J . Phys. Chem. 1990,94,4067-4068
4067
Electron Paramagnetic Resonance Studies of Pyrimidinones Yi Lu, Tien-Sung Lin,* and John-Stephen Taylor Department of Chemistry, Washington University, St. Louis, Missouri 63130 (Received: November 13# 1989)
EPR studies of 1,4,6-trimethylpyrimidin-2-ones were reported at 77 K. The EPR signals were attributed to the 3 ~ 7 r * phosphorescent state based on the measured zero-field splittings (D/hc = f0.0977 cm-' and E/hc = 70.0204 f 0.0005cm-I) and the triplet decay lifetime (0.2 s).
Introduction Recently one of us reported the structure and biological significance of TpT3.I Here we report our EPR studies of a simpler (TMPO). pyrimidonone, namely, 1,4,6-trimethylpyrimidin-2-one T M P O was known to undergo photoisomerization to the Dewar structure as indicated in reaction I at wavelengths greater than 300 nm.2 The yield of the Dewar form was reported as high as 33%. However, the Dewar isomer could be reversibly converted back to the original pyrimidinone if the system was irradiated with light at 254 nm. The Dewar isomer was reported to be indefinitely stable at room temperature in the absence of U V irradiation.2
The objectives of our EPR studies of pyrimidinones are 2-fold: ( I ) to investigate the possible formation of free radical and/or biradical intermediates involved in the photoisomerization and ( 2 ) to examine whether an excited triplet state of T M P O is involved in the photoisomerization. Furthermore, the EPR studies enable us to characterize and to determine the paramagnetic properties of these transient species. We performed our EPR experiments on samples containing TMPO in organic solvents and in mixed crystal forms at 77 K. No EPR signals were observed at room temperature.
Experimental Section The pyrimidinone was synthesized, purified, and kindly provided by M. P. Cohrs of this department. The solvents used in rigid glass experiments were octafluorotoluene and benzophenone, and the solid used in the mixed crystal was durene. All chemicals were purchased from Aldrich Chemical Co. Benzophenones and durenes were further zone-refined before use. The solution samples in standard EPR quartz tubings were deoxygenated by freeze-thaw cycles under vacuum. The mixed crystals of T M P O in durene host were grown in a Bridgman furnace. To prepare the melt, 50 mg of T M P O was added to 5 g of durene solids. But the concentration of TMPO in the mixed crystal used in the actual experiment was not known. I n some mixed durene crystals we also introduced 5 mg of naphthalene-d8 in the mixed crystal preparation. Previous EPR experiments showed that the principal axes of naphthalene are parallel to the symmetry axes of the durene molecule in the c r y ~ t a l . We ~ thus used naphthalene as a reference molecule to determine the principal axes of T M P O in the durene crystal. The durene crystal shows a distinct cleavage along the ab plane. The crystallographic axes were identified by the conoscopic technique. Three wedges for mounting crystals were designed according to ref 3. The cleavage faces of the crystals were mounted on the ruled surface of wedges by means of halocarbon ( 1 ) Taylor, J.-S.; Cohrs, M. P. J. Am. Chem. Soc. 1987,109,2834-2835. (2) (a) Nishio, T.; Kato, A.; Omote, Y.; Kashima, C. Tetrahedron Lett. 1978, 1543-1544. (b) Nishio, T.; Kato, A,; Kashima, C.: Omote, Y. J . Chem. Soc., Perkin Trons. I 1980, 607-610. (3) Hutchinson, Jr., C. A.: Mangum, B. W. J . Chem. Phys. 1961,34, 908.
grease. The b axis of the durene crystal was parallel to the ruled lines on the wedge. The experiments were performed at 77 K using a Varian E-4 (X-band) EPR spectrometer. The excitation source was an 150-W xenon lamp. Light modulation and a lock-in amplifier were also employed in recording transient spectra. Band filters were used to isolate the proper wavelength for irradiation (Schott UG-1) (transmission maximum 300 nm). A high-pressure Hg lamp (PEK) was also used in the photochemical recovery experiments.
Results and Discussion We will divide this section into two parts: pyrimidinones in rigid glasses and pyrimidinones in mixed crystals. Pyrimidinones in Rigid Glasses. We have not observed any EPR signals that can be attributed to radicals or biradicals derived from TMPO in the g = 2 region during or after light irradiation at 77 K. This implies there may be no radical or biradical formation in the process of photoisomerization. It may also imply that the intermediate radicals or biradicals are so short-lived and reactive that our spectrometer is not capable of detecting these transient species. However, we observed a triplet EPR signal at 1500 G for T M P O samples. The signals were assigned to Am = f 2 transitions. These signals were light sensitive: they disappeared when the excitation light source was turned off. Thus, these signals must arise from the photoexcited triplet state. The Am = fl transitions in rigid glasses were not observed (within our noise level). However, Am = f 1 transitions were clearly displayed in the mixed crystal studies (see below). The transitions at 1500 G yielded the combined zero-field splittings (zfs) D* = (D2 + 3E2)'/2 where D and E are zfs due to the spin dipolar interaction. The D* value was evaluated from the following equation4
D* = [3/(hv)' - 3(g@Hmi,)2]'/2 where hu is the microwave resonance frequency and Hmin is the minimum resonance magnetic field. The best-fit D* value obtained in octafluorotoluene solvent for TMPO is D * / h c = 0.1043 f 0.0005 cm-'
The D* value is of a typical value for a 317r* state.4 The intensity of the Am = f 2 transition diminished as a function of irradiation time. The signal disappeared eventually after a prolonged irradiation. This is presumably due to the conversion of T M P O to other photoproducts (see below). The lifetime of the photoexcited triplet states of T M P O measured from the decay of Am = f 2 signals is 0.2 s. Pyrimidinones in Mixed Crystals. As mentioned above, the observed EPR signals at 1500 G were attributed to the photoexcited triplet state. These triplet signals should be very anisotropic due to the nature of spin dipolar interaction of an S = 1 system. The spin Hamiltonian used to interpret the observed spectrum is (4) McGlynn, S.P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewocd Cliffs, NJ, 1969; Chapter 10.
