Quenching of Singlet Oxygen by Nickel Complexes
The Journal of Physical Chemistry, Vol. 83, No.
5, 1979 591
Quenching of Singlet Oxygen by Nickel Complexes Bruce M. Monroe* and Joseph J. Mrowca Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, Delaware 19898 (Received October 2, 1978) Publication costs assisted by E. I. du Pont de Nemours and Company
Singlet oxygen quenching rate constants (k,) were measured for 24 nickel complexes in chloroform solution. Diamagnetic nickel complexes are excellent quenchers with rate constants which vary from the diffusion-controlled rate limit to about one-tenth of this value. Paramagnetic complexes quench singlet oxygen at 1/100 to 1/1000 this value or less. Diamagnetic nickel complexes are also excellent quenchers of rubrene fluorescence, quenching at or near the diffusion-controlled rate limit. Typical rate constants are as follows: bis(N-phenyldithiocarbamato)nickel(II) (l),1.1 X 1O1O L/M s; bis(di-n-butyldithiocarbamato)nickel(II) (6), 8.1 X lo9; bis(Nn-butylsalicylaldiminato)nickel(II)(15), 2.8 X lo9; [2,2'-thiobis-4-(1,1,3,3-tetramethylbutyl)phenolato](n-butylamine)nickel(II) (18), 1.7 X lo8; and bis(salicylaldehydato)nickel(II) dihydrate (22), 4.6 X lo7.
Introduction Nickel complexes are efficient quenchers of singlet oxygen (loz), a property which is believed to contribute to their activity as photo stabilizer^.^-^ Although several studies of IO2 quenching by nickel complexes have been carried out, all but one were limited to a few compounds, primarily those which are important commerical photostabilizers. Based on these limited data, it was concluded that the spin properties of the complex were not important in determining that rate constant for IO2 quenching and that the relationship between structure and IO2quenching ability was u n k n o ~ n . l ,In~ ~ order ~ ~ ~to elucidate this relationship, we measured k, for 24 nickel complexes of widely varying structures and correlated these rate constants with the geometry and magnetic properties of the complexes. Experimental Section Materials. Rubrene (Aldrich) and chloroform (Fisher Certified Reagent) were used as received. 5-Bromosalicylaldehyde, 5-methoxysalicylaldehyde,and ammonium dicyclohexyldithiophosphinate were obtained from the Aldrich Chemical Co. Complexes 6 (Aldrich), 18 (Aldrich), and 23 (Ciba-Geigy) were commerical materials. Complex 17 was obtained from Professor D. Busch, complexes 7,9, and 11 were provided by Dr. W. R. McClellan. Complexes 1,I0 2,11 3,1° 4,12 5,13 8,14 14,15 15,1616,1719,1822,15and 2419 were prepared by literature procedures. Bis(dicyclohexyldithiophosphinato)nickel(II) (10). To a filtered solution of 12.00 g (45 mmol) of ammonium dicyclohexyldithiophosphinate in 400 mL of water was added a solution of 5.00 g of nickel chloride hexahydrate (21 mmol) in 100 mL of water. The mixture was stirred for 1 h, and the resulting blue solid was filtered, washed with water, and dried at 100 "C in vacuo: yield 11.2 g (92%); mp 284-288 "C dec. X,(CHC13): 335 nm (E 17400), 570 nm (E 99), 730 nm (E 83). Anal. Calcd for Cz4HA4P2S4Ni:C, 49.57; H, 7.63; S, 22.06; P, 10.65. Found: C, 49.50; H, 7.53; S, 22.98; P, 10.55. Bis(5-bromosalicylaldehydato)nickel(II) Dihydrate (21). To a filtered solution of 2.47 g (10 mmol) of nickel acetate tetrahydrate in 125 mL of 1:l ethanol-water was added 4.02 g (20 mmol) of 5-bromosalicylaldehyde. After standing overnight, the reaction mixture was heated to reflux for several minutes and allowed to cool. The yelContribution No. 2580. 0022-365417912083-0591$01.00/0
