ISOMERIZATION OF DIAZINES n--?F* and p a * triplet states could not be calculated due to the lack of accurate values for the phosphorescence and SrT intersystem crossing yields. No conclusion can be drawn concerning the nature of the lowest triplet state from the phosphorescence polarization data due to the wide bandwidths needed to observe the emission. Tentative identification of the lowest triplet states of these molecules as a-a* is consistent with their low yields of photoreduction in alcohol solvents at ambient temperatures.26 The order and nature of the low-lying electronic states of the furanquinones I and I1 are therefore identified as So, TIyX*,Tzn-,*,S1*,*, S2'-,*, . . . in slr-r*, ~ ~ n - , * , nonpolar solvents and So, SZn-s* , . . . in polar solvents. This order appears most
2245 consistent with the observed photophysical and photochemical behavior of these quinones. Further, the latter order is similar to that reported for other substituted p-quinones having low-lying intramolecular charge-transfer state^.^ It is of interest to note that fluorescence and a weak phosphorescence are also observed from single crystals of the furanquinones.26 Acknowledgment. The authors wish to acknowledge discussions on this work with Drs. A. R. Monahan and G. E. Johnson, and computer-programming assistance from A. Wilson. (25) M. S. Walker, M. A. Abkowitz, R. W. Bipelow, and J. H. Sharp, to be published. (26) M. S. Walker, R. L. Miller, C. H. Grifliths, and P. Goldstein, Mol. Cryst. Liq. Cryst., 16,203 (1972).
Mercury Sensitization of the Isomerization of Diazines by F. Lahmani and N. Ivanoff* Laboratoire de Physb-chimie des Ragonnements, Universitt5 de Paris-Sud, Centre d'Orsau, 91 Orsay, France (Received December $0, 1 O Y l ) Publication costs assisted bu Centre National de la Recherche Scientijique
The Hg(6 8P,)-sensitized isomerization of pyrimidine, 5-methylpyrimidine, 4,6-dimethylpyrimidine, pyraaine, and 2,5-dimethylpyrazine has been investigated; the quantum yields of the sensitized isomerization are respectively (3.8 f 0.4) X lod2, (5.2 i 0.5) X lo+, (3.6 f 0.4) X (2.08 f 0.2) X loF2,and (2.1 f 0.2) X 10-2. Quenching rate constants of the mercury *Pi atom by pyrimidine have been determined by physical and chemical methods; the value of k, found is 9 X 10'0 l./(mol sec). The addition of argon decreases the quantum yield of sensitized isomerization in the same way as that of direct isomerization. The phosphorescence of biacetyl sensitized by pyrimidine alone is low : it is increased in the presence of mercury and this effect is enhanced by the additionif argon. These results suggest that a high triplet state can be involved in the photoisomerization of diazines.
Introduction The photochemical formation of unstable isomers leading to a shift of ring atoms seems to be a general process in the photochemistry of benzene compounds. We have found that an analogous reaction takes place in the case of the diazines,l i.e.
N
citation and is about 100 times higher at 253.7 nm (excitation of the second excited singlet state ST,*) than a t 313 nm (excitation of the first excited singlet state Sn,*). (2) The formation of the isomer is in competition with the collisional deactivation of a vibrationally excited species; in the case of the excitation of pyrazine *, absorption band the quantum yield of isomer in the S decreases when the pressure of an added inert gas increases while the phosphorescence Tn,*,-+ Soof pyrazine increases without any change in the fluorescence. As the lifetime of the S*, singlet state can be sup-
gasphase
A previous investigation2 on the mechanism of this reaction has shown the following points. (1) The quantum yield of isomer depends on the wavelength of ex-
(1) F. Lahmani and N. Ivanoff, Tetrahedron Lett., 3913 (1967). (2) M. Magat, N. Ivanoff, F. Lahmani, and M. P. Pilleni, paper
presented at the 208me Reunion de la Societe de Chimie Physique, Paris, May 27, 1969; F. Lahmani, These, Facut16 des Sciences d'Orsay, Universite de Paris, 1970. The Journal of Physical Chemistry, Vol. 76,No. 16,1972
F. LAHMANI AND N. IVANOFF
2246 Swmor
Figure 1. Schematic energy level diagram for diazine photoisomerization,
posed to be shorter than the time of collisions in the pressure range used, it was concluded from this study that the process competing with isomerization could be the vibrational relaxation of a highly excited triplet state according to the energy diagram of Figure 1. I n order to elucidate further the mechanism of the reaction, we have performed experiments on the mercury sensitization of diazines.