0022-3654/90/2094-4067$02.50/00 1990 American Chemical Society
4068
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 ?t = gPHS
+ DS,' + E(S,Z - SY2)
S = 1
(2)
T o measure the zfs accurately and to determine the paramagnetic properties of the corresponding triplet state, we have performed the mixed crystal experiments. EPR spectra have been obtained in three principal planes-xy, yz, and xz planes-of the durene molecule by using the naphthalene triplet signals as references. We have not observed any hyperfine splittings in any orientations at 77 K. We found that the z axis of T M P O (out-of-plane) is parallel to the z axis of the naphthalene molecule; so are the relative guest-host alignments of the two in-plane axes. Here we assume that TMPO is a planar molecule and the three methyl groups of T M P O are aligned parallel to the four methyl groups of the durene molecule. We further assume that a substitutional crystallization takes place in the diluted mixed crystal preparation. The principal values of the photoexcited triplet state of T M P O are evaluated from the stationary resonance fields by the following equation (for HoIl.x):s (3) where H,, and H , , are the high-field and the low-field resonance signals, respectively, and 6 is the microwave frequency. W e have assumed an isotropic g value (=2.0023) in our calculation. The best-fit principal values are
Z / h c = f0.0651
f
0.0003 cm-'
Y / h c = r0.0121
f
0.0003 cm-l
X / h c = -0.0530 f 0.0003 cm-' The corresponding zfs are
D / h c = f0.0977
f
0.0005 cm-l
E / h c = r0.0204
f
0.0005 cm-'
These values are the average of several independent measurements on several different crystal samples. We further calculated the D* value to compare with the result obtained from the rigid glass experiments: D* = (D2 3E2)Il2. W e obtained D* = 0.1039 cm-I which is slightly smaller than that obtained in the rigid glass experiments. The slight difference in D* could arise from the solvent effect. The zfs are of typical values for a 3ar*state of aromatic hydrocarbons; e.g., for naphthalene, D = +0.1012 and E = -0.0141 Also, the values are comparable to the zfs of the lowest triplet state of tetramethylpyrazine: D = f0.0963 and E = r0.0135 ~ m - ' . ~ , The ' observation of the magnetic z axis being
+
( 5 ) Kottis, Ph.; Lefebvre, R. J . Chem. Phys. 1963, 39, 393; 1964, 41, 379. (6) de Groot, M. S.; Hesselmann, I . A. M.; Reinders, F. J . : van der Waals, J . H . Mol. Phys. 1975, 29, 37-48. (7) Antheunis, D. A.; Botter, B. J.: Schmidt, J.; van der Waals, J. H. Mol. Phys. 1975, 29, 49-59.
Lu et al. perpendicular to the molecular plane further supports the aa* state assignment. The involvement of the nonbonding electrons in the carbonyl group and the nitrogen atoms in the a-a* transition is minimal as judged from the magnitude of zfs. This is also consistent with the measured lifetime: 0.2 s, a typical value for a 3aa*state. Two electronic absorption bands have been observed in pyrimidinones: 3 10 and 220 nm. The 3 10-nm band is assigned as the Ina* So transition based on an observation of the spectral blue shift in polar solvents.* The 220-nm band is assigned as the IKK* So transition. However, the nature of the phosphorescent state is unambiguously a 37ra*state based on the magnitude of zfs and the measured lifetime. The reverse order of states in the triplet manifold has also been observed in other nitrogen-containing molecules, such as tetramethylpyrazines. We observed that the triplet EPR signals in the mixed crystals diminished as a function of irradiation time. The intensity reduced to one-half of its original value in about 1 h. The decrease in EPR signals may be due to the photoisomerization of the Dewar structure or other photoproducts. It has been indicated that the Dewar structure can be reversibly converted back to pyrimidinone upon 254-nm irradiation., To test whether the diminish in EPR signal is mainly due to the conversion to the Dewar structure, we carried out the following experiment: we monitored the change in EPR signal intensity of a prolonged irradiated sample subjected to a high-pressure Hg source with proper solution filters to isolate the 254-nm band.9 We observed a recovery of EPR signal intensity upon further sample irradiation with the 254-nm source. Thus, the reversibility of photoisomerization is unambiguously established. However, this further imposes a question: why is the EPR signal of the Dewar triplet not observed? The nonobservance may be due to either its very short lifetime or inaccessible excited states; i.e., the employed excitation light source may not be able to excite the Dewar form to the triplet state effectively. Since the photoconversion takes place upon an irradiation of X > 300 nm, we may infer that the photoconversion takes place = 310 nm) and not the Ira* (Ama = 220 via the Ins* state (A, nm). Thus, the excitation is first to the ha* and then intersystem crossing over the observed )na*. During the excitation process, some of the pyrimidinone is also converted to the Dewar structure which accounts for the diminish in the EPR signal intensity. This is consistent with triplet sensitization experiments that the photoisomerization occurs mainly via an excited singlet channel as reported in ref 2b.
-
-
Acknowledgment. This work was supported partly by the Missouri State Higher Education Research Fund and partly by the donors of the Petroleum Research Fund, administered by the American Chemical Society. (8) Cohrs, M. P. Ph.D. Thesis, Washington University, 1990. (9) Calvert, J. G.; Pitts. Jr., J. N. Photochemistry; Wiley: New York, 1966: p 729.
