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Triplet State of Fluorobenzene
by I. Unger' Department of Chemistry, University of Texas, Austin, Texas
(Received July 1%,2966)
The sensitized emission of biacetyl and the cistrans isomerization of butene-2 have been applie! to fluorobenzene. Approximately 90% of the fluorobenzene molecules excited by 2537-A. radiation cross over to the triplet state. The effect of wave length has also been studied.
Introduction Although the triplet state of benzene was detected by Lewis and Kasha in 1944,2by its strong phosphorescence in a rigid medium a t low temperatures, it was not found either in solution or in the gas phase until much later. Ishikawa and Noyes3 developed a technique based on the sensitized emission of biacetyl which clearly demonstrated the existence of triplet benzene. These authors also showed that, within experimental error, all of the benzene molecules which had absorbed 2537-A, radiation and did not fluoresce crossed over to the triplet state. Cundall and c o - w ~ r k e r s ~used - ~ the sensitized cistrans isomerization of butene-2 to detect the triplet state of benzene. Their estimate of the amount of triplet produced is in agreement, within experimental error, with that given by Ishikawa and Noyes. It was thought of interest to apply the above-outlined techniques to some halogenated derivatives of benzene to detect indirectly their triplet states and to try to observe a McClure effect.8 Fluorobenzene is the only monohalogenated benzene which does not decompose when irradiated at 2537 11.9 (Loeff, Revetti, and Steinlo have noted a very small quantum yield of decomposition ofo fluorobenzene in solution when irradiated at 2537 A,) Fluorobenzene shows strong absorption over the same region as benzene. Its triplet state (detected by oxygen perturbation) lies very close to that of benzene.ll It has a weak fluorescent, emission in all phases but does not show any triplet state emission even in a glassy matrix.12 It seemed therefore that this molecule would be very well suited for the type of study outlined above. The Journal of Physical Chemistry
Experimental Section Materials. The fluorobenzene used was Eastman White Label, purified by bulb-to-bulb distillation in vucuo. The cyclohexane was Matheson Coleman and Bell Chromatoquality reagent used without, further purification; benzene was Matheson Coleman and Bell Spectroquality reagent purified in the same way as the fluorobenzene ; biacetyl was also obtained from Matheson Coleman and Bell and purified by bulb to bulb distillation in vacuo. Research grade cisand trans-butene-2 were obtained from the Phillips Petroleum Co., and vapor phase chromatography showed them to be essentially pure. A conventional high-vacuum, grease-free line was employed. A11 experiments were carried out in a 60 mm. long T-shaped cell. The horizontal windows of (1) The University of New Brunswick, Saint John, New Brunswick, Canada. (2) G. N. Lewis and M. Kasha, J . Am. Chem. Soc., 66,2100 (1944). (3) (a) H. Ishikawa and W. A. Noyes, Jr., ibid., 84, 1502 (1962); (b) H. Ishikawa and W. A. Noyes, Jr., J . Chem. Phys., 37, 583 (1962). (4) R. B. Cundall and T. F. Palmer, Trans. Faraday Soc., 56, 1211 (1960). (5) R. B. Cundall and D. G. Milne, J. A m . Chem. SOC.,83, 3902 (1961). (6) R.B. Cundall, F. J. Fletcher, and D. G. Milne, J . Chem. Phys., 39, 3536 (1963). (7) R.B. Cundall, F. J. Fletcher, and D. G. Milne, Trans. Faraday SOC., 60, 1146 (1964). (8) D.S. McClure, J . Chem. Phys., 17, 905 (1949). (9) 0.P. Semenova and G. S. Tsiknnov, Zh. Fiz. Khim., 18, 311 (1944). (10) I. Loeff,L. M. Ravetti, and G. Stein, Nature, 204, 1300 (1964). (11) D.F. Evans, J . Chem. Soc., 2753 (1959). (12) E. H. Gilmore, G. E. Gibson, and D. 8. McClure, J. Chem. Phys., 20, 829 (l952),and correction t o this publication: ibid., 23, 399 (1956).
