J. Phys. Chem. 1995,99, 11854-11859
11854
Interaction of Singlet Oxygen with Peroxy and Acylperoxy Radicals Alexander P. Darmanyan and Christopher S. Foote* Department of Chemistry and Biochemistry, University of California, Los Angeles, Califomia 90024-1569
Pierre Jardon LEDSSN/ (Photochimie), Universiti Scientijque et Midicale de Grenoble, BP 68, 38402 St. Martin d'lreres Cedex, France Received: January 31, 1995; In Final Form: May 24, 1995@
Quenching of the lowest n,n* triplet states of various diketones and acetone by molecular oxygen was studied by time-resolved ' 0 2 luminescence and nanosecond laser photolysis in solution. The singlet oxygen production efficiency SAin benzene was 0.28 for acetone and 0.28-0.58 for a-diketones. The quenching rate constants of a-diketone triplets by oxygen (kT)are very low, (2-9) x lo8 M-' s-l. A decrease in I 0 2 lifetime was observed upon excitation of a-diketones at laser pulse energies 21 mJ and is explained by quenching of singlet oxygen by acylperoxy radicals. Rate constants of ' 0 2 quenching by different peroxy and acylperoxy radicals kq were estimated to be "(1-5) x 1O'O M-' s-l, near the diffusion-controlled limit.
Introduction Generation of singlet oxygen by different sensitizers has been intensively studied' because '02plays an increasingly important role in cancer and antiretroviral ph~totherapy.~,~ Determination of the efficiency of IO2 formation from triplet sensitizers (SA) is necessary for an understanding of the general mechanism of triplet quenching by molecular oxygen. In the present work, we measure SA values and rate constants of triplet state quenching by oxygen for acetone and various diketones. The mechanism of photosensitized oxidation of different substrates by a-diketones has also been extensively studied."-6 It is known that for benzil and biacetyl, two processes compete: triplet quenching followed by I 0 2 production and oxygen addition to the triplet a-diketone, resulting in production of two acylperoxy radicals, (R(C0)02*.7-10In this paper, we describe the competitive pathways of decay of the triplet a-diketone leading to production of single oxygen and acylperoxy radicals and estimate the rate constant of I 0 2 interaction with peroxy radicals.
Experimental Section Azoisobutane from Lancaster and trans-stilbene, tetraphenylporphine (TPP), and various ketones from Aldrich were used as purchased. Solvents from Fisher Scientific were purified by distillation. Experiments were carried out at 20 & 1 "C. Absorption spectra were recorded on a Beckman DU-25 spectrophotometer. The decay kinetics and sensitizer triplet state absorption were studied by nanosecond laser photolysis; the amplitude (IO)and decay kinetics of IO2 luminescence at 1.27 p m were measured with an IR laser fluorimeter, as described elsewhere."-'3 Sensitizer solutions, except for acetone, were excited in a 1-cm quartz cell (laser beam diameter 1 cm) at 355 nm by 1-50 mJ 7-11s pulses from a Quanta-Ray Nd:YAG laser. Acetone was excited at 308 nm by a 1-10 mJ 10-ns pulse from a Lambda Physik EMG-100 excimer laser (Xe/HCl gas mixture). The quenching of triplet biacetyl by trans-stilbene was studied using @Abstractpublished in Advance ACS Absfracfs,July 1, 1995,
430 nm, 5-12 mJ,7-ns pulses from a dye laser. Trans-stilbene does not absorb this radiation. Optical densities were -0.3 at the excitation wavelength, corresponding to concentrations of acetone, biacetyl, and camphorquinone of -lo-' M; of benzoin and azoisobutane of M; of benzil of (5-9) x M; of dimethoxybenzil and acenaphthenequinoneof (5-7) x M; of phenanthrenequinone and vitamin K3 of -2 x M; and of 1,2-naphthoquinoneof -1 x M. The laser pulse energy was measured by a Scientech 362 power meter. Quantum yields of I 0 2 generation (@A) were measured in air- and 02-saturated solutions at laser pulse energies 1 mJ. The standard was benzophenone in air-saturated benzene, using the averaged literature value = 0.3.11-13The extrapolation procedure for @A- values ([02] w), oxygen solubilities, and '02 relative luminescence rate constants were published previously.I - I 3 Triplet decay rate constants in degassed (hT)and in air-saturated (klT) solutions were determined at -800 nm for biacetyl and camph~rquinone;'~ at -480 nm for benzil, dimethoxybenzil, phenanthrenequinone, and vitamin K3;I5 and at -550 nm for acenaphthenequinoneand 1,2-naphthoquinone. The quenching rate constants of triplet diketones by molecular oxygen (kT)were obtained from eq 1.