Experimental Section Pyrazine, pyrimidine, and other diazines (Aldrich or Koch Light) were dried over BaO and distilled in vucuo. Argon, NzO, butane, and propane were obtained from Air Liquide. Benzene (Prolabo RP) was purified by recrystallization, and biacetyl (Fluka) , by fractional distillation. Mercury Sensitization. Silica cells (Supracil; optical path 1 cm; volume -10 ml; closed with a Teflon valve) were filled using a grease-free vacuum system. Mercury vapor was supplied by a small mercury droplet contained in the cell All photolyses were carried out a t room temperature using a low-pressure mercury resonance lamp (Claude Paz & Visseaux) and a chlorine filter. The relative light intensity absorbed by mercury was measured with a Zeiss spectrophotometer, the deuterium lamp being replaced by a low-pressure mercury lamp. This provided a relative means of measuring reproducibility of light absorption and permitted us to correct the effect of pressure broadening of the absorption. The absolute measurement of light intensity was achieved either by using cis-trans isomerization of 2butene3 or by measuring the yield of nitrogen formed in the mercury-sensitized decomposition of nitrous oxiden-but ane mixtures. I n the “physical” quenching experiments the cells were filled under a vacuum system with mercury vapor from a reservoir maintained at 0” and the experimental arrangement used a Zeiss spectrofluorimeter. Analysis. Following the photolysis, the sample cell was connected to an aliquot of water cooled to liquid nitrogen temperature. The sample cell and the aliquot were isolated by a valve. The aliquot was evacuated to Torr using a standard vacuum system. After evacuation the pumps were valved off from the aliquot and the valve to the sample cell was opened, allowing the products of the photolysis to be collected on the The Journal of Physical Chemistry, Vol. 76, No. 16, 1972
aliquot of refrigerated water. The vessel containing the mixture of water and products was then sealed. Under these conditions all the contents of the sample cell, except polymer, were collected in the water. After warming and agitating, the cell was opened and the sample was analyzed by gas chromatography on a Perkin-Elmer F6 apparatus using a flame ionization detector with a Carbowax 1500 column. The identification of the products of photolysis has been previously done by trapping the new products after chromatographic separation and by comparing the uv absorption spectra of the eluted products with that of an authentic sample.2 The quantitative analysis was performed by reference to standard aqueous solutions of the diazines at concentrations in the same order of magnitude as that of the sample to be analyzed, the standard solutions being injected just before and after the sample of the photolysis products.
Results When pyrimidine was irradiated at 253.7 nm in the presence of mercury under conditions such that no direct isomerization could be detected, the formation of pyrazine was easily observed. The mercury-sensitized isomerization is also observed in the case of all the diazines that have been proved to isomerize in direct photolysis at the same wavelength (Table I). I n the case of 2,5-dimethylpyrazine the same isomeric pyrimidines are obtained in the same ratio for mercury sensitization as for the direct photolysis. The quantum yields of mercury-sensitized isomerization are higher in all cases than those obtained in direct photolysis, as shown in Table I, while the ratio @isomer/ @polymer 5= 0.2 seems to be the same. To show that isomerization occurs via the mercurysensitized pathway, we have determined the quenching rate constants of Hg(3Pl) excited atoms by pyrimidine, by the following physical and chemical methods. (a) The intensity decrease of the resonance emission of Hg(3P1) atoms was followed as a function of pyrimidine pressure. The experimental data were found to be consistent with the Stern-Volmer equation modified by Yang5 (Figure 2) &o/(&o
-
&I
=
CY.