TRIPLET STATE O F l?LUOROBENZBNE
the cell were 30 mm. in diameter while the vertical or emission window was 20 mm. in diameter. The cell was encased in an aluminum block which served a dual purpose in that it reduced stray light and could be used as a furnace. High-vacuum Teflon stopcocks (Delmar Corp.) were installed close to the cell in order to diminish the ((dea$d space.” The light source was a Hanovia 5-100 medium-psessure mercury arc (Alpine Burner) whose eniission was focused by two qurartz lenses on che entrance slit of a Bausch and Lomb grating monochromator, Model 33-86-45. The reciprocal linea: dispersion of this monochromator is given as 6.6 A./mm., and the slit widths used in this study were entrance slit 4 mm. and exit slit 1 mm. The light from the monochromator was focused by a quartz lens onto the window of the cell. The light transmitted through the cell was measured with an RCA 935 phototube connected to a Keithley Micro-Microammeter, Model 410. The emission intensity was measured by an RCA 11’28 photomultiplier. The associated eleatronics have been previously des~ribed.1~ Fluorobenzene and benzene emission was viewed through a Corning No. 9863 filter which transwhile biacetyl emission was mits from 2400 to 4000 viewed through a Corning C.S. 373 filter which transmits wave length longer than about 4400 A. All materials were thoroughly degassed before admission to the cell. Mixing was accomplished by flash vaporization, and in the cases where the components had widely separated boiling points the flash vaporization procedure was repeated several times. Pressures below 3 mm. were measured by allowing a relatively large pressure into the cell and then expanding into large calibrated flasks. I n order to measure fluorobenzene fluorescence and the sensitized emission of biacetyl, the following technique was used. Benzene or benzene biacetyl mixtures were introduced into the cell and photolyzed a t 2537 A. The absorbed and emitted intensities were noted a t several pressurea for which quantum yields of emission and sensitized emission have been given by Ishikawa and Xoyes. ab These quantum yields were then set equal to
w.,
Q
=
Ie
-z
4285
obtained in this way could then be used to obtain quantum yields of fluorescence either of fluorobenzene or of biacetyl emission sensitized by fluorobenzene. These Z values were reproducible and were checked prior to and a t the end of each set of experiments. When measuring emission of biacetyl sensitized by fluorobenzene the pressure of fluorobenzene was kept at about 3 mm. This pressure of fluorobenzene has an absorption equal to about 20 mm. of benzene. To keep these experiments consistent with those of Ishikawa and Noyes, cyclohexane was introduced to make up the difference in pressure. The added cyclohexane has no effect on the fluorobenzene but does inhibit biacetyl decomposition. The potassium ferrioxalate actinometer described by Parker14 and by Hatchard and Parker16 was used in conjunction with the Cundall technique. cis- and trans-butene-2 were analyzed on a Wilkens HyFi Aerograph Chromatograph, Model 600-C equipped with a 7.6-m. benzyl ether (2001,) on firebrick (60-80 mesh) column followed by a 3.05-m. silicone gum rubber column which acted as a scrubber. Both columns were at room temperature. The retention time for trans-butene-2 under these conditions was 12 min. 19 see. and for cis-butene-:! was 14 min. 2 sec. Although Beer’s law is not obeyed for fluorobenzene (see Table I) absorption by fluorobenzene, particularly a t the pressures used in this study, is so much higher than that of biacetyl that, for most of the mixtures used, effectively all of the light was absorbed by fluorobenzene. ~~
~~
Table I: Variation of Apparent Extinction Coefficient e
with Fluorobenaene Pressure a t 2537 A. and 27’“ Fluorobenzene press., mm.
C, 10kM
43.5 33.5 24.5 9.0
23.272 17.922 13.107 4.815
128 129 156 202
’
a Length of cell: 6 em. E = 1/CZ log Io/Itr where Io is the incident intensity and It, is the transmitted intensity at C moles/ liter of fluorobenzene in the cell.
