J . Phys. Chem. 1985, 89, 948-951
948
Lifetimes of Dissociation-Relaxed Triplet States of Pyrazlne and Pyrimidine Joseph Kneet and Philip Johnson* Department of Chemistry, State University of New York, Stony Brook, New York 1 1 794 (Received: October 12, 1984)
The lifetimes of various triplet energy levels of pyrazine and pyrimidine have been measured under collision-freeconditions. The energy levels were prepared via the excitation of van der Waals complexes of these molecules with rare gases or with themselves. Upon intersystem crossing, dissociation occurs, populating triplet states which are no longer isoenergetic to the pumped singlet levels. Excited-state populations were measured by delayed ionization. It is found that in contrast to previous work on aniline a complex dissociation process is taking place whereby the resulting triplet levels are not simply related to van der Waals bond energies.
Introduction It has recently become possible to measure the lifetime of the triplet states of large molecules under collisionless conditions in the environment of a supersonic From these measurements an entirely new set of data is emerging which enables some quantitative tests of radiationless transition theory and ideas of intramolecular energy evolution. A decay curve for a given triplet energy is measured by ionizing the molecule in a two-step process.2 The first step of the ionization populates an excited singlet vibronic level which evolves in time, crossing over isoenergetically into the triplet manifold. The excited-state population is measured by delaying the ionizing laser with respect to the pump laser, and from the decay curve both the singlet and triplet lifetimes are obtained (along with some quantum yields). Hot ground-state population is not measured because of poor Franck-Condon factors. It was found that the lifetimes of the hot triplets populated by intersystem crossing (ISC) are as much as 9 orders of magnitude shorter than the liftimes of relaxed triplet^.'-^-^ There was also a smaller than expected variation in the triplet lifetime over the energy range which is accessible by crossing over from the singlet, indicating the presence of a very dramatic change in the lifetimes at low vibrational energy in the triplet. It is a challenge to theories of radiationless transitions to quantitatively aceount for the variation in the rate of crossing to the ground-state manifold, which controls the lifetime. The triplet states measured by the pump-probe ionization technique are rather special ones, being isoenergetic with a singlet vibronic level which is optically coupled to the ground state. It is not known how much this perturbs the lifetime. They are also quite some distance above the triplet origin, a typical tripletsinglet gap being 6000-8000 cm-l. In a recent study on aniline we were able to probe states which were not isoenergetic to singlet states by taking advantage of the dissociation of van der Waals comp l e x e ~ . In ~ this work, complexes were made in the supersonic expansion which could be selectively excited because of well-defined spectral shifts. When the molecules cross over into the triplet manifold their vibrational energy content increases dramatically and the van der Waals bond is broken as the resulting monomer drops down in vibrational energy by approximately the bond strength. For the aniline-argon complex it was found that the above model is quite adequate, giving triplet lifetimes which were appropriate to a very reasonable bond energy of -275 cm-I. The success of using complex dissociation to probe below the iscenergetic SIorigin in the triplet of aniline suggests the possibility of using a variety of complex partners, with varying bond strengths, to obtain the T So decay rate as a function of TI vibrational excitation below the SI origin. Molecules such as aniline and benzene are not good candidates for this experiment because the SI to TI origin spacings are large (7600 and 8600 cm-I, respectively). Presumably, a prohibitively large amount of relaxation
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‘Present address: Department of Chemistry, California Institute of Technology, Pasadena, CA 91 125.
0022-3654/85/2089-0948$01 .50/0
from the TI states isoenergetic to the SIorigin would be necessary to probe the lower portion of the TI manifold where the steep excess energy dependence of the TI decay rate is expected. The two molecules chosen for this study are pyrazine and pyrimidine whose SI to TI spacings are 4056 and 2543 cm-I, respectively.’ These molecules are classified in the intermediate case for SI TI intersystem crossing. This means that instead of considering the optically prepared SI state to decay irreversibly to a large number of coupled T, states the system is considered as coupling to a discrete set of TI states reversibly. This is a consequence of the larger spin-orbit coupling in these azines as well as a lower density of TI states (due to lower S1-T1 spacing) which prevents the decay from being irreversible. It will be shown that this somewhat different preparatiqn of the TI states can be taken into So decay as a function of excess energy account and that T can be obtained as in the statistical limit molecules, benzene and aniline. There is a fairly extensive literature on the photodissociation of van der Waals complexes as probed by fluorescence. Among aromatic molecules, the most thoroughly studied are tetrazine-Ar,8 aniline-He: and benzene-HeI0 complexes. From this work there is information about binding energies in the first singlet state and about branching ratios in vibrational/dissociational relaxation. These studies concentrated on the lower lying vibronic bands which were not as affected by intrastate vibrational relaxation. It was found that relaxation upon dissociation is not completely selective in the terminating vibration when the density of states is low. Thus it is possible in the present study which concerns molecules with small singlet-triplet gaps that there is a distribution of triplet states following the dissociation in the triplet manifold. In principle this could lead to a distribution of triplet lifetimes being measured in a single experiment and a nonexponential decay curve. All of our measured lifetimes were fairly exponential, but should be considered as a composite of the decays from a distribution of final states fairly close in energy until further information is available about branching ratios.
