J . Phys. Chem. 1988, 92, 5288-5292
5288
Pulse Radiolysls of Poly(styrenesu1fonate) in Aqueous Solutions David Behar*,+and Joseph Rabani*,* Energy Research Center and Department of Physical Chemistry, The Hebrew University, Jerusalem 91 094, Israel, and Department of Radiation Chemistry, Soreq Nuclear Research Center, Yavne 70600, Israel (Received: December 15, 1987)
Various radical species have been identified in the pulse radiolysis of aqueous solutions of poly(styrenesu1fonate) (PSS). OH radicals react with PSS ( k = (7.5 f 1) X los M-ls-' ) t o produce predominantly a mixture of OH adducts. An intrapolymer biradical decay of the OH adducts ( 2 k / t = (3.0 i 1) X IO5cm s-') takes place when several OH radical adducts are produced on the same polymer molecule. This decay reaction competes with a first-order formation of a positive radical ion ( k = (160 i 40) s-' at pH 5.6). The formation reaction is acid catalyzed. It is slowed down by addition of inert salts. The positive radical ion decays in the time range of hours, obeying a first-order rate law ( k = (5.4 i 0.5) X s-]). This decay is attributed to an intramolecular conversion of the positive radical ion to a benzyl type radical, which subsequently decays to produce stable recombination products. The spectra and kinetics are discussed in light of the known properties of various aromatic and polymer systems. Long-lived polymeric free radicals are useful models for photochemical storage systems.
Introduction The role of polyelectrolytes in fast reaction kinetics, in general, and in photochemical electron-transfer reactions in particular, has been extensively investigated in the last decade.' Photochemical electron-transfer reactions are ordinarily followed by fast thermal back-electron-transfer processes with the subsequent formation of the original ground-state reactants. Such reactions are the main obstacle for achieving photochemical energy storage in purely chemical systems. Relatively very slow back reactions have been first observed in our laboratory when the redox products were linked to polyelectrolyte molecules.'4 The photochemical products are formed so that the oxidant and reductant species are linked to different polymer molecules that possess the same type of charge. Inhibition of the back reactions between such species was particularly effective when the reactive segments were covalently linked to the charged polymer molecule^.^^^ In this case photochemical energy Storage could be observed for several minutes. Similar observations have been reported by Margerum et aL5 Recently an ionizing radiation method for the production of chromophores linked covalently to polymers has been developed.6 Pulse radiolysis is known to be a powerful tool in creation and kinetic investigation of highly reactive species. Polymeric free radicals can be easily produced and serve as models for the more complicated photochemical systems. Indeed the development of the first polyelectrolyte energy storage was based on preliminary work that was carried out by pulse radiolysis.' Although our main interest remains the application of polyelectrolytes to photochemical conversion and storage systems, we find it useful to extend the earlier work on neutral and charged to additional compounds, using the pulse radiolysis technique. The following questions are still open: How does the polymer charge density affect the rates of reactions of species linked to the polymer? What are the relative contributions of the electric field repulsion and the slower diffusion of polymer molecules to the inhibition of radical-radical reaction rates? How does the polymer configuration in solution (coil versus rod) affect the reaction rates? What is the role of segmental diffusion? How does the polymer field affect the reaction of specific groups? Obviously, an undertaking to answer the above questions is beyond the scope of an ordinary single publication. In the present work, we chose to investigate the poly(styrenesulfonate) negative polyelectrolyte with average molecular weight of 70000. As will be seen later, the present results, when compared with the earlier research on the low molecular weight positively charged p ~ l y b r e n e show , ~ a much greater complexity. This is
'Soreq Nuclear Research Center. $The Hebrew University.
0022-3654/88/2092-5288$01.50/0
apparently due to the specific properties of the styrene repeating units.