Ia
where Q is the absolute quantum yield of either Auorescence or sensitized emission, I, is the photocurrent due to emitted light, I , is the differeince in photocurrent due to transmitted light when the cell is empty and when it is full, and 2 is a dimensionless constant of the system which allows the conversion of these arbitrary photocurrents into quantum yields. 2 values
Results A . FluorobenxeneoFluorescence. Fluorobenzene when excited at 2537 A. shows a relatively strong (13) J. Heicklen, Ph.D. Thesis, University of Roohester, 1958. (14) C. A. Parker, Proc. Roy. SOC.(London), A220, 104 (1953). (15) C.G.Hatohard and C. A. Parker, ibid., A235,518 (1956).
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4286
I. UNGER
fluorescence, and the fluorescence yield is almost as great as that of benzene. The absolute fluorescence yield based on benzene extrapolates to 0.235 a t zero pressure. It should be emphasized that this extrapolation to zero pressure cannot be used to predict behavior at very low pressures. Plots of 1/Qi us. P are probably not linear a t low pressures.16 Self-quenching for fluorobenzene is small but larger than that of benzene. Since fluorobenzene absorbs very strongly at 2537 A., it is possible that the observed self-quenching is mainly due to a geometrical effect and that actual self-quenching is much smaller. Fluorescence quenching by biacetyl is stronger than self-quenching while cyclohexane has little o r no effect on the fluorescence of fluorobenzene. A plot of 1/Qfus. (fluorobenzene) at room temperature follows the equation 1/Qr
=
4.22
+ 2.05 X 103(FPh)
(21
where (FPh) is the fluorobenzene concentration in moles per liter. A plot of 1/Qf us. (biacetyl) follows the equation 1/Qf = 4.44
+ 6.1 X 103(B)
Fluorescence yield of fluorobenzene
cis-Butene-2 press., mm.
0.23 0.24 0.21 0.20 0.23 0.23 0.25 0.24 0.24 0.24
54 48 42 36 30 24
17.5 12 6 0 Fluorobenzene pressure:
3 mm.
The Journal of Physical Chemistry
13
. 11
8
2
9
7 5
0
10 20 30 Fluorobenzene or biactyl pressure, mm.
40
Figure 1. Variation of l/(fluorescence yield of fluorobenzene) with increasing fluorobenzene pressure, 0; at 3.5 111111. of fluorobenzene and increasing pressure of biacetyl, @; and the effect of 40 mm. of cyclohexane on 1/( fluorescence yield of fluorobenzene) a t various fluorobenzene pressures, 9. Temperature 27".
(3)
where (B) is the biacetyl concentration in moles per liter. (All concentrations in this paper are expressed in moles per liter.) The data mentioned above are summarized in Figure 1. The addition of cis-butene-2 has no effect on the fluorescence of fluorobenzene (see Table I1 and Figure 1). There is a considerable amount of scatter in the data. This is probably due to the inefficient mixing of the two gases. R. Sensitized Emission of Biacetyl. A sensitized emission of biacetyl is observed when fluorobenzene is excited in the presence of biacetyl. The sensitized
Table 11: Effect of cis-Butene-2 on the Fluorescence Yield of Fluorobenzene a t 2537 8. and 27""
15
1 50
d 2
-
-1 10
10 20 Biacetyl pressure, mm.
30
Figure 2. Variation of l/(sensitized emission yield of biacetyl) with biacetyl pressure a t constant fluorobenzene pressure. Temperature 27".
emission yield of biacetyl passes through a maximum at approximately 4.5 mm. of biacetyl. The sensitized emission yield a t 3 mm. of fluorobenzene and room temperature follows the equation (see Figure 2 )
+
i / =~5.5~ 4.4
x
103(~)
(4)
Equation 4 is not obeyed a t very high and very low values of (B). Increasing the pressure of fluorobenzene decreases the sensitized emission yield. The (16) P.Sigal, J. Chem. Phys., 42, 1953 (1965).
