Photochemistry and Phorobiology, 1976, Vol 23, pp 209-21 I
Pergamon Press
Printed ~n Great Britain.
RESEARCH NOTE
THE QUENCHING EFFECT OF IODIDE ION ON SINGLET OXYGEN I. ROSENTHAL and A. FRIMEK Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot, Israel (Received 17 October 1975 trccepteil 4 Noremher I975
While the interaction of singlet oxygen (loz)with organic compounds has been extensively investigated, much less attention has been paid to inorganic substrates. Thus, NaN, was shown to deactivate '0, by a mixed chemical-physical mechanism (k, = 2.2 x lo8 / M - ' S-' ) most probably due to N; anion (Foote et al., 1972; Hasty et al., 1972). Conversely the observed quenching effect of NiCI2 (k, = 3.1 x 10' / M - ' s-I) and CoCI, (k, = 4.8 x lo7 / M - ' s-') (Carlsson et a/., 1974) has been attributed to the hydrated cations. We have recently found that superoxide radical anion, 0; physically deactivates '0, molecules with a k, = 3.5 x lo7 / M-' s - ' (Rosenthal, 1975). We wish to report that iodide ion in aprotic medium compares favorably in its quenching efficiency to '02-quenchers of organic nature.
media. In this respect the self-sensitized photoperoxidation of rubrene is particularly convenient since this reaction, which proceeds through a 'Oz mechanism (Wilson, 1966),can he easily monitored by the disappearance of initial rubrene absorption in the visible region (A = 520 nm). The ability of 1- to quench singlet oxygen can be calculated from the amount it inhibits this reaction (Eq. 2) (Carlsson rt lil., 1974).
In Eq. 2 li,, is the rate constant for reaction of rubrene with lo2:[Re]: initial rubrene concentration, k,: rate constant for the decay of '02,[Q]; quencher concentration and d[R]/dt initial rate of rubrene reaction in the absence (subscript 0 ) and presence (subscript Q) of quencher. The results of the representative experiments are summarized in Table 1. The results indicate that the MATERIALS AND METHODS iodide ion in aprotic solvents quenches '02with a Commercial iodide salts were dried in an oven at tem- k , higher than 10' / M - ' s - ' . Several bromide and peratures up to 250°C under vacuum for 12 h. Dicyclohex- chloride salts were also tested, but a quenching effect yl-18-crown-6 polyether was prepared according to could be detected only for the KBr-crown ether comPedersen (1972). All solvents were rigorously dried. The irradiations were performed with a DC operated Osram plex. Even in this case, however, the efficiency was 200 W lamp housed in a Wild reflector in connection with much lower than that of the iodide complex. It was a Pyrex filter, a filter solution of Na2Cr,0, (1 g/100m/) noted that in all cases the quenching effect totally and a yellow Schott GG 498 cut-off filter. Alternatively, ' 0 , disappears i!i protic solvents. Such a drastic solvent was generated with a 2450 MHz Raytheon microwave effect in a singlet oxygen reaction has been reported generator operated at about 80 W input power with an oxygen flow rate of 600m//s at a pressure of 6 torr. The for the photooxidation of sulfides: in that case, howgas stream was passed over mercury ahead of the discharge ever, an efficient chemical reaction replaces the physiand a HgO deposit was allowed to form in order to sup- cal quenching when switching from aprotic to protic press oxygen atom contamination and ozone formation. media (Foote and Peters, 1971). In this context it is The absence of NOz airglow below the HgO deposit testi- noteworthy that the eosin-sensitized photooxidation fied for the absence of atomic oxygen in the gas flow (Gleason rt ul., 1970). A reaction system composed of twinned of I- to Ij in aqueous media has been claimed to reaction cells as described by Carlsson (1974) was proceed through a singlet oxygen mechanism with a employed. The absorption spectra were recorded with a rate constant of 7 x lo4 / M - ' s-' (Kepka and Cary 14 spectrophotometer. Grossweiner, 1973). Since this value is comparable with the decay rate of '02 in methanol (lid = 8.8 x lo4 s- Young et a[., 1973) this is consisRESULTS AND DISCUSSION tent with the negative results in protic solvents The effect of physical quenching of '02(Eq. 1) is observed in the present work. In addition, no quenching occurred when the halide is covalently bonded obser\nble in the inhibited like in C2H51. This observation is in agreement with 'O2('A8) Q h O2 Q (1) the already notcd lack of a heavy atom effect on the oxidation of other 'OZ substrates present in the same lifetime of 'OZ (Foote and Denny, 1971: Merkel and 209
';