-
-
kg' = (kIT- k(3/[02] Benzil abstracts the hydrogen atom from ethanol efficiently. The T-T absorption spectrum and ketyl radical absorption spectrum overlap strongly, as in the cases of benzophenone, biacetyl, and camphr~quinone.'~The hT value for benzil in ethanol took into account the ketyl radical buildup (see, for example, ref 12). The T-T extinction coefficient of benzil (BZ) was measured in comparison with a benzophenone (BP) standard in benzene with cTmax(532.5 nm) = 7630 M-' cm-l,I6 assuming that the quantum yield of the triplet state is unity for both ketones. The optical density at the excitation wavelength (ODW) and the energy of the laser pulse (El = 5 mJ) were equal for both solutions. The value of cTmax(BZ)was evaluated from eq 2,
0 1995 American Chemical Society 0022-3654/95/2099-11854$09.00/0
Interaction of
IO2
with Peroxy Radicals
J. Phys. Chem., Vol. 99,No. 31, 1995 11855
TABLE 1: Photophysical Properties of Diketones and Acetone @A
sensitizer
solvent
acetone camphorquinone phenanthrenequinone acenaphthenequinone biacetyl benzil
benzene benzene benzene benzene benzene benzene THF cyclohexane eth an o1 acetonitrile 4,4'-dimethoxybenzil benzene 1,2-naphthoquinone benzene benzene vitamin K$
air-sat. 02-sat. 0.083 0.41 0.26 0.22 0.31 0.58 0.43 0.67 0.66 0.49 0.45 0.53 0.065
0.185 0.41 0.35 0.265 0.31 0.58 0.475 0.72 0.71 0.52 0.46 0.53 0.22
SA koT x 0.28 0.41 0.39 0.28 0.31 0.58 0.49 0.74 0.73 0.53 0.46 0.53 0.58
s-I
kqT x
1306 5.3 120 11.2 10.3 3.3 19.0 4.3 26.0 6.6 4.0 25.0 1240
M-] s-I
ET,' cm-l
rpph(77K),ms ket/k-d,f k,,/k-d,r
-300' 2.2 4.7 3.4 4.1 5.0 2.7
27 800 17 900' 17 400' 17 800' 19 700 18 700
3.6 7 .O 9.3 18.4 24.7
19 OOO
4.9 4.6 2.5
20 100'
g
-1Od 2.5e 7.6 154' 2.2 4.4 4.2
1.7e
' Reference 18. Reference 20 (in hexane). Reference 21. Reference 22. e Reference 17. f In ethanol. Not observed.
@A
0.03 0.06 0.03 0.04 0.1
0.013 0.03 0.025 0.03 0.02
0.1
0.012
0.18 0.41 0.75
0.05 0.1 0.12
= 0.1 1 and 0.15
in benzene bubbled by OdN2 mixture containing 39.8% and 59.6% 02,respectively. I
comparing the optical density of the ketone triplet absorption (ADT) at the of the T-T absorption maximum.I3
The phosphorescence lifetimes of diketones (Zph) were measured on a laser fluorimeter. The solutions in a quartz tube were placed in a liquid nitrogen Dewar and excited by a 355nm laser pulse. Decay kinetics were recorded at the phosphorescence spectrum m a ~ i m a . ' ~ , ' ~ Quenching of biacetyl and benzil fluorescence by molecular oxygen was studied on a Spex fluorimeter (5,= 355 nm, OD355 I 0.1) at the maxima of the fluorescence spectra, 470 and 505 nm,I9 respectively. The solutions were air-saturated or bubbled with 0 2 or 0 2 / N 2 (39.8% and 59.6% 0 2 ) mixtures for 20 min. The accuracy of @ A and SA determinations are &15%; hT,kqT,and t p h values are f20%.
