J. Phys. Chem. 1984,88, 4380-4384
4380
-
-
TABLE VI: Specific Rate Constants k(E,J=O) for the Reactions H2C0 H2 CO and D2C0 D2 CO Including Tunneling Contributions H2CO + Hz C O DZCO D2 C O Elcm-I k(E,J=O)ls-l Elcm-l k(E,J-0) /s-I 24 300 9.9 x 102 24 730 4.6 25 000 9.0 x 103 8.3 X 10’ 25 430 26 130 25 700 8.0 x 104 1.4 x 103 26 400 7.1 x 105 26 830 2.2 x 104 27 100 6.2 X lo6 27 530 3.4 x 105 27 800 5.1 x 107 28 230 5.0 X IO6 28 500 3.9 x 108 6.7 x 107 28 930 29200 (=Eo) 1.8 X lo9 29630 (=Eo) 5.5 x 108 29 900 4.7 x 109 30430 1.8 x 109 30 600 9.6 x 109 31 130 4.5 x 109
+
+
+
+
that our rate constants include a factor of 2 for rotational symmetry of the molecule. They do not include the additional mode-specific effects due to a planar reaction path which were discussed in ref 8 and 9. Our rate constants listed in Table VI essentially agree with the results of ref 7 (Table IV, column
“scaled”, corrected by a factor of 2 for symmetry) for HICO at J = 0. Our rate constants are about a factor of 1.3 lower than the results of ref 7. This discrepancy is due to the anharmonicity contribution included here, but neglected in ref 7 .
Conclusions Not unexpectedly the calculations show simple bond fission to be dominant except very close to or below the threshold energy of this channel. The specific rate constants are markedly larger than fluorescence decay rate constant^.^ The J dependence of the rate constants is complicated showing channel switching. This behavior poses particularly problems in the analysis of photolysis product yields and IR multiphoton and thermal dissociation branching ratios. Further analysis of these phenomena should be based on k(E,J) calculations of the present kind. Acknowledgment. Financial support of our work by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich93 “Photochemie mit Lasern”) and discussion of this problem with W. H. Miller and C. B. Moore are gratefully acknowledged. Registry No. H,CO, 50-00-0; D,CO, 1664-98-8.
Photochemical Debromination of Meso-Substituted Bromoanthracenes Studied by Steady-State Photolysis and Laser Photolysis Kumao Hamanoue,* Shigeyoshi Tai, Toshiharu Hidaka, Toshihiro Nakayama, Masaki Kimoto, and Hiroshi Teranishi Department of Chemistry, Faculty of Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku. Kyoto 606, Japan (Received: January 5, 1984)
Debrominations of 9-bromoanthracene (BA) and 9,lO-dibromoanthracene (DBA) in acetonitrile containing triethylamine (TEA) or N,N-dimethylaniline (DMA) have been studied by means of steady-state photolysis and laser photolysis. By the addition of TEA, the decay constants of the lowest excited singlet states of BA and DBA increase and the maximum yields of the triplet states decrease. The singlet quenching rate constants by TEA are calculated to be of the order of 1OloM-’ s-], showing that the reactions are diffusion controlled. Compared with the result of y-radiolysis and pulse radiolysis, it is suggested that the photochemical debrominationsof BA and DBA in the presence of amines take place via the anion radicals which are produced through exciplexes between amines and the lowest excited singlet states of bromoanthracenes.
Introduction The nonradiative processes of the fluorescing states in anthracene derivatives have been studied extensively,’S2and it is well-known that the fluorescence quantum yields of 9- and 9,lO-substituted anthracenes in fluid media decrease as the temperature is increased. This temperature dependence has been attributed to thermally activated intersystem crossing (isc) from the lowest excited singlet state SIto an adjacent higher excited triplet state T,. Our direct measurements of the temperature dependence of the fluorescence lifetimes of 9-bromoanthracene (BA) and 9,lO-dibromoanthracene (DBA)3 have allowed the determination of the rate constant for isc in the form kisc= Aiscexp(-AE/kT), with activation energies of 600 and 1100 cm-’ for BA and DBA in 3-methylpentane, respectively. We have also reported that the solvent dependence of the nonradiative transition rates of the lowest excited singlet states in BA and DBA4is due to the different extent (1) J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley-Interscience, New York, 1970, Chapters 4 and 5. (2) K. Hamanoue, S. Hirayama, T. Nakayama, and H. Teranishi, J. Phys. Chem., 84, 2074 (1980), and references cited therein. (3) M. Tanaka, I. Tanaka, S. Tai, K. Hamanoue, M. Sumitani, and K. Yoshihara, J . Phys. Chem., 87, 813 (1983).