Experimental Section Materials. PSS, average molecular weight 70 000, from Polyscience was purified by dialysis against 0.2 mM EDTA sodium salt. The EDTA was removed by additional dialysis against water. All other chemicals were of the best available purity and were used without further purification. PSS solutions were prepared in syringes and saturated with N 2 0 by bubbling. In some experiments controlled concentrations of O2 were employed by injection of appropriate volumes of 02-saturated PSS solutions. Water was distilled and passed through a Millipore Milli-Q water purification system. PSS concentrations are given in monomer units. Apparatus. The pulse radiolysis setup consisted of Varian V77 15 linear accelerator fitted with optical detection and computerized data processing systems as previously described.l0 The pulse duration varied from 0.1 to 1.5 bs with a dose ranging from 0.2 to 3.5 krad per pulse. Dosimetry was performed with N 2 0 saturated KSCN solutions taking G((SCN)-,) = 6 and t((SCN)-,) = 7600 M-l cm-' at 480 nm. The light path was 12.3 cm. Decay kinetics and absorption spectra of very long-lived radicals were investigated in a 1-cm quartz cell. The solutions were saturated with N 2 0 with the aid of a thin Teflon tubing that was passed through the stopcock connected to the quartz cell. The cell was irradiated by one or two electron pulses and quickly (within less than 30 s) transferred to a Kontron spectrophotometer (Uvikon Model 860) for recording absorption spectra and kinetics of the radicals. Unless otherwise stated, the measurements were carried out at 300 nm, at 24 i 2 OC, in air-free N20-saturated aqueous solutions. Results and Discussion The primary free radicals produced by the irradiation of aqueous solutions, in the presence of N 2 0 , are mostly O H radicals: (1) The field has been recently reviewed by: Rabani, J., In Photoinduced Elecrron Transfer; Fox, M. A,, Ed.; Elsevier: Holland, 1988. (2) Sassoon, R. E.; Rabani, J. J . Phys. Chem. 1985,89, 5500. (3) Rabani, J.; Sassoon, R. E. J . Photochem. 1985, 29, 1. (4) Sassoon, R. E. J . A m . Chem. SOC.1985, 107, 6133. (5) Margerum, L. D.; Murray, R. W.; Meyer, T.J. J . Phys. Chem. 1986, YO, 728. (6) (a) Neta, P.; Silverman, J.; Markovic, V.; Rabani, J. J . Phys. Chem. 1986, 90,703. (b) Behar, D.; Neta, P.; Silverman, J.; Rabani, J. Radiat. Phys. Chem. 1987, 29, 253. (7) Sassoon, R. E.; Rabani, J. J . Phys. Chem. 1984, 88, 6389. (8) Behzadi, A,; Borgwardt, U.; Henglein, A,; Schamberg, E.; Schnabel, W. Ber. Bunsen-Ges. Phys. Chem. 1970, 74, 649. (9) Matheson, M. S.; Marnou, A,; Silverman, J.; Rabani, J. J . Phys. Chem. 1973, 77, 2420. (10) Nahor, G. S.; Rabani, J. Radiat. Phys. Chem. 1987, 29, 79
0 1988 American Chemical Society
Poly(styrenesu1fonate) in Aqueous Solutions H20
--
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5289
e- aq (2.6), H (0.61, O H (2.6), H202 ( O J ) , H2 (0.45), H30’ (3.6), OH- (0.8) (1)
The numbers in parentheses represent the respective G values. The hydrated electrons react efficiently with N 2 0 according to eq 2, so that the actual yield of O H radicals is doubled. e-aq
+ N20
H20
N2
+ O H + OH-
(2)
Under our conditions the yield is even higher due to the scavenging of e-aq from the spurs at the saturated NzO solutions. In the following, calculations are based on G(0H) = 6.0. When PSS is also present, O H radicals as well as hydrogen atoms are expected to react with it. The hydrogen yield amounts to only 10% of the total radical concentration, and no distinct absorption that can be specifically attributed to the reaction of H atoms has been observed. In the following we will discuss the results in terms of the O H products only:
PSS
+ OH
-
PSSOH.