+ PW1-l
Qoand Q are the intensities of the resonance emission in the absence and presence of [MI mol/l of pyrimidine; OL and are experimental parameters whose ratio a/@ = &,; k , is the quenching rate constant of Hg(3P1) by pyrimidine; and 7 is the mean lifetime of mercury atoms. (3) R. B. Cundall and T. F. Palmer, Trans. Faraday Soc., 1211 (1960). (4) R. J. Cvetanovic, J . Chem. Phys., 23, 1208 (1955). ( 5 ) K. Yang, J . Amer. Chem. Soc., 88, 4575 (1966).
ISOMERIZATION OF DIAZINES
2247
Table I : Quantum Yields of Mercury-Sensitized and Direct Isomerization of Diazines Initial oompd
Isomer formed
Pyrimidine 4,&Dimethylpyrimidine 5-Methylpyrimidine Pyrazine 2,5-Dimethylpyrazine
Pyrazine 2,5-Dimethylpyrazine 2-Methylpyrazine Pyrimidine 4,6-Dimethylpyrimidine 2,5-Dimethylpyrimidine
*
%ma
direot isom
(at 253.7 nm)
isom
( 3 . 8 i:0.4) x 10-2 (3.6 4 0.4) x 10-2 (5.2 f 0.5) x (2.08 4 0.2) x 10-2 (7 i:0.7) x 10-3 (1.4 i:0.15) x
x 10-3 (4.6 i: 0.4) X (7 =k 0.7) x (7 0 . 7 ) x 10-3 (1.5 A 0.15) x 10-3 (3 i:0.3) x 10-3 ( 1 . 7 4 0.1)
*
e
I
b$(ToPP-’)
1
5
3
Figure 2. Quenching of Hg( 3P1)resonance radiation by diazines, Qo/(Qo - Q) against the reciprocal pressure of diazine: *, pyrimidine; 0 , pyrazine; ., 2,5-dimethylpyrazine ; , water.
-._
Figure 3. (A) (--) plot of ~ / @ ( N Iagainst ) the pyrimidine to nitrous oxide ratio; (B) (- -) plot of l/@(pyrazine) x 2 X againt the ratio of the mixture of nitrous oxide and plot of l/@(pyrazine) x butane to pyrimidine; (C) (-A-) 2 X against the propane to pyrimidine ratio.
-
The values of a and P are, respectively, 1.284 and 0.255;4 the ratio [C4H1o]/[NzO]is 0.5 under our experimental conditions, while the value of y (l/@pyr&zine in the absence of added compounds) is close to 25 (Table I); P’ = J C C ~ N ~ N ~ / ~ N is ~ O the ratio of quenching efficiencies of the mercury a P state ~ by pyrimidine and nitrous oxide; the partial pressures of pyrimidine, butane, and NzO are, respectively, 6-8) 2.5-10, and 5-20 Torr. The values of P’ obtained from the slopes of plots of 1 / @and ~ ~l/@pyraeine are, respectively, 0.520 and 0.850 (Figure 3); assuming ~ N to ~ Obe 14 X loio l./(mol sec),6 the relative value of k p y r i m i d i n e would be 7.2 X loio or 11 X 1Olo l./(mol sec). (In this calculation, we have neglected collisional deactivation of excited pyrimidine molecules by foreign gases. In fact, the value of P’ would be even larger.) Considering that the 1 / @ ~=, a(1 -t P[C4HioI/[PJzO] P ’ [ C ~ H ~ N Z I / / N ~ O Idiscrepancy ) in the values of S’ arises from the complexity of the ternary system and analytical difficulties, llapyrazine = Y{ 1 ~/P’([PJ~OI
The experimental value of &,(pyrimidine) was compared to the value obtained in the same geometrical arrangement with a standard molecule (H2O) for which the quenching rate constant is knowna (Figure 2). From the slope of the Stern-Volmer plot, one deduces k,(pyrimidine) ‘v 12 X 1O1O l./(mol sec), the corresponding cross section being 11 A2. Some experiments done with pyrazine and with 2,5-dimethylpyrazine have shown that the cross sections of these compounds approach that of pyrimidine. (b) On the other hand, we have studied the mercury photosensitization of a ternary mixture of nitrous oxide, butane, and pyrimidine, according to the method of C v e t a n ~ v i c . ~On the basis of the mechanism described by this author, the reciprocals of the quantum yields of nitrogen and pyrazine produced are
+
+ +
(6)
R. J. Cvetanovic, Progr. React. Kinet., 2 ,
66 (1964).