+
+
I. R~SENTHAL and A.
210
FRlMER
Table 1. Quenching rates of singlet oxygen by iodide ion
Quencher nBu NI 4 ( 1 x 1 ~ - 3 ~1. , 5 x 1 ~ - 3 ~ 2, x 1 ~ - 3 ~ )
Crown-KI
9.120.1
A
8.1'0.1
A
28kO.l
A
t
( L ~ ~ O - ~1. M 5, x 1 ~ - 3 ~ z, ~ ~ o - ~ M )
(nBu4N, Li,K)I
No detectable quenching
B
(Z ~ ~ O - ~ M )
C2H51 (10-2M)
Crown-KBr $
No detectable quenching
%0.12
(10-2M)
Dicyclohexyl-18-crow-6
No detectable quenching
(10-2M)
nBu NBr,nBu NC1,LiBr 4 (1042M)
No detectable quenching
* I\, was calculated using Eq. 2. k , was computed from the solvent composition and the singlet oxygen lifetimes in each of the pure solvent [kd (C6H5Br)= 2.94 x 104sC1 (Stevens and Perez, 1974),k,(acetone) = 3.84 x 104s-' (Merkel and Kearns, 1972)l.Under identical photolysis conditions k, was virtually independent on [ R J (2 x 10 -4A4. 1.5 x 10-4M, 1 x l V 4 M ) , k,,, was taken as 4 x 10' / mol- Is-'; the minor possible modification of this value due to the solvent (Stevens and Perez, 1974) should insignificantly affect the k,. t A = bromobenzene-acetone ( 2 : l ) ; B = bromobenzenemethanol (2:l). 1Complexes of dicyclohexyl-18-crown-6 with KI and KBr were prepared according to Sam and Simmons (1974). Kearns, 1972; Young et d., 1973). It is noteworthy that the possible initial contamination of 1- salt with Iz cannot be held responsible for the quenching effect observed. Iodine was shown to act as a sensitizer for production of '02by virtue of its higher triplet state, and no chemical reaction between I, and '02was observed (Olmsted and Karal, 1972). Control experiments ruled out the possible restoration of rubrene chromophore by a dark reaction between rubrene endoperoxide and iodide salts. It is generally recognized that transannular peroxides are not reduced by iodide ion even o n prolonged refluxing (Mair and Hall, 1971). Several observations confirmed that the inhibition
observed was due to quenching of 'OZ rather than of the rubrene excited states. Thus, fluorescence intensities of rubrene solutions remained unchanged when 1- salts were added (ruhrene l W 4 hf. 1- wit M ) . In addition [he rate of rubrcne photooxidation in the presence of I was found to be unaffected when pure oxygen was replaced by air, while the other conditions were kept unchanged. This makes it unlikely that I - is competing significantly with oxygen for rubrene triplets. Finally, a solution of rubrene (2 x M ) in a 2 : l mixture of bromobenzene and dimethyl forniamide was reacted with singlet oxygen generated by direct excitation in a microwave discharged stream of oxygen at low pressure. The
The quenching cffcct of iodidc ion on singlet oxqgen
presence of I- (LiI or n-Bu,NI, M ) inhibited the oxidation of rubrene after the same cxposure time, to one fifth as compared to the quencher-free solution. No ecidence was found for a chemical reaction between 1 and '02in aprotic media. This fact was ascertained bq photooxidizing a solution of rubrene (0.1 mmol) and 1 salt (0.5 mmol) in a 2:l mixturc benzene-acetone (100 m/) to total conversion. After removal of solvent under vacuum the residue was partitioned between water and benzene. The aqueous layer which was malysed for 1- (Volhard's method) showed complete recovery. In addition no incorporation of iodine into rubrene endoperoxide could be detected by the mass-spectrometric analysis of the organic residue. It is tempting to suggest that the observed yuenching occurs through a mechanism which involces the formation of a complex between '02and I (Eq. 4) ~
