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C O M M U N I C A T I O N S TO T H E E D I T O R
Apparent Triplet Yields of Pyrazine Publication costs assisted b y the Robert A . Welch Foundation
two isomerization yields cannot be equated to the triplet yield.8
Sir: Pyrazine (1,4-diazine) has been shown to have very low quantum yields of fluorescence, of phosphorescence, and of decomposition in the vapor phase.1,2 Thus it becomes important to know the triplet yield of pyrazine so as to determine whether or not the low fluorescent yield reflects an efficient internal conversion, or intersystem crossing process, or both. We have attempted to determine the triplet yield of pyrazine by two of the more common methods, namely the photosensitized isomerization of trans-2-butene3 and the Photosensitized phosphorescence of biacetyl. The two methods lead to significantly different results. Figure 1 displays the photosensitized isomerization Figure 1, Quantum yield of isomerization of trans-2-butene of trans-2-butene as a function of trans-2-butene presin the pyraxine-butene system (irradiated at 313 mm and 25') sure, for a fixed pyrazine pressure, at room temperature, as a function of the butene pressure in millimeters. with an exciting wavelength of 313.0 nm. Figure 2 is a plot of the reciprocal of these variables under the same The inverse of the sensitized biacetyl phosphorescent experimental conditions. Figure 2 also shows a recipyield as a function of added biacetyl, at a fixed pyrazine rocal plot of the results obtained with the cis isomer. pressure, at room temperature irradiated at 313.0 nm The experimental quantum yields were calculated by is plotted in Figure 3. From the accepted kinetics of standardizing the system using potassium ferrioxalate the r e a c t i o n ~ , *it~ ocan ~ ~be ~ shown that the intercept in a ~ t i n o m e t r y . ~From Figure 1, the isomerization rate Figure 3 (Le., emission at infinite biacetyl pressure) is appears to level off at about 10 mm pressure of added proportional to 1/&$~where $P is the quantum yield of olefin, indicating a quantum yield of isomerization of phosphorescence of biacetyl and (PT is the triplet yield of 0.28. The same quantum yield calculated from Figure pyrazine. Thus using benzene-biacetyl mixtures to 2 by an extrapolation to infinite olefin pressure is 0.34. standardize the system, and taking the benzene triplet (Likewise, from the extrapolated intercept for the cis yield to be 0.72,Qthe triplet yield of pyrazine is calcudata, a value of 0.53 is obtained.) To be consistent lated to be 1.1 0.1. In addition to these studies, the with accepted mechanisms of photoisorneri~ation,~~~ phosphorescent spectra of biacetyl, and biacetyl-pyra+isom must be calculated at infinite olefin pressure. zine mixtures, irradiated at both 313.0 and 435.8 nm The estimate of the asymptote in Figure 1 gives a significantly lower yield. (1) K. Nakamura, J. Amer. Chem. SOC., 93,3138 (1971). In addition, the ratio of trans to cis isomers of 2(2) M.IMagat, N. Ivanhoff, F. Lahmani, and M .Pileni, J . Chim. Phys., 212 (1970). butene at the photostationary state has been deter(3) R. B. Cundall and T. F. Palmer, Trans. Faraday, Soc., 56, 1211 mined as 0.9 0.1. This was achieved by irradiating (1960). systems containing near-equilibrium mixtures of both (4) H. Ishikawa and W. A . Noyes, Jr., J . Chem. Phys., 37, 583 (1962). isomers, so that the final photostationary state could (5) C . G. Hatchard and C. A . Parker, Proc. Roy. Soc., Ser. A , , 235, 518 (1956). be approached from both sides. This way, the need for (6) E. K. C. Lee, H.0. Denschlag, and G . A . Haninger, Jr., J . Chem. long periods of irradiation of either pure isomer, with Phys., 48,4547 (1968). the attendant possibility of complicating side reactions, (7) M . Tanaba, M. Kato, and S. Sato, Bull. Chem. Soc. Jap. 39, was obviated. Thus naively if a simple energy transfer 1423 (1966). mechanism is assumed,6 the above results imply a trip(8) F. Lahmani (private communication) has also found the branching ratio to be greater than unity, although her cis +trans let yield (PT = 0.87, calculated from the sum of (PCT and figure is somewhat higher than ours. However, this slight discrep~ T C . However, since the branching ratio of unity ancy does not detract from the point made above; i.e., the triplet yield based on the Cundall mechanism may well be invahd. found for a wide variety of compounds is not obtained (9) W. A. Noyes, Jr., and I. Unger, Aduan. Photochem., 4,49 (1960). here, the mechanism of the energy transfer process is (10) S. H . Jones and T. Brewer, submitted for publication in J . Amer. complex in this case, and thus a simple addition of the Chem. SOC.
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The Journal of Physical Chemistrv. Vol. 76, No. g.4, 1972
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COMMUNICATIONS TO THE EDITOR
t
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t Figure 2. Inverse of isomerization quantum yield as a function of the inverse of trans-2-butene pressure, 0 , and cis-2-butene pressure, d (conditions same as Figure 1).
were recorded on an Aminco-Bowman spectroffuorometer. The fact that these emission profiles appear to be identical suggests that biacetyl is probably solely responsible for the luminescence in both cases. At the present time, the discrepancy between the two methods and the two isomers is not fully understood, but it certainly indicates the caution that should be exercised in using these methods to determine triplet yields since it is clear that at least one and possibly both methods are unreliable. However, other workersll using certain benzenoid aromatic sensitizers have found the two methods to give results essentially in agreement.
The Journal of Physical Chemistrg, Vol. 76,N o . $4, 1971
Figure 3. The inverse of sensitized emission of biacetyl in the pyrazine-biacetyl system (conditions as in Figure I ) as a function of biacetyl pressure in millimeters.
In order to shed more light on the general applicability of these methods, further work is in progress in this laboratory on various other compounds, including a more detailed study of pyradne.
Acknowledgment. We wish to thank Dr. W. A. Koyes, Jr., for his advice and support during the course of this work. Also, financial assistance by the Robert A. Welch Foundation and the Camille and Henry Dreyfus Foundation is gratefully acknowledged. (11) W. A . Noyes, Jr., and C. S. Burton, Rer. Runsenges. Phys. Chem., 72, 146 (1968), and references cited therein.
DEPARTMEXT OF CHEMISTRY THEUNIVERSITY O F TEXAS .4T AUSTIN AUSTIN,TEXAS78712
RECEIVED APRIL 30, 1971
TERRYBREWER S. H. JONES*