The Journal of Physical Chemistry, Vol. 76, No. 16,1978
2248
F. LAHMANI AND N. IVANOFF
i@
I
I
5
4
3
2
1
Figure 4. For the sake of comparison, Stern-Volmer plots of the quenching of isomerization by argon, in the case of direct excitation at 253.7 and 300 nm and for mercury sensitization: __, mercury-sensitized isomerization of pyrimidine (a); , mercury-sensitized isomerization of pyrazine (b); , isomerization of pyrazine at 253.7 nm (direct irradiationz) (c); - - -, isomerization of pyrazine at 310 nmz (direct irradiationz) (d).
-.-.-.---_-___
an additional investigation was performed with a binary mixture of propane and pyrimidine. The value of kpyrimidine found was 9 X lolo l./(mol sec) (Figure 3). The quenching rate constants obtained by physical and chemical methods are in relatively good agreement, especially in view of the uncertainty of the absolute values of kp for the standard molecule, and show that under our experimental conditions, the isomerization of pyrimidine to pyrazine in the presence of mercury at 253.7 nm may involve sensitization by excited mercury atoms. Effect of Argon. When mercury sensitization of pyrimidine or pyrazine is performed in the presence of increasing pressures of argon, a decrease in formation of isomer is observed. This in turn leads to a SternVolmer linear relationship between l / @ d l a a i n e isomer and argon pressure (Figure 4). Argon does not quench excited mercury atoms and this effect observed can only
The Journal of Physical Chemistry, Vol. 76, N o . 16, 1979
be explained by the collisional deactivation of the excited diazine molecule. A similar effect was obtained in direct photolysis. However the efficiency of argon in reducing isomer formation is greater in the mercurysensitized photolysis than in the direct one at the same wavelength. This shows that Hg(6 3P1) does not transfer all its energy to diazine and is in agreement with the results of Montague and Rowland for the mercury-sensitized decomposition of cyclobutanone,I where the average energy transferred from mercuryexcited atoms has been evaluated to 105 lrcal/mol. Effect of Biacetyl. Since pyrimidine does not emit phosphorescence in the gas phase, the presence of a triplet state of pyrimidine can be detected by the addition of biacetyl and the observation of the sensitized phosphorescence of biacetyl. When biacetyl (0.6 Torr) is added to pyrimidine (1.5 Torr), the sensitized phosphorescence of biacetyl is very low for the excitation a t 253.7 nm. I n the presence of mercury, the sensitized phosphorescence of biacetyl is strongly increased and the effect is enhanced by the addition of argon. No phosphorescence of biacetyl is observed for mercury and biacetyl alone.8 This experiment seems to indicate the presence of an excited triplet state of pyrimidine arising from mercury 3P1energy transfer, and the increase of sensitized phosphorescence of biacetyl with pressure of added argon can be related to the decrease of isomer formation.
Conclusion The results converge to show that Hg(3Pl) atoms can produce via triplet-triplet energy transfer an excited triplet state of the diazine from which isomerization of the ring can occur. (0-Xylene has been shown to behave differently from diazine: Hg(3Pl) atoms are inefficient in sensitizing the formation of rn- or p-xylene.) Attempts to sensitize the reaction from triplet benzene (ET = 85 kcal/mol) or Cd(3Pl) atoms (ET = 87.5 kcal/mol) have failed, showing that the reaction of isomerization is energy dependent. Finally it can be pointed out that the role of a highly excited triplet state in the reaction of isomerization of diazine does not exclude a possible role of hot ground-state molecule in a subsequent step. (7) D. C. Montague and F. S. Rowland, J . Amer. Chem. SOC.,91, 7230 (1969). (8) A. G. Harrison and F. P. Lossing, Can. J . Chem., 37, 1478 (1959); H. Ishikawa and W. A. Noyes, Jr., J . Chem. Phus., 37, 683 (1962).