l o 2
+I
---$
,i 3+ '02 . . . 1 - --t " 0 ,+ I
(4)
Among the halide ions, 1- possesses the highest polari-
211
xability which means that its electron cloud can be most easily distorted by electrophilic ccnters (Breslow, 1969). Consequently the I requires the lowest actication energy for the formation of the complex with ' 0 2The . retardation in quenching efficiency in methanolic solution is explained by the relative deactivation of the anion by hydrogen bonding. I t is to be expected that the cation counterpart should have no effect on the nucleophilic anion as long as it is freely dissociated in the solvcnt. When the ions exist as "ion pair" tightly bound electrostatically, the electron donation properties of the anion should be seriously impaired. This may explain why among the bromide salts only the complex with crown ether showed some quenching activity. This effect is less obvious for iodide salts which are equally and extensively dissociated in contradistinction to the corresponding bromides (Winstein er u/., 1960). Acknowledgenienf-This investigation was supported by the Israel Commission for Basic Research. A. F. thanks the Weizmann Institute or Science for a Postdoctoral Fellowship.
REFERENCES
Breslow, R. (1969) Organic Reaction Meckanis,ns. 2nd Ed. p. 85. W. A. Benjamin, New York. Carlsson, D. J., J. Suprunchuk and D. M. Wiles (1974) Can. J . Cheni. 52, 3728-3737. Foote, C. S., and R. W. Denny (1971) J . Am. Chem. SOC.93, 5168-5171. Foote, C. S.. and J. W. Peters (1971) 1.Am. Chwn. Soc. 93. 3795-3796. Foote, C. S., T. T. Fujimoto and Y. C. Chang (1972) Tetruhedron Letters 45-48, Gleason, W. S., A. D. Broadbent, E. Whittle and J. N. Pitts.. Jr. (1970) J . Am. Chem. Soc. 92. 2068-2075. Hasty, N., P. B. Mcrkel, P. Radlick and D. R. Kearns (1972) Tcircthcdron Letters 49-51. Kepka, A. G., and L. I.. Grossweincr (1973) Photochem Photohiol. 18. 49-61. Mxir. K. D.. and R. T. Hall (1971) In Orqaizic Peroxides (Edited by D. Swern) Vol. 11. p. 535, Wilc) Iiitcrscience. New York. Merkel. P. B., and D. R. Kearns (1972) J . Am. Chem. Soc. 94. 7244-7253. Olmsted, J., and G. Karal (1972) J . Am. Chem. Soc. 94, 3305-3310. Pedersen, C. J. (1972) In Orgunic S.yrithesis (Edited by H. 0. House) Vol. 52, pp. 66 71. Wilt). New York. Rosenthal. 1. (1975) Isrrrcd J . ('Iwn. 13. Nr. 2. Sam, D. J.. and H. E. Simmons (1974) J . Am. Choni. Soc. 96. 2252-2253. Stevens, B., and S. R. Perez (1974) Mol. Photochem. 6. 1-7. Wilson, T. (1966) J . Am. Chem. Soc. 88. 2898-2902. Winstein, S., L. G. Savedoff and S. Smith (1960) Tetrahedron Letters 24--30. Young, R . H.. D. Brewer and R. A. Keller (1973) J . Am. Chem. Soc. 95. 375--379.
