- Email: [email protected]
3CTC as the main step in the quenching of 3BP by electron and hydrogen donors
J. Photochem.
Photobioi.
A: Chem., 56 (1991) 159-163
3CTC as the main step in the quenching electron and hydrogen donors
159
of 3BP by
P. Jacques Laboratoire de Photochimie Gt+zt+ale associt! au CNRS, Ecole Nationale de Mulhouse, 3 rue A. Werner, 68200 Mulhouse (France)
Superieure de Chimie
(Received June 29, 1990)
Abstract It is shown that the quenching of triplet benzophenone by electron and hydrogen donors involves a charge transfer state (‘CTC) as the governing step. Moreover, it is shown for the first time that the aliphatic donors follow fully the simple model of Rehm and Weller for electron transfer: the rate of quenching increases as the ionization Potential (IP) of the donor decreases and reaches a limiting value when IP is 9 eV or less, i.e. for most amines. It is confirmed that alkyl benzenes lie on another line. However, the slopes of the rr and n lines are nearly the same in contrast with previous reports. The different behaviour of n- and n quenchers is tentatively addressed.
1. Introduction On photoexcitation, benzophenone (BP) undergoes intersystem crossing rapidly (within a few tens of picoseconds) and efficiently (#r= 1) to populate its lowest triplet state (3BP). This triplet state is responsible for most of its photochemistry: electronic energy transfer, addition to unsaturated compounds, charge transfer reactions (where BP can behave as an electron donor or acceptor) and hydrogen abstraction. These last two processes have been extensively investigated in an attempt to discover the intimate mechanism of deactivation of the triplet [l-6]. For electron donors, the participation of a charge transfer state (3CIC) is well documented [6]; the pioneering work of Guttenplan and Cohen [2], where the logarithm of the deactivation rate was linearly related to the ionization potential of the donors infers a charge transfer mechanism. With hydrogen donors, e.g. tetrahydrofuran (THF), 2-propanol and cyclohexane, the situation is not so clear. It has been suggested that a charge transfer state (3CTC) is involved in the first step of deactivation, followed by the formation of a ketyl radical with various quantum yields & depending on the substrate [3]. Recently, it has been proposed, on the basis of studies of the primary process of BP photoreduction under pressure, that the transition state is not polar, suggesting that triplet benzophenone is radical-like in reactivity [5]. It can be seen from the study of Formosinho and coworkers [4] that the reactivity of 3BP cannot be explained by consideration of only the C-H or O-H bond energy (see Table 1). In this paper, we tentatively address this question through a critical examination of the literature and new quenching data; a clfarge transfer state (3CTC) can explain the reactivity of 3BP towards electron donors and hydrogen donors.
Q Elsevier Sequoia/Printed in The Netherlands
160 TABLE
1
Quenching of 3BP in benzene Compound
1 2-Propanol
log k,’
.IPb
DWf) (kcal mol-‘)
10.40 10.30
88 94 91
2
Cyclohexane
6.26 5.56d
3 4
THF Di-n-propylether
6.63 7.20
9.80’ 9.49
2-Butylamine t-Butylamine Cycloheqlamine N-Methyl-2-butylamine Di-n-propylamine Triethylamine Tri-n-butytamine DABCO’
8.40 7.81 8.51 9.16 9.54 9.49 9.54 9.52
9.30 9.25 9.16 8.63 8.54 8.10 7.98 7.70
4.25 5.90 6.40 8.42 9.10
9.25 8.90 8.60 8.05 7.90
5 6 7 8 9 IO 11 12
13 Benzene 14 Toluene 15 p-Xylene 16 Durene 17 Hexamethylbenzene
94
aFrom refs. l-3 and 7 and references cited therein except for 4, 11 and 14-17 (this work). bFrom refs. 7-9. Worn ref. 4. dNeat solvent. ‘From ref. 10. fl,4-Diazabicyclo[2.2.2]octane. 2. Results
and discussion
The choice of quenchers and the use of the vertical ionization potential (JP) as the thermodynamic parameter are based on the observations reported in ref. 7: JP values are more common and reliable than the adiabatic ionization potential (JP) or the redox potential (Em). Indeed, the use of JP or Em would not have affected the conclusions of this work. 2. I Aromatic quenchers
From previous studies it is known that benzene derivatives are less efficient quenchers than amines [2, 111. However, information on 3BP quenching by alkyl benzenes is rather scarce. Guttenplan and Cohen [2] reported data for benzene and toluene and recently Wagner et al. [ll] in a thorough study of the photoreduction of phenyl ketones by alkyl benzenes, considered toluene andp-xylene in acetonitrile. Our extended data clearly indicate that all@ benzene compounds lie on a v line with a slope of 0.126 mol kcal-’ which is considerably less than the value of 0.74 mol kcal-’ corresponding to full charge transfer [12]. Moreover, this rr line enables us to substantiate the self-quenching mechanism of triplet benzophenone proposed by Wolf et al. [13]. Indeed it has been proposed that self-quenching of 3BP involves the formation of an exciplex, wherein the
161
half-filled n orbital of ‘BP is directed towards the r electron density of the aromatic ring of ground BP. This proposition is confirmed here: the ,IP value of benzophenone is 9.05 [8] and the average value of kso is 3 X ld M-’ s-l [13,14]; thus the corresponding
point lies on the 71‘line (Fig. 1) and benzophenone with the above-mentioned mechanism.
