5791
J . Phys. Chem. 1991, 95, 5791-5797 reactions only in the presence of oxygen. The RB radical cation has been proposed as an active intermediate in promoting phot~bleaching.'~Scheme I1 shows a plausible route to photobleaching of RB and eosine.
SCHEME I1 'D 'D 'D
+D + 'D
-. + D+
D-
(1)
D+
+ D-
(2)
D+
+ 02-
(3)
+ + Oz
D++D--2D D-
02
D+ + O2/OZ-
D
(4)
+ Oz--
(5)
colorless products
(6)
During photolysis, O2may accept an electron from the dye (D) radical anion (eq 5) thereby preventing the fast cage recombination of this radical anion with the geminate radical cation (eq 4). An alternative route yielding radical cation is direct electron transfer from the dye triplet to molecular oxygen (eq 3). Such a process, if it accounts for photobleaching, should occur more easily in the aqueous phase than in the nonpolar micelle interior. Thus, the electron-transfer process that is essential for photobleaching (eqs 1-3) is diminished in micelles resulting in a significantly higher photostability of the micellar form of both dyes. In the ion-pair
aggregates, electron transfer ( q s 1-3) can contribute to rapid photobleaching.
Conclusions We have observed the formation of neutral hydrophobic ion pairs between RB or eosine and CPC in water which undergo "premicellar aggregation". These ion pairs make it possible to solubilize RB or eosine in nonpolar polarizable solvents. We postulate that RB/eosine-CPC micelles contain a neutral ion pair hosted deep inside and that the aprotic nonpolar surroundings alter the spectral properties of both dyes. Such a model may also help explain similar spectral changes previously reported for other ionic dyes interacting with oppositely charged surfactants. We have correlated the photoreactivity of the dyes in aerobic aqueous solutions to the nature of the aggregates and micelles. The proximity of the dye molecules in the aggregates results in the rapid deactivation of excited states, which leads to low IO2 yield and rapid dye photobleaching. In contrast, the excited states of RB and eosine molecules encapsulated in micelles are protected from bimolecular deactivation observed in the absence of detergent. This also diminishes dye photobleaching that arises from electron-transfer processes. The dye triplet states are deactivated only by molecular oxygen, which promotes efficient IOzformation over a wide pH range for RB in the cationic micelles. Acknowledgment. We thank Dr. A. G. Motten for helpful discussions. We are also grateful to Dr. Don Davis for carrying out the NMR experiments on the RB(CPC)2 ion pair.
Spectrum and Mutual Klnetlcs of HOCH2CH202Radicals C.Anastasi,* D.J. Muir, V . J. Simpson, Department of Chemistry, University of York, Heslington, York, YO1 5 0 0 , UK
and P. Pagsberg Environmental Science and Technology Department, Riso National Laboratories, DK-4000 Roskilde, Denmark (Received: September 7, 1990; In Final Form: March 5, 1991)
8-Hydroxyethyl peroxy radicals have been studied by using pulse radiolysis to generate the radicals and kinetic absorption to monitor their formation and decay. The ultraviolet absorption spectrum assigned to HOCH2CH2OZis broadband in nature with a maximum absorption cross section of 3.5 (f0.6) X cm2 molecule-' at 230 nm. An overall rate constant for the self-reaction 2HOCHzCH20z HOCHzCH20H HOCH2CH0 + O2 (3a), 2HOCHzCHzO2 2HOCH2CHz0+ O2 (3b) of k3 = 7.7 ( f l . 2 ) X cm3 molecule-' s - ~was measured at room temperature together with an estimation of the branching ratio, k3,/k3= 0.75 (f0.1).
-
Introduction Hydroxyethyl radicals play an important role in the gas-phase oxidation of alcohols and alkenes in the atmosphere and in combustion chemistry. The 6-hydroxyethyl radical, CHzCH20H,is known to be an important intermediate in the low-temperature photooxidation of ethene, and the a-hydroxyethyl radical, CH3CHOH, has been identified as a key radical intermediate in the oxidation of ethanol. In atmospheric chemistry, 0-hydroxyethyl radicals are removed by reaction with O2to form a peroxy adduct: CH2CH2OH 0 2 (+M) HOCH2CHz02 (+M) (1)
+
+
Very recently, Miyoshi et al.' reported a room-temperature rate constant of k l = 3.0 X 10-l2cm3 molecule-' s-' measured using flash photolysis coupled with photoionization mass spectrometry. ~
-
+
~~~
(1) Miyoclhi, A.; Matsui, H.; Warhida, N. Chem. Phys. k i t . 1989, 160, 291.
