J. Phys. Chem. 1987, 91, 2791-2794
2791
Quenching of Triplet Benzophenone by 2,4,6-Tri-ferf-butylphenol and Formation of I t s Phenoxy Radical Yoshizumi Kajii, Masato Fujita, Hiroshi Hiratsuka, Kinichi Obi,* Yuji Mori, and Ikuzo Tanaka Department of Chemistry. Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan (Received: July 18, 1986; In Final Form: November 17, 1986)
Quenching of triplet benzophenone in benzene by 2,4,6-tri-terf-butylphenolis studied by the nanosecond laser flash photolysis. The quenching rate constant of triplet benzophenone has been determined to be 6.2 X lo8 M-' s-l, which is slightly smaller than the diffusion-controlled limit. The quenching reaction produces benzophenone ketyl and 2,4,6-tri-tert-butylphenoxy radicals simultaneously. The transient spectrum of the latter radical shows characteristic structured bands around 400 nm and a broad absorption between 600 and 700 nm. The extinction coefficient at the peak A, = 670 nm of the red absorption has been estimated to be 1200 M-' cm-I.
Introduction Extensive investigations have been carried out on the electronic structure and excited-state dynamics of benzyl and its isovalence radicals. Visible absorption spectra of benzyl radicals were first observed in low-temperature rigid matrices by Porter and Strachan.' The fluorescence lifetimes and quantum yields were determined by Okamura et al.2-4 A detailed spectroscopic investigation was carried out by Cossart-Magos and Leach.5 Recently, Ikeda et a1.6 reported formation and relaxation of hot benzyl radical in the gas phase by the ArF excimer laser photolysis. Porter and Wright reported the absorption spectra of thiophenoxy radical^.^ The emission and excitation spectra and the emission lifetimes of thiophenoxy radical were obtained in low-temperature matrices by Jinguji et a1.* Phenoxy radical is considered to be an important intermediate in oxidation of benzene at high temperature."' Porter et al. first reported absorption spectra of phenoxy radical^.^ Violet and UV absorptions were obtained by the flash photolysis, and in addition to these, weak absorption was observed in the red region by chemical oxidation of phenol. The violet and UV spectra were confirmed by the photolysis in the low-temperature matrix and the pulsed r a d i ~ l y s i s . ' ~ JWard14 ~ reported the red absorption by the flash photolysis of phenol, but the absorption maximum shifted to blue by 40 nm compared with the absorption prepared by chemical oxidation. Photochemical reaction of benzophenone with 2,6-di-tert-butylphenol was reported by Becker.Is Turro and EngelI6 studied the quenching of biacetyl phosphorescence by phenols. Das et al.17investigated the carbonyl photosensitized reaction of phenols by the laser flash photolysis. They reported absorption spectra of phenoxy radicals in the violet region and obtained quenching rate constants of triplet carbonyls. In order to reinvestigate the transient spectrum of phenoxy (1) (2) (3) (4) (5) (6) 5803. (7)
Porter, G.; Strachan, E. Spectrochim. Acta 1958, 12, 299. Okamura, T.; Obi, K.; Tanaka, I. Chem. Phys. Len. 1973, 20, 90. Okamura, T.; Obi, K . ; Tanaka, I. Chem. Phys. Lett. 1974, 26, 218. Okamura, T.; Tanaka, I. J . Phys. Chem. 1975, 79, 2728. Cossart-Magos, C.; Leach, S.J . Chem. Phys. 1976, 64, 4006. Ikeda, N.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1984, 88,
Porter, G.; Wright, F. J. Trans. Faraday SOC.1955, 51, 1469. (8) Jinguji, M.;Imamura, T.; Obi, K.; Tanaka, I. Chem. Phys. Lett. 1984, 109, 31. (9) Sibener, S. J.; Buss, R. J.; Casavecchia, P.; Hirooka, T.; Lee, Y. T. J . Chem. Phys. 1980, 72, 4341. (10) Lin, C.-Y.; Lin, M. C. Int. J . Chem. Kinet. 1985, 17, 1025. (11) Lin, C.-Y.;Lin, M. C. J . Phys. Chem. 1986, 90, 425. (12) Roebber, J. L. J . Chem. Phys. 1962, 37, 1974. (13) Schuler, R.H.; Buzzard, G. K. Int. J . Radiat. Phys. Chem. 1976,8, 563. (14) Ward, B. Spectrochim. Acta 1968, 24, 813. (15) Becker, H. D. J. Org.Chem. 1967, 32, 2115, 2124, 2140. (16) Turro, N. J.; Engel, R. J. Mol. Phorochem. 1969, I , 143,235; J . Am. Chem. Soc. 1969, 91, 7113. (17) Das, P.K.; Encinas, M. V.; Scaiano, J. C. J. A m . Chem. SOC.1981, 103.4154.