TRIPLET STATEOF FIJJOROBENZENE
4287
with caution because the low-incident intensity at this wave length will magnify any error. It is interesting to contrast these data with those obtained for the benzene-sensitized emission of biacetyl. Benzenesensitized emission of biacetyl drops off rapidly with wave length and is zero, within experimental error, a t 2420 8. even in the presence of large pressures of a quenching gas such as cyclohexane. 10
20 30 Fluorobenzene pressure, mm.
40
Table I11 : Variation of Sensitized Emission Yield of Biaoetyl with Wave Length"
Figure 3. Variation of lr/(sensitized emission yield of biacetyl) with increasing fluorobenxene pressure, biacetyl pressure remaining constant a t 5 mm. Teimperature 27".
A.
Sensitized emission yield
2350 2420 2480 2537 2537 2537 2640
0.066 0.139 0.121 0.148 0.123 0.137 0.125
Wave length,
30
20
6'
\
a Pressure: biacetyl, 5 mm.; fluorobenzene, 2-3 mm.; cyclohexane, 20 mm. Temperature 27".
i
10
20
10
30
40
cis-Butene-2 pressure. mm.
Figure 4. Variation of l/(sensitixed emission yield of biacetyl) with cis-butene-2 pressure. Fluorobenzene pressure 2 mm.; biacetyl pressure 6 mm. Temperature 27".
equation for l/Qs for a constant biacetyl pressure of 5 mm. and room temperature is given by (see Figure 3) l/QB
=
5.5
+ 4.67 X 103(FPh)
(5)
The sensitized emission yield is strongly quenched by cis-butene-2, and the data may be represented by (see Figure 4) l/Qs
=
6 $. 1.02 X 104(cis-butene-2)
(6)
In this connection it should be noted that cis-butene-2 was found to have no effect on the emission ofobiacetyl when biacetyl was excited at 4050 and 4358 A. This would be expected because of the low energy values for the singlet and triplet states of biacetyl and is in agreement with data in the 1iterature.l' C. Variation of Sensitized Emision with Inxident Wave Length. The sensitized emission yield of biacetyl is essentially constant from 2640 to 2420 8. but appears to drop off sharply at 2350 8.; the data are summarized in Table 111. The value at 2350 8. must be regarded
D. cis-trans Isomerizatiop of cis-Butene-9. Fluorobenzene excited at 2537 A. sensitizes the cis-trans isomerization of cis-kutene-2. This was found to be true even at 2420 A. where there is no longer any observable fluorescence from fluorobenzene. I n marked contrast, no cis-trans isomerization was cbserved when benzene was used as sensitizer at 2420 A., even under prolonged irradiation. The experimental technique used in this study, in conjunction with the Cundall type of experiments, was not precise enough to allow accurate estimates of the amount of triplet state molecules of fluorobenzene produced, mainly because of difficulty with the chromatography. It was, however, sufficient to show unequivocally that isomerization of cis-butene2 had occurred and that it did not occur in blank runs. One of the better results indicated that 90% of the fluorobenzene molecules which adsorbed radiation ended up in the triplet state.
Discussion There are several ways of determining whether or not one is dealing with a triplet state. These methods must be used with caution since triplet states of different molecules will not respond to any given test in the same manner.ls The experimental observation of a strong, ~~
(17) N. A. Coward and W. A. Noyes, Jr., J. Phys. Chem., 22, 1207 (1954). (18) D. W. Sester, D. W. Placeek, R. J. Cvetanovic, and B. S. Rabinovitch, Can. J. Chem., 40, 2179 (1962).