low-green crystals were filtered off, washed with alcohol, and dried in vacuo at room temperature: yield 3.6 g (73%). ,A, (CHC13): 394 nm (E 51001,380 nm (sh) (E 3500). urn,: 3400 cm-I (broad). Anal. Calcd for Cl4Hl2O6Br2Ni:C, 33.99; H, 2.44; Br, 32.30; Ni, 11.87. Found: C, 33.90; H, 2.55; Br, 32.19, Ni, 11.87. Bis(5-methoxysalicylaldehydato)nickel(II)Dihydrate (20) was prepared from 5-methoxysalicylaldehyde by a similar procedure. A,(CHC13): 413 nm ( E 5100), 380 nm (sh) (E 4700). vmB,: 3400 cm-l (broad). Anal. Calcd for Cl6Hl8O8Ni:C, 48.04; H, 4.57; Ni, 14.79. Found: C, 48.33; H, 4.38; Ni, 14.69. Bis(N-n-butyl-5-bromosalicylaldiminato)nickel(II) (12). To 495 mg (1mmol) of 19 suspended in 10 mL of methanol was added 1 mL of n-butylamine. After stirring the solution overnight, dark green crystals were filtered off and dried in vacuo: yield 493 mg (86%); mp 190-191 "C. Xrn,,(CHC13): 618 nm (e 87), 425 nm ( E 4300), 336 nm (E 9000). Anal. Calcd for C22H26N2Br202Ni: C, 46.44; H, 4.61; N, 4.92; Br, 28.09; Ni, 10.32. Found: C, 46.61; H, 4.72; N, 5.11; Br, 27.88; Ni, 10.11. Bis(N-n-butyl-5-methoxysalicylaldiminato)nickel(II) (13) was prepared from 18 by a similar procedure: mp 176 "C. h,,(CHC13): 635 nm (E 1041,438 nm ( E 5200), 340 nm (E 10000). Anal. Calcd for C24H32N204Ni:C, 61.17; H, 6.84; N, 5.94; Ni, 12.46. Found: C, 61.21; H, 6.75; N,-5.85; Ni, 12.47. Rate Constants Measurement. Rate constants were determined as previously described.20 Solutions of rubrene (8 X M) in chloroform with and without added quencher were irradiated simultaneously on a "merrygo-round' apparatusz1 and the final rubrene concentration measured spectrophotometrically. Optical densities were measured at 440 nm unless the quencher was absorbing strongly a t this wavelength and then optical density measurements were taken at 553.5 nm. Quenching constants were calculated by substitution of the quencher concentration and the initial and final rubrene concentrations into eq 7.20 Fluorescence Lifetimes. Rubrene fluorescence lifetimes were measured on nondegassed chloroform solutions by phase shift using a modulated nitrogen lamp.2z Magnetic Measurements. Measurements on Complexes in chloroform solution were made by NMR.23 Solid-state measurements were carried out by ESCA.z4sz5
0 1979 American Chemical Society
592
The Journal of Physical Chemistry, Vol. 83, No. 5, 1979
B. M. Monroe and J. J. Mrowca
Results and Discussions Rubrene is an orange hydrocarbon which reacts cleanly with lo2to form a colorless photoperoxide (eq A). Ph
I
\
Ph
1
/ Ph
Ph
I
+
/
-
‘02
Ph
I
/’
\
Ph
Ph
Ph
(A)
When a nondegassed solution of rubrene and a singlet oxygen quencher is irradiated, the following reactions occur:26 R hv R1 (1)
+
R1 + O2 R3
+ ‘02
+
k k,
+
kd
102
+Q
R
+ ‘02
,OZ
+ R --%R 0 2 302+ Q (and/or Q02)
k,
R3 + ‘02
(2) (3) (4)
(5)
(6) where R is rubrene, Rl is the rubrene singlet, R3 is the rubrene triplet, R 0 2 is the rubrene photoperoxide, Q is the quencher, and QO, is the oxidation product of the quencher. Note that rubrene is both the sensitizer and lo2 acceptor. In the previous study of the quenching of IO2 by amines,2oit was shown that, if two chloroform solutions of equal volume, one containing quencher and one without quencher and each having the same initial concentration of rubrene are each exposed to the same amount of lo2, the rate constants for singlet oxygen quenching (k,) can be calculated from k, = 5.3 x 1O7([R]pQ - [R]po) + 1.7 x lo4 In ([R]pQ/[R]po) IO2
(7) where [R] is the initial concentration of rubrene, [R]FQ the final concentration of rubrene in the quenched solution, [R]$ the final concentration of rubrene in the unquenched solution, and [Q] the quencher concentration. Since this technique only measures the rate at which an added material removes lo2from the system, not how it removes it, the measured rate constant is actually the sum of the rate constants for quenching and for oxidation. For olefins29and sulfides,3O which undergo oxidation with IO2, this technique has been used to measure lo2addition rate constants. However, the possibility that there is an oxidation component to the measured rate constant cannot be eliminated