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No pressure dependency was observed over the total pressure range (6.5-22.7) X 10l6 molecules cm-' suggesting kl is at the highpressure limit. In addition, Niki et al.z.3identified the products formed following photolysis of RONO-NCkC2H4-02 (R = C2HS or s-C4HIO)using FTIR and proposed a series of elementary reactions for the oxidation of CH2CH20H radicals in air; formation of the peroxy radical via redction 1 is followed by conversion of NO to NOz HOCH2CH20z + NO HOCHZCHZO + NO2 (2) and unimolecular dissociation of the substituted alkoxy radical to yield C H 2 0 and H 0 2 . However, no direct kinetic studies have been reported on the P-hydroxyethyl peroxy radical, HOCH2CH201, and the work
-
(2) Niki, H.; Maker, P. D.; Savage, C. M.; Brietenbach. L. P. J . Phys. Chem. 1978.82, 135. (3) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chcm. Phys. Lett. 1981, 80,499.
0 1991 American Chemical Society
5792
The Journal of Physical Chemistry, Vol. 9.5, No. 1.5, 1991
presented here includes both the ultraviolet (UV) spectrum of this species, and the overall rate constant for the self-reaction, k 3 : 2HOCH2CH202 HOCH2CH20H + HOCHzCHO + 02 (3a) 2HOCH2CH2O + 02 (3b) In addition, a value for the branching ratio, k3a/k3a+3b, has been obtained by studying changes in the kinetic absorption at different wavelengths. Previously, we have used a pulse radiolysis/kinetic absorption (PR/KA) technique to measure the spectra and kinetics for the self-reactions of the a-and Bhydroxyethyl radicals,43 and we have now extended this study to include the analogous 8-hydroxyethyl peroxy radical. Two chemical systems have been used: the radiolysis of reaction mixtures containing SF,, H 2 0 , C2H4, and 02 and SF6,CH3CH20H,and 02.The first system provides a clean source of HOCH2CH202radicals from which the UV spectrum and kinetics of this radical could be derived, while both the a- and /3-hydroxyalkyl radicals are formed in the second system; analysis of this more complex system was used as independent confirmation of the results.
-
+
Experimental Section Pulse radiolysis was used to generate the radicals of interest in a static system and UV absorption spectrometry to monitor their formation and decay. The apparatus has been described in detail elsewhere! and only a brief outline is given here. The reaction gases are mixed in a 1-Lstainless steel reaction vessel, and the radicals are produced by a single pulse of 2-MeV electrons from a field emission accelerator (Febetron 705B). The short pulse length of 30 X IO4 s gives good kinetic resolution, and the small surface-to-volume volume ratio of the cell ensures that wall effects are negligible. Analyzing light from a pulsed Xe lamp passes through the cell that contains a multipass system of mirrors giving a maximum optical path length of 120 cm. After the light beam has been monitored by using a monochromator/photomultiplier assembly, the output signal is digitized and transferred to a minicomputer for storage and data analysis. The partial pressures of gases are measured by using a Baratron electronic manometer giving a sensitivity of bar, and the temperature of the reaction mixture is monitored by using a platinum resistance thermometer. Source of F Atoms. High yields of F atoms are obtained by pulse radiolysis of SF6, and in the presence of small amounts of additives (RH), reaction 4 provides a useful source of many F + RH R + HF (4) free-radical species. For example, it is possible to obtain high yields of CH, or O H radicals by using CH4 or H 2 0 as additives. The formation of F atoms by the radiolysis of SF6 must be accompanied by the formation of SF, fragments such as SF4and SFS, although the exact ratio of these products is not known: 2-MeV e- + SF6 SF4 + 2F + e(5a)
-.
-.