(3)
O H radicals are not very selective and might react at various sites of the PSS molecule. However, it has been generally found that O H addition to an aromatic ring is several times faster than hydrogen abstraction. Thus, it has been shown in pulse radiolysis of phenylacetic acid solutions that more than 90% of the O H radicals add to the aromatic ring.” Only 3% of the OH radicals abstract hydrogen from toluene, while 97% add to the aromatic part.12 Similar results have been observed in methylated benze n e as ~ well ~ ~ as other aromatic cornpounds.”J2 It is therefore expected that the O H radicals attack the PSS at the aromatic parts mainly and produce O H adducts. Rates of Reaction of PSS with OH and 0-. The rate of formation of the O H adduct has been measured at 300 nm by following the increase of absorption after an electron pulse. The measurements were carried out at relatively low PSS concentrations (0.1-0.2 mM) and low pulse intensities (2-6 WMO H radicals per pulse), at pH 5.5. The kinetic measurements yield k3 = (7.5 f 1) X IO8 M-I SKI.Addition of 4 mM MgS04 to 0.2 mM PSS has no effect on k3. A buildup of optical absorption at 300 nm is observed also when the solutions described above are pulse irradiated in 0.2 M NaOH. Under such conditions, the gxidizing radicals are predominantly in the form of 0- radical ions. The absorption observed in 0.2 M NaOH is identical in shape (but not in size) with the absorption observed in near-neutral solutions. We suggest that despite the low relative concentration of O H radicals (compared with 0-) in 0.2 M NaOH, the former account for the buildup of absorbance. Indeed, the kinetic measurements show that the absorbance in 0.2 M N a O H builds up considerably slower than in the nearneutral solutions. The rate of formation of the absorbance can be accounted for if we attribute it to the small concentration of O H radicals in 0.2 M NaOH, neglecting the reaction of 0- with PSS (reaction 3’). Our results yield an upper limit k’3 < 5 X or
PSS
PSS
+ 0-
+ 0-
+ OHPSSOH. + OH-
PSS.
+
H20
(3’)
lo6 M-’ s-l. Under our conditions in 0.2 M NaOH, considerable OH/O- radical radical reaction takes place in competition with reactions 3 and 3’. The slowness of reaction 3’ is attributed to the strong electrostatic field of the polyelectrolyte, resulting in repulsion of the negatively charged 0- radical ions. Intramolecular Reactions of PSSOH-. Reaction 3 is followed by a first-order buildup process that in near-neutral solutions (pH 5.6) has a rate constant of (1.6 & 0.4) X lo2 s-l. Typical computer traces are shown in Figure 1. As will be seen later, the rate of (11) Neta, P.; Hoffman, M. Z.; Simic, M. J . Phys. Chem. 1972, 76, 847. (12) Christensen, H. C.; Sehested, K.; Hart, E. J. J . Phys. Chem. 1973, 77, 983. (13) Sehested, K.; Corfitzen, H.; Christensen, H. C.; Hart, E. J. J . Phys.. Chem. 1975, 79, 310.
010 -
I
I
I
1
,D
000 006c
/
0.041
ID’
O
o
2
1
1
Oo6
OI2
0.08
l
i
t
1
t
0.04
0
0
16
24
32
40
t(ms)
Figure 1. Optical density traces of PSS radicals at 300 nm: (a) 0.6 mM PSS, total OH radicals [OH], = 1.3 pM per pulse; (b) 0.1 mM PSS, [OH], = 1.3 pM per pulse; (c) 0.1 mM PSS,[OH], = 12 pM per pulse.
270
300
330 360 390 X (nm) Figure 2. Absorption spectra of transients (1 mM PSS, neutral solution, N20-saturated, [OH], = 4.6pM): (a) spectrum of the OH adduct 0.1 ms after the pulse; (b) spectrum of the cation radical 30 ms after the pulse.