Results The experimental data are presented in Table 1. According to the definition,Ii-l3 the efficiency of singlet oxygen generation can be estimated from eq 3, where @T is the quantum yield of the triplet state. Since for all diketones in this study and acetone @T = 1'8-22, SA = @Am from eq 3.
(3) Biacetyl and benzil exhibit small S I*-TI splittings, -2500 cm-', and long fluorescence lifetimes: T"fl = 11.5 and 2 ns,I8 respectively. We therefore studied the fluorescence quenching by molecular oxygen. From the Stem-Volmer equation, @O,/CDfl
=1
+ K[O,]
(4)
the constants are very low (K= 7.2 and K2.1 M-' (f50%) for biacetyl and benzil, respectively, in benzene). We can estimate the rate constants of fluorescence quenching by 0 2 SI*)) using the T"fl values from ref 18 to be 6.2 x lo8 and < 1 x lo9 M-' s-l for biacetyl and benzil, respectively. These rate constants are significantly lower than values for aromatic hydrocarbons with lowest n,n*excited singlet and triplet states and various SI*-TI splittings (-(1-4) x 1Olo M-' s-1).22-25 The inefficient fluorescence quenching by oxygen is probably caused by the n,n*lowest excited singlet and triplet states of a-diketones2* and requires further investigation. We established in a laser photolysis experiment that @T is constant (&lo%) for all diketones in Ar- and 02-saturated solutions. We also established that there is no triplet-triplet
O
450
100 wavelength. nm
Figure 1. Triplet-triplet absorption spectrum of benzil in benzene.
absorption for benzil between 550 and 1000 nm (Figure l), while, for biacetyl and camphorquinone, strong characteristic absorption bands between 630 and 1100 nm are observed. These triplet bands have been attributed to the 3n*(31) 3n*(311) transition (13B, I13A, for biacetyl).I4 The splittings of the two triplet n* levels depends on the intercarbonyl dihedral angle (0)between the two carbonyl groups and decreases when 0 decreases from 180" to 90", and the infrared absorption band is displaced toward longer wavelength at this occurs.14 We assume that this triplet infrared band for benzil is in the region A > 1000 nm in spite of the trans-planar configuration (0 = 180") of triplet benzil in liquid solution^.^^-^^ The IO2 decay rate constant at laser pulse energies 5 1 mJ ( k ~was ~ ) in agreement with literature data2 for all carbonyls. However, increasing the laser pulse energy (El)leads to a sharp increase in the IO2 decay rate constant ( k ~with ) a-diketones. The maximum effect was observed for biacetyl: for example, in benzene k~ = 2 x lo5 s-l at E1 = 50 mJ (Figure 2), compared to 3.25 x lo4 s-l at E1 0. This effect depends strongly on the ketone and is small for phenanthrenequinone and acenaphthenequinone ( k ~= 4.5 x lo4 s-l) and very small for /3-naphthoquinone and vitamin K3 ( k = ~ 4 x lo4 s-I) at El = 50 mJ in benzene. The yield and decay kinetics of singlet oxygen for biacetyl, benzil, and camphorquinone are the same in air- and 02-saturated benzene. The decay kinetics of IO2 luminescence for a-diketones at high pulse energies are biexponential (Figure 2). The IO2 luminescence lifetime at limiting long times does not depend on the laser pulse energy for the a-diketones, suggesting that the decrease is caused by interaction with an intermediate that decays. The sharp decrease of the IO2 lifetime at high pulse energies cannot be explained by singlet oxygen quenching by the
-
-
-
Darmanyan et al.
11856 J. Phys. Chem., Vol. 99, No. 31, 1995 0.8
0.6
2
2
b 00 0.0 2 0
I .o
50 * .2 -4
I
'20.0
I
I
ll
.l.O
2.0
600
%
400
me
$
1.0
$ a
I
44 200
0
3.0 10
0
20
30
50
40
0
El, ml Figure 2. Dependence of decay rate constant and luminescence amplitude of singlet oxygen on laser pulse energy with biacetyl (OD3ss = 0.22) in benzene. Upper panel: (a) fast exponential decay rate constant of ' 0 2 luminescence at EI = 51 mJ; (b) exponential approximation of luminescence tail with t = 19 ps. Lower panel: plot of fast decay rate constant (left axis) and maximum intensity (right axis) vs laser power.