-
SFS+ F + e(5b) While SF4 is a stable molecule, the SF5 radical would be expected to react with other species present in the gas mixture, complicating the chemistry involved. To establish the SF? species formed from SF,, we looked for transient absorption signals due to SFS. The UV absorption spectrum of SFShas been'observed by pulse radiolysis of SF6in methanol and consists of a broad continuum with a maximum absorption cross section of 5.0 X 1OI8 cm2 molecule-' at 300 nmS7 If reaction 5b is the source of F atoms in our experiments, then the concentrations of SF, and F formed by the radiolysis of 1.2 (4) Anastasi. C.; Pagsberg, P.; Munk, J.; Simpson, V.J. Chem. Phys. Leu. 1989, 164, 18. ( 5 ) Anastasi, C.;Simpson, V. J.; Munk, J.; Pagsberg, P. J . Phys. Chem. 1990, 94,6327. (6) Munk, J.; Pagsberg, P.; Ratajczak, E.;Sillesen, A. J. Phys. Chem. 1985, 89, 2752. (7) Johnson, D.W.; Salmon, G. A. J . Chem. Soc., Faraday Trans. I 1977, 73, 2031.
Anastasi et al.
x loi8 molecules cm4 pure SF6 are equivalent, and we would expect to observe a peak absorption signal (Amx)of 0.47 due to SFS. In fact, no transient absorption signal was observed a t 300 nm (A, 50.009), i.e., [SFs]/[F] I0.019, and we can conclude that the degradation of SF6 proceeds via reaction 5a. To establish further support for this conclusion, we can consider the chemistry initiated by pulse radiolysis of SF6/CH4 mixtures in which the F atoms are converted rapidly and quantitatively to CH3 radicals: F + CH4- H F CH3 (6)
+
Transient absorption signals due to CH, radicals were monitored at 216.4 nm and the yield of CH, was found to approach a maximum value at [CH,] > 1.2 X 10'' molecules an-,.The decay kinetics of CH3 were studied at varying radiation doses (yielding varying concentrations of the radical), and in all cases, the decay was found to be second order in accordance with the combination reaction: 2CH3 (+M) C2H6 (+M) (7)
-
Values of k 7 / u were derived from the decay kinetics and combined with the consensus value of uCH = 4.12 X lo-'? cm2 moleculzi8 to obtain k7 = 5.6 X lo-" cm-$molecule-' s-l, which is in very good agreement with the recommended value for the rate of reaction 7 a t 298 K and a total pressure of 1 atm. The effect of the cross reaction of CH, radicals with SFS (reaction 8) would be to increase the overall decay rate of CH3 radicals: CH3 + SF5 (+M) CH3SFj (+M) (8)
-
+
2SFS (+M)
S2Flo(+M)
(9) Furthermore, as k7 >> k9,9J0the concentration ratio [SFs]/[CH3] should increase with time, and consequently reaction 8 should be rate controlling at longer times resulting in the tail end of the CH3 decay curves being lower than that of the simple second-order curve of reaction 7. These results provide further evidence that radiolysis of SF6 yields much lower concentrations of SF5 than of F atoms. SF4has an absorption spectrum in the region X = 240-340 nm." The cross section is 3.3 X em2 molecule-' at 250 nm and would give an absorption signal of A, < 0.005 under our conditions. The experimental observations on radiolysis of pure SF6 are consistent with this calculation yielding a small (A, < 0.01) signal, stable over the time scale of our experiments. In conclusion, the most likely pathway following the radiolysis of SF6 is reaction 5a, with reaction 5b making a negligible contribution (51%). Chemical System 1. HOCHzCH20zradicals were generated in a series of conversion reactions following the radiolysis of mixtures containing SF6, H 2 0 , C2H4, and 02.Radiolysis of SF6 in the presence of H,O yields OH radicals via reactions 5 and 10
2-MeV e-
+ SF6
+
-
SF, + 2F + e-
+ eOH + H F
SFS -t F
(54 (5b)
F + H2O(10) which add to the double bond in C2H4 to produce CH2CH20H radicals: O H + C2H4 (+M) --c CH2CH2OH (+M) (1 1) Subsequent addition of O2 forms the radical of interest: CHzCHzOH + 0 2 (+M) -.* HOCH2CH202 (+M) (1) Concentrations of [SF,] = 2.4 x [H20] = 1.2 x [02] (8) MacPhcrson, T.; Pilling, M. J.; Smith, M. J. C. J . Phys. Chem. 1985, 89, 2268. (9) Tait, J. C.; Howard, J. A. Cab. J . Chem. 1975, 53, 2361. ( 1 0) Herron, J. T. Int. J . Chem. Kinet. 1987, 19, 129. (11) Modica, A. P. J . Phys. Chem. 1973, 77, 2713.