Pergamon Press
Printed ~n Great Britain.
RESEARCH NOTE
THE QUENCHING EFFECT OF IODIDE ION ON SINGLET OXYGEN I. ROSENTHAL and A. FRIMEK Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot, Israel (Received 17 October 1975 trccepteil 4 Noremher I975
While the interaction of singlet oxygen (loz)with organic compounds has been extensively investigated, much less attention has been paid to inorganic substrates. Thus, NaN, was shown to deactivate '0, by a mixed chemical-physical mechanism (k, = 2.2 x lo8 / M - ' S-' ) most probably due to N; anion (Foote et al., 1972; Hasty et al., 1972). Conversely the observed quenching effect of NiCI2 (k, = 3.1 x 10' / M - ' s-I) and CoCI, (k, = 4.8 x lo7 / M - ' s-') (Carlsson et a/., 1974) has been attributed to the hydrated cations. We have recently found that superoxide radical anion, 0; physically deactivates '0, molecules with a k, = 3.5 x lo7 / M-' s - ' (Rosenthal, 1975). We wish to report that iodide ion in aprotic medium compares favorably in its quenching efficiency to '02-quenchers of organic nature.
media. In this respect the self-sensitized photoperoxidation of rubrene is particularly convenient since this reaction, which proceeds through a 'Oz mechanism (Wilson, 1966),can he easily monitored by the disappearance of initial rubrene absorption in the visible region (A = 520 nm). The ability of 1- to quench singlet oxygen can be calculated from the amount it inhibits this reaction (Eq. 2) (Carlsson rt lil., 1974).
In Eq. 2 li,, is the rate constant for reaction of rubrene with lo2:[Re]: initial rubrene concentration, k,: rate constant for the decay of '02,[Q]; quencher concentration and d[R]/dt initial rate of rubrene reaction in the absence (subscript 0 ) and presence (subscript Q) of quencher. The results of the representative experiments are summarized in Table 1. The results indicate that the MATERIALS AND METHODS iodide ion in aprotic solvents quenches '02with a Commercial iodide salts were dried in an oven at tem- k , higher than 10' / M - ' s - ' . Several bromide and peratures up to 250°C under vacuum for 12 h. Dicyclohex- chloride salts were also tested, but a quenching effect yl-18-crown-6 polyether was prepared according to could be detected only for the KBr-crown ether comPedersen (1972). All solvents were rigorously dried. The irradiations were performed with a DC operated Osram plex. Even in this case, however, the efficiency was 200 W lamp housed in a Wild reflector in connection with much lower than that of the iodide complex. It was a Pyrex filter, a filter solution of Na2Cr,0, (1 g/100m/) noted that in all cases the quenching effect totally and a yellow Schott GG 498 cut-off filter. Alternatively, ' 0 , disappears i!i protic solvents. Such a drastic solvent was generated with a 2450 MHz Raytheon microwave effect in a singlet oxygen reaction has been reported generator operated at about 80 W input power with an oxygen flow rate of 600m//s at a pressure of 6 torr. The for the photooxidation of sulfides: in that case, howgas stream was passed over mercury ahead of the discharge ever, an efficient chemical reaction replaces the physiand a HgO deposit was allowed to form in order to sup- cal quenching when switching from aprotic to protic press oxygen atom contamination and ozone formation. media (Foote and Peters, 1971). In this context it is The absence of NOz airglow below the HgO deposit testi- noteworthy that the eosin-sensitized photooxidation fied for the absence of atomic oxygen in the gas flow (Gleason rt ul., 1970). A reaction system composed of twinned of I- to Ij in aqueous media has been claimed to reaction cells as described by Carlsson (1974) was proceed through a singlet oxygen mechanism with a employed. The absorption spectra were recorded with a rate constant of 7 x lo4 / M - ' s-' (Kepka and Cary 14 spectrophotometer. Grossweiner, 1973). Since this value is comparable with the decay rate of '02 in methanol (lid = 8.8 x lo4 s- Young et a[., 1973) this is consisRESULTS AND DISCUSSION tent with the negative results in protic solvents The effect of physical quenching of '02(Eq. 1) is observed in the present work. In addition, no quenching occurred when the halide is covalently bonded obser\nble in the inhibited like in C2H51. This observation is in agreement with 'O2('A8) Q h O2 Q (1) the already notcd lack of a heavy atom effect on the oxidation of other 'OZ substrates present in the same lifetime of 'OZ (Foote and Denny, 1971: Merkel and 209
';