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Experimental Procedures The experimental design is the same as was used previously for benzene and aniline.5 Briefly, the SIresonant state is first (1) Duncan, M. A.; Dietz, T.G.; Liverman, M. G.; Smalley, R. E. J . Phys. Chem. 1981,85, 7. (2) Dietz, T. G.; Duncan, M. A.; Smalley, R. E. J . Chem. Phys. 1982, 76, 1227. (3) Dietz, T. G.; Duncan, M. A.; Puiu, A. C.; Smalley, R. E. J . Phys. Chem. 1982, 86,4026. (4) Otis, C. E.; Knee,J. L.; Johnson, P. M. J. Chem. Phys. 1983, 78,2091; J . Phys. Chem. 1983,87, 2232. (5) Knee, J. L.; Johnson, P. M. J . Chem. Phys. 1984, 80, 13. (6) Morse, M. D.; Puiu, A. C.; Smalley, R. E. J. Chem. Phys. 1983, 78, 3435. (7) Innes, K. K.; Bryne, J. P.; Ross, I. G. J . Mol. Spectrosc. 1967, 22, 125. (8) Brumbaugh, D. V.; Kenney, J. E.; Levy, D. H. J. Chem. Phys. 1983, 78, 3415. (9) Bernstein, E. R.; Law, K.; Schauer, M. J. Chem. Phys. 1984,80, 207, 634. (10) Stephenson, T. A.; Rice, S. A . J . Chem. Phys. 1984, 81, 1083.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 6,1985 949
Triplet States of Pyrazine and Pyrimidine
PYRIMIDINE
TABLE I: Pyrimidine Triplet State Lifetimes and Decay Rates as a Function of Excess Eneerw in T,
band origin argon complex krypton complex 6a; argon complex krypton complex .1
energy in TI, cm-I 2543 3156 3485 4172 4568
triplet lifetime, ns 910 6300 6430 1096 4730 4690 1076 765 563
I4
decay rates, s-1 x 106 1.10 0.16 0.16 0.91 0.21 0.21 0.93 1.31 1.78
populated with a frequency-doubled pulsed dye laser, and then at a variable delay time the excited-state population is ionized with the 193-nm output of an ArF excimer laser. The 193-nm radiation is of sufficiently short wavelength that in addition to ionizing the SI population it is also able to ionize any molecules in the T, state formed by intersystem crossing from SI.However, when a T So nonradiative transition occurs even 193-nm radiation cannot ionize the extremely vibrationally hot So molecules because of poor Franck-Condon factors, and therefore this decay results in a loss of ion signal. The ions created by the excimer laser are accelerated into a time of flight mass spectrometer. This allows signal collection on individual mass peaks and avoids stray ions formed by the strong ArF excimer laser. The time delay between the two lasers is controlled by a programmable time-delay generator and the actual delay is measured on each shot by a time-to-digital converter. A decay curve of the combined excited singlet- and triplet-state populations is obtained by stepping cyclically through the appropriate time delays while averaging approximately 10 0oO laser shots. Spectral features corresponding to the van der Waals complexes were found by scanning the doubled dye laser while the ion signal was monitored. It was necessary to use both lasers in these scans because the first singlet states (IB3, in pyrazine (DZh)and 'B1in pyrimidine (C,)) are less than half way to the ionization potential so the photons which populate SIcannot subsequently ionize the molecule. Of course in the pump-probe ionization experiment the second photon is the 194-nm probe which can ionize both SI and TI. In this case the lack of ionization from the dye laser alone is a benefit. The conditions necessary to observe maximum complexation with pyrazine and pyrimidine were found after a variety of gas concentrations and pressures were used. For pyrazine, Ar- and Xe-containing complexes were observed, with the Ar conditions being similar to the aniline case. For xenon it was found that 20 psi of a 25:l mixture of He:Xe as a carrier gas yielded the highest percentage of the pyrazine-Xe complex. For pyrimidine, argon and krypton complexes were observed, with the krypton complex maximized by 20 psi of a 25:l Kr:He mixture. It must be emphasized that the pulsed nature of the supersonic beam makes it almost impossible to reproduce exactly a given set of operating conditions because of the temporal evolution of the beam. In addition to these rare-gas complexes several resonances have been assigned to the pyrazine dimer.