Spectrum and Mutual Kinetics of HOCH2CH202Radicals
7’he Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5193
Q‘
zio
zip
iio
2 i o wavml.2:fth
“260
270
110
190
300J
Figure 2. UV spectrum of HOCH2CH2O2derived from analysis of
SF6/H20/C2H4/02mixtures. Spectral band-pass = 0.8 nm.
which react with O2 either by addition in the case of CH2CH20H2.3 CH2CH2OH + 02 (+M) HOCH2CH202 (+M) (1) 4
or by abstraction of an H atom in the cases of CH3CHOH’4*’S and CH3CH2016radicals: CHSCHOH + 0 2 CHSCHO + H02 (15)
-
+
CH3CH20
0
2 3 1 0 ~ 1i 1017 =iacui.
1
b
5
Figure 1. (a, top) Absorption signal measured following radiolysis of 2.4 X IOl9 SF6, 1.2 X IO1*H20, 2.5 X IO’’ 02,1.2 X 10l6molecules cm-3 CzH4,respectively. X = 250 nm, T = 31 5 K,A, = 0.084. (b, bottom) A, measured following radiolysis of 2.4 X IOI9 sF6,1.2 X 10” H20, 1.2 X 1 0I6molecule C2H4plus varying concentrations of OF X = 240 nm.
+ 02
CHSCHO
+ H02
(16) Radiolysis of [SF,] = 2.4 X lOI9, [CH3CH20H] = 2.5 X loi7, and [O,] = 4.9 X 1017molecules cm-3 yielded 2.7 X 10ls molecules cm-3 F atoms (the greater yield obtained here relative to the previous system was attained by using a higher electron flux). Experiments were carried out at ambient temperature (295 K) and 1 atm total pressure. Materiala Research grade gases SF,, C2H4, and o2(Matheson) were used directly from the cylinders without further purification. CH3CH20H (James Burroughs, 99.7%) was thoroughly outgassed, and H 2 0 was distilled several times and then outgassed before use.
= 2.5 X lOI7, and [C2H4]= 1.2 X 10I6molecules cm-3 were used, and an example of the absorption due to HOCH2CH202measured at 250 nm is given in Figure la. A total pressure of 1 atm was used throughout, and calibration of the initial F atom yield by the normal methodI2 gave a yield of 8.6 X 1014molecules ~ m - ~ . Results (a) Chemical System 1. UV Spectrum of HOCH2CH202 Experiments were carried out at 315 (f2) K to allow a high Radicals. Figure 2 shows the UV spectrum assigned to HOCvapor pressure of H 2 0 to be used, thus ensuring quantitative H2CH202measured in the range X = 210-300 nm. Values for conversion of F atoms to O H radicals while maintaining a temthe absorption cross sections ( u ) were calculated from the peak perature close to ambient conditions. Care was taken to minimize absorption signal, the corresponding radical density derived from direct H atom abstraction from C2H4 by F atoms to form vinyl simulations of the absorption traces (qv), and the optical path radicals as vinylperoxy radicals also absorb in the UV region:I3 length of 120 cm. Maximum absorption occurs approximately F C2H4 H F + C2H3 (12) 12 X 1O-a s after the electron pulse (Figure la) due to the relatively slow reactions leading to the formation of HOCH2CH202radicals, C2H3 + 0 2 (+M) CzH3O2 (+M) (13) in particular the addition of O H to C2H4. These processes occur The relative concentrations of H 2 0 and C2H4 where arranged such over a time scale comparable with decay processes (qv), and thus that reaction 12 accounts for 2200 nm, which indicates a stability of ca. 2.4 kcal mol-' less than that of the full-overlap type. The intermolecular pyrene (Py) dimer radical cation had a CR band at 1450-nm peak and was ca. 0.9 kcal mol-' more stable than the full-overlap structure of 1PylPy and 2Py2Py dimer radical cations. This conformation of intermolecular Py dimer radical cation was ascribed to a distorted conformation.