0022-3654/84/2088-4380$01.50/0
to which S1 and T, are stabilized by the solvent, in accordance with the result of Wu and Ware for DBA.S On the other hand, it is well-known that photoinduced dehalogenations of halogenated aromatic compounds are accelerated by the addition of several amines. And it has been suggested that radical anions of halogenated compounds which are produced through exciplexes are the reaction intermediates which break down to give aryl radicals and halogen anion.6-’2 (4) K. Hamanoue, T. Hidaka, T. Nakayama, H. Teranishi, M. Sumitani, and K. Yoshihara, Bull. Chem. SOC.Jpn., 56, 1851 (1983). (5) Kam-Chu Wu and W. R. Ware, J. Am. Chem. Soc., 101, 5906 (1979). (6) 0. M. Soloveichik and V. L. Ivanov, J . Org.Chem., 10,2416 (1974); translated from Zh. Org. Khim., 10, 2404 (1974). (7) 0. M. Soloveichik V. L. Ivanov, and M. G. Kuzmin, J . Org. Chem., 12, 860 (1976); translated from Zh. Org. Khim., 12, 859 (1976). (8) N. J. Bunce, P. Pilon, L. 0. Ruzo, and D. J. Sturch, J . Org. Chem., 41, 3023 (1976). (9) N. J. Bunce,Y . Kumar, L. Ravanal, and S. Safe, J. Chem. Soc., Perkin Trans. 2, 880 (1978). (10) K. Tsujimoto, S. Tasaka, and M. Ohashi, J. Chem. SOC.Chem. Commun., 758 (1975); M. Ohashi, K. Tsujimoto, and K. Seki, ibid., 384 (1973). (1 1) B. Chittin, S. Safe, N. Bunce, L. Ruzo, K. Olie, and 0. Hutzinger, Can. J . Chem., 56, 1253 (1978).
0 1984 American Chemical Society
Photochemical Debromination of Bromoanthracenes
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4381
As an extension of our studies on the photophysics and photochemistry of bromoanthracenes, this paper deals with the debromination reactions of BA and DBA in acetonitrile containing amines, in order to estimate the intermediates involved, including the multiplicity of the reactive states of bromoanthracenes.
Experimental Section Chemicals. Zone-refined anthracene and BA (EP grade) were purchased from Tokyo Kasei Kogyo Co., Ltd. DBA was synthesized by bromination of anthracene.13 After repeated crystallization from methanol, both bromoanthracenes were purified by vacuum sublimation. Spectral-grade acetonitrile (Dojin) was used as a solvent without further purification. 2-Methyltetrahydrofuran (MTHF) (Aldrich) was refluxed over sodium metal and distilled under nitrogen atomsphere. G.R.-grade triethylamine (TEA) and N,N-dimethylaniline (DMA) (Wako) were refluxed over calcium hydride, followed by distillation under nitrogen atmosphere. Steady-State Photolysis. A 500-W super high-pressure mercury lamp was used for the photolysis of DBA. Light of 404-nm monochromatic wavelength was selected by the combination of two Toshiba color glass filters (UV-39, V-V40) and a filter solution (CuS04.5H20, 100 g dm-3, path length 3 cm). Irradiation of BA was carried out with light of longer wavelength than 390 nm which was selected by the combination of a 500-W xenon lamp, a UV-39 glass filter, and a filter solution (loc. cit.). The sample solutions were degassed by several freeze-pumpthaw cycles. All the experiments were carried out at room temperature. Absorption and fluorescence spectra were taken by using a Hitachi 200-20 spectrophotometer and a Shimazu RF-502 fluorescence spectrophotometer, respectively. The quantum yield of the debromination of DBA was determined by the Hatchard-Parker potassium ferrioxalate actinometry method.I4 Laser Flash Photolysis. For the measurements of the transient absorptions of BA and DBA at room temperature in acetonitrile, the sample solutions were excited by using the second harmonics (347.2 nm) from a picosecond mode-locked