products of the photochemical oxidation of a-diketones. We established, for example, that the oxidation products of benzil (benzoic acid, biphenyl, and phenyl benzoate) quench singlet oxygen with very low rate constants, < 104 M-' s-l (using TPP sensitizer, 530-nm laser pulse, addition of these compounds led to no decrease in ' 0 2 lifetime). Superoxide anion quenches singlet oxygen very efficiently with a rate constant of 1.6 x lo9 M-' In this work we attempted to detect 01.- using a pulsed conductivity method." However, the formation of charged intermediates was not observed upon excitation of any of the carbonyls. In principle, the sharp decrease of singlet oxygen lifetime could be caused by singlet-singlet annihilation of ' 0 2 molecules: '0,
+ '0, - 0, + lo2*- 0, + lo2
(5)
To test this hypothesis, we studied ' 0 2 qeneration by TPP at laser pulse energies from 1-50 mJ (Figure 3). At a TPP concentration of 1.7 x M in benzene, k A = kAo = 3.25 x 104 s-l, independent of pulse energy. We also studied a more concentrated solution ([TPP] = 3 x l W 4 M in a 2-mm cell) and observed photobleaching of TPP at E1 = 50 mJ along with only -6% decrease of ' 0 2 lifetime, probably from ' 0 2 quenching byproducts of sensitizer decomposition. When pulse energies are decreased to -1 mJ, the I 0 2 lifetime was less than 30 ps due to quenching by impurities, in contrast to a-diketones. Thus, in any case we can neglect spin-forbidden reaction 5 . Singlet oxygen is rapidly quenched by triplet sensitizer
10
20
30
40
50
El, ml Figure 3. Dependence of decay rate constant and luminescence amplituqe of singlet oxygeqon laser pulse energy with TPP in benzene (OD355 = 0.29, [TPP]= 1.7 x M). Upper panel: exponential decay rate constant of '02 luminescence at E1 = 50 mJ. Lower panel: plot of decay rate constant (left axis) and maximum intensity (right axis) vs laser power.
where IS* and 3S*2 are the excited singlet and second triplet state of sensitizer, and in the liquid phase, the rate constant of reaction 6 should be diffusion controlled. This annihilation of ' 0 2 and triplet sensitizer can lead to a decrease of ' 0 2 lifetime at large pulse energies, and this process must be taken into account. We can neglect reaction 6 for TPP as sensitizer because the ' 0 2 lifetime does not depend on the laser pulse energy (Figure 3). The yield and decay kinetics of ' 0 2 luminescence for diketones are constant in air- and 01-saturated solvents as mentioned above. We next analyze the case of biacetyl in benzene. According to the data in Table 1, the lifetime of biacetyl triplet is 260 ns in 02-saturated benzene. This means that the concentration of triplet biacetyl should be -lo-* M through 2 ps after the laser pulse at an energy of 20 mJ. Thus, the contribution of reaction 6 should lead to an increase in the ' 0 2 decay rate constant of about -1% if the rate constant of reaction 6 is diffusion controlled. The same estimate holds for other diketones. In principle, annihilation of ' 0 2 and triplet diketone can contribute to the ' 0 2 decay rate constant only during the first 1-2 ps after the laser pulse. We conclude that the sharp decrease of ' 0 2 lifetime for diketones at high pulse energy after a 1-2-ps delay cannot be due to any of the above mechanisms and suggest that it is caused by quenching of singlet oxygen by acylperoxy radicals formed on decomposition of a-diketones in air-saturated solutions. Photolysis of benzil and biacetyl are given by reaction 7?-1°
(3s*)33-36
'0,
+
-
3 ~ *
'S*(or
+
3~*2)
3
~
,
(6)
where R = Ph or Me for benzil and biacetyl, respectively. The
Interaction of
'02
with Peroxy Radicals
J. Phys. Chem., Vol. 99,No. 31, 1995 11857
TABLE 2: Singlet Oxygen Quenching by Peroxy and Acylperoxy Radicals in Benzene sensitizer a, lo3 s-'/mJ OD355 biacetyl benzil
5.2 0.7 2.9' 1.05 0.3 1.46 0.78 1.45 9.5
4,4'-dimethox ybenzil camphorquinone
9,lO-phenanthrenequinone acenaphthenequinone azoisobutane benzoin e
0.22 0.266 0.3W 0.274 0.038 0.156 0.163 0.344 O.O7(TPP)e 0.25 0.042(TPP)'
+ +
0.23
[R(CO)O,'] = [R(CO)O,'], exp(-t/t,) The
I02
(8)
decay in the presence of R(CO)O2' radicals is given
by lo,
-
(9)
3 ~ 2
kAo
The kinetic equations for reactions 9 and 10 give the 0
lo2 + R
Y O p
k,
following expression for
0
302
I02
+ RYOp
(10)
0.051d 0.014
0.44f
- kq[R(CO)O,'],[l -
[IO2] = [102], exp{-k/t
exp(-r/zR>lzR) ( l l ) For long-lived radicals, when
t~ >
(kh0)-', eq 11 becomes
= [1021rJexp(-k,t)
@ ~ ( 0 2 )= 0.46
from this work.