radical and to clarify the kinetics of phenoxy formation in benzophenone photosensitized reaction, we carried out the nanosecond laser flash photolysis. The special radical studied was 2,4,6tri-tert-butylphenoxy radical (TBPO), which is expected to be considerably stable. The phenoxy radical was produced by two methods: the direct photodissociation of phenol by a KrF excimer laser and the benzophenone photosensitized reaction. Experimental Section
2,4,6-Tri-tert-butylphenol(TBPOH) and benzophenone were obtained from Tokyo Kasei. TBPOH was purified by column chromatography with silica gel and was recrystallized several times in n-hexane. Benzophenone was purified by recrystallizing several times in ethanol. n-Hexane and benzene (Merck) were used as solvents. n-Hexane was carefully purified by column chromatography with silica gel followed by distillation in order to eliminate benzene derivatives. Uvasol benzene was used without further purification. All samples were deaerated by bubbling nitrogens for about 1 h before use. All measurements were carried out at room temperature. A XeCl excimer laser (Lambda Physik EMG52 MSC; 308 nm, 70 mJ pulse-', 15-11s duration) was used as an excitation light source for the photosensitized reaction. In the case of the direct photodissociation, we used a KrF excimer laser (Lambda Physik EMG103E; 248 nm, 200 mJ pulse-', 20-11s duration). A xenon flash lamp (Ushio UXL-l50DS, 150 W) was synchronously fired with the excimer laser and used as a monitoring light for the transient signals. A monochromator (Nikon P-250)/photomultiplier (Hamamatsu R928) combination was used to obtain transient spectra. Signals from the photomultiplier were measured by a digital memory (Iwatsu DM-901) on-line with a personal computer (NEC PC-9801F) and were accumulated for a few tens of shots. A flow system was employed to eliminate the influence of photoproducts. The flow rate was approximately 0.5 mL s-I. A rectangular cell made of Suprasil was used. The excimer laser light was focused with a cylindrical lens in order to irradiate along the long axis of the cell. Results and Discussion Figure 1 shows the transient absorption spectra obtained at 50 and 700 ns after the laser excitation of TBPOH in benzene with benzophenone. The spectrum obtained at 50 ns has a broad peak near 530 nm and a red tail. This spectrum was assigned to the T-T absorption of benzophenone.'* At the delay time of 700 ns, the absorption in 450-590 nm still remained but its peak shifted to red by about 10 nm. This absorption profile is identical with the spectrum of benzophenone ketyl radical reported by Topp.19 (18) Land, E. J. Proc. R . SOC.London, A 1968, 305, 457. Ledger, M.; Porter, G. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 539. (19) Topp, M. R. Chem. Phys. Lett. 1975, 32, 144; Chem. Phys. Lett. 1976, 39, 423.
0022-36S4/87/2091-2791$01.50/00 1987 American Chemical Society
2792 The Journal of Physical Chemistry, Vol. 91, No. 11, I987
0‘360
400
500 Wavelength
600
Kajii et al.
70(
nm
Figure 1. Transient absorption spectra of the benzophenone (0.002 M)-TBPOH (0.012 M) system in benzene obtained at (0)50 ns and (m) 700 ns after the laser shot.
w V z
a
g!
.5-
m 0 m
Wavelength
6
nm
Figure 2. Transient absorption spectra of benzophenone (0.002 M) in ethanol obtained at (0)50 ns and (a) 500 ns after the laser shot.
100
0
200
4 00
300
CHANNEL
( 10
500
nslch )
Figure 4. Time profiles of transient absorbance of the benzophenone (0.001 M)-TBPOH system in benzene monitored at 520 nm. Concentration of TBPOH: (a) 0.001 M, (b) 0.004 M, and (c) 0.006 M.
0.02 W 0 2
a g 0.01
SIm
2!
g
U
C 300
.5’
v)
400
500 Wavelength
600
700
m
a
nm
Figure 3. Transient absorption spectrum of TBPOH in benzene obtained at 1 ~s after the KrF excimer laser excitation.