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iY
0
3
13-
2000 3500 4000 EXCESS ENERGY T, (cm-'1
Figure 1. Plot of the pyrimidine triplet decay rates as a function of TI
excess energy. The values for the argon and krypton complexes are included.
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Results Pyrimidine. For pyrimidine five vibronic bands have been investigated (origin, sa;, lh, 6ah12;, 12:).l2 In this case no resonance was observed which could be ascribed to the dimer, but complexes with Ar and Kr occurred at the origin and 6aA bands. The results are listed in Table I. The triplet lifetimes for the monomer bands of pyrimidine display a behavior unlike that of other aromatics. As can be seen in Figure 1 the lifetimes actually increase on going from the SI origin to the 6ah band, while the 1; and higher bands show the behavior that is expected. This has been observed previously by Smalley et aL3 and explained by the fact that the density of TI states at the SIorigin (TI origin + 2543 cm-l) is so low that the SI state can easily recur. The SI state fluoresces strongly, which
*=MONOMER x = ARGON 0 : KRYPTON
1
PYRAZINE
WAVELENGTH (NM)
Figure 2. 1 + 1 ionization spectrum of the pyrazine SI origin band
showing the shifted dimer resonances.
TABLE 11: Pyrazine Triplet-State Lifetimes and Decay Rates as a Function of Excess Energy in TI
band origin argon complex xenon complex dimer 6ah argon complex xenon complex dimer 8a;
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energy in T,, cm-I 4036
4639
5439
triplet lifetime, ns 503 840 1156 7240 350 600 970 7240 245
decay rates, s-I
X
lo6
1.99 1.19 0.87 0.14 2.86 1.67 1.03 0.14 4.08
leads to a significant decay channel which competes with the T So nonradiative decay. Thus the overall decay of these triplet So states is faster than the higher lying bands, but the T nonradiative decay is as expected and can even be extrapolated from the values at higher energy. Pyrazine. For this molecule the spectra and lifetimes were recorded in the vicinity of three main vibronic peaks: the origin, 6ah (+583,cm-'), and 8ah (+1383 cm-I). The spectral position of these bands as well as the position and identity of the van der Waals complex resonances are listed in Table 11 along with the measured triplet lifetimes. There are several interesting things to be noted in addition to the triplet lifetimes. One is the appearance of the two bands to the red of the origin (shown in Figure 2) and the 6ah band when pyrazine is expanded in pure helium. They have been determined to be due to the dimer of pyrazine by observing their intensity when the pyrazine concentration is changed. The appearance of two bands is probably due to two possible configurations of the dimer. The lifetimes of the two bands agree, and so the details of the geometry are not critical to the lifetime study.
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950
The Journal of Physical Chemistry, Vol. 89, No. 6, 1985
15.
/ PY R AZINE
ORIDIN
+ 14.
Y
;
1312. IIJ
0
= MONOMER x=ARGON a =XENON 0 =DIMER
0
5000
Figure 3. Plot of the pyrazine decay rates as a function of T, excess energy. The values for the argon and krypton complexes are included.
Another observation is that no complexes were observed at the 8aA level 1383 cm-' above the origin. It appears to be a general phenomenon that bands above the 6: (525 cm-I) in aniline, pyrazine, and pyrimidine have no shifted complex resonances. It is possible that this is above the dissociation of the van der Waals bond in the singlet, but even if dissociation occurred directly in S1 the dissociation products would still be ionized and yield a resonant signal. In fact these resonances and dissociation in SI have been observed in fluoresence by other groups,'b12 and our inability to observe them remains a mystery. The triplet lifetimes of the reported bands were obtained by a nonlinear single exponential fitting program.
