J . Phys. Chem. 1984,88, 1157-1 159
1157
Photochemistry of 1,I,1-Trifluoro-3-bromoacetone. 2. Dual Fluorescence in the Gas Phase John R. Majer and Zeki Y. AI-Saigh* Department of Chemistry, University of Birmingham, Birmingham B1.5 2TT, England (Received: September 21, 1982; In Final Form: September 6, 1983)
Two fluorescence peaks of gaseous 1,1,1-trifluoro-3-bromoacetone have been observed when excited at a wavelength close to the S2absorption band, while excitation in the S1 absorption band produces one fluorescence peak. These emissions have been identified as S1 So and Sz So fluorescence with quantum yields of l O : l , respectively. Assignment of the latter is verified by an excitation spectrum. The quenching of these emissions has been studied by using both vibrational relaxers such as nitrogen and sulfur hexafluoride and selective energy transfer agents such as oxygen, biacetyl and trans-butene-2.
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Introduction In a previous communication' we have reported the wavelength effect upon the photodecomposition of trifluorobromoacetone. The primary process observed depended markedly upon the wavelength of irradiation and different modes of photodecomposition were observed upon irradiation in the S1 and S2 absorption bands (44450and 32 800 cm-l). Our interest in the photochemistry of trifluorobromoacetone led us to investigate the emission of this ketone. To our surprise, two well-separated emission maxima are observed, one of which appears to be anti-Kasha fluorescence (AKF). Although the SI fluorescence of ketones in the gas phase is relatively weak, a number of studies have been undertakenS2-' The SI fluorescence of acetone vapor is complicated due to the fact that one of the products of decomposition has a relatively intense8 fluorescence which competes with that of acetone itself. This complication is not apparent in the case of hexafluoroacetone because its simple mode of photodecomposition leads predominantly to the production of hexafluoroethane and carbon monoxide.3 The emission spectrum of hexafluoroacetone is due to both fluorescence and phosphorescence processes, the latter being quenched by traces of oxygen.2 The emission is quenched also by small amounts of biacetyl which itself exhibits sensitized fluorescence. The fluorescence spectrum of hexafluoroacetone extends from 26 300 to 14 300 cm-' with a broad maximum at 22 200 cm-I. While the spectrum of decafluorodiethyl ketone is less extensive, ranging from 28 600 to 20000 cm-l with a maximum at 24400 cm-I, its fluorescence quantum yield decreases at higher concentrations. Although the introduction of chlorine atoms into the ketone molecule has a distinct effect upon the absorption spectrum there is no great difference observed in the emission spectra." The emission of chloropentafluoroacetoneat 23 800 cm-' is unaffected by the presence of mercury vapor and traces of oxygen. It has been shown that the fluorescence quantum yield increases with increasing p r e s s ~ r e . Similar ~ results have been observed with 1,3-dichlorotetrafluoroacetone.5 Azulene,12 which fluoresces with quantum yield (S2 So) = 0.034 in solution,13is the best documented example of AKF. Other nonalternate hydrocarbons behave ~ i m i l a r l y . ' ~ - 'While ~ S2 So fluorescence has only been observed in thiocarbonyl compounds in the gas phase and in condensed media,'8-22 it has not been reported for carbonyl compounds. The electronic excited-state decay of thiocarbonyl compounds has been recently reviewed by Steer.23 -+
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Experimental Section All fluorescence studies were made with a Farrand spectrofluorimeter (Optical Co. Ltd.). The sample cell was a quartz, cylindrical vessel of diameter 20 mm and length 7 mm. It had a side arm leading to a tap (Young, teflon tap) and a B14 socket. *Address correspondence to this author at the Department of Chemistry, University of Texas, Austin, Texas 78712.