behaves as a r donor in accordance
2.2. Alipha tic compounds If, in the first instance, we do not consider 2-propanol and cyclohexane (see later), we see that the aliphatic compounds lie on two lines: one corresponds to the limiting rate of reaction between 3BP and quenchers (log k,=9.6), and the second shows an increase in log k, as .IP decreases with a slope somewhat less than that of the v
line: 0.107 mol kcal-‘. Therefore the quenching of 3BP by aliphatic compounds follows the expected plot from the Rehm-Weller [12] theory on electron transfer; the unusual feature of this result is that it has been overlooked hitherto, since all previous reports mention only one line. Moreover, our interpretation leads to a ratio between the r and n slopes (1.18), which is nearer to unity than the value of 1.57 reported in ref. 2, suggesting nearly the same percentage of charge transfer for n and v donors.
-8
-6
-4
10
VIP
Fig. 1. The n and VTlines (log kq vs. the vertical ionization potential (JP) of the donors). The numbers refer to the compounds in Table 1.
162
It is the scatter of the points reported in refs. 2 and 7 that induced these workers to consider only one regression line. There are two main reasons for this scatter: (i) a steric effect can occur in the case of amines substituted by bulky groups (consider, for example, the fluorescence quenching of arylbenzo[b]thiophenes by aliphatic amines [15]; such an effect is well illustrated by tribenzylamine (log k,, = 9.05; lowest ,IP - 8.05 [7]) where the rate is substantially lower than the limiting rate (log kg= 9.6)reported here); (ii) for some aliphatic donors, a contribution from hydrogen abstraction to the quenching rate cannot be completely discarded. The high rates for 2-propanol and, to a less extent for cyclohexane, in view of their JP values, suggest that direct hydrogen abstraction competes with charge transfer. Very recently it has been shown by Scaiano and coworkers [16] that decafluorobenzophenone triplet quenching involves hydrogen transfer and/or charge transfer in the case of phenol, 2-propanol and cyclohexane. It remains to explain why, in the case of 3BP, aromatic quenchers are considerably less efficient than aliphatic quenchers, whereas slower electron transfer quenching by aliphatic amines than aromatic amines has been observed for Cr(bpy),3+ and Ru(bpy)32+ in acetonitrile (bpy = bipyridine) [17]. We suggest that the coulombic term t?/~a of the Rehm-Weller relation plays a key role [18] since benzene is a non-polar solvent with a dielectric constant E of approximately 2.3 and the centre-to-centre point-charge separation a is certainly different for aromatic and aliphatic quenchers. Therefore solvent effects and geometrical considerations need to be investigated more thoroughly in order to be able to explain these apparent discordances.
3. Conclusions A critical examination of the literature and new data on benzophenone triplet quenching in benzene has enabled us to propose a unique mechanism: both electron and hydrogen donors act through a charge transfer state (3CTC) as the first step. Even cyclohexane behaves mainly as an electron donor. However, for strong hydrogen donors, direct hydrogen abstraction may contribute substantially to the quenching rates. New aspects of the different behaviour of n and ‘TTdonors are revealed through a treatment of the experimental data which differs from previous points of view. Moreover, this report illustrates the difficulty of finding a convenient set of donors in order to demonstrate that a quenching mechanism is of the charge transfer type, especially in the case of 3BP due to possible hydrogen abstraction. In a forthcoming paper we present further examples of this important problem for photochemists.