by these measurements alone. Chloroform was chosen for these measurements for several reasons. The lifetime of IO2 in chloroform, needed for the kinetics, had been determined.31 Since ‘02has a relatively long lifetime in chloroform, low concentrations of quenchers, which minimize errors from competitive absorption of the exciting light by the quencher and quenching of rubrene excited states by the quencher, could be used. This also allowed quenchers of limited solubility to be investigated. Because of the method of determining rate constants, the complexes measured were limited to those which were stable in air, stable in chloroform solution, and soluble in
TABLE I : Rubrene Fluorescence Quenched by Nickel Complexes
complex [(cyclohexyl),PS,],Ni (10) [(n-C,H,),NCS,I,Ni (6) [(C,H,O),PS,I,Ni ( 5 )
h f , L/mol s
(10.4t 0.5)x l o 9 (9.6F 0.6)x 109 (9.2F 0.4)x loy (9.1 F 0.4)x l o y (6.7 F 0.7)x lo8 (11.2-I: 0.4)x 109 (2.1F 0.1)x 108
15 18
19 23
chloroform. This eliminated all complexes with phosphine ligands since they formed precipitates when dissolved in chloroform. The solubility requirement ruled out common nickel compounds such as nickel(I1) chloride, nickel(I1) benzoate, and bis(dimethylglyoximato)nickel(II) and tris(ethylenediamine)nickel(II) sulfate. Errors in the measured k , arise if the quencher absorbs a significant portion of the exciting light or if it quenches rubrene excited states. The absorption spectrum of each quencher was determined and the amount of exciting light absorbed by the quencher calculated. Since the samples were irradiated at 546.1 nm, a region where most of the compounds are not absorbing strongly and because low concentrations of quencher were used (10-4-5 X M), generally less than 1% and never more than 10% of the exciting light was absorbed by the quencher. In the few cases in which more than 1% was absorbed by the quencher, a correction was made in the rate constant calculation. Quenching of rubrene singlet by added quencher should not be a significant problem since the concentration of complex necessary to quench ‘02( T = 60 X s in CHClJ is much less than that needed to significantly quench rubrene singlet (7 = 13.4 X s in nondegassed CHCl,). As a check, rubrene fluorescence quenching rates were determined for seven nickel complexes (Table I). Although some better quenchers quench at or near the diffusion-controlled rate limit, even one this efficient would only quench about 0.1% of rubrene singlets at M. Quenching of the rubrene triplet state is another possible source of error. Since quencher is in competition with oxygen for the rubrene triplet, rubrene triplet can disappear by two routes: R3
-
+ 302kt
R
+ IO2
(2)
kR
R3 + Q - R +
Q (8) The importance of triplet quenching as a source of error depends on the relative values of kJ302] and kR[Q]. The concentration of oxygen in air-saturated chloroform is about 2.1 X M.32 Although the rate of rubrene triplet quenching by oxygen has not been measured, the rate of aromatic hydrocarbon triplets has been shown to depend on the triplet energy.33 From the rubrene triplet energy (-29 kcal/mo1),34a rate of at least 3 X lo9 L/mol s can be estimated so that = 6 X lo6 k J 3 0 2 ]= (3 X 109)(2.1X (9) Except for complexes which were very poor ‘02 quenchers, concentrations of m4-l 0-5M or less were used. Studies on the quenching of anthracene phosphorescence have shown that certain diamagnetic nickel chelates are “strong” quenchers of anthracene phosphorescence while other paramagnetic complexes are only “moderate” q~enchers.,~ Measurements by Furue and Russell8 on the rate of quenching of pentacene phosphorescence by nickel complexes in benzene solution at room temperature gave the following values: bis(dithiobenzyl)nickel(II) (2), 4.7
Quenching of Singlet Oxygen by Nickel Complexes
The Journal of Physical Chemistry, Vol. 83, No.