and a nanosecond Q-switched ruby laser. The details of our picosecond transient absorption spectrometer with the mean pulse width of 26 psI5J6 and the nanosecond Q-switched ruby laser with the half-peak duration of 22 nsl' have been given elsewhere. The concentrations of BA and DBA in the picosecond photolysis were 1.2 X and 7.4 X lo4 M, respectively, in a cell of 2-mm path length. The concentration of TEA was 0.05 M. For the measurements of the transient spectra between 400 and 550 nm by nanosecond photolysis, the concentrations of BA and DBA were 2.5 X and 1.8 X M, respectively, in a cell of 10-mm path length. For the measurements of the transient spectra between 550 and 750 nm, the concentrations of BA and DBA were 6.0 X lo4 and 3.7 X M, respectively, and the concentration of TEA or DMA was 1 M. By the excitation of the samples using the third harmonic (355 nm) with a pulse width of 15 ps from a mode-locked Nd3+:YAG laser,3 the decays of SI So fluorescences of BA and DBA were measured. The concentrations of the samples in a cell of 10-mm M for BA and DBA in acetonitrile. path length were 1 X Pulse radiolysis was carried out at room temperature by using the single electron pulse generated from a 35-MeV linear accelerator. The details of the linear accelerator have been described e l ~ e w h e r e . ' ~ The * ' ~ pulse width in the present study was 10 ns. -+
~
~~~~
(12) R. S . Davidson, and J. W. Goodwin, Tetrahedron. Lett., 22, 163 (1981). (13) H.Gieman, Ed.,'Organic Syntheses", Wiley, New York, 1967,p 207. (14) C.G. Hatchard and C. A. Parker, Proc. R . SOC.London, Ser. A , 235, 518 (1956). (15) T. Nakayama, S . Tai, K. Hamanoue, and H. Teranishi, Mem. Fac. Ind. Arts, Kyoto Tech. Univ., Sci. Technol., 29, 46 (1980). (16) K. Hamanoue, T. Hidaka, T. Nakayama, and H. Teranishi, Chem. Phys. Lett. 82, 55 (1981). (17) T. Nakayama, T. Miyake, M. Okamoto, K. Hamanoue, and H. Teranishi, Mem. Fac. Ind. Arts, Kyoto Tech. Uniu., Sci. Technol., 29, 35 (1980).
\
I
30
350
I
,
.
I
I
400 Wavelength 'nm
:..
4
I
4
Figure 1. Absorption spectral change of DBA (8.9 X M)-DMA (0.03 M ) in deaerated acetonitrile upon irradiation with 404-nm light at room temperature: (A) 0, (B) 0.5, (C) 1.0, and (D) 4 min.
3
Figure 2. Absorption spectral change of BA (1.3 X lo4 M)-DMA (0.03 M ) in deaerated acetonitrile upon irradiation with X > 390 nm light a t room temperature: (A) 0, (B) 5, (C)15, and (D)30 min.
The concentrations of the samples in a cell of IO-mm path length M in M T H F for anthracene, BA, and DBA. were 5 X
Results Steady-State Photolysis. Upon room-temperature irradiation of DBA with 404-nm light in acetonitrile containing 0.03 M DMA, the absorption spectrum of DBA changed as shown in Figure 1 . The photoproduct (spectrum D) was identified as BA by comparison of the absorption and fluorescence spectra with those of the authentic sample of BA. Irradiation of BA in acetonitrile containing 0.03 M DMA with light of longer wavelength than 390 nm gave spectrum C in Figure 2. This spectrum was assigned to anthracene. Similar results were also obtained when 0.14 M TEA was added instead of DMA. Namely, BA and anthracene were obtained upon irradiation of DBA and BA, respectively. The quantum yield of the formation of BA from DBA (5 X 10" M) in the presence of 0.28 M TEA was 0.07 f 0.01, which is nearly equal to the value (18) H. Kobayashi, T. Ueda, T. Kobayashi, M. Washio, Y. Tabata, and S . Tagawa, Radiat. Phys. Chem., 21, 13 (1983). (19) K. Hamanoue, M.Kimoto, T. Nakayama, H. Teranishi, S.Tagawa, and Y.Tabata, Radiat. Phys. Chem., in press.