The photolysis of benzophenone in the absence of TBPOH was carried out in ethanol for reference, and the results are shown in Figure 2. These spectra confirm the assignment described above. In addition to the absorption of the ketyl radical, a new absorption with a peak at 670 nm and a structured absorption around 400 nm appeared. Land et aLzo have reported absorption spectra of phenoxy radicals produced by photodissociation and chemical oxidation of phenol, anisol, and their derivatives. Their spectra also show structured absorption around 400 nm and broad absorption in the red region. The transient absorption in violet and red regions observed here are identical with those reported by Land et al. and is assigned to the TBPO radical. The transient spectra observed at three different delay times (50,70, and 700 ns) showed isosbestic points at 407, 645, and 685 nm. This indicates that the decay of triplet benzophenone is accompanied by the stoichiometrical formation of the phenoxy and benzophenone ketyl radicals. In order to confirm the absorption spectrum of the phenoxy radical, the direct photodissociation of TBPOH was carried out. The photolysis of phenols is known to yield phenoxy radicals.*O Figure 3 shows the transient spectrum obtained by the KrF excimer laser photolysis of TBPOH. This spectrum rose immediately (20) Land, E. J.; Porter, G.; Strachan, E. Trans. Faraday SOC.1961, 57, 1885.
a
Y6 .5-
m 0 m 6
0
c.
100
200 CHANNEL
300
LOO
I 500
( 10 nslch
Figure 5. Time profiles of transient absorbance of the benzophenone (0.001 M)-TBPOH system in benzene monitored at 400 nm. Concentration of TBPOH: (a) 0.001 M, (b) 0.004 M, and (c) 0.006 M.
after the laser excitation and did not decay within a time range of I O ps. The characteristic structured bands in the violet region
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2193
Quenching of Triplet Benzophenone TABLE I Estimated Values of the Coefficients at 520 nm [TBPOH]
M
Cn X lo2'
C1 X lo2'
Ci/Co
Cl/Cob
I .O
1.21
2.5
4.0 6.0
1.39 1.52
3.02 2.50 2.54
2.6 2.1 2.0
X lo',
1.8
1.7
.OD@) = Co + C , exp(-t/rT). bEstimated from eq 12. and the broad absorption in the red region shown in Figure 1 were also observed by the direct photodissociation. In order to elucidate the kinetics of the hydrogen abstraction reaction in the benzophenone-TBPOH system, we observed time profiles of the transient absorption at 400, 520, and 670 nm. Figures 4 and 5 show results observed at 520 and 400 nm, respectively. Time profiles at 670 nm were the same as those at 400 nm, but the S/N ratio was poor because of the low absorbance at 670 nm. In accordance with Birk's scheme, the following reaction mechanism is given:
hu
'BP
'BP*
(1)
ISC
'BP*
3BP* 3BP*
+ ITBPOH
ko ki
3BP*
(2)
'BP
(3)
2BPH
+ *TBPO
(4)
Immediately after the laser excitation, excited singlet benzophenone yields triplet benzophenone within a few tens of picoseconds.2' The decay of formed radicals does not need to be included in the time range studied here because of relatively slow combination rate of the ketyl radical22and low reactivity of the phenoxy radical. The molar concentrations of triplet benzophenone [BP*] and the ketyl radical [BPH] are given as [BP*] = [BP*], exp(-(ko
+ k,[TBPOH])t)
(5)
[BPHI = kl [TBPOH]
[BP*loko + kl[TBPOH]
[l - exp(-(ko
+ k,[TBPOH])t)] (6)
where [BP*Io is the initial concentration of triplet benzophenone. The absorbance in the observed spectrum is expressed as
O D x ( t )= thp[BP*]d
+
+
ehp,[BPH]d &po[TBPo]d (7) where e$, and represent the extinction coefficients of triplet benzophenone and the ketyl and phenoxy radicals, respectively, and d is the optical path length. Using eq 5-7 and equimolecular formation of the phenoxy and ketyl radicals, we can express the absorbance as follows
O D X ( t )= Ck
+ C$ exp(-t/TT)
(8)
where superscript h denotes the observation wavelength of absorption. T~ and the coefficients of Cb and Ct are given by TT
= (ko
+ k,[TBPOH])-'
(9)
The absorbance ODX(t)was used to fit the data by least-squares procedures. The time profiles of absorption intensities observed at 520 nm for different TBPOH concentrations were simulated by treating C;,Ct,and T~ as adjustable parameters. The obtained ODA(t) is shown in Figure 4 by solid lines. The decay coefficients, Ct (21) Hochstrasser, R.M.; Lutz, H.; Scott, G . W. Chem. Phys. Lett. 1974, 24, 162. Damschen, D. E.; Merritt, C. D.; Perry, D. L.; Scott, G . W.; Talky, L. D. J . Phys. Chem. 1978, 82, 2268. (22) Beckett, A.; Porter, G . Trans. Faraday SOC.1963, 59, 2038.