5, 1979 593
s; bis(diethyldithiocarbamato)nickel(II),7.0 were diffusion controlled triplet quenchers. [2,2’-thiobis-4-(1,1,3,3-tetramethylbutyl)- Analogy with t h e quenching of pentacene phosphorescence8 would indicate that the compounds phenolato](n-butylamine)nickel(II) (18), 2.3 X lo7. If we X lo9 L/mol X lo8; and
assume that the values for rubrene phosphorescence quenching are similar to the values reported for pentacene, for complex 2, which because it is an excellent IO2quencher was used at 1 X M and less, we find kR[$] = (5 x io9)(i x ion5)= 5 x io4 S-l (10) Therefore
~R[QI .--=----.-
5 X lo4 - o,oo8 k,[302] 6 X lo6 This indicates that triplet quenching would be responsible for less than 1% of the observed lo2quenching and, therefore, not a significant source of error. Since the triplet quenching rate used in this calculation (5 X lo9 L/mol s) is within a factor of 2 of the diffusion rate (-1.1 X 1O1O L/mol s), triplet quenching by the nickel complex cannot be a significant source of error for any of the good lo2 quenchers (compounds 1-17), all of which were measured R
R1s>Ni/2 R
G
S
2, R = C,H, 7, R = CF,
\
I
N N‘’ i
/
2
‘-4/“Ni/2
I
/4
C=N
R
‘R’
H ’
9, R = CH, 12,R = Br;R’ = n-C,H, 11, R =: p-CH,C,H, 13, R = OCH,;R’ = n-C,H, 14,R = R’ = H 15, R = H;R’ = n-C,H, 16,R = H; R’ = i-C,H,
(q n -C4 HgNH 2
I
3.7
CH3-C-CH3
CH3-C-CH3
I
I
iH2rH2
CH3-C-CH3
CH3-C-
I
CH3
I
CH3
CH3
18
[-&:+ NI*2H20
/JQ ~
N
20, R = OCH, 21, R = Br 22, R = H
J
l
N
/
2
t
Yj
which are poor lo2quenchers (such as 18) are also poor phosphorescence quenchers. Since it was necessary to measure the poorer IO2 quenchers (18-24) at higher concentrations ( M), a significant error in a measured rate constant could be present if one of these compounds should unexpectedly be an excellent triplet quencher. This error would make the measured h, higher than the actual rate. Since the net effect would be to make a poor lo2 quencher appear to be slightly better than it actually is, the conclusions of this study would not be altered. Singlet oxygen rate constants (h,) were measured for 24 nickel complexes. These are tabulated in Table I1 with the magnetic state of the complex either in the solid state or in chloroform solution. For some complexes, whose magnetic properties had not been reported, magnetic measurements were carried out in the solid state by ESCA24s25or in chloroform solution by NMR.23 Compounds with no measurement indicated have close structural analogues of known magnetic state. For those compounds which are equilibrium mixtures of diamagnetic and paramagnetic isomers in solution, the amount of paramagnetic isomer present is given. The values in Table I1 are in good agreement with those of previous studies. Bis(di-n-buty1dithiocarbamato)nickel(I1) (6) and [2,2’-thiobis-4-(1,1,3,3-tetramethylbutyl)phenolato](n-butylamine)nickel(II) (18), two commercial photostabilizers, were extensively studied previously, and the k,s reported are in Table 111. The quenching rate constants for the Schiff-base complexes (12-16) are close to literature values for similar comp l e ~ e s The . ~ ~ rates ~ measured for the salicylates (20-22) agree with the rate of 5 X lo7 L/mol s measured in