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I. UNGER
4288
sensitized emission of biacetyl when fluorobenzene is excited a t 2537 A. is not in itself sufficient evidence for postulating the existence of a triplet state of fluorobenzene. Only if the ratio of the integrated green to the integrated blue is much larger than 60:119920 can biacetyl emission be used as conclusive evidence of triplet sensitization. To obtain good values of the blue-to-green ratio requires exposures of up to 12 days a t the intensities available. I n the present study, rather than go to such long exposures, the cis-trans isomerization of butene-2 was used as confirming evidence for triplet fluorobenzene. The evidence for the formation of triplet fluorobenzene may now be summarized as follows. (1) The sensitized emission yield of biacetyloreaches a maximum value of 0.136 f 0.013 at 2537 A. The value for the phosphoresce y e efficiency when biacetyl is excited directly a t 4358 A. has been given by Almy and Gillette21 as 0.15 f 0.03. The data on sensitized emission therefore indicate that 80 to 97% of all the initially excited fluorobenzene molecules transfer their energy to biacetyl. The fluorescence yield of fluorobenzene with the pressure of biacetyl at the maximum is about 0.16. The total lies between 0.96 and 1.13. With the rather large errors in the data, all of the initially excited fluorobenzene molecules either fluoresce or cross over to the triplet state. (2) One plate of the sensitized emission was taken, and it showed the integrated green-to-blue ratio to be much larger than the 60: 1 value obtained when biacetyl is excited directly. (3) Michael and Noyes22have given a derivation which shows that, if one takes slope over intercept from a plot of biacetyl concentration, over sensitized emission vs. biacetyl concentration, (B)/Qs vs. (B), it will be equal to slope over intercept of a plot of l / Q (where Q is the quantum yield of any measurable quantity for some process such as fluorescence or decomposition) vs. biacetyl concentration if both processes occur from the same spin state. I n our case, slope over intercept for sensitized emission is 0.72 whereas slope over intercept for fluorescence quenching of fluorobenzene by biacetyl is 0.13, or that state of fluorobenzene which undergoes fluorescence cannot be the same one which passes on its energy to biacetyl. The kinetics shown below elucidate this point. (4) The observations in the presence of cis-butene-2 are in themselves quite strong evidence that a large fraction of the excited fluorobenzene molecules reaches the triplet state. The singlet state of fluorobenzene cannot transfer energy to the olefin since the singlet state of the former is lower in energy than the singlet state of the latter.23 The olefin has no effect on the The Journal of Physical Chemistry
emission of biacetyl, but it does strongly quench the sensitized emission. It is, therefore, removing triplet fluorobenzene molecules before they can sensitize the emission of biacetyl. Moreover cis'trans isomerization of the olefin is observed when fluorobenzene is excited in its presence. This is typical of triplet state behavior.24 The following mechanism is consistent with the experimental results and the photochemical behavior of biacetylzK FPh
+ hv +lFPhI
'FPh' +FPh
lFPhT(+M) lFPhI
3FPhT(+M)
-+
+ FPh +2FPh
3FPh1 (+M)
-+
(8) (9) (10) (11)
+'FPh 3B1+ FPh
(12)
lBII
3FPh1 -I-B
(7)
(+M)
-3 FPh
+B +
lFPhI
+ hv
I,
(13)
-+dissociates
(14)
(+nq +IBI (+M)
(15)
lBII 1 ~ x 1
1BI 43BI
(16)
+ hv 3B' +hv + B
(18)
(+M) +B (+MI
(19)
IBI +B
(17)
I n the above scheme FPh represents a ground-state fluorobenzene molecule, B is a ground-state biacetyl molecule, M is any molecule capable of removing vibrational energy, the superscripts refer to singlet and triplet states, and the Roman numerals refer to the first and second excited states. All radiationless processes are shown as possibly being bimolecular. It is difficult to conceive of any of the occurring without the removal of vibrational energy although this may not be the rate-determining (19) H. Okabe and W. A. Noyes. Jr., J. Am. Chem. SOC.,79, 801 (1958). (20) H. L. J. BLokstrom and X. Sandros, Acta Chem. Scand., 14, 48 (1960). (21) G. M. Almy and P. R. Gillette, J. Chem. Phys., 11, 188 (1943). (22) J. L. Michael and W. A. Noyes, Jr., J. Am. Chem. Soc., 85, 1027 (1963). (23) P. G. Wilkinson and R. S. Mulliken, J. Chem. Phys., 23, 1895 (1955). (24) H. M. Frey, J. Am. Chem. Soc., 82, 5947 (1960). (25) W. A. Noyes, Jr., G. B. Porter, and J. E. Jolley, Chem. Rev., 56, 49 (1956).