energy (Figure 2) using the equation [R(CO)O,'], = 20decEl(1 - 10-0D355)CDdec (14) where @&c is the quantum yield of a-diketone decomposition resulting in the formation of free radicals in bulk solvent, i.e., the quantum yield of acylperoxy radicals avoiding geminate recombination and escaping the bulk solvent is 2@dec. We obtain from eqs 13 and 14
where a is a parameter describing the efficiency of quenching by radicals
a = (kA - k : ) / ~ ~
I02
(16)
The a parameter equals the initial linear slope of the dependence of k~ on the energy of the laser pulse, for example, for biacetyl in Figure 2, lower panel, left axis. The experimental parameters a and the quenching rate constants (k,) of reaction 10 obtained from eq 15 for diketones are presented in Table 2. The assumption t~ > (kh0)-' is followed well for acylperoxy and peroxy radicals. For example, in ref 9 the lifetime of PhC03' radical in benzene was estimated to be -5 x s. The rate constant for rert-butylperoxy radical recombination is 4 x lo3 M-' s-' in benzene;39this would give a half-time for radical decay of 250 ms if the initial concentration of radicals after the laser pulse is M. We also studied azoisobutane (Figure 4),known to give tertbutyl radicals.40 No I 0 2 luminescence was observed upon excitation of this compound. Tert-butylperoxy radicals were generated in air-saturated benzene: t-BuN=NBu-t
decay:
1.2 1.5 0.13/@decc 0.9 5.5 o.l/@dec 0.05/@dec o.o7/@dec 1.1
0.022
a Reference 7. &30%. In cyclohexane. Calculated from the ratio @dec/@&(Oz)= 10/90 from ref 7 and taking Adding correction for TPP absorption. fReference 38.
free radical RCO' is efficiently scavenged by oxygen: the reaction of different alkyl radicals with oxygen was shown to occur with a rate constant in benzene of -(2-8) x lo9 M-' s-1.37Taking this value into account, we estimate the lifetime of RCO' is 1300 ns in air-saturated benzene; Le., it decays very rapidly, and the observed effect cannot be explained by its reaction with l 0 2 . We also studied the quenching of triplet benzil by trunsstilbene and established that, in benzene, the quenching rate constant is 3 x lo9 M-' s-l. The triplet energy (ET)of trunsstilbene is 17 300 cm-' I*, less than for benzil, and quenching probably occurs by energy transfer to the triplet state of trunsstilbene. The addition of trans-stilbene leads to an increase in the I 0 2 lifetime upon excitation of benzil at EI = 50 mJ (data not shown). This result confirms the suggested mechanism of I 0 2 quenching by acylperoxy radicals: the quenching of benzil triplet by trans-stilbene leads to a decrease in the acylperoxy radical concentration, leading to a decrease in the quenching efficiency and an increase in the I 0 2 lifetime. We will assume an exponential decay of acylperoxy radicals in order to simplify the theoretical approach:
k,,b 1OloM-' s-]
@deca
; 2t-Bu'
)02
2t-BuOO'
(17)
For ' 0 2 production we added a small quantity of TPP to the solution. The energy of the laser pulse absorbed by azoisobutane (AZ)was estimated from the expression
Eabs= EIOD3,,(AZ)[1 - 10-oD35S(AZ+TPP)]/OD355(AZ+TPP)
(12)
(18)
where the rate constant of '02decay in the presence of R(C0)0 2 . radicals is given by
We could estimate only the lower limit of the k, value for ' 0 2 quenching by tert-butylperoxy radicals (Table 2) because the @&c value for 2,2'-azoisobutane is absent in the literature. For a-diketones and azoisobutane, we observed biexponential decay of IO2 at high laser pulse energies, but the laser-powerdependent decay is relatively small (Figure 4). Singlet oxygen has a high diffusion coefficient (D(O2) = 5.7 x lo-, cm2 s-I in benzene23)and should diffuse very rapidly into unirradiated regions (the laser beam is usually not uniform) where the concentration of peroxy radicals is low or absent. Diffusion
r'021
kA = k /
+ k,[R(CO)O,'],
(13)
Thus, it is necessary to know the R(CO)O2' radical concentration immediately after the laser pulse in order to estimate the quenching rate constant of singlet oxygen by acylperoxy radicals (reaction 10). We can estimate the value of [R(CO)02.10 from the initial linear dependence of k~ on the laser pulse
Darmanyan et al.