Discussion An analysis of the results quickly reveals that the behavior of these azines upon complex dissociation is not as straightforward as in aniline. However, the results for the pyrazine dimer and pyrimidineargon and pyrimidine-krypton complexes suggest that the triplet mainfold has been relaxed past some critical point where a dramatic increase in the lifetime takes place. Pyrimidine. Figure 3 shows a plot of the pyrimidine T So decay rate as a function of excess energy in the triplet, with the points above the S1 origin having been extrapolated to lower energy. If the higher triplet decay rates can be linearly extrapolated, then an estimate for the bond strength of the complex could possibly be obtained by determining the TI energy for which the measured decay rate for the complex falls on the line, There is a dramatic change in the lifetimes of the argon (6300 ns for origin, 4730 ns for 6ah) and krypton (6430 ns at origin, 4690 ns at 6aA) complexes compared to the monomer (1 850 ns for origin, 1300 ns for 6aA- these values are extrapolated from higher bands to account for their fluorescence). Following the above procedure based solely on bond strength, the van der Waals bonds would have to be greater than 2000 cm-I, a highly unlikely possibility. It can then be assumed that a critical point in the triplet has been passed and the lifetime values of the complexes reflect behavior of the lower portion of T1. It is quite possible that even the monomer origin and 6a; bands display this behavior, but it is obscured experimentally by the competing fluorescence decay. If one assumes bond strengths for the complexes based on previous singlet-state experience (Le., 300-600 cm-I for Ar and 400-700 cm-' for Kr), then it can be inferred from the data that the change in the lifetime behavior occurs in the region of 2000-3500-~m-~ excess energy. This corresponds to the region where intramolecular vibrational randomization should occur, the implication of which will be discussed later. The results for pyrimidine show several unexplained peculiarities. The argon and krypton complexes of the same band yield almost the same triplet lifetime (origin-Kr = 6.43 ps, origin-Ar = 6.30 ps, 6aA-Kr = 4.69 ps, 6aA-Ar = 4.73 p s ) . This is counter to the expectation that the krypton-pyrimidine bond should be
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(1 1) Kenney, J. E.; Johnson, K. E.; Sharfin, W.; Levy, D. H. J . Chem. Phys. 1980, 72, 1109. ( 1 2 ) Kenney, J. E.; Russell, T. D.; Levy, D. H. J . Chem. Phys. 1980, 73, 3607.
stronger than that of argon-pyrimidine. (Although little data are available on van der Waals bond strengths, they have always been seen to increase as He:Ne:Ar:Kr:Xe in complexes with aromatics.) Why is the krypton complex triplet not relaxed more in breaking the van der Waals bond, with a subsequently longer triplet lifetime, especially when one would predict a steep dependence on excess energy in this region? The leading possibility is that the complex dissociations of both the Ar and Kr species terminate in the same final state because of the low density of states, with resulting equal lifetimes. However, more information about the bond strengths and the actual dependence of T So decay on excess energy in this lower portion of the triplet would have to be known before a certain explanation could be formulated. Pyrazine. The most interesting result for pyrazine is the observation and triplet-lifetime measurement of dimer resonances at the origin and 6aA bands. The lifetime change is dramatic in going from the monomer origin (503 ns) to the dimer origin (7240 ns) and from the 6aA band (350 ns) to the dimer 6aA (7240 ns). It appears that the situation is analogous to that of pyrimidine in that the triplet has been relaxed past some critical point by the dimer dissociation. The situation, however, is less clear in this case because the dimer binding energy is even more indefinite than that of the rare-gas complexes. Furthermore, there exists the possibility that when the dimer dissociates the pyrazine molecule which is not excited in T, may contain energy in internal degrees of freedom, causing a further and unpredictable relaxation of the other molecule's triplet. A complicating result is that the triplets which result from dissociation of the origin and 6aA bands yield identical lifetimes. Again, as in the pyrimidineargon and pyrimidinekrypton results, this can be explained by assuming the dissociation dynamics are such that both dimer bands have similar final triplet states. Pyrazine also exhibited argon and xenon complexes at the origin and 6ah level. The lifetime of these species exhibited more predictable behavior in that the xenon-containing species of each band yielded a longer lifetime than those of argon. Also the 6aA complex lifetimes were shorter than those at the origin, as expected, unlike the dimer case. However, the problem with the pyrazine-rare-gas complex results is presenting a self-consistent explanation of all the observations. Using Figure 3 as a guide the following alternative analyses can be made: (1) Using the lifetimes to predict the complex bond strengths one would obtain
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6000 EXCESS ENERGY T, (cm-')
4000
Knee and Johnson
6aA argon = 914-cm-l bond origin argon = 993-cm-' bond 6aA xenon = 1826-cm-' bond origin xenon = 1620-cm-' bond (2) If one assumes a bond strength for the argon complexes of about 400 cm-l and plots their lifetimes as a function of excess energy in TI, the result is a line parallel to that of the monomer results but displaced toward lower lifetimes. The same is true for xenon, where a bond strength of 700 cm-I is assumed. These plots are displayed in Figure 3. There