+
+
I. R~SENTHAL and A.
210
FRlMER
Table 1. Quenching rates of singlet oxygen by iodide ion
Quencher nBu NI 4 ( 1 x 1 ~ - 3 ~1. , 5 x 1 ~ - 3 ~ 2, x 1 ~ - 3 ~ )
Crown-KI
9.120.1
A
8.1'0.1
A
28kO.l
A
t
( L ~ ~ O - ~1. M 5, x 1 ~ - 3 ~ z, ~ ~ o - ~ M )
(nBu4N, Li,K)I
No detectable quenching
B
(Z ~ ~ O - ~ M )
C2H51 (10-2M)
Crown-KBr $
No detectable quenching
%0.12
(10-2M)
Dicyclohexyl-18-crow-6
No detectable quenching
(10-2M)
nBu NBr,nBu NC1,LiBr 4 (1042M)
No detectable quenching
* I\, was calculated using Eq. 2. k , was computed from the solvent composition and the singlet oxygen lifetimes in each of the pure solvent [kd (C6H5Br)= 2.94 x 104sC1 (Stevens and Perez, 1974),k,(acetone) = 3.84 x 104s-' (Merkel and Kearns, 1972)l.Under identical photolysis conditions k, was virtually independent on [ R J (2 x 10 -4A4. 1.5 x 10-4M, 1 x l V 4 M ) , k,,, was taken as 4 x 10' / mol- Is-'; the minor possible modification of this value due to the solvent (Stevens and Perez, 1974) should insignificantly affect the k,. t A = bromobenzene-acetone ( 2 : l ) ; B = bromobenzenemethanol (2:l). 1Complexes of dicyclohexyl-18-crown-6 with KI and KBr were prepared according to Sam and Simmons (1974). Kearns, 1972; Young et d., 1973). It is noteworthy that the possible initial contamination of 1- salt with Iz cannot be held responsible for the quenching effect observed. Iodine was shown to act as a sensitizer for production of '02by virtue of its higher triplet state, and no chemical reaction between I, and '02was observed (Olmsted and Karal, 1972). Control experiments ruled out the possible restoration of rubrene chromophore by a dark reaction between rubrene endoperoxide and iodide salts. It is generally recognized that transannular peroxides are not reduced by iodide ion even o n prolonged refluxing (Mair and Hall, 1971). Several observations confirmed that the inhibition
observed was due to quenching of 'OZ rather than of the rubrene excited states. Thus, fluorescence intensities of rubrene solutions remained unchanged when 1- salts were added (ruhrene l W 4 hf. 1- wit M ) . In addition [he rate of rubrcne photooxidation in the presence of I was found to be unaffected when pure oxygen was replaced by air, while the other conditions were kept unchanged. This makes it unlikely that I - is competing significantly with oxygen for rubrene triplets. Finally, a solution of rubrene (2 x M ) in a 2 : l mixture of bromobenzene and dimethyl forniamide was reacted with singlet oxygen generated by direct excitation in a microwave discharged stream of oxygen at low pressure. The
The quenching cffcct of iodidc ion on singlet oxqgen
presence of I- (LiI or n-Bu,NI, M ) inhibited the oxidation of rubrene after the same cxposure time, to one fifth as compared to the quencher-free solution. No ecidence was found for a chemical reaction between 1 and '02in aprotic media. This fact was ascertained bq photooxidizing a solution of rubrene (0.1 mmol) and 1 salt (0.5 mmol) in a 2:l mixturc benzene-acetone (100 m/) to total conversion. After removal of solvent under vacuum the residue was partitioned between water and benzene. The aqueous layer which was malysed for 1- (Volhard's method) showed complete recovery. In addition no incorporation of iodine into rubrene endoperoxide could be detected by the mass-spectrometric analysis of the organic residue. It is tempting to suggest that the observed yuenching occurs through a mechanism which involces the formation of a complex between '02and I (Eq. 4) ~