4382 The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 na.
061 04
Hamanoue et al.
__
I L
n
,I
A
n
0
a
y0
" " ' " 01 ' " ' ' ' 02 ' " " ' ~ 03 '
OL,
2 ""I . .e...B
0 400
450 500 Wavelength / nm
550
Figure 3. Time-resolved absorption spectra of BA (a) and DBA (b) in acetonitrile at 0.5 (A) and 500 (B) ns delays. Spectrum C is the experimental base-line spectrum for our picosecond laser system, taken with no excitation.
estimated by Soloveichik and Ivanov, i.e., 0.02.6 (In the presence of TEA, the absorptions which may be due to some byproducts were clearly observed around 320 nm. Upon irradiation of anthracene (1 X lo4 M) with 366-nm light in acetonitrile containing 0.07 M TEA, the absorption spectrum of anthracene decreased and no absorptions around 320 nm were observed. Since it is reported that irradiation of anthracene and dimethylaniline in acetonitrile led to small quantities of photodimer, 9,lO-dihydroanthracene, and tetrahydrobianthryl, and to a good yield of 9(pdimetbylamin0pheny1)-9,10-dihydroanthracene,~~ and that irridation of anthracene with DMA or TEA in acetonitrile led similarly to 9,1O-dihydroanthracene, tetrahydrobianthryl, and aminated anthracenes?l the byproducts mentioned above may be different from those in anthracene-amine systems.). All the results so far obtained indicate that the photmhmical reactions of bromoanthracene in acetonitrile containing amines are interpreted in terms of the following consecutive reactions: DBA
2BA + byproduct
hv'
anthracene
" " " 05 " " ' " ' 10 ~ " " ~ 15 "
-
Delay timelns
2
Figure 4. Time evolutions of T' T1 absorptions of BA (a) and DBA (b) in acetonitrile at 424 nm. The smooth curves are the theoretical ones with ko-' = 0.15 ns for BA and 1.0 ns for DBA.
By nanosecond laser photolysis at 77 K in EPA (etherlisopentane/ethanol = 5:5:2 in volume ratio),25we have observed similar absorption spectra which have two absorption maxima at 405 and 430 nm with a lifetime of 200 ks for BA, and at 400-406 and 427 nm with a lifetime of 22 1 s for DBA. These spectra are very similar to the T' T, absorption of anthracene with absorption maxima at 402 and 425 nm. Moreover, the 0-0 band of the phosphorescence spectrum of BA was observed at 693 nm at 77 K in EPA, while that of anthracene was observed at 670 nm. On the basis of these results, the observed spectra in Figure 3, a and b, can be assigned to the T' T1absorptions of BA and DBA, respectively. Although the lifetimes of the T l states of bromoanthracenes at 77 K are much shorter than those of 9nitroanthracene and 9-benzoyl- 10-nitroanthracene, which are about a half of the TI states of anthracene, Le., 17.7 ms for 9-nitroanthracene and 16.0 ms for 9-ben~oyl-lO-nitroanthracene,~ this may be due to the internal heavy-atom effect of bromine. By the addition of amines, no spectral change could be observed in the region of 400-550 nm. The typical time evolutions of T' TI absorptions of BA (a) and DBA (b) are shown in Figure 4. The experimental points give the average A(OD(t))/A(OD(a)) of all shots at each delay time, and the error bars give the deviation from the average. A(OD(-)) was determined by averaging data at t = 0.6 ns for BA and 2 ns for DBA. The smooth curves correspond to the theoretical absorbances calculated with a well-known convolution method,26 by assuming a single-exponential population of the absorbing triplet states and 26-ps Gaussian probe and excitation pulse shapes. Our direct measurements of the temperature and solvent dependences of the fluorescence lifetimes of BA and DBA3s4have led us to the conclusion that isc is attributed to that from the lowest excited singlet state Sl to an adjacent higher excited triplet state T,, and that the total rate constant (ko) of the SI decay process is expressed as +-