References 1 J. C. Scaiano, .I. Photochem., 2 (1973/1974) 81. 2 J. B. Guttenplan and S. G. Cohen, Z, Am. Chem. Sec., 94 (1972) 4040. 3 S. Inbar, H. Linschitz and S. G. Cohen, .I. Am. Chem. Sot., I02 (1980) 4
5 6 7
8
1419; 103 (1981) 1048. L. G. Amaut, S. J. Formosinho and A. M. da Silva, J. Photochem., 27 (1984) 185. M. Okamoto and H. Teranishi, J. Am. Chem. Sot., I08 (1986) 6378. M. Hoshino and H. Shizuka, in M. A. Fox and M. Chanon (eds.), Photoinduced Electron Transfer, Part C, Elsevier, Amsterdam, 1988, p, 313. P. Jacques, Chem. Phys. Lett., 142 (1987) 96. Ionization Potential and Appearance Potential Measurements 1971-1981, 1982 (NSRDS, U.S. Department of Commerce).
163 9 M. Takahashi, I. Watanabe and S. Ikeda, J. Electron Spctrosc., 37 (1985) 275. 10 G. Levy and P. de Lath, C. R Acud. Sci., Ser. C, 279 (1974) 931. 11 P. J. Wagner, R. J. Truman, A. E. Puchalski and R. Wake, J. Am. Chem. SK, 108 (1986) 7727. 12 D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 73 (1969) 834; Isr. J. Chem., 8 (1970) 259. 13 M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer and J. H. Parks, J. Am. Chem. Sot., 97 (1975) 4490. 14 D. I. Schuster and T. M. Weil, J. Am. Chem. Sot., 95 (1973) 4091. 15 A. Brehon, A. Couture, A. Lablache-Combier and A. Pollet, N. J. Chim., 5 (1981) 243. 16 D. R. Boate, L. J. Johnston and J. C. Scaiano, Can. J. Chem., 67 (1989) 927. 17 R. Ballardini, G. Varani, M. T. Indelli, F. Scandola and V. Balzani, J Am. Chem. Sot., 100 (1978) 7219. 18 P. Suppan, .I. Chem. Sot., Faraday Trans., I, 82 (1986) 509.
Photobioi.
A: Chem., 56 (1991) 159-163
3CTC as the main step in the quenching electron and hydrogen donors
159
of 3BP by
P. Jacques Laboratoire de Photochimie Gt+zt+ale associt! au CNRS, Ecole Nationale de Mulhouse, 3 rue A. Werner, 68200 Mulhouse (France)
Superieure de Chimie
(Received June 29, 1990)
Abstract It is shown that the quenching of triplet benzophenone by electron and hydrogen donors involves a charge transfer state (‘CTC) as the governing step. Moreover, it is shown for the first time that the aliphatic donors follow fully the simple model of Rehm and Weller for electron transfer: the rate of quenching increases as the ionization Potential (IP) of the donor decreases and reaches a limiting value when IP is 9 eV or less, i.e. for most amines. It is confirmed that alkyl benzenes lie on another line. However, the slopes of the rr and n lines are nearly the same in contrast with previous reports. The different behaviour of n- and n quenchers is tentatively addressed.
1. Introduction On photoexcitation, benzophenone (BP) undergoes intersystem crossing rapidly (within a few tens of picoseconds) and efficiently (#r= 1) to populate its lowest triplet state (3BP). This triplet state is responsible for most of its photochemistry: electronic energy transfer, addition to unsaturated compounds, charge transfer reactions (where BP can behave as an electron donor or acceptor) and hydrogen abstraction. These last two processes have been extensively investigated in an attempt to discover the intimate mechanism of deactivation of the triplet [l-6]. For electron donors, the participation of a charge transfer state (3CIC) is well documented [6]; the pioneering work of Guttenplan and Cohen [2], where the logarithm of the deactivation rate was linearly related to the ionization potential of the donors infers a charge transfer mechanism. With hydrogen donors, e.g. tetrahydrofuran (THF), 2-propanol and cyclohexane, the situation is not so clear. It has been suggested that a charge transfer state (3CTC) is involved in the first step of deactivation, followed by the formation of a ketyl radical with various quantum yields & depending on the substrate [3]. Recently, it has been proposed, on the basis of studies of the primary process of BP photoreduction under pressure, that the transition state is not polar, suggesting that triplet benzophenone is radical-like in reactivity [5]. It can be seen from the study of Formosinho and coworkers [4] that the reactivity of 3BP cannot be explained by consideration of only the C-H or O-H bond energy (see Table 1). In this paper, we tentatively address this question through a critical examination of the literature and new quenching data; a clfarge transfer state (3CTC) can explain the reactivity of 3BP towards electron donors and hydrogen donors.
