1 in benzene. With the absolute value kp = 1.5 s1 in benzene [2] the independent term yields kp = 6 · 102 s1, which has to be compared, if it is really independent, with kp = 2.3 · 104 s1 [2] of the unperturbed O2(1Dg) molecule. Therefore the kind of analysis of first-order rate constants kp performed in [1] provides no insight into the perturbation mechanism of the a1 Dg ! X3 R g emission in solution. Instead it produces unnecessary confusion in an area of photophysics already well understood, vide infra. The strictly forbidden a1 Dg ! X3 R g transition is enhanced in the gas phase by perturbations excerted in
316
R. Schmidt / Chemical Physics 304 (2004) 315–316
bimolecular collisions [6]. A further strong increase of kp is observed in going to the solution phase [2]. Apparently anomalous dependences of kp on the refractive index n or solvent polarizability P = (n2 1)/(n2 + 2) similar to those reported by Bilski et al. [3], which are the object of the investigation of [1], have been observed for the solvent mixtures water/acetone, acetone/benzene and methanol/chloroform [7]. These differing dependences kp = f(P) have all quantitatively been described and realized by simply using the second-order rate constants k caX and the molar concentrations of the mixture components for calculation of kp, and the molar concentrations and molar refractions R of the mixture components for calculation of the mixture polarizabilities P [7]. These results prove that the a1 Dg ! X3 R g emission is a bimolecular process in the liquid phase as well. That is the reason why second-order rate constants k caX should be analyzed to achieve an understanding of the perturbation mechanism in different solvents [2,7]. A theory on the perturbation of the radiative transitions of singlet oxygen developed by Minaev [8,9] demands the direct proportionality of the enhancement 1 þ 1 of the a1 Dg ! X3 R g and b Rg ! a Dg radiative transitions. Two effects cooperate: (1) The asymmetric shift of electron density from the collider into the antibonding pg,x and pg,y molecular orbitals of O2 induces transient dipole moments Mx and My of different magnitude. Hereby, electric dipole character is induced 3 1 1 into the b1 Rþ g ! a Dg transition. (2) The a Dg ! X Rg transition profits proportionally from the enhancement 1 of the b1 Rþ g ! a Dg transition by intensity borrowing due to the strong spin–orbit-coupling of O2. The transient shift of electron density into the MOs of O2 correlates with the molecular polarizability a of the collider, which is directly proportional to its molar refraction R. Thus, the second-order rate constants of both collision-induced radiative transitions should
mainly depend on the value of R of the collider [7]. This was proven to hold true by means of the analysis of second-order radiative rate constants for the a1 Dg ! X3 R g emission in 64 different pure solvents and the 1 b1 Rþ g ! a Dg emission in nine different pure gases [10]. After having removed for means of comparison the effects of refractive index, collision frequency and the collider size from the second-order radiative rate constants, it was discovered that the transition moments of both collision-induced radiative transitions of singlet oxygen are directly proportional to the molecular polarizability of the collider. Moreover, the experimentally determined ratio of enhancement of both radiative transitions agrees with the value predicted by MinaevÕs theory [8,9]. These findings have been recently confirmed by the results of an independent and complementary study of Andersen and Ogilby [11], who investigated the b1 Rþ a1 Dg absorption in solution. g
References [1] J. Catalan, C. Diaz, L. Barrio, Chem. Phys. 300 (2004) 33. [2] C. Schweitzer, R. Schmidt, Chem. Rev. 103 (2003) 1685. [3] P. Bilski, R.N. Holt, C.F. Chignell, J. Photochem. Photobiol. A: Chem. 109 (1997) 243. [4] N. Dam, T. Keszthelyi, L.K. Andersen, K.V. Mikkelsen, P.R. Ogilby, J. Phys. Chem. A 106 (2002) 5263. [5] T.D. Poulsen, P.R. Ogilby, K.V. Mikkelsen, J. Phys. Chem. A. 103 (1999) 3418. [6] R.M. Badger, A.C. Wright, R.F. Whitlock, J. Chem. Phys. 43 (1965) 4345. [7] R. Schmidt, F. Shafii, M. Hild, J. Phys. Chem. A 103 (1999) 2599. [8] B.F. Minaev, S. Lunell, G.I. Kobzev, J. Mol. Struct. (Theochem) 284 (1993) 1. [9] B.F. Minaev, H. Agren, J. Chem. Soc., Faraday Trans. 93 (1997) 2231. [10] M. Hild, R. Schmidt, J. Phys. Chem. A 103 (1999) 6091. [11] L.K. Andersen, P.R. Ogilby, J. Phys. Chem. A 106 (2002) 11064.