TRIPLET STATEOF FLUOROBENZENE
4289
step.26 Reaction 14 is much more rapid than (15).27 We may therefore neglect steps 15, 16, and 17. For the scheme shown above the expression for fluorescence efficiency of fluorobenzene can be shown to be
For the case of no biacetyl present, thLe last term on the right-hand side of (20) vanishes, and one should have a straight line with slope equal to klo/k8. When the fluorobenzene is kept constant, we should again have a straight line, and this time the slope will be kl2/k8. The experimental results show that this is indeed the case and that klo/k8 = 2.05 X lo3 whereas klz/k8 = 6.1 X lo3; these data indicate that k12 is about three times as large as klo. Slope over intercept for constant fluorobenzene in eq. 20 will be given bsy slope .intercept
kl2
k8
(21)
-I k g f ka(FPh)
If reaction 13 does not occur and sensitized emission is due to (12) followed by (15)-(19), then one obtains for (B)/Qs
f kl9)(kl6 f k17)(k14 f k15) x ( h f k 9 f kio(FPh) $. kiz(B))
(k18
(B)
-
Qs
~18kl6~lSklZ
(22,)
At constant (FPh) the slope of (22) will be slope =
klZ(k18
-tklB)(kl6 f kl;)(k15 f
___
k14)
kl8kl6klbkl2
(23)
+
.-I- kio(FPh))(kis f h)X
and the slope over the intercept is the same as (21). On the other hand, if (13) does occur, then (B)/Qa will be given by (kl8 4- k19)(kl3(B) $. kll) x (B) - (k8 f kg f ho(FPh) f kiz(B)) -QS
k18k13k9
The above yields 8.77 X lo6 sec.-l as the limiting value'of k8. From eq. 2 and 20 and k8, k g is found to be 2.82 X lo7 sec.-l. From the slope of eq. 20 a t zero biacetyl pressure, klo can be calculated and is . ~ found to be 2.98 X 10-l1 molecule-1 ~ m sec.-l. This leads to a self-quenching cross section of 2.07 X 10-16 cme2for fluorobenzene. The ratio of kl2 to k10 is 3. This leads to a value of 8.8 X molecule-1 ~ m sec.-l . ~ for k12. We may now use 1~12to estimate an effective cross section for quenching of singlet fluorobenzene by biacetyl which turns out to be about 6.0 X cm.-2. Should this be the correct value, then one would be forced to conclude that biacetyl quenches the singlet state of fluorobenzene very effectively. Earlier in this discussion it was noted that kll is not negligible with respect to kvd (B). If, however, this assumption is made and the appropriate values are k19), i e . , inserted into eq. 25, we can obtain k18/(k18 the fraction of triplet state biacetyl molecules which emits. On going through the above procedure, kls/ (kl8 1 % ~ ) is found to be about 0.24 which is rather high but not too far out of agreement with the value obtained by Almy and Gillette.2l
+
+
and the intercept of (22) will be (k8
state of fluorobenzene is not the one which causes the sensitized emission of biacetyl since slope over intercept can in no way equal (21). An examination of eq. 25 shows that kll is not negligible with respect to k13 (B). If this were the case, the slope over intercept of 1/QS vs. (B) should equal the slope over intercept for 1/Qf vs. (B) at constant fluorobenzene pressure. The integrated molar absorptivity of fluorobenzene has been given as 2.03 X lo628; this leads to a fluorescence lifetime ( r ) of 1.14 X sec.
(25)
From the above we can conclude that the singlet
Acknowledgments. The author is deeply indebted to Dr. W. A. Noyes, Jr., for many helpful discussions and suggestions during the course of this work. Financial support from the Robert A. Welch Foundation is also gratefully acknowledged. (26) W. A. Noyes, Jr., Proe. Acad. Sei. Lisbon, 3 (1964). (27) A. G. Hmison and F. P. Lossing, Can. J. Chem., 37, 1478 (1959). (28) M. Ballester, J. Palau, and J. Riera, J. Quant. Spectry. Radiative Transfer, 4, 819 (1964).
Volume 69, Number 13 December 1966