0
2
6
L
E TBPOH3
Figure 6. Stern-Volmer plots of centration in benzene.
$mol
TTO/TT
dfi3
as a function of TBPOH con-
TABLE 1 1 Quenching Rate Constants of Triplet Benzophenone by Phenols in Benzene TBPOH'
2.4 X los
6.2 X lo8 1.3 x 109
phenolb 'This work. bSee ref 17.
TABLE III: Estimated Values of the Coefficients at 400 nm
[TBPOH] x 103, M
cox
1.o
4.0 6.0
1024
cIx
1.28 1.29 1.47
1024
-1.20 -2.82 -3.80
c,/co
c,/c:
-0.09 -0.22 -0.26
-0.08 -0.27 -0.29
.OD(r) = Co + CI exp(-t/rT). *Estimated from eq 12.
and C :, are determined by the simulation, as listed in Table I. The decay rates, ko + k,[TBPOH] = l / r T , of triplet benzophenone obtained by the simulation are plotted against TBPOH concentration. The Stern-Volmer plots of T T ' / T T vs. TBPOH concentration give a good linear relation, as shown in Figure 6 , where TT' and TT denote the triplet lifetimes of benzophenone without and with the quencher, respectively. The quenching rate constant kl for triplet benzophenone was determined from the slope of the Stern-Volmer plots and is listed in Table 11. In another manner, the ratio of Ct/Ct is derived from eq 10 and 11 as follows: ebp
c:/c; = €hPH
+
CPBPO
ko
+ kl[TBPOH] - 1
kl[TBPOH]
(12)
Using reported extinction coefficients of triplet b e n ~ o p h e n o n e , ~ ~ eBp(532nm) = 7630 M-' cm-', and the ketyl radical,24 tgpH(540 nm) = 3300 M-I cm-', and the rate constant obtained above, we estimated the ratios of C:/Cb as listed in the last column of Table I. The ratios of C:/Ct estimated show good agreement with those obtained by the fitting within the experimental error. This indicates that the quenching rate constant obtained is reliable. The fittings for the rise at 400 nm were carried out by using eq 8 and the quenching rate constant obtained, where Cox and C: were used as fitting parameters. The best-fit curves are shown in Figure 5 as solid lines. The parameters obtained are listed in Table 111. The ratios of Ct to C; were also estimated by eq 12 in a similar manner as above. The extinction coefficients used were 1980 and 2450 M-' cm-' for the ketyl and phenoxy radic a l ~ . ~ The ~ ratios , ~ ~ estimated agree well with those obtained by the fitting. The agreement between decay and rise kinetics (23) Bensasson, R.;Land, E. J. Trans. Faraday SOC.1971, 67, 1904. (24) Hodgson, B. W.; Keene, J. P.; Land, E. J.; Swallow, A. J. J . Chem. Phys. 197563, 3671. (25) Nickel, B.; Mausel, H.; Hezel, V. 2. Phys. Chem. (Munich) 1967, 54, 214.
J . Phys. Chem. 1987, 91, 2794-2800
2794
confirms the scheme through reactions 1-4 and the assignments of transient spectra. The quenching rate constant of triplet benzophenone by TBPOH in benzene is slightly smaller than the diffusion-controlled rate.26 Das et reported quenching rate constants by several substituted and nonsubstituted phenols. The quenching rate by nonsubstituted phenol seems to be a limit of the diffusion controlled, as seen in Table 11. The smaller value for TBPOH would be caused by obstruction of approach of triplet benzophenone by the bulky tert-butyl groups and reduction of the Arrhenius factor. In the red region we detected the buildup signals appearing with the same kinetics at 400 nm. Since benzophenone ketyl radical has no absorption in the region between 600 and 700 nm as seen in Figure 2 and at 500 ns after the laser irradiation triplet benzophenone is completely quenched, the transient spectrum at 700 ns in the red region is attributed to the phenoxy radical. In this time range, there was no appreciable decay of both the ketyl and phenoxy radicals and the concentrations of these radicals
Registry No. BP, 119-61-9; TBPOH, 732-26-3; BPH, 16592-08-8; TBPO, 2525-39-5.
(26) Birks, J. B.; Dyson, D. J.; Munro, I. H. Proc. R. Sot. London, A 1963, 275, 575.