toluene for the 3,5-diisopropylsalicylateby Zweig and Henderson5 The quenchers fall into two distinct groups corresponding to their magnetic states. Contrary to photochemical lore, the diamagnetic complexes are better IO2 quenchers. All these complexes have rate constants within an order of magnitude of the calculated diffusion-controlled rate limit (1.1X 1O1O L/mol s in chloroform at room temperature) while all paramagnetic complexes have rate constants about two orders of magnitude or less below the limit. Previous work had indicated that “the magnetism and coordination of the chelate appears to be unimportant factors for the lo2quenching effi~iency”.~Furue and Russell did not limit their comparisons to nickel complexes, but based their conclusion on the comDarison of diamagnetic nickel and zinc complexes with paramagnetic nickel and copper complexes.’ Since a change of metal changes factors other than the magnetic state of the complex, this comparison is invalid. Zweig and Henderson found that the quenching rate constants for the diamagnetic bis(salicyladiminto)nickel(II) complexes 25 and 26 did not differ greatly from those of the paramagnetic N
19
C=N
H’
1 23 at a concentration of about 1 X
I
I1
\ 24
M or less, even if they
k
16,R = i-C,H, 25, R = cyalohexyl
26, R = n-C,,H,, 27,R = s-C,H,
complexes 16 and 27.6 Interpretation of the data was complicated by two factors. They reversed the assignment
594
The Journal of Physical Chemistry, VoL 83, No.
B. M. Monroe and J. J. Mrowca
5, 1979
TABLE 11: Singlet Oxygen Quenching Rates for Nickel Complexes compound
k,:
[C,H,NHCS,],Ni (1) 2 [p-CH,C,H,NHCS, ],Ni (3) [ (C$sO)zPSz IzNi ( 4 ) [(C,H,O),PS,I,Ni (5) [ (n-Cd-4 ),NCS, 1,Ni (6) 7 [ (C6Hs)zNCSzlzNi ( 8 ) 9 [ (cyclohexyl),PS,],Ni (10) 11 12 13 14 15
L/mol s
1.1x 10’O 1.1 x 10’0 1.1x 10’O 1.1 x 1 O ’ O
k,d
magnetic moment (method)b
(1.00) diamagnetic ( G o ~ y ) ~
1.00 1.00 1.00
diamagnetic ( G o ~ y ) ~ 0.86 diamagnetic ( G o ~ y ) ~ 8.1 X l o y 0.74 diamagnetic (Gouy)f 8.1x 109 0.74 diamagnetic in CHCl, ( G o ~ y ) ~ 6.3 x 109 0.57 diamagnetic (ESCA) 6.1 x 109 0.55 0.5% paramagnetic isomer in CHCl, (NMR)P 5.7 x 109 0.52 diamagnetic (ESCA) 5.6 x 109 0.51 -26% paramagnetic isomer in CHC1, (NMR)g 3.7 x 109 0.34 3.4 x l o y 0.31 3.2 x 109 0.29 diamagnetic ( G ~ u y ) ~ 2.8 x 109 0.25 -7% paramagnetic isomer in CHC1, (Gouy)’ 16 2.6 x 109 0.24 -37% paramagnetic isomer in CHCl, (NMR) 17 1.6 x 109 0.15 diamagnetic (ESCA) 18 1.7 X l o 8 0.015 3.23 p g in CHCl, (NMR) 19 1.6 x l o 8 0.015 3.1 p~ in CHCl, (NMR) 20 1.2 x 108 0.011 21 5.3 x 107 0.005 22 4.6 x 10’ 0.004 3.1 P B ( 9 0 ~ ~ ) ~ 23 2.2 x 10’ 0.002 3.19pg in CHCl, (NMR) , 24 2.1 x l o 6 0.0002 3.24 pg in CH,Cl, (NMRY a Values +-15%. Magnetic moment of t h e solid com lex unless otherwise indicated. A. Davidson, N. Edelstein, R. H. Holm, and A. H. Maki, Inorg. Chem., 2, 1227 (1963). CPReference12. e Reference 13. L. Cambi and C. Coriselli, G a m . Chim. It& 66,779 (1936);L. Malatesta, ibid., 67,738 (1937). g D. R.Eaton, W. D. Phillips, and D. J. Caldwell, J. A m . Chem. Soc., 85, 397 (1963). Reference 15. * Reference 16. J. P.Jesson, S. Trofimenko, and D. R. Eaton, J. A m . Chem. Soc., 89,3148 (1967).