l,l,l-Trifluoro-3-bromoacetonewas prepared as described previously.' Nitrogen and oxygen were obtained from British Oxygen Co. and Sulfur hexafluoride from Imperial Chemical Industries Ltd. Benzene was analar grade from Hopkin and Williams Ltd., biacetyl was obtained from British Drug Houses Ltd., and trans-butene-2 from Cambrian Chemicals Ltd. All reagents were subjected to bulb-to-bulb distillation before use. The quartz sample cell was attached to a conventional vacuum line and filled with the vapor of trifluorobromoacetone at an appropriate pressure. the ketone was frozen into a side arm with liquir air and the appropriate pressure of quenching gas introduced into the vacuum line and the cell. The tap was closed and the contents of the cell mixed thoroughly by alternately freezing and thawing several times. After 30 min the emission spectrum of the contents of the cell was recorded. Results and Discussion Gaseous trifluorobromoacetone shows fluorescence over the wavelength range of 3 1 000 to 20 000 cm-I with a maximum at 25 000 cm-' when the ketone is excited in the SI absorption band at room temperature (Figure 1). This fluorescence is similar to that exhibited by all halogenated ketone^^-^ and can be readily identified as the SI So fluorescence. Upon excitation in the S2absorption band the spectrum appears more complex (Figure 1) with the appearance of an additional weaker emission at 35 500 cm-' which can be identified as S2 So fluorescence resulting -+
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(1) J. R. Majer, J. C. Robb, and Z. Y. Al-Saigh, J . Chem. SOC.,72, 1697 (1976). (2) P. Bowers and G. B. Porter, J . Phys. Chem., 68, 2982 (1964). (3) G.Giacommette, H. Okabe, S. J. Price, and E. W. R. Steacie, Can. J . Chem., 38, 104 (1960). (4) H. S. Samant and A. J. Yarwood, Can. J . Chem., 48, 2611 (1970). ( 5 ) P. A. Hackett and D. Phillips, Trans. Faraday SOC.,68, 323 (1972). (6) G. Giacommette, H. Okabe, S. J. Price, and E. W. R. Steacie, Proc. R . SOC.London, Sec. A , 250, 287 (1959). (7) A. Gandini and K. C. Kutschke, Can. J . Chem., 44, 1720 (1966). (8) M. S.Matheson and W. A. Noyes, Jr., J. Am. Chem. SOC., 60, 1857 (1938). (9) P. A. Hackett and D. Phillips, Trans. Faraday SOC.,68, 329 (1972). (10) W. R. Ware and S. K. Lee, J . Chem. Phys., 49, 217 (1968). (1 1) P. A. Hackett and D. Phillips, Trans. Faraday SOC.,68, 335 (1972). (12) J. B. Birks, E. C. Esterly, and L. G. Christophorou, J. Chem. Phys., 66, 4231 (1977). (13) G. D. Gillispie and C. E. Lim, J . Phys. Chem., 65, 4315 (1976). (14) D. L. Philen and R. M. Heddges, Chem. Phys. Lett., 43, 358 (1976). (15) G. Viswanath and M. Kasha, J . Chem. Phys., 24, 574 (1956). (16) I. B. Berlman, H. 0. Wirth, and 0. J. Steingraber, J. Am. Chem. SOC.,90, 566 (1968). (17) V. Rehak, A. Novak, and M. Titz, Chem. Phys. Lett., 52,39 (1977). (18) D. J. Clothier, A. R. Knight, and R. P. Steer, J . Chem. Phys., 71, 5022 (1979). (19) T. Oka, A. R. Knight, and R. P. Steer, J . Chem. Phys., 66, 699 (1977). (20) D. Phillips and R. P. Steer, J . Chem. Phys., 67, 4780 (1977). (21) J. R. Huber and M. Mahaney, Chem. Phys. Lett., 30, 410 (1975). (22) D. J. Clothier, A. R. Knight, and R. P. Steer, Chem. Phys. Letl., 59, 62 (1978). (23) R. P. Steer, Rv. Chem. Intermed., 4, 1-41 (1981).
0022-3654/84/2088-1157$01.50/00 1984 American Chemical Society
1158 The Journal of Physical Chemistry, Vol. 88, No. 6, 1984
Majer and Al-Saigh
.7/ - 0.7 - 0.6
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Figure 1. The spectra of CF3COCH2Br: absorption (--); S2 emission (- ); SI So emission (- - -).