+ -
+-
+ byproduct
These reactions were strongly retarded by the presence of dissolved oxygen or by changing the solvent to a nonpolar solvent such as cyclohexane. No debromination could be observed in the absence of amines. The effect of oxygen in the present study contrasts with that reported by Soloveichik and Ivanov,6 who observed no oxygen effect on the dehalogenation of 9,lO-dichloroanthracene and DBA in heptane containing diethylamine. Laser Flash Photolysis. Figure 3 shows the transient absorption spectra in the region of 400-550 nm, observed by nano- and picosecond laser photolyses of BA (a) and DBA (b) in acetonitrile without amines. Spectrum C is a typical base-line spectrum for our picosecond laser system. The systematic deviation from zero was everywhere less than 0.02 absorbance unit. The lifetimes of these absorptions were 19.5 p s for both comko = k, + kiso (1) pounds. Moreover, these absorptions were quenched by the addition of oxygen (-2 X M) and ferrocene (1 X M) with quenching rate constants of 2.8 X lo9 M-I s-] for O2and (5.4-5.8) (24) K. Hamanoue, S. Hirayama, M. Okamoto, T. Nakayama, and H. X lo9 M-' s-l for ferrocene. Teranishi, Mem. Fac. Ind. Arts, Kyoto Tech. Uniu., Sci. Technol., 27, 69 (1978). (The T' TI absorption of 9-benzoylanthracene in benzene with A, The spectral profiles of these absorptions are very similar to N 440 nm was taken by a photographic method and its spectral accuracy was TI absorptions of anthracene,** 9-acetylanthra~ene,~~~~~ the T' not calibrated sufficiently. Recently we have improved our picosecond and 9-ben~oylanthracene.~~ spectrometer by using multichannel detectors of photodiode array and have +-
+ -
(20) C. Pac and H. Sakurai, Tetrahedron Lett., 3829 (1969). (21) R. S. Davidson, Chem. Commun., 1450 (1969). (22) M. V. Alfimov, N. Ya. Buben, V. L. Glagalev, E. S. Kuyumdzhi, Yu. V. Pomazan, and V. N. Shamshev, Opt. Spectrosc., 42, 267 (1977). (23) K. Hamanoue, K. Nakajima, T. Hidaka, T. Nakayama, and H. Teranishi, Laser Chem., 4, 287 (1984).
confirmed that the T' + T, absorption has two band maxima at 408 and 434 nm.) (25) Y. Kajiwara, T. Hidaka, T. Nakayama, K. Hamanoue, H. Teranishi, M. Sumitani, and K. Yoshihara, to be submitted for publication; preprint of the Symposium on Molecular Structure and Electronic State, Tokyo, Japan, 1982, p 458. (26) R. W. Anderson, Jr., R. M. Hochstrasser, H. Lutz, and G. W. Scott, J . Chem. Phys., 61, 2500 (1974); Chem. Phys. Lett., 28, 153 (1974).
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4383
Photochemical Debromination of Bromoanthracenes TABLE I: Buildup Times ( k c ] )of T'
-
TI Absorptions of BA and DBA and Quenching Rate Constants (k,)by TEA
ruby laser 10-10k M-I
10-Iok M-1
s-l
9'
s-I
9'
10-9kn,s-l
kn?, ns
from ea 4
from ea 5
static fluorescence"
NdSC:YAGlaserd
6.7 1.o
0.15
4.6
1.o
2.8
1.8
4.4,b 4.1' 4.0,b3.1'
2.3 1.8
BA
DBA
1.o
"[TEA] = 1.2 X 2.2 X and 3.3 X M for BA (5 X M), and 5.8 X 8.1 X 1.0 X 1.6 X and 2.4 X M for DBA (3 X M). *Calculated with values of ko. 'Calculated with the SIlifetimes of BA and DBA in ref 4. d[TEA] = 4.7 X 1.12 X and 1.40 X lo-* M for BA, and 4.7 X 9.4 X and 1.56 X M for DBA.