l o 2
+I
---$
,i 3+ '02 . . . 1 - --t " 0 ,+ I
(4)
Among the halide ions, 1- possesses the highest polari-
211
xability which means that its electron cloud can be most easily distorted by electrophilic ccnters (Breslow, 1969). Consequently the I requires the lowest actication energy for the formation of the complex with ' 0 2The . retardation in quenching efficiency in methanolic solution is explained by the relative deactivation of the anion by hydrogen bonding. I t is to be expected that the cation counterpart should have no effect on the nucleophilic anion as long as it is freely dissociated in the solvcnt. When the ions exist as "ion pair" tightly bound electrostatically, the electron donation properties of the anion should be seriously impaired. This may explain why among the bromide salts only the complex with crown ether showed some quenching activity. This effect is less obvious for iodide salts which are equally and extensively dissociated in contradistinction to the corresponding bromides (Winstein er u/., 1960). Acknowledgenienf-This investigation was supported by the Israel Commission for Basic Research. A. F. thanks the Weizmann Institute or Science for a Postdoctoral Fellowship.
REFERENCES
Breslow, R. (1969) Organic Reaction Meckanis,ns. 2nd Ed. p. 85. W. A. Benjamin, New York. Carlsson, D. J., J. Suprunchuk and D. M. Wiles (1974) Can. J . Cheni. 52, 3728-3737. Foote, C. S., and R. W. Denny (1971) J . Am. Chem. SOC.93, 5168-5171. Foote, C. S.. and J. W. Peters (1971) 1.Am. Chwn. Soc. 93. 3795-3796. Foote, C. S., T. T. Fujimoto and Y. C. Chang (1972) Tetruhedron Letters 45-48, Gleason, W. S., A. D. Broadbent, E. Whittle and J. N. Pitts.. Jr. (1970) J . Am. Chem. Soc. 92. 2068-2075. Hasty, N., P. B. Mcrkel, P. Radlick and D. R. Kearns (1972) Tcircthcdron Letters 49-51. Kepka, A. G., and L. I.. Grossweincr (1973) Photochem Photohiol. 18. 49-61. Mxir. K. D.. and R. T. Hall (1971) In Orqaizic Peroxides (Edited by D. Swern) Vol. 11. p. 535, Wilc) Iiitcrscience. New York. Merkel. P. B., and D. R. Kearns (1972) J . Am. Chem. Soc. 94. 7244-7253. Olmsted, J., and G. Karal (1972) J . Am. Chem. Soc. 94, 3305-3310. Pedersen, C. J. (1972) In Orgunic S.yrithesis (Edited by H. 0. House) Vol. 52, pp. 66 71. Wilt). New York. Rosenthal. 1. (1975) Isrrrcd J . ('Iwn. 13. Nr. 2. Sam, D. J.. and H. E. Simmons (1974) J . Am. Choni. Soc. 96. 2252-2253. Stevens, B., and S. R. Perez (1974) Mol. Photochem. 6. 1-7. Wilson, T. (1966) J . Am. Chem. Soc. 88. 2898-2902. Winstein, S., L. G. Savedoff and S. Smith (1960) Tetrahedron Letters 24--30. Young, R . H.. D. Brewer and R. A. Keller (1973) J . Am. Chem. Soc. 95. 375--379.