buildup as well as its magnitude (&), relative to the initial absorption Do)depend on the experimental conditions (pH, pulse intensity, and PSS concentration). In the following this process will be referred to as “the first process”. The absorption spectra at the beginning and at the end of the first process are presented in Figure 2. The initial spectrum (a) is undoubtedly that of the mixture of the OH adducts at positions 2, 3, 5 , and 6 in the benzene ring. The spectrum at the end of the first process (the plateau in Figure l a ) is attributed to a product that is produced from the OH adduct. First-order formation of free radicals from OH adducts has been previously observed in several systems. l-I3 Neta et al.” studied the effect of pH on the absorption spectra of several aromatic radicals produced by reactions of O H and 0-. In alkaline solutions, aliphatic hydrogen abstraction by 0-radical
Behar and Rabani
5290 The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
ions is preferential to ring addition. The product is a benzyl type radical. On the other hand, at near-neutral solutions O H additions predominate. The immediate product is a mixture of OH adducts that undergo an intramolecular reaction to produce the benzyl type free radicals. Sehested et al.13 found similar conversion processes in methylated benzenes. From the effect of acid, which enhances the rate of conversion of the O H adducts into benzyl radicals, the following mechanism has been proposed:I3 ti+ I
0 (4)
A similar reaction mechanism with certain modifications may also account for the PSS results. It will be discussed later. Effect of Pulse Intensity and [PSS]. The ratio D,/Do increases with PSS concentration and decreases with increasing pulse intensity. The limiting value at the highest PSS concentration used (1 mM) and lowest pulse intensity (0.8 WMtotal O H radical concentration) is 1.83. Increasing the pulse intensity by 8-fold results in a decrease of the ratio D,/Do to 1.75. However, at considerably lower PSS concentration, with relatively high pulse intensities, smaller D,/Do ratios are observed, and in some cases of low [PSS],decay of absorbance is observed instead of formation. These results can be interpreted by competition between a first-order process that leads to increase of the absorbance and a radical-radical decay reaction that decreases the absorbance. The biradical decay is favored at higher pulse intensities and lower PSS concentrations. It cannot be attributed to an interpolymer radical-radical reaction between the O H adducts (eq 5) because 2PPSOH.
-
products
(5)
of the following reasons: First, such polyelectrolyte radical reactions have been found to be very taking place under our conditions in the time range of several seconds rather than milliseconds as observed here. Second, increasing the PSS concentration gradually eliminates the biradical decay. The rate of an interpolymer radical-radical reaction is expected to depend only on the pulse intensity and not on [PSS]. At very low polymer concentrations and high pulse intensities several radicals per chain are produced. This facilitates the intrapolymer radical-radical reactions (eq 6). Upon increase of the [PSS], the number of O H -CH-
CH-
I
I
SO3-
SO3-
adducts on the same polymer molecule decreases, the bimolecular reaction is slowed down, and the first-order formation reaction predominates. When there is only one radical per polymer molecule, no intrapolymer decay is possible and only the first-order formation process takes place in the millisecond time range. It should be noted that we were unable to achieve conditions where the first-order formation is completely suppressed, and no pure second-order kinetics could be observed. We calculated an average apparent 2k6/t = (3.0 A 1.0) X lo5 cm s-l. This value is based not on the local concentration of the radicals in the polymer volume but on the average concentration in solution. The spectra in Figure 2 were taken under conditions where the second-order decay could be neglected. Reaction of the OH Adduct with 02. Pulse irradiation in the presence of oxygen eliminates the buildup of absorbance in the millisecond time range, and instead a relatively fast decay of the initial absorbance is observed. The rate of decay is proportional to the oxygen concentration (Figure 3): PSSOH.
+ O2
-
product (peroxy radical)
(7)
1 2 3 [Oz] a IO4, M
4
Figure 3. Reaction rate of PSSOH. radical with 02:[PSS] = 1 mM; [OH], = 7 fiM per pulse.
TABLE I: Dependence of the First-Order Rate of Formation (the First Process) on DHand Inert Salts" molar PH k(formation),b s-I added 0.67 0.2 3.6 x 103 HC104 0.05 1.3 3.3 x 10' HCIO~ 2.8 X 10' ~ ~ 1 0.01 0 ~ 2 5.6
NaOH NaOH Na2S04 Na2S04 MgS04
0.10 0.05
0.10 0.002
11.5 13
neutral neutral neutral
1.6 X lo2
0.6 1.6 0.9 3.0 7.3
[PSS] = 1mM; pH adjusted with HC104or NaOH. k(formation) = k8 (see text).