-
-
where k, and k,,, are the rate constants for the S, So fluorescence, and the S, T, isc, respectively. In the case that the laser pulse shape is a &function, the concentration of the TI state at time t ([T,]) is given by [Tll = k1sc [S110-{1 k0
- exP(-kotN
+
k1sc ~
~
1
1Iexp(-k,ct) 0 ~ 0-
-
~ ~ l l o ~ ~ , s exP(-koOI c / ~ o ~ ~ ~
(3)
Since the obtained data could be fitted with high precision to convoluted curves, we have determined the buildup times k0-l of T' TI absorptions as shown in Table I. The obtained buildup times correspond to the decay times of the S, states, Le., 0.16 ns for BA and 1.27 ns for DBA in the same s01vent.~Thus, one can say that the rate constant for internal conversion in the triplet manifold is very large compared with that of isc. This is consistent with the result of Kokubun et a].,*' who have estimated that the T, lifetimes of several anthracenes are tens of picoseconds. In the presence of TEA, the rate constant (k) of the SI decay process can be expressed as k = ko k,[TEA] (4)
-
+
where [TEA] is the concentration of TEA, and k, is the rate constant for the fluorescence quenching by TEA. Addition of 0.05 M TEA caused by the decrease not only in the S1 lifetimes of bromoanthracenes but also in the maximum yields of the triplet states. From eq 3 and 4 the relative maximum yield is given by D(t=m)/D'(t=m) = 1 ik,[TEA]/ko (5)
-
where D(t=m) and D'(t=m) are the maximum absorbances of T' TI absorptions in the absence and presence of TEA, respectively. The value of kq can also be obtained by the Stern-Volmer plots of QF0/aF vs. [TEA] for BA and DBA in acetonitrile, where aFo and aFare the fluorescence quantum yields in the absence and presence of TEA, respectively. From the slopes of straight lines, one can calculate the values of k,T, where T = ko-' are the lifetimes of the SIstates of BA and DBA in the absence of TEA. In Table I we show the values of k, obtained by eq 4 and 5 and the Stern-Volmer plots. We also show the values of k, which were determined by the Stern-Volmer plots by measuring the fluorescence lifetimes in the presence and absence of TEA by using a Nd3+:YAG laser. In the absnece of amines, no absorption was observed in the wavelength region longer than 550 nm. However, we could observe the transient absorption spectra in the spectral region of 500-750 nm by the addition of amines. Figures 5a and 6a shows the results observed by nanosecond laser photolyses of BA and DBA, respectively, in the presence of 1 M amines. For the sake of comparison, we also show the absorption spectra of anion radicals of BA and DBA (BA-. and DBA-.) which were taken by pulse rad io lyse^.'^ The spectra observed in the presence of TEA are ~
I
exp(-kot)l
(2) where [Silo is the concentration of the S1 state at t = 0, and k,, is the rate constant for the internal conversion in the triplet manifold. If k,, is very large compared to ko, eq 2 becomes [Til =
0151
~
~~~
(27) S . Kobayashi, K. Kikuchi, and H. Kokubun, Chern. Phys., 27, 399 ( 1978).
0 ~ ~ " ' ~ " " ~ ' " " " " " " " 600 650 700
500
550
750
Wavelength / nm
Figure 5. Time-resolved absorption spectra of BA (6.0 X lo4 M)-TEA or DMA (1 M) in deaerated acetonitrile by nanosecond laser photolysis at 200-11s delay (a) and decay of the absorbance at 705 nm (b): (A) BA-TEA, (B) BA-DMA, (C) by pulse radiolysis in ref 19.
at 685 nrn
C2
01,
36
08
+:A
10
Delay Time I ps
"
2 0.05 -
Wavelength / nm
Figure 6. Time-resolved absorption spectra of DBA (3.7 X lo4 M)-TEA or DMA (1 M) in deaerated acetonitrile by nanosecond laser photolysis at 200-11sdelay (a) and decay of the absorbance at 685 nm (b): (A) DBA-TEA, (B) DBA-DMA, (C) by pulse radiolysis in ref 19.
relatively weak compared with those observed in the presence of DMA. As shown in Figures 5b and 6b, the observed spectra decayed by the single-exponential functions with decay constants of k = 2.2 X lo6 s-l for BA and 2.3 X lo5 s-l for DBA. These values are consistent with those of BA-- and DBA--, Le., 2.1 X lo6 s-l for BA-. and 2.7 X lo5 s-l for DBA-..19 Thus, one can safely conclude that transient spectra observed in the presence of amines are due to the BA-. and DBA-., respectively.