9.5 x 109
J
TABLE 111: Comparison of k , with Literature Values h, x lo9 L/mol s solvent
18 0.17 4.0 0.15 17.0 0.20, 0.27 1.0 0.10 4.3 0.27 Reference 3. Reference 1. Refer-
chloroforma carbon disulfideb isooctaneC hexadecaneC toluened a This work. ence 6.
6
8.1
of the magnetic state of the complexes indicating that 25 and 26 are paramagnetic and 16 and 27 are diamagnetic when the reverse is the correct assignment.17 Secondly, although the complexes with branched-chain N-alkyl groups (16 and 27) are paramagnetic in the solid state, in solution the tetrahedral, paramagnetic isomer and the square-planar, diamagnetic isomer are in conformational eq~i1ibrium.l~ The measured rate constant is not the k, for a paramagnetic complex but contains contributions from both the paramagnetic and diamagnetic isomers. The measured magnetic moment of the N isopropyl isomer (16) in chloroform at room temperature, 2.01 p ~ , shows that 63% of the diamagnetic isomer and 37% of the paramagnetic isomer are present. The measured k,, 2.6 X lo7 L/mol s, is only slightly less than those of similar complexes which are nearly or completely in the diamagnetic form in chloroform solution. This seemingly “high” rate constant for a “paramagnetic” complex is readily understood since most of the molecules are in the square-planar diamagnetic form in chloroform. Because quenching by the diamagnetic complexes is so much more efficient than by paramagnetic complexes, it is necessary to compare diamagnetic complexes with paramagnetic complexes which are not in equilibrium with a diamagnetic isomer. Similarily “high” values for k, were observed for 9, 11, and 15 which are in conformational equilibrium. The correlation of k , with magnetic state does not necessarily indicate that the magnetic state governs k,; the
geometry of the complex is most likely the controlling factor. Diamagnetic nickel complexes have coordinating atoms arranged in a square-planar geometry. Singlet oxygen would have a relatively unhindered approach to the nickel atom from above or below the plane formed by the coordinating atoms and could easily interact with an orbital perpendicular to this plane. In the paramagnetic complexes the approach to the nickel atom is blocked by the tetrahedral or octahedral arrangement of the ligands. The highly hindered pyrazolyl borate complex (24) has a k, an order of magnitude less than any of the other complexes studied. Previous studies of amines:’ 0lefins,2~ and sulfides30demonstrated that IO2 interaction constants are sensitive to steric effects. The factors which determine the relative position of a complex within each group are less obvious. The best quenchers have nickel coordinated to four sulfur atoms and throughout the diamagnetic group the order S4 > N4 > N z 0 2 is generally followed. This corresponds to a nephelauxetic series indicating that k increases as the polarizability of the ligands increaseq3lThis observation is consistent with quenching occurring by interaction of an orbital on nickel with one of lo2followed by electronic energy transfer.s In general, correlation is seen between k, and kf, the rate constant for quenching of rubrene fluorescence. One exception is the paramagnetic complex 19 which is an excellent fluorescence quencher, quenching rubrene fluorescence at the diffusion-controlled rate, but a relatively poor IO2quencher (see Tables I and 11). The reasons for this difference are not apparent, but may indicate that the mechanisms of IO2quenching and rubrene fluorescence quenching are fundamentally different, a t least for this compound, Strong correlation between k, and the quenching rate for triplet pentacene has been observed for a number of metal complexes.8
