3138
Emission Studies of Pyrazine in the Gas Phase Kenichiro Nakamura' Contribution from the Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712. Received September 30, 1970 Abstract: The emission of pyrazine in the gas phase has been studied. Pyrazine emits both fluorescence and
phosphorescence, and their yields are determined to be (1.74 f 0.08) X 10+ and (1.70 k 0.08) X at 313 nm, respectively. Both emission yields decrease with decreasing wavelength of excitation and are zero in the a-a* band. However, the ratio of fluorescentyield to phosphorescent yield is independent of the exciting wavelength. Pyrazine emits phosphorescence when sensitized by benzene and can be used for triplet yield measurements of benzene. This sensitized-phosphorescencemethod for pyrazine shows the benzene triplet yield to be 0.69 0.07 at (252 5 1)
*
nm.
E
mission and spectroscopic studies of pyrazine (1,4diazine) in the solid and liquid phases have been discussed by many authors. 2-4 However, few studies are reported for pyrazine in the gas phase and no emission yield of pyrazine in the gas phase has been reported. In this paper both triplet and singlet emission yields of pyrazine have been determined, and the sensitized emission from pyrazine is also discussed. Electronic states of pyrazine have been calculated by many Pyrazine has two absorption bands, an n--K* and a -K--T* t r a n s i t i ~ n . ~ Goodman2* ,~ has found a weak absorption (f= 10-6-10-7) at 370 nm in pyrazine and assigns it to a spatially allowed electronic dipole transition. He3 has also calculated the rates of radiationless transitions, internal conversion, and intersystem crossing, and shows that the fast intersystem crossing in pyrazine is due to the S(n--K*) (30,175 cm-l) -+ T(-K-T*) transition (28,238 cm-I). Phosphorescent emission comes from the lowest triplet state, T(n-n*) (26,545 cm-I), after internal conversion T(n--K*) + T(w-n*). Intersystem crossing in pyrazine is sufficiently fast4 to give a triplet yield of almost unity, as determined by the biacetyl method. lo Pyrazine is a relatively stable compound compared to biacetyl, which emits phosphorescence but also dissociates at short wavelengths and at higher temperatures. 1 1 , 1 2 Both isomerization and polymer formation have been found in previous photochemical investigations of pyrazine. l S The quantum yields of these processes are negligibly small in the n-T* band but are of the order of in the P - K * band. (1) Supported by the Robert A. Welch Foundation. (2) (a) L. Goodman and M. Kasha, J . Mol. Specrrosc., 2, 58 (1958); (b) L. Goodman, ibid., 6, 109 (1961). (3) M. J. Cohen and L. Goodman, J . Chem. Phys., 46, 713 ( 1967). (4) M. H . El-Sayed, ibid., 36, 573 (1962); 38, 2834 (1963). (5) J. W. Sidman, Chem. Reu., 58, 689 (1958). (6) R. McWheeny and T. E. Reacock, Proc. Phys. Soc., London, Secr. A , 70, 41 (1957). (7) L. Goodman and R. W. Harrell, J . Chem. Phys., 30, 1131 (1959). ( 8 ) I amount of absorption by pyrazine. No sensitized fluorescence from pyrazine could be observed, although strong bands from the fluorescent emission of benzene were superimposed on the same region as that of pyrazine fluorescence. The benzene-sensitized emission of pyrazine can be described as follows.
+ hv +'B 'B +B + hv
(1)
--+
(3)
B
'B
3B
IB
+ P +B + 'P,
3B
+ P +B + 3p
(2)
3B +B 'P, 3P
(6 )
+x
+P + h v +X
B and P are benzene and pyrazine molecules, respectively, and 'P, is a pyrazine molecule in an upper singlet state in a high vibrational level. It should be noted that 'P, must be the upper state of the T-T* transition. Reactions 7 and 9 represent internal conversion or isomerization. The fluorescent yield of benzene in the presence of pyrazine is
+ + k@))
kz/(k? k , and in the absence of pyrazine Qf
=
Q?
(IO)
+
kz/(kz k3) (1 1) The triplet yield of benzene monitored by the triplet emission of pyrazine is =
where is the yield of reaction 6 and Qp is the emission yield from the triplet state of pyrazine when it is sensi-
Journal ojthe American Chemical Society 1 93:13
(15)
where (15) is the true triplet yield of pure benzene. Since Qp0 = 1.7 X Q p = 0.96 X and Q+/'Qr = 1.23 when benzene at 20 Torr is used in the presence of 0. I Torr of pyrazine. @:
=
(0.96/1.7)(1.23)
=
0.69 k 0.07
(16)
where the limit of error is a rough estimate. Any error in the fluorescent yield of benzene used as a standard is not included. Phosphorescent and fluorescent yields of pyrazine are both very low and this tends to reduce the accuracy. However, since they are nearly equal instead of differing by a factor of nearly 60 as in biacety1,l4 relative values are more easily determined. Agreement of (15) with values reported by Cundall is satisfactory.22 Two other facts must be kept in mind. (a) Reaction 4 is assumed to lead to a T-T* transition in pyrazine and hence to no singlet emission. (b) An Hg-Xe high-pressure lamp was used so that there is no emission between 252.9 and about 258 nm. Thus the triplet yield of benzene should be slightly lower than at 253.6 nm. Systematic errors in Qp tend to cancel since Qf'O, = Qpo. The low phosphorescent yield in the gas phase compared to that in a glassy matrix is of interest. This is not an unusual phenomenon. Benzene, for example, shows no phosphorescence in the gas phase but a strong one in a glassy matrix. A good explanation of these facts is not apparent. Acknowledgment. The author wishes to thank Professor W. A. Noyes, Jr., for reviewing this manuscript and for many helpful suggestions. This work was supported by a grant to W. Albert Noyes, Jr., by the Robert A. Welch Foundation. A grant from the Camille and Henry Dreyfus Foundation was helpful for the purchase of supplies. (22) R. B. Cundall, I