Introduction Many studies have been made on photophysical and photochemical properties of intramolecular diary1 compounds such as 1,3-diarylpropanes, bis( 1-arylmethyl) ethers, 2,4-diarylpentanes, and bis( I-arylethyl) These compounds have two aromatic chromophores separated by a three-carbon and or oxygen flexible chain which satisfies Hirayama's n = 3 rule. At this chain length, two aromatic chromophores can take a sandwich conformation where the overlap of two *-orbitals of both chromophores is maximized, and hence when two chromophores are identical, the intramolecular excimer is readily formed by photoexcitation. The analysis of the intramolecular excimer emission
1.
(1) Reviews: (a) Semerak, S. N.;Frank, C. W. Ada Polym. Sci. 1984, 54,31. (b) Guillet. J. Polymer Photophydcs and Photochemistry; Cambridge University Press: London, 1985. (c) Roberts, A. J.; Soutar, I. In Polymer Photophysics;Phillips, D., Ed.;Chapman and Hall: London, 1985; Chapter 5 . (d) Winnik, M. Photophysical and Photochemical Tools in Polymer Science; Reidel: Dordrecht, 1986. (e) De Schryver, F. C.; Collart, P.; Vandendriache, J.; Gadeweeck. R.; Swinnen, A,; Van der Auweraer, M. Acc. Chem. Res. 1987,20, 159. (f) Ushiki, H.; Horie, K. In Handbook offolymer Science and Technology;Cheremisinoff, N. P., Ed.; Marcel Dekker: New York, 1989; Vol. 4. (2) References are also cited in: Tsuchida, A.; Tsujii, Y.; Ito, S.;Yamamoto, M.;Wada, Y. J. Phys. Chem. 1989, 93, 1244.
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1:
spectra and decays of these bichromo horic compounds such as N-carba~olyl,~ phenyl?.6 1-naphthyl!. 2-naphthyl!** 9-anthryl,9 bipheny1,'O 9-phenanthry1," l-pyrenyl,12and 2-pyrenyl" moieties (3) Hirayama, F. J. Chem. Phys. 1965, 42, 3163. (4) Chandrm, E. A,; Dempster, C. J. J. Am. Chem. Soc. 1970,92,3586. ( 5 ) Carbazolyl: (a) Klapffer, W. Chem. Phys. Left. 1969, 4, 193. (b) Johnson, G . E.J. Chem. Phys. 1974,61,3002. (c) Itaya, A.; Okamoto, K.; Kusabayashi, S.Bull. Chem. Soc. Jpn. 1976,49,2082. (d) De Schryver, F. C.; Vandendricssche, J.; Toppet, S.;Demeyer, K.; Boens, N. Macromolecules 1982, 15, 406. (e) Evers, F.; Kobs, K.; Memming, R.; Terrell, D. R. J . Am. Chem. Soc. 1983,105,5988. (f) Vandendriessche, J.; Palmans, P.;Toppet, S.;Boens, N.;De Schryver, F. C.; Masuhara, H. 1. Am. Chem. Soc. 1984, 106,8057. ( 6 ) Phenyl: (a) Bokobza, L.; Jaasc, B.; Monnerie, L. Eur. Polym. 1.1977, 13,921. (b) De Schryver, F. C.; Mocns, L.; Van der Auweraer, M.; Boens, N.; Monnerie, L.; Bokobza. L. Macromolecules 1982, IS,64. (c) ltagaki, H.; Horie, K.; Mita, 1.; Washio, M.; Tagawa, S.;Tabata, Y.; Sato, H.; Tanaka. Y. Macromolecules 1987, 20,2774. (7) I-Naphthyl: (a) Goldenberg. M.; Emert, J.; Morawetz, H. J. Am. Chem. Soc. 1978, 100, 7171. (b) Itagaki, H.; Obukata, N.; Okamoto, A,; Horie, K.; Mita, 1. J. Am. Chem. Soc. 1982,104,4469. (c) De Schryver, F. C.; Demeyer, K.; Toppet, S.Macromolecules 1983, 16, 89. (8) 2-Naphthyl: Ito, S.;Yamamoto, M.; Nishijima, Y. Bull. Chem. Soc. Jpn. 1981, 54, 35. (9) 9-Anthryl: Becker, H. D.; Andersson, K. J. Org. Chem. 1982,47,354. (IO) Biphenyl: Zachariasse, K. A,; Ktlhnle, W.; Weller, A. Chem. Phys. L e t . 1978, 59, 375.
0 1991 American Chemical Society