(27) Chang, H. M.; Jaffe, H. H.; Masmanidis, C. A. J . Phys. Chem. 1975, 79, 1118.
should be equal to each other. The ratio of the extinction coefficient of the phenoxy radical to that of the ketyl radical, therefore, is represented by the ratio of absorbances of these transients. Using the value 3300 M-I cm-' of the extinction coefficient of the ketyl radical at 540 nm, we determined the extinction coefficient of the phenoxy radical at 670 nm to be 1200 M-I cm-'. Chang et carried out CNDO/S calculations of aromatic radicals and predicted that the n--R* transition of nonsubstituted phenoxy radical lay lower than 25 000 cm-I. WardI4 assigned the red absorption to the n--R* transition. In the case of thiophenoxy radical, Jinguji et aL8 reported the absorption maximum of 590 nm by the laser-induced emission and tentatively assigned it to the n-x* transition. The rather low extinction coefficient in this study suggests the n--R* character of the red absorption though the value of 1200 M-' cm-' seems to be slightly large for the pure n--K* transition.
One-Photon Infrared Photodissociation of Polyatomic Ions in a Fast Beam M. J. Coggiola,* P. C. Cosby, H. Helm, J. R. Peterson, Molecular Physics Laboratory, S R I International, Menlo Park, California 94025
and Robert C. Dunbar+ Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: August 13, 1986; In Final Form: January 21, 1987)
Photodissociation of 22 vibrationally excited polyatomic ions, ranging in size from four to thirteen atoms, has been observed following absorption of a single C02-laserphoton. In a number of cases the infrared wavelength dependence of the process shows well-defined peaking, which can be interpreted as absorption at the normal-mode frequencies of the ion. Kinetic energy release and order-of-magnitudefragmentation rate information have also been obtained for both photon-induced and metastable decomposition of a number of the ions. RRKM theory modeling indicates that these data are compatible with a quasi-equilibrium theory description of the unimolecular decomposition of highly vibrationally excited molecular ions. Considered as resulting from the last photon absorption of a multiphoton dissociation process, these results are relevant to understanding infrared multiphoton photochemistry of polyatomic molecules.
Introduction In its early stages, the investigation of multiphoton excitation and multiphoton dissociation (MPD) was confined to studies of molecules that were initially in their ground electronic state and vibrationally cool. These conditions were especially enhanced in those experiments which utilized a thermal energy neutral molecular beam formed by supersonic expansion.' While these experiments continue to provide a great deal of insight into the dynamics of MPD, a new class of experiments has appeared recently involving infrared absorption by initially excited species. These experiments have employed a number of excitation techniques and have pr >beda wide range of initial excitation. For example, Welge a. co-workers first observed the MPD of a vibrationally hot S ,+ ion beam produced in a plasma source.2 They were subsequelitly able to measure the lifetime distribution of the dissociating ions using a time-of-flight m e t h ~ d . ~An RRKM analysis of the results showed that the majority of ions absorbed 4-6 photons in excess of the dissociation energy. In a later experiment, the same group studied the MPD of NO2 which was initially excited by a one-photon visible absorption! Although 'John Simon Guggenheim Fellow, 1978-79.
0022-3654/87/2091-2794$01.50/0
the visible photon supplied >90% of the dissociation energy and only 2-3 infrared photons (10 km) were needed to observe product NO formation, they found the dissociation to be very fast and fully consistent with a statistical description. Several variations of this visible IR dissociation scheme have also been reported recently. Heller and WestS prepared vibrationally hot Cr02C1, molecules via a nonradiative decay following visible excitation from the ground state. Subsequent multiphoton IR absorption was found to be efficient, even off resonance, as might be expected for molecules initially in the quasi-continuum. Even in the presence of competing relaxation processes (V-V, V-T), they found that efficient IR absorption persisted. A similar sequence of visible (electronic) excitation followed by internal conversion to a vi-
+
( I ) Schulz, P. A.; S u d b ~Aa. , s.;Kranovich, D. J.; Kwok, H. s.;Shen, Y. R.; Lee, Y. T.Annu. Rev. Phys. Chem. 1979, 30, 319. (2) von Hellfeld, A.; Feldman, D.; Welge, K. H.; Fournier, A. P. Opt. Commun. 1979, 30, 193. (3) von Hellfeld, A.; Anndt, B.; Feldman, D.; Fournier, A. P.; Welge, K. H. Appl. Phys. 1980, 21, 9. (4) Feldman, D.; Zacharias, H.; Welge, K. H. Chem. Phys. Lett. 1980,69, 466. ( 5 ) Heller, D. F.; West, G. A. Chem. Pbys. Lett. 1980, 69, 419.
0 1987 American Chemical Society