Acknowledgment. We thank Professor Daryl Busch, Dr. William McClellan, and Dr. Rawls Frazier for some complexes, Dr. Robert Swingle for ESCA measurements
The Journal of Physical Chemjstty, Vol. 83,No. 5, 1979 595
Formylmethyl Radical Reaction with Phenolates
on several of the complexes, and Dr. George Parshall and Dr. Leo Manzer for discussions of this work. References and Notes (1) D. J. Carlsson, T. Suprunchuk, and D. M. Wiles, Can. J. Chem., 52, 3728 (1974); J . Polym. Chem., Polym. Lett. Ed., 11, 61 (1973); D. J. Carlsson, G. D. Mendenhall, T. Suprunchuk, and D. M. Wiles, J . Am. Chem. SOC., 94, 8960 (1972). (2) A. Farmilo and F. Wilkinson, Photochem. Photobbl., 18, 447 (1973). (3) J. Flood, K. E. Russell, and J. K. S.Wan, Macromolecules, 6, 669 (1973). J. P. Guillory and C. F. Cook, J . Polym. Sci., Polym. Chem. Ed., 11. . , 1927 ._(1973). A. Zweig and W. A. Henderson, Jr., J . Polym. Sci., Polym. Chem. Ed., 13, 717 (1975). A. Zwela and W. A. Henderson, Jr., J. Polym. Sci.. Polvm. Chem. Ed., 13,693 (1975). D. Bellus in “Singlet Oxygen: Reactions with Organic Compounds and Polymers”, B. Ranby and J. F. Rabek, Ed., Wiley-Interscience, New York, 1978, p 61. H. Furue and K. E. Russell, Can. J. Chem., 56, 1595 (1976). V. Ya. Shlyapintokh and V. B. Ivanov, Russ. Chem. Rev., 45, 99 (1976) [Usp. Khem., 45, 202 (1976)]. D. Coucouvamos and J. P. Fackler, Jr., Inorg. Chem., 6, 2047 (1967). G. N. Schrauzer and V. P. Mayweg, J. Am. Chem. Soc., 87, 1483 (1965). L. Malatesta and R. Pizzatti, Chim. Ind. (Milan), 27, 6 (1945). S.E. Livingstone and A. E. Kihkelson, Inorg. Chem., 9, 2545 (1970). M. Ahmed and R. J. Magee, Anal. Chim. Acta, 75, 431 (1975). G. N. Tyson, Jr., and S.C. Adams, J . Am. Chem. SOC., 62, 1228 (1940). L. Sacconi, P. Paoletti, and G. Del Re, J. Am. Chem. Soc., 79, 4062 (1957). L. Sacconl, P. Paoletti, and M. Ciampolini, J . Am. Chem. SOC.,85, 411 (1963). %
- I
(18) M. A. Robinson, S.I. Trotz, and T. J. Hurley, Inorg. Chem., 6, 392 (1967). (19) S. Trofimenko, J. Am. Chem. Sac., 89, 3171 (1967). (20) 8. M. Monroe, J. Phys. Chem., 81, 1861 (1977). (21) F. 0. Moses, R. S.-H. Llu, and B. M. Monroe, Mol. Photochem., 1, 245 (1969). (22) B. M. Monroe, C.-G. Lee, and N. J. Turro, Mol. Photochem., 6, 271 (1974). (23) D. F. Evans, J. Chem. Soc., 2003 (1959). (24) L. J. Matienzo, L. I. Yin, S. 0. Grlm, and W. E. Swartz, Jr., Inorg. Chem., 12, 2762 (1973). (25) C. A. Tolman, W. M. Riggs, W. J. Linn, C. M. King, and R. C. Wendt, Inorg. Chem., 12, 2770 (1973). (26) The conclusion of Stevens and Om2’ that ‘0,Is generated from the quenchlng of rubrene slnglet as well as the quenching of rubrene triplet by 302 has been questioned by Merkel and Herkstroeter.28Since this method only examines the fate of IO2 rather than its source, the rates reported here do not depend upon which of these conclusions is correct. (27) B. Stevens and J. A. Ors, J . Phys. Chem., 80, 2164 (1976). (28) P. B. Merkel and W. G. Herkstroeter, Chem. Phys. Lett., 53, 350 (1976). (29) B. M. Monroe, J. Phys. Chem., 82, 15 (1978). (30) B. M. Monroe, Photochem. Photobiol., in press. (31) P. B. Merkel and D. R. Kearns, J. Am. Chem. Soc., 94, 7244 (1972). (32) B. M. Monroe, unpublished results. (33) L. K. Patterson, G. Porter, and M. R. Topp, Chem. Phys. Left., 7, 612 (1970). (34) Estimated from the tetracene triplet energy (29.3 kcal/mol). S.L. Murov, “Handbook of Photochemistry”, Marcel Dekker, New York, 1973, p 21. (35) P. J. Briggs and J. F. McKellar, Chem. Ind. (London), 622 (1967); J. Appl. Pokm. Sci., 12, 1825 (1968); D. J. Harper and J. F. McKelbr, ibid., 18, 1233 (1974). (36) C. K. Jorgensen, Prog. Inorg. Chem., 4, 73 (1962).