Comment on ‘‘Analysis of mixed solvent effects on the properties of singlet oxygen (1Dg)’’ [Chem. Phys. 300 (2004) 33–39] Reinhard Schmidt
*
Institut fu¨r Physikalische und Theoretische Chemie, J.W. Goethe-Universita¨t, Marie-Curie-Str.11, D60439 Frankfurt am Main, Germany Received 17 May 2004; accepted 23 June 2004 Available online 28 July 2004
Recently [1], literature data of mixed solvent effects on the rate constant of the charge transfer (CT) induced quenching and on the rate constant kp of the a1 Dg ! X3 R g radiative deactivation of singlet oxygen O2(1Dg) have been used to demonstrate the benefits of linear complexation energy relationships. Analysis was done in linear multi-parameter fits using quantities as independent variables, which characterize the solvent with respect to dipolarity/polarizability (SPP), basicity (SB), and acidity (SA). This analysis may provide some physical insight for the CT induced quenching of O2(1Dg). But this is not the case for the solvent dependence of the radiative rate constant kp. Different explanations have been presented for the strong solvent effect on kp [2], as is mentioned in [1]. But in the last years a consistent and quantitative understanding of the perturbation effects on the radiative transition of singlet oxygen has been gained by contributions of different groups [4–11]. This development is concealed in [1]. Instead, it is claimed to contribute by the investigation of [1] to the open question how solvent affects the a1 Dg ! X3 R g radiative transition of singlet oxygen. Using values of kp measured by Bilski et al. [3] for the binary solvent mixtures propylene carbonate/1,4-dioxane, water/1,4-dioxane, and water/acetonitrile and the corresponding SPP, SB and SA data it was found that the variation of kp in these three mixtures is well described by Eq. (1) log k p ¼ ð0:703 0:11ÞSPP þ ð1:10 0:07ÞSB ð0:60 0:02ÞSA ð1:38 0:11Þ
ð1Þ
*
Tel.: +49-69-798-29448; fax: +49-69-798-29445. E-mail address: [email protected] (R. Schmidt).
0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.06.043
This correlation, resting on the rather small data basis of only three solvent mixtures, led to the assertion that the variables SPP, SA, and SB can be used to rationalize the solvatochromism and spectroscopic properties of singlet oxygen in different types of molecular environment [1]. It was furthermore concluded that not only polarizability governs the changes of kp but also more specific interactions such as acidity and basicity have an important role on the kinetic of phosphorescence. The proof of these assertions is missing. The extremely weak solvatochromism of the a1 Dg ! X3 R g phosphorescence, that has been explained by Ogilby and coworkers [4,5] in experimental and theoretical studies which consider the importance of equilibrium and nonequilibrium solvation, is not at all treated in [1]. Furthermore, not even one of the three coefficients 0.70, 1.10, and 0.60 of the variables SSP, SB and SA of Eq. (1) is discussed. Thus, their possible physical meaning remains unclear. The independent term 1.38 of Eq. (1) is interpreted as value of log kp for the gas phase (kp = 4 · 102), which is claimed to be in good agreement with that calculated for the perturbed O2 molecule [1]. But that is not true. The kp data of [1] are relative numbers related to kp = 1 in benzene. With the absolute value kp = 1.5 s1 in benzene [2] the independent term yields kp = 6 · 102 s1, which has to be compared, if it is really independent, with kp = 2.3 · 104 s1 [2] of the unperturbed O2(1Dg) molecule. Therefore the kind of analysis of first-order rate constants kp performed in [1] provides no insight into the perturbation mechanism of the a1 Dg ! X3 R g emission in solution. Instead it produces unnecessary confusion in an area of photophysics already well understood, vide infra. The strictly forbidden a1 Dg ! X3 R g transition is enhanced in the gas phase by perturbations excerted in