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from the direct excitation in the second absorption band. It is not due to emission from a photolysis product because of the slow rate of photodecomposition due to the low a b ~ o r b a n c e . ~The ~ 0-0 band was close to the wavelength expected from the absorption spectrum and the excitation spectrum was in good agreement with the absorption spectrum in the S2 region. Similar S2 emission has been observed around 33 300 cm-' from thiophosphene ~ a p o r . 2 ~ SI So fluorescence has also been observed when l,l,l-trifluoro-3-bromoacetone is excited in the S2band. It is evident that the radiationless process S2 SI is occurring since the energy gap in this ketone AE(S2-Sl) = 9700 crn-'. According to the criteria given by Eber et a1.26for azulene derivatives, compounds with AE(S2-Sl) lower than 10 000 cm-I may emit dual fluorescence or S1 So exclusively when the molecule is excited in the S2 state. In this work, no detectable emission S2 S1 was observed. The radiative rate constants Kf(Sl)and Kf(S2)were calculated by integration of the first and second absorption bands using Strickler-Berg equation as follows2'
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40 KETONE PRESSURE IN TORR
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Figure 2. The effects of CF,COCH,Br pressure on Sz So emission: (0)Sl So; (a) emission; values for Qf(S,)and QAS,) are in arbitrary units. I
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and were found to be 0.25 X lo4 and 0.22 X lo5 s-l, respectively. The apparent quantum yield of S? fluorescence has been found to be ten times lower than the S1 fluorescence quantum yield. Some of the S2fluorescence radiation may be reabsorbed by the ketone to re-emit SI fluorescence, thereby increasing the SI fluorescence quantum yield and decreasing the S2 fluorescence quantum yield, because the S2fluorescence peak overlaps the S1 absorption peak. It is difficult to determine the SI and S2 fluorescence quantum yields accurately. Due to this difficulty, S1 and S2fluorescence quantum yields were taken arbitrarily as a peak area in this work. Our comparison of S1 So and S2 So quantum yields is made only when trifluorobromoacetone is excited in the S2state. It has been shown2*that the introduction of a carbonyl group into the azulene molecule causes a drastic reduction in the S2 So fluorescence intensity relative to that of azulene because of the large decrease in the S2 S1 energy gap. In this study it was shown that the fluorescence output was linearly dependent upon light intensity. Following this, the variation in fluorescence yield with ketone pressure was investigated. The ketone was excited at a number of different pressures in the second absorption band. Figure 2 shows a plot of the
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(24) Z. Y . AI-Saigh, Ph.D. Thesis, University of Birmingham, 1973. (25) T. Oka, A. R. Knight, and R. P. Steer, J . Chem. Phys., 63,2414 (1975). (26) G. Eber, F. Gruneis, S . Schneider, and F. Dorr, Chem. Phys. Lett., 29, 397 (1974). (27) S. J. Stricker and R. A. Berg, J . Chem. Phys., 37, 814 (1962). (28) H. J. Griesser and U. P. Wild, Chem. Phys., 52, 117 (1980)
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Figure 3. Typical Stern-Volmer plot: S, So emission (benzene, @; oxygen, e ) ;S1 So emission (benzene, A; oxygen, m).
reciprocal of the fluorescence yield in arbitrary units against ketone pressure. The fluorescence yield increases in the region below 15 torr due to an increase in optical density so that more molecules are excited and emit fluorescence. However, above this pressure a self-quenching process becomes important. This is not an instrumental artifact. Correction has been made for the effect on the viewing system of increasing concentration, as reported by Phillips et al?9 Similar effects have been reported in other studies upon the fluorescence of halogenated ketone^.^^^ It is interesting to note that the slope of Stern-Volmer plot for Sl is about five So times greater than the slope €or S2, suggesting that Sl fluorescence is more efficiently quenched than S2 So fluorescence. This may be a result of decreasing the coupling between S, and S2states by increasing concentration, so that SI fluorescence can be affected by this factor in addition to the collisional deactivation. In a further series of experiments, nitrogen and sulfur hexafluoride were added to the ketone. Both gases quenched SI and S2fluorescence over a wide range of pressures up to 150 torr when the ketone is excited in the S2state. However, at a low pressure
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(29) D. Phillips, D. Gray, and K. Al-Ani, J . Chem. SOC.A , 905 (1971).