Discussion In summary, irradiation of a solution of DBA and TEA or DMA in acetonitrile yielded BA, which was then converted into anthracene by further photolysis. No debromination could be observed in the absence of amines. The presence of oxygen practically inhibited the reaction. Moreover, this reaction was strongly retarded by decreasing the polarity of solvents. In the presence of amines, the absorption spectra of BA-- and DBA-. were observed by nanosecond laser photolyses. In our previous study19we found that y-radiolyses of DBA and BA in acetonitrile gave rise to just the same reactions as those by photolyses, though it was found that the reactions also proceeded in the absence of amines. In radiation chemistry, organic halides are frequently used as effective electron scavengers. This is based on the fact that halogenated compounds are highly reactive toward electrons, and when an electron attaches to the
4384
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984
molecule it dissociates into a neutral radical and a halide anion by a so-called dissociative electron attachment Actually we have observed the transient absorption spectra of BA-a and DBA-. by means of pulse radiolyses at room temperature and y-radiolyses at 77 K.I9 Since anion radicals which are produced through exciplexes are thought to be the reaction intermediates in the photoinduced dehalogenation of halogenated aromatic compounds in the presence of amines,6-12 and since the SI lifetimes and the maximum yields of the triplet states of bromoanthracenes decreased by the addition of TEA with the diffusion-controlled quenching rate constants of the order of 1OIo M-' s-l, that is, the average values of k, are 3.3 X 1 O l o M-' s-' for BA and 2.7 X 1OloM-' s-l for DBA, our present observations suggest that the photolysis of bromoanthracenes in the presence of amines gives rise to anion radicals of solute molecules which are produced via exciplexes between the the lowest excited singlet states of bromoanthracenes and amines. However, no exciplex emissions were observed in a nonpolar solvent such as cyclohexane containing TEA. In the system of BA-DMA, very weak emission was observed. These results are consistent with those observed by Soloveichik et aL63' for DBA in the presence of TEA or DMA. As described before, debromination reactions of bromoanthracenes were inhibited by dissolved oxygen. Thus, it is expected that the electron transfer from anion radical to oxygen molecule occurs very effectively and the lifetimes of anion radicals should be more than 100 ns. This is consistent with our results of pulse radiolysis at room t e m p e r a t ~ r e , 'indicating ~ the rate constants of electron transfer from anion radicals to oxygen to be -1.3 X 1 O l o M-l s-l for BA-. and -6.5 X lo9 M-' s-l for DBA-.. Thus, the retardation of the debromination reactions by dissolved oxygen on photolysis and y-radiolysis is explained in terms of the effective electron transfer from solute anion radical to oxygen. In order to estimate the absorbances of BA-. and DBA; which were produced by nanosecond laser photolyses, we have carried out the following pulse radiolyses in MTHF. (1) Absorbances of anthracene anion radical (A--), BA-., and DBA-. were measured under the same experimental conditions. (2) Taking the molar extinction coefficient for A; to be emax N 1.0 X lo4 M-' cm-' (ref 32) and assuming the efficiency of the formation of A-. to be equal to those of BA-. and DBA--, we calculated the molar extinction coefficients (e) for BA-e and DBA; by = €maxD/DA;
Hamanoue et al. +-
[SI]= tT-l@iS;lDT
(28) S. Arai, S. Tagawa, and M. Imamura, J . Phys. Chem., 78, 519 (1974). (29) P. Neta and D. Behar, J . Am. Chem. SOC.,102, 4798 (1980); 103, 103 (1981). (30) D. Behar and P. Neta, J. Phys. Chem., 85,690 (1981); J. Am. Chem. SOC.,103, 2280 (1981). (31) H . Kigawa, S. Takamuku, S. Toki, N. Kimura, S. Takeda, K. Tsumori, and H. Sakurai, J . Am. Chem. SOC.,103, 5176 (1981). (32) K. H. J. Buschow, J. Dielman, and G. J. Hoijink, J. Chem. Phys., 42, 1993 (1965). (33) T. Shida, J. Phys. Chem., 73, 4311 (1969). (34) T. Shida and S. Iwata, J. Chem. Phys., 56, 2858 (1972).