316
R. Schmidt / Chemical Physics 304 (2004) 315–316
bimolecular collisions [6]. A further strong increase of kp is observed in going to the solution phase [2]. Apparently anomalous dependences of kp on the refractive index n or solvent polarizability P = (n2 1)/(n2 + 2) similar to those reported by Bilski et al. [3], which are the object of the investigation of [1], have been observed for the solvent mixtures water/acetone, acetone/benzene and methanol/chloroform [7]. These differing dependences kp = f(P) have all quantitatively been described and realized by simply using the second-order rate constants k caX and the molar concentrations of the mixture components for calculation of kp, and the molar concentrations and molar refractions R of the mixture components for calculation of the mixture polarizabilities P [7]. These results prove that the a1 Dg ! X3 R g emission is a bimolecular process in the liquid phase as well. That is the reason why second-order rate constants k caX should be analyzed to achieve an understanding of the perturbation mechanism in different solvents [2,7]. A theory on the perturbation of the radiative transitions of singlet oxygen developed by Minaev [8,9] demands the direct proportionality of the enhancement 1 þ 1 of the a1 Dg ! X3 R g and b Rg ! a Dg radiative transitions. Two effects cooperate: (1) The asymmetric shift of electron density from the collider into the antibonding pg,x and pg,y molecular orbitals of O2 induces transient dipole moments Mx and My of different magnitude. Hereby, electric dipole character is induced 3 1 1 into the b1 Rþ g ! a Dg transition. (2) The a Dg ! X Rg transition profits proportionally from the enhancement 1 of the b1 Rþ g ! a Dg transition by intensity borrowing due to the strong spin–orbit-coupling of O2. The transient shift of electron density into the MOs of O2 correlates with the molecular polarizability a of the collider, which is directly proportional to its molar refraction R. Thus, the second-order rate constants of both collision-induced radiative transitions should
mainly depend on the value of R of the collider [7]. This was proven to hold true by means of the analysis of second-order radiative rate constants for the a1 Dg ! X3 R g emission in 64 different pure solvents and the 1 b1 Rþ g ! a Dg emission in nine different pure gases [10]. After having removed for means of comparison the effects of refractive index, collision frequency and the collider size from the second-order radiative rate constants, it was discovered that the transition moments of both collision-induced radiative transitions of singlet oxygen are directly proportional to the molecular polarizability of the collider. Moreover, the experimentally determined ratio of enhancement of both radiative transitions agrees with the value predicted by MinaevÕs theory [8,9]. These findings have been recently confirmed by the results of an independent and complementary study of Andersen and Ogilby [11], who investigated the b1 Rþ a1 Dg absorption in solution. g
References [1] J. Catalan, C. Diaz, L. Barrio, Chem. Phys. 300 (2004) 33. [2] C. Schweitzer, R. Schmidt, Chem. Rev. 103 (2003) 1685. [3] P. Bilski, R.N. Holt, C.F. Chignell, J. Photochem. Photobiol. A: Chem. 109 (1997) 243. [4] N. Dam, T. Keszthelyi, L.K. Andersen, K.V. Mikkelsen, P.R. Ogilby, J. Phys. Chem. A 106 (2002) 5263. [5] T.D. Poulsen, P.R. Ogilby, K.V. Mikkelsen, J. Phys. Chem. A. 103 (1999) 3418. [6] R.M. Badger, A.C. Wright, R.F. Whitlock, J. Chem. Phys. 43 (1965) 4345. [7] R. Schmidt, F. Shafii, M. Hild, J. Phys. Chem. A 103 (1999) 2599. [8] B.F. Minaev, S. Lunell, G.I. Kobzev, J. Mol. Struct. (Theochem) 284 (1993) 1. [9] B.F. Minaev, H. Agren, J. Chem. Soc., Faraday Trans. 93 (1997) 2231. [10] M. Hild, R. Schmidt, J. Phys. Chem. A 103 (1999) 6091. [11] L.K. Andersen, P.R. Ogilby, J. Phys. Chem. A 106 (2002) 11064.