Photochemistry of 1,l , l -Trifluoro-3-bromoacetone TABLE I: Rates of Quenching Constants of Trifluorobromoacetone as a Slope of Stern-Volmer Plots (Eq 2), A, = 4 0 000 cm-' 1 O2 x fluorescence, L mol-' quenchers
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nitrogen sulfur hexafluoride oxygen benzene biacetyl trans-butene-2
6.29 2.85 2.59 2.87 7.77 4.70
4.26 1.96 0.69 0.74 2.66 1.67
of added sulfur hexafluoride and up to 20 torr, the curve of fluorescence yields against sulfur hexafluoride pressure passes through a maximum and becomes constant above a certain pressure. The enhancement of S2 fluorescence, due to the reduction in the number of excited ketone molecules suffering decomposition by the removal of excess vibrational energy, results in the enhancement of S2 S, radiationless transition and therefore in the enhancement of S1 fluorescence. The results of quenching studies for SF6 and nitrogen are illustrated more clearly by Stern-Volmer plots of the form
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where Qp and Qf are the arbitrary fluorescence quantum yields in the absence and presence of quencher molecules, respectively. In the case of SFs, data are plotted only for the quenching region and the enhancement is ignored for this purpose. [Q] represents the concentration of quenchers in terms of mol ~ m - The ~ . values of the quenching rate constants for SF6 and nitrogen as slopes of Stern-Volmer plots are given in Table I. Similar results were observed for the quenching of S2 emission from thiophosgene by Oka et al.I9 When the sulfur hexafluoride or nitrogen was replaced by oxygen, it was found that once again both the S2 and SI fluorescence yields were reduced. No enhancement was observed at low pressure of oxygen. When the results of the studies of fluorescence quenching by oxygen were plotted in Stern-Volmer form, the values for the quenching rate constants could be deduced and the values obtained are given in Table I. It can be seen that the quenching is more efficient for SI than for S2 fluorescence. Earlier studies1 on the photolysis of l,l,l-trifluoro-3-bromoacetone suggested that the predominant mode of decomposition from the S2states was by carbon-bromine bond fission and that this mode of decomposition was completely quenched by the addition of 780 torr of nitrogen. In contrast, the mode of decomposition from the S, state was carbon-carbon bond fission and this was not quenched in the presence of 780 torr of nitrogen. When 10 torr of oxygen was added decomposition by carboncarbon bond fission was strongly quenched, while that for carbon-bromine bond fission remained substantially unaltered. This suggests intersystem crossing S, T,before decomposition and that some of the emission at 25 000 cm-' is phosphorescent in nature. Similary, oxygen strongly quenches S1 fluorescence, while less efficiently quenching S2fluorescence. The value of the slope of the Stern-Volmer plot appears to be ca. four times greater for SI fluorescence than for S2 fluorescence. Attempts were made to study energy transfer between the excited ketone and other specific quencher molecules. Benzene was selected for this study, and the ketone excited at 35 500 cm-' to avoid the absorption of benzene itself. The S1 So fluorescence at 25 000 cm-' was quenched by increasing pressures of benzene. The effect upon excitation at 40000 cm-' is even less marked. In an attempt to study the reverse process, Le., transfer from benzene to the ketone, the excitation wavelength was changed to 37 500 cm-' where the absorption due to ketone is at a minimum. All ketone emissions were greatly reduced and were quenched by increasing pressure of benzene, so that no transfer of energy between benzene and ketone was evident. There is thus little interaction between excited ketone molecules and benzene in the form of charge transfer complexes30and no indication of chemical