Q Elsevier Sequoia/Printed in The Netherlands
160 TABLE
1
Quenching of 3BP in benzene Compound
1 2-Propanol
log k,’
.IPb
DWf) (kcal mol-‘)
10.40 10.30
88 94 91
2
Cyclohexane
6.26 5.56d
3 4
THF Di-n-propylether
6.63 7.20
9.80’ 9.49
2-Butylamine t-Butylamine Cycloheqlamine N-Methyl-2-butylamine Di-n-propylamine Triethylamine Tri-n-butytamine DABCO’
8.40 7.81 8.51 9.16 9.54 9.49 9.54 9.52
9.30 9.25 9.16 8.63 8.54 8.10 7.98 7.70
4.25 5.90 6.40 8.42 9.10
9.25 8.90 8.60 8.05 7.90
5 6 7 8 9 IO 11 12
13 Benzene 14 Toluene 15 p-Xylene 16 Durene 17 Hexamethylbenzene
94
aFrom refs. l-3 and 7 and references cited therein except for 4, 11 and 14-17 (this work). bFrom refs. 7-9. Worn ref. 4. dNeat solvent. ‘From ref. 10. fl,4-Diazabicyclo[2.2.2]octane. 2. Results
and discussion
The choice of quenchers and the use of the vertical ionization potential (JP) as the thermodynamic parameter are based on the observations reported in ref. 7: JP values are more common and reliable than the adiabatic ionization potential (JP) or the redox potential (Em). Indeed, the use of JP or Em would not have affected the conclusions of this work. 2. I Aromatic quenchers
From previous studies it is known that benzene derivatives are less efficient quenchers than amines [2, 111. However, information on 3BP quenching by alkyl benzenes is rather scarce. Guttenplan and Cohen [2] reported data for benzene and toluene and recently Wagner et al. [ll] in a thorough study of the photoreduction of phenyl ketones by alkyl benzenes, considered toluene andp-xylene in acetonitrile. Our extended data clearly indicate that all@ benzene compounds lie on a v line with a slope of 0.126 mol kcal-’ which is considerably less than the value of 0.74 mol kcal-’ corresponding to full charge transfer [12]. Moreover, this rr line enables us to substantiate the self-quenching mechanism of triplet benzophenone proposed by Wolf et al. [13]. Indeed it has been proposed that self-quenching of 3BP involves the formation of an exciplex, wherein the
161
half-filled n orbital of ‘BP is directed towards the r electron density of the aromatic ring of ground BP. This proposition is confirmed here: the ,IP value of benzophenone is 9.05 [8] and the average value of kso is 3 X ld M-’ s-l [13,14]; thus the corresponding
point lies on the 71‘line (Fig. 1) and benzophenone with the above-mentioned mechanism.
behaves as a r donor in accordance
2.2. Alipha tic compounds If, in the first instance, we do not consider 2-propanol and cyclohexane (see later), we see that the aliphatic compounds lie on two lines: one corresponds to the limiting rate of reaction between 3BP and quenchers (log k,=9.6), and the second shows an increase in log k, as .IP decreases with a slope somewhat less than that of the v
line: 0.107 mol kcal-‘. Therefore the quenching of 3BP by aliphatic compounds follows the expected plot from the Rehm-Weller [12] theory on electron transfer; the unusual feature of this result is that it has been overlooked hitherto, since all previous reports mention only one line. Moreover, our interpretation leads to a ratio between the r and n slopes (1.18), which is nearer to unity than the value of 1.57 reported in ref. 2, suggesting nearly the same percentage of charge transfer for n and v donors.
-8
-6
-4
10
VIP
Fig. 1. The n and VTlines (log kq vs. the vertical ionization potential (JP) of the donors). The numbers refer to the compounds in Table 1.