Oxidation of Phenolates and Phenylenediamines by 2-Alkanonyl Radicals Produced from 1,2-Dihydroxy- and 1-Hydroxy-2-alkoxyalkyl Radicals S. Steenken Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received August 16, 1978) Publication costs assisted by the U.S. Deparfment of Energy
Using pulse radiolysis with optical detection it is shown that the formylmethyl radical CH2CH0 reacts with phenolates XC6H40- (X = H, alkyl, OCH3, OH) by one electron oxidation to yield XC6H40.. With parasubstituted phenolates the rate constants for formation of XCBHIO.increase with increasing electron-donating power of X (Hammett p = -7.9). For X = OH (hydroquinone anion) the rate constant for the electron transfer reaction is 2.2 X lo9 M-I s-l . Replacement of H in CHzCHO by alkyl or hydroxyalkyl groups leads to a decrease in the rate constants for oxidation of the hydroquinone anion. CHzCHO is also able to oxidize o- and p phenylenediamine and N,N’-tetramethyl-p-phenylenediamine( k = 7.3 X lo7, 4.0 X lo*, and 2.0 X lo9 M-’ s-l, respectively). The 2-alkanonyl radicals were produced by elimination of OH- or alkoxide from ionized 1,2dihydroxyalkyl or 1-hydroxy-2-alkoxyalkylradicals, respectively. For cyclic 1-hydroxy-2-alkoxyalkylradicals elimination of alkoxide results. in ring opening. The rate constants for elimination of OH- from ionized 1,2-dihydroxyalkyl radicals RlC(O-)CH(OH)R2 are between 3 X lo6 s-l (for R1 = R2 = H and R1, R2 = H, hydroxyalkyl) and 2 8 X lo6 s-l (for R1 = R2 = CH3). Introduction The p r o d ~ c t i o n l -of~ 2-alkanonyl radicals 2 from 1,2dihydroxyalkyl radicals 1 formally involves elimination of water, as shown in reaction 1. The rate of formation of OH OH I
I
R,-C--C. I
I
Rz R, 1
Rl
0 Ii
-H,O
R,-C-C-R, I
R, 2a
*
0. \ /
I
C=C
RZ
(1) \
R3
2b
The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-1909 from the Notre Dame Radiation Laboratory. 0022-3654/79/2083-0595$01 .OO/O
2 from 1 is enhanced in the presence of H+1-3t6or Formation of 2 from 1 is analogous to conversion of OH adducts of phenols to phenoxy1 radical^,^ a reaction the rate of which is also increased by H+ or ESR spectroscopic studies have shown that with 2, for the case R1 = R2, R1 and R2 are magnetically nonequivalentlOJ1and that for R1 # R2 and R1 = R3 2 exists in cis and trans forms.’lJ2 This indicates that allylic structures characterized by localization of the unpaired spin on the oxygen (2b) contribute to the distribution of the unpaired e1ectr0n.l~ Radicals of this type can be expected to have oxidizing properties and it was therefore decided to investigate this possibility using phenolates and phenylenediamines as reducing agents.
0 1979 American Chemical Society