11858 J. Phys. Chem., Vol. 99,No.31, 1995
r -
I
1
\
I
24
9 '
-
100
*a
b
S '
40
i
N
WQ
> E
I 0
50 20
3 .
0
I
0
O
10
20
40
30
0
50
10
20
30
40
El, mJ
El,mJ
Figure 4. Dependence of decay rate constant and luminescence amplitude of singlet oxygen on laser pulse energy following excitation of TPP (OD355 = 0.073) and azoisobutane (OD355 = 0.344) in benzene. Upper panel: (a) fast exponential decay rate constant of IO2 luminescence at EI = 32 mJ; (b) exponential approximation of luminescence tail with t = 18.2 ps. Lower panel: plot of fast decay rate constant (left axis) and maximum intensity (right axis) vs laser power.
Figure 5. Dependence of decay rate constant and luminescence amplitude of singlet oxygen on laser pulse energy following excitation of TF'P (OD355 = 0.04) and benzoin (OD355 = 0.25) in benzene. Upper panel: (a) fast exponential decay rate constant of '02 luminescence at El = 40 mJ; (b) exponential approximation of luminescence tail with t = 30.0 ps. Lower panel: plot of fast decay rate constant (left axis) and maximum intensity (right axis) vs laser power.
coefficients of the peroxy radicals are unknown but should be lower than that of singlet oxygen. For instance, D = 2.09 x cm2 s-I 42 in benzene for methyl ethyl ketone, which has a comparable size to terr-butylperoxy radicals. The analogous experiment was also carried out with benzoin (Table 2). Benzoin is not a I02 generator because it has very short triplet lifetime (-0.3 n ~ ) For . ~ example, ~ in 02-saturated benzene, @A 5 5 x a-Cleavage of the central C-C bond in the triplet state results in formation of benzoyl and a-hydroxybenzyl radicals, which are trapped rapidly by oxygens-10~22~37*38-42 The results are shown in Figure 5 .
of Q A ( 0 2 ) = 0.84 was obtained for camphorquinone, considerably higher than the value of SAfound in the present work, 0.41. Probably, the error is caused by the indirect method of '02measurement, based on the analysis of oxidation products of 1,2-dimethylcyclohexannein reaction with IO2 and acylperoxy radicals, which may produce a large error. In the work of Gijzeman et al.," the classical scheme of triplet quenching by molecular oxygen was suggested. Using this scheme and the values of kqT and SAobtained, we evaluated the rate constants of the energy transfer (ket) '(3S**-02) I (SO-* *'02) and internal conversion (IC,,) 3(3S.*-02) 3 ( S ~ *-02) * in Table 1. Table 1 shows that there is no correlation between kt or k,, and the triplet energy (Table 1). The analogous conclusion was reached in ref 44 for different ketones and aromatic hydrocarbons. Moreover, evaluation gives negative values of kt and k,, for acetone because the triplet state quenching rate constant assumed is too high, kqT RZ kdlf (Table 1). This means that the simplified classical scheme43does not correctly describe the quenching mechanism. In ref 12, we suggested a modified quenching scheme which takes into account intersystem crossing between singlet and triplet states of the encounter complex, which can have a structure similar to an exciplex, and which depends on the energy of the charge transfer (CT) state. Probably this scheme is also oversimplified, because it gives a maximum quenching rate constant hT= 4/9k,f. However for acetone, the compound with the highest triplet state, kqT k d l f . This fact can be explained only by taking into account the additional intersystem crossing channels between quintet and singlet and triplet states of the encounter complex. It is necessary to evaluate the CT interaction, which leads to effective coupling of encounter complex states with different multiplicites via an acetone.