(7)
+ k,[TEA])
- -
D = tk,[SI] [TEA]/(ko
(8)
where ai,is the quantum yield of S1 T, isc, and tT is the molar extinction coefficient of T' TI absorption. At room temperature in acetonitrile, ais,can be estimated to be 0.99 for BA and 0.90 for DBA from our measurements of the S1 So fluorescence decay^.^ For BA, eTBA is 4.7 X lo4 (in dioxane)-6.4 X lo4 M-' cm-' (in benzene) at 425 and 445 nm, re~pectively.~~ From the average of eTBA, eTDBA can be estimated by
-
where x1 and x2 are the absorbances of BA and DBA at 347.2 nm, respectively. The estimated value is eTDBA E 5.2 X lo4 M-' cm-'. Taking [TEA] = 1 M, eq 8 gives D = 0.28 for BA-- and 0.33 for DBA--. These values are about 5.6-1 1 times greater than experimental ones. This may be due to the efficiency of the formation of anion radical being smaller than unity. The maximum quantum yield of DBA-. can be estimated to be 0.88 which is 1 order of magnitude greater than the experimental quantum yield for the formation of BA from DBA. This may be due to the smaller efficiency of anion radical formation and some side reactions of DBA-e. We have suggested that BA-- and DBA-- are produced via the exciplexes between amines and the SI states of bromoanthracenes. Although the triplet states of bromoanthracenes were quenched by the addition of TEA (0.28 M) with quenching rate constants of 2.7 X lo4 M-' s-l for BA and 5.5 X lo4 M-' s-I for DBA, the conclusion that BA-- and DBA-- are not produced via the triplet states is strongly confirmed by the following experimental facts: ( I ) There was no essential change in the photochemistry of BA and DBA by the addition of ferrocene ( 5 X M), though the Tl absorptions were almost quenched by ferrocene. (2) If T' BA-- and DBA-. are produced via the exicplexes between amines and the T, states of bromoanthracenes, the calculated absorptions of BA-- and DBA-. are given by
-
(6)
where DA-.and D are the absorbances of A-- and BA-* (or DBA-*), respectively. The obtained values were e = 1.1 X lo4 M-I cm-I for BA-. at 700 nm and 1.9 X lo4 M-' cm-' for DBA-. at 670 nm. These values are consistent with those estimated from G values of solvated electrons in MTHF (G = 2.5533,34)and absorption spectra of BA-. and DBA; in MTHF at 77 K by y-radiolyse~,'~ Le., e = 1.7 X lo4 M-l cm-' for BA-. and 2.2 X lo4 M-' cm-I for DBA-.. By nanosecond laser photolyses in acetonitrile without TEA at room temperature, we have measured the maximum absor-
-
bances (DT) of the T' TI absorptions of BA and DBA. AsTI suming that the efficiency of the internal conversion of T, is unity and that the e is the same for all the solvents studied, the SI yields ([S,])and the absorbances of BA-- and DBA-. are given by
D=
t [ S , ] ( k ,- k,) k,[TEAI ko + k,[TEA] ki, + &[TEA]
(10)
where the rate constant for the T, state quenching by TEA was assumed to be equal to that for the fluorescence quenching. Taking kiclN 10 psZ7and k, = 7.81 X lo7 s-l for BA and 8.20 X lo7 s-I for DBA: one can get D = 0.014 for BA-. and 0.002 for DBA--. Moreover, the maximum quantum yield of DBA-. is estimated to be 0.007. The values estimated so far are about 1 order of magnitude smaller than the experimental ones. Acknowledgment. We express our sincere thanks to Professor Keitaro Yoshihara and Dr. Minoru Sumitani of Institute for Molecular Science for the help in the measurements of fluorescence lifetimes using a Nd3+:YAG laser. Thanks are also due to Professors Yoneho Tabata and Seiichi Tagawa of the University of Tokyo, for the use of a linear accelerator. Registry No. BA, 1564-64-3; BA-., 5491 1-51-2; DBA, 523-27-3; DBA-e, 80780-55-8; DMA, 121-69-7; TEA, 121-44-8; anthracene, 12012-7; cyclohexane, 110-82-7; acetonitrile, 75-05-8; oxygen, 7782-44-7. (35) See also ref 1, p 316.