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The Journal of Physical Chemistry, Vol. 88, No. 6, 1984 1159 reactions such as abstraction of hydrogen or addition to the aromatic system. In the study of the S1 So fluorescence of acetone it has been shown8 that the fluorescence of biacetyl could be sensitized by the excited acetone molecule. On the other hand, Phillipsg has shown that biacetyl quenches the S1 So fluorescence of 1,3dichlorotetrafluoroacetone at all wavelengths. In order to study the effect of biacetyl on the fluorescence of trifluorobromoacetone, mixtures of the ketone (20 torr) and biacetyl at various pressures were made and their emission spectra recorded. Biacetyl quenched both the S, So and S2 So fluorescence, and no enhancement was observed at low pressure. A linear relationship was obtained when the results were plotted in the Stern-Volmer form, again the value of the Stern-Volmer plot (Table I) for SI So fluorescence is about three times greater than that of S2 So fluorescence. Obviously, biacetyl is about three times as efficient as benzene for the quenching of SI So and S2 So fluorescence, but nitrogen was found to be the most efficient quencher for S2 So fluorescence. Finally, the effect of trans-butene-2 on the emission of the ketone was studied. It has been suggested that this molecule can isomerize on transfer of energy from excited states of molecules." In the present work no evidence for such an isomerization was obtained when mixtures of the ketone (20 torr) and various pressures of trans-butene-2 were mixed and irradiated at 40 000 and 32 800 cm-' due to the short irradiation time and low intensity of light. On the other hand, measurements of the emission spectrum of the mixture showed that trant-butene-2 behaved as a quenching agent for both SI and S2 fluorescence, and no enhancement was evident at low pressure. Linear relationships were obtained when the results were plotted in Stern-Volmer form and the values for the energy transfer rate constants are given in Table I. All these results are in accord with those observed by previous workers on S1 fluorescence of other halogenated ketones. Oxygen, nitric oxide, olefins, and benzene were shown by Ware and Leelo to quench the fluorescence of hexafluoroacetone at all wavelengths and Phillips has shown similarly" that the fluorescence of 1,3dichlorotetrafluoroacetone is quenched by biacetyl, olefins, and benzene.
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Conclusions l,l,l-Trifluoro-3-bromoacetone has an energy gap aE(S2-S1) smaller than 10000 crn-'. Therefore it exhibits an anomalous fluorescence from the S2 state in addition to the normal fluorescence (S, So). To our knowledge, this is the first observation of S2 So fluorescence from compounds containing carbonyl groups. In our experiments, when only the S2state was excited, the observation of both S1 So and S2 Sofluorescences revealed the competition between S2 So radiation and S2 S1 radiationless processes. However, due to the overlap of the S2 So fluorescence with So S1 absorption band, we were unable to calculate the relative transition rates for S2 So and S2 S1 processes. The increased concentration of trifluorobromoacetone decreases the coupling between Sl and S2 states. Consequently, the S2 So fluorescence is affected by an increase of concentration less than the SI So fluorescence. Nitrogen is a known vibrational So fluorescence. In our experiments, it was relaxer for the S, efficient in quenching both SI So and S2 So fluorescences which showed its efficiency for quenching excited states higher than S1. On the other hand, biacetyl quenched effectively the S1 Sofluorescence of trifluorobromoacetone, but was a less efficient quenching agent for the S2 So fluorescence. We conclude that the primary mechanism of the quenching of biacetyl molecule is in the transfer of energy into the S1state of the acceptor biacetyl molecule (migration of the S1 excitation).
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Registry No. l,l,l-Trifluoro-3-bromoacetone,431-35-6; nitrogen, 7727-37-9; sulfur hexafluoride, 255 1-62-4; oxygen, 7782-44-7; benzene, 7 1-43-2; biacetyl, 43 1-03-8; trans-2-butene, 624-64-6. (30) R.G. Brown and D.Phillips, J . Photochem., 3, 337 (1975).