0 H OH
Hph hv- PhCO. Ph
+ PhEHOH
3 4
0
II
PhC02.
00.
I
+ PhCHOH
(19)
We estimated the k, value for benzoin in Table 2, assuming that the peroxy radicals formed in reaction 19 have identical reactivity toward singlet oxygen. In all the experiments, the lifetime of '02luminescence in the tail is close to that in pure benzene (Figures 2, 4, and 5 ) and does not depend on the laser pulse energy.
Discussion All ketones studied in this work except for 1,2-naphthoquinone are carbonyls with lowest n,n* triplet states.17,22,26-31 The low values of the phosphorescence lifetimes (Table 1) confirm this conclusion. The photochemistry of the naphthoquinones has not been reported; for vitamin K3, only the phosphorescence lifetime has been p~b1ished.l~In ref 7, a value
- -
Interaction of
'02
with Peroxy Radicals
J. Phys. Chem., Vol. 99, No. 31, 1995 11859
CT state. It was recently shown that CT interactions strongly influence the quenching mechanism, leading to a sharp increase of kqTand decrease of SAvalues of substituted naphthalene^.^^ To provide a complete analysis of the quenching mechanism of triplet ketones by oxygen, it will be necessary to establish the elementary rate constants of energy transfer with production of oxygen in the 'E,+,'A, and states, as has been done recently in a different system46 in order to evaluate the contribution of CT interaction and intersystem crossing rate constants within the encounter complex. The rate constants of singlet oxygen quenching by acylperoxy radicals (Table 2) in benzene are close to the diffusion-limited rate constant, kdif = 3.0 x 10'' M-'s-'.'~ Quenching by energy transfer is probably not possible because the lowest excited double states of peroxyls are very high, since the absorption bands in peroxy radicals lie in the far W r e g i ~ n . ~ ' Quenching .~~ probably occurs via the electron exchange interaction in a spinallowed reaction
In ref 48, ' 0 2 quenching by stable nitroxyl radicals was studied, and it was established that quenching similar to reaction 20 takes place and that charge transfer interactions do not play a significant role. The quenching rate constants are -lo5M-' s-' for aliphatic radicals with a strong steric screening of the > N - O reactive center, while for aromatic radicals, steric hindrance around the >N-O' center is significantly lower, leading to higher rate constants. For example, for di-tertbutyldiphenyl nitroxide, the quenching rate constant is 1.6 x lo7 M-' We assume that, in the case of peroxy radicals, CT interactions also should not make a significant contribution to quenching and that steric hindrance of the -0-0' reactive center is practically absent, so that the rate constant of reaction 20 is close to the diffusion-controlled limit. Disproportionation of secondary alkylperoxy radicals is known to follow the Russell m e ~ h a n i s m : ~ ~ - ~ l alcohol
R02'
+
R02'
+
ketone(So)
+
'02
/" \ alcohol + ketone(T1) + 02alcohol
+
ketone(So)
+
IO2 (21)
To estimate IO2 yields correctly, it is necessary to add reaction 20, which becomes important, for example, in benzene at peroxy radical concentrations I M, to this scheme. In studying oxidation reactions with participation of both singlet oxygen and peroxy radicals, reaction 20 must also be taken into account for accurate estimation of the yield of products; at high concentrations of peroxy radicals, the radical pathway should be dominant. The decrease of ' 0 2 lifetimes at high laser pulse energies should be a general phenomenon for various organic compounds because photodecomposition of sensitizers in air-saturated solution can lead to the production of various peroxy radicals which could quench singlet oxygen with rate constants up to the diffusion limit.
Acknowledgment. Supported by National Science Foundation Grant No. CHE89-11916. References and Notes (1) Wilkinson, F.; Helman, W. P.; Ross, A. B. J . Phys. Chem. Ref. Data 1993, 22, 133.
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