162
It is the scatter of the points reported in refs. 2 and 7 that induced these workers to consider only one regression line. There are two main reasons for this scatter: (i) a steric effect can occur in the case of amines substituted by bulky groups (consider, for example, the fluorescence quenching of arylbenzo[b]thiophenes by aliphatic amines [15]; such an effect is well illustrated by tribenzylamine (log k,, = 9.05; lowest ,IP - 8.05 [7]) where the rate is substantially lower than the limiting rate (log kg= 9.6)reported here); (ii) for some aliphatic donors, a contribution from hydrogen abstraction to the quenching rate cannot be completely discarded. The high rates for 2-propanol and, to a less extent for cyclohexane, in view of their JP values, suggest that direct hydrogen abstraction competes with charge transfer. Very recently it has been shown by Scaiano and coworkers [16] that decafluorobenzophenone triplet quenching involves hydrogen transfer and/or charge transfer in the case of phenol, 2-propanol and cyclohexane. It remains to explain why, in the case of 3BP, aromatic quenchers are considerably less efficient than aliphatic quenchers, whereas slower electron transfer quenching by aliphatic amines than aromatic amines has been observed for Cr(bpy),3+ and Ru(bpy)32+ in acetonitrile (bpy = bipyridine) [17]. We suggest that the coulombic term t?/~a of the Rehm-Weller relation plays a key role [18] since benzene is a non-polar solvent with a dielectric constant E of approximately 2.3 and the centre-to-centre point-charge separation a is certainly different for aromatic and aliphatic quenchers. Therefore solvent effects and geometrical considerations need to be investigated more thoroughly in order to be able to explain these apparent discordances.
3. Conclusions A critical examination of the literature and new data on benzophenone triplet quenching in benzene has enabled us to propose a unique mechanism: both electron and hydrogen donors act through a charge transfer state (3CTC) as the first step. Even cyclohexane behaves mainly as an electron donor. However, for strong hydrogen donors, direct hydrogen abstraction may contribute substantially to the quenching rates. New aspects of the different behaviour of n and ‘TTdonors are revealed through a treatment of the experimental data which differs from previous points of view. Moreover, this report illustrates the difficulty of finding a convenient set of donors in order to demonstrate that a quenching mechanism is of the charge transfer type, especially in the case of 3BP due to possible hydrogen abstraction. In a forthcoming paper we present further examples of this important problem for photochemists.
References 1 J. C. Scaiano, .I. Photochem., 2 (1973/1974) 81. 2 J. B. Guttenplan and S. G. Cohen, Z, Am. Chem. Sec., 94 (1972) 4040. 3 S. Inbar, H. Linschitz and S. G. Cohen, .I. Am. Chem. Sot., I02 (1980) 4
5 6 7
8
1419; 103 (1981) 1048. L. G. Amaut, S. J. Formosinho and A. M. da Silva, J. Photochem., 27 (1984) 185. M. Okamoto and H. Teranishi, J. Am. Chem. Sot., I08 (1986) 6378. M. Hoshino and H. Shizuka, in M. A. Fox and M. Chanon (eds.), Photoinduced Electron Transfer, Part C, Elsevier, Amsterdam, 1988, p, 313. P. Jacques, Chem. Phys. Lett., 142 (1987) 96. Ionization Potential and Appearance Potential Measurements 1971-1981, 1982 (NSRDS, U.S. Department of Commerce).
163 9 M. Takahashi, I. Watanabe and S. Ikeda, J. Electron Spctrosc., 37 (1985) 275. 10 G. Levy and P. de Lath, C. R Acud. Sci., Ser. C, 279 (1974) 931. 11 P. J. Wagner, R. J. Truman, A. E. Puchalski and R. Wake, J. Am. Chem. SK, 108 (1986) 7727. 12 D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 73 (1969) 834; Isr. J. Chem., 8 (1970) 259. 13 M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer and J. H. Parks, J. Am. Chem. Sot., 97 (1975) 4490. 14 D. I. Schuster and T. M. Weil, J. Am. Chem. Sot., 95 (1973) 4091. 15 A. Brehon, A. Couture, A. Lablache-Combier and A. Pollet, N. J. Chim., 5 (1981) 243. 16 D. R. Boate, L. J. Johnston and J. C. Scaiano, Can. J. Chem., 67 (1989) 927. 17 R. Ballardini, G. Varani, M. T. Indelli, F. Scandola and V. Balzani, J Am. Chem. Sot., 100 (1978) 7219. 18 P. Suppan, .I. Chem. Sot., Faraday Trans., I, 82 (1986) 509.