The resulting product of reaction 7 does not absorb light at 300 nm. From the slope of Figure 3, k7 = (1.04 f 0.05) X lo7 M-I s-l is calculated. Effect of p H and Inert Salts on the First Process. The rate of the first process depends on both pH and inert ion concentration. The rate is considerably enhanced by the addition of acid. The results are summarized in Table I. Enhancement is observed upon changing the pH from 13 to 2, while in the pH range 0.2-2 the reaction rate remains almost constant. No detailed investigation of the pH profile was carried out. Addition of an alkali (NaOH) or salt (Na2S04or MgS04 at neutral pH) shows retardation of the rate. The effect of the addition of NaOH may be explained in part by the effect of the Na ions. This effect will be discussed later. Decay of Radicals in the Time Range of Hours. In the pulse radiolysis experiments, it has been found that the absorption present at the end of the first process ( D , in Figure 1) decays in the time range of several minutes. This decay has been found to be due to a photochemical effect of the analyzing light. Very little decay is observed in the time range of several minutes in the dark. Therefore the absorption present after the pulse could be investigated by using a conventional spectrophotometer. It has been observed that the decay of this absorption takes place in the time range of hours, as can be seen from Figure 4. The reaction obeys a strict first-order rate law k = (5.4& 0.5) X s-l. No residual absorption remains at 300 nm after 24 h. The absorption spectrum of the decaying species remains unchanged through the entire decay time, showing that the final products of the reaction do not absorb light in the measured range (270-450 nm). A typical spectrum taken with the Kontron spectrophotometer is shown in Figure 5. This spectrum is more accurate than the one taken with the pulse radiolysis setup (Figure 2). The small differences are due to more accurate wavelength calibration and better wavelength resolution in the commercial spectrophotometer setup. The long-lived absorbing species react with 0,. The reaction rate was not measured; however, when the reaction cell is opened to air, the absorption disappears immediately. Tests were carried out to ensure that the slow decay is not due to photochemistry
The Journal of Physical Chemistry, Vol. 92, No. 18. 1988 5291
Poly(styrenesu1fonate) in Aqueous Solutions
I 0.35
I
c\
-CH-CH2-
@H
so< -CH
-CH*-
I
0.15' 0
I
200
I 400
I
600
I 000
1 1000
1 1200
t (rnin)
+ -CH-CHz--
+
OH-
(8)
SO3-
-C-CHz-
I
-C-CH2-
I
Figure 4. Decay of the long-lived cation radical at 300 nm; two consecutive pulses each producing 12 pM OH, 10 mM PSS pH 5.5. The
decay was measured with the Kontron spectrophotometer. Insert: first-order plot of the decay.
0 270
300
330 X (nm)
360
8 390
Figure 5. Absorption spectrum of the long-lived cation radicals measured with the Kontron spectrophotometer 120 min after irradiation.
induced by the analyzing light. Thus, solutions that were kept in the dark showed the same decay as solutions that were continuously exposed to the analytical light of the spectrophotometer. Mechanism. The initial absorption (Doin Figure 1) observed several microseconds after the pulse is undoubtedly due to the OH adducts formed according to reaction 3. We suggest that in the absence of additives which may react with the OH adducts, reaction 3 is followed by reaction 8. At relatively high doses, reaction 6 competes with reaction 8. The product of reaction 8, a positive ion radical, further reacts according to eq 9. The product of reaction 9 is a benzyl radical, produced by an intramolecular electron transfer and deprotonation, which take place in the time range of hours. Reaction 9 was invoked to explain the first-order decay rate of the long-lived absorption. A radical-radical process would have resulted in a second-order rate law, contrary to the observations. The lack of observation of any absorption that might be attributed to the free radical product of reaction 9 can be accounted for, if we assume that reaction 10 is fast in comparison with reaction 9. The relatively high stability of the positive radical ion is quite unusual. However, some of the aromatic positive radical ions, such as several methoxybenzene radical^,'^*'^ are known to be relatively stable. For example, di(14) ONeill, p.; Steenken, S.;Shulte-Frohlinde, D. J . Phys. Chem. 1975, 79, 2113. (1 5 ) Brandys, M.; Sassoon, R. E.; Rabani, J. J. Phys. Chem. 1987,91,953.
so,and tetramethoxybenzenes can be oxidized to free radical cations, which under our conditions can be observed during several hundreds of milliseconds. it is conceivable that a radical cation, which is a part of a polyelectrolyte molecule, may live longer, for hours or even days. The formation of the positive ion radical from the O H adduct (reaction 8) is probably assisted by the negative electric field of the polymer. This is supported by the effect of inert salts (Table I), as the inhibition of reaction 8 upon addition of Na2S04 or MgS04 can be attributed to partial neutralization of the negative electric field of the polymer. Photochemical systems in which reactive segments were covalently linked to polyelectrolyte molecules showed up to 6 orders of magnitude inhibition of back reactions between the photochemical electron-transfer p r o d u c t ~ .With ~ ~ ~ the polyelectrolyte microenvironments, photochemical energy was stored for up to several minute^.^,^ The present results indicate that longer inhibition may be possible and that energy storage can perhaps be endured for periods of hours. N o comparison of the PSS results with the respective monomer radical reactions could be made. Attempts to use toluenesulfonate as a monomer model show that the initially formed OH adduct decays by a second-order process, competing efficiently with a possible formation of a positive radical ion. Therefore, direct comparison of the polymer and monomer is not possible. It should be mentioned that in the case of p toluenesulfonate a direct H abstraction from the CH3 group with 0- can be observed.I6 Our assumption that the benzyl radical in PSS decays much faster than the positive ion radical could be verified if the formation of the benzyl radical could be carried out directly, as was found in the case of p-toluenesulfonate. Unfortunately, in highly alkaline solutions the reaction between 0- and PSS is severely inhibited by the strong electrostatic repulsion between 0- and the polyelectrolyte as discussed above, and direct formation of the benzyl type radical of poly(styrenesulfonate) was not feasible. Several radical cations have been found to absorb light in the visible range,14 in contrast to the long-lived intermediate reported here. However, the optical absorptions of the cation radicals depend strongly on the nature of the cation radical and are affected by the functional groups. Therefore, the lack of absorption in the visible range cannot be used for identification of the transient radicals. Reaction of the Long-Lived Radical with Cu2+Ions. Addition of 0.2 mM Cu2+ (as perchlorate) to a preirradiated solution containing 10 mM PSS and saturated with N,O eliminated the long-lived absorption. On the other hand, 0.2 mM methylviologen (MV2+) and 0.2 mM Fe(CN),4- did not react with the long-lived species (all additives were added after the formation of the radical by the electron pulse). The experiments described above indicate that the polymeric positive ion radical is a moderate reductant and a poor oxidant. (16) Behar, D.; Rabani, J., to be submitted for
publication.
5292
J . Phys. Chem. 1988, 92, 5292-5297
This is in contrast with the observations in the monomeric methoxybenzene positive ion radicals, which in some cases have been found to be powerful oxidants, with no reducing proper tie^.'^,^^ The difference is attributed to the structure of the polymeric radical ion, which enables redox reactions involving the removal of the tertiary hydrogen from the C H group adjacent to the benzene ring.
Acknowledgment. This research was supported by the Israel US BNSF and by the Balfour Foundation. We are indebted to E. Gilead for assistance with the maintenance of the pulse radiolysis setup. Registry No. PSS, 50851-57-5; HO', 3352-57-6; Na2S04,7757-82.6; MgSO4, 7487-88-9; 0 2 , 7782-44-7; Cu(CIOJ,, 13770-18-8.
Thermolysis of a Polymeric Endoperoxide: The Yield of Singlet Oxygen Released into the Gas Phase Allen Twarowski* Rockwell International Science Center, Thousand Oaks, California 91 360
and Phan Dao Air Force Geophysics LaboratorylLID, Hanscom AFB, Massachusetts 01 731 (Received: December 21, 1987)
The fraction of oxygen that is electronically excited upon release from thin films of 1,4-dimethyl-2-poly(vinylnaphthalene 1,4-endoperoxide)was measured by reaction with 2,5-dimethylfuran, DMF. The amount of DMF that reacted with singlet oxygen was assayed by mass spectrometry and the dependence of the singlet oxygen yield on film thickness was found to be in substantial agreement with previous experimental results. Comparison of these measurements with model calculations is reported.
Introduction Thermolysis of the endoperoxides of many polycyclic aromatic hydrocarbons has been shown to result in efficient conversion of thermal energy to electronic excitation of the product oxygen molecule.' Recently, solid films of 1,4-dimethy1-2-poly(vinylnaphthalene 1,4-endoperoxide), 2PVNE, were reported to quantitatively release oxygen into the gas phase upon heating. The fraction of released oxygen which was electronically excited to the IAe was found to vary markedly with 2PVNE film thickness in agreement with a simple model of molecular diffusion and quenching in the polymer solid. The O2 'Ag fraction was determined from luminescence measurements at 1270 nm using a liquid nitrogen cooled germanium photodiode.2 There are difficulties with the determination of singlet oxygen yields from photometric data. First, the photometric signal is proportional to the number of luminescing species present at any one time rather than the integral amount of species produced. This problem was circumvented in our previous work2 by releasing the singlet oxygen quickly into a 10 Torr buffer gas which served to confine the excited oxygen to the viewing region of the detector for a period of time comparable to that required for thermolysis of the 2PVNE and release of O2from the polymer film. Under these conditions the peak germanium detector signal is proportional to the total amount of O2 IAg released into the gas phase. Another difficulty with photometric assay of O2 derives from the large number of physical constants required to convert the germanium detector signal to an absolute measure of luminescing species population. Uncertainties associated with the special distribution of O2 lAg in the sample chamber, the effective f number of the optical setup, the responsivity of the detector, and the radiative rate constant for the 02(1A,-3Z,) transition all contribute to the uncertainty of the calculated singlet oxygen yield. A more direct measure of the O2 'A, yield can be obtained by preferentially reacting the singlet oxygen molecule with another chemical species and assaying the reactants or products. This
'$
(1) Turro, N. J.; Chow, M. F.; Rigaudy, J. J . Am. Chem. SOC.1981, 103,
7218. (2) Twarowski, A. J.; God, L.; Busch, G. E. J . Phys. Chem. 1988,92, 396.
0022-3654/88/2092-5292$01.5O/O
method has been used to measure singlet oxygen density in the gas flow from a microwave discharge source. Gleason et aL3 measured the rate constants for the reaction of O2 lAg with tetramethylethylene (TME) and with 2,s-dimethylfuran (DMF) using a gas-liquid chromatograph to separate reactants from products and quantify the amount of material reacted. Their findings suggested that DMF physically quenched O2 'A, at about the same rate with which it reacted, whereas TME, though having a reaction rate with O2 lA, which was about 4 times slower than DMF, reacted quantitatively. The rate constants reported by these workers for the reaction of O2 'Ag with DMF and TME are 3.7 X lo8 and 1 X lo8 cm3 mol-' s-I, respectively. Huie and Herron4" measured the rate constants for the same reactions of singlet oxygen with D M F and TME using mass spectrometry to detect the reactants and products. D M F was found to react quantitatively with O2 lA,, in contrast with results reported by Gleason et al.3 The rate constants which Huie and for DMF Herron repod are 1.5 X 1Oloand 7.7 X IOs cm3 mol-I and TME, respectively. These rate constants are significantly larger than earlier reported values and the differences have been attributed to the presence of atomic oxygen in the gas flow in earlier studies. The effects of atomic oxygen were minimized in the studies reported by Huie and Herron by addition of NO2 to the singlet oxygen flow.5 In this paper we report the fractional yield of O2 lABreleased from 2PVNE films determined by reaction of the excited oxygen with TME and DMF. Using mass spectrometry to assay the quantity of D M F reacted, we found the singlet oxygen yield to be in substantial agreement with previous photometric determinations. Experimental Section The preparation of ZPVNE, its deposition as thin films on resistive glass plates, and the heating of the endoperoxide films (3) Gleason, W. S.; Broadbent, A. D.; Whittle, E.; Pitts, Jr., J. N. J . A m . Chem. SOC.1970, 92, 2068. (4) Herron, J. T.; Huie, R. E. Ann. N . Y . Acad. Sci. 1970, 171, 229. (5) Huie, R. E.; Herron, J. T. In?. J . Chem. Kine?. 1973, 5, 197.
0 1988 American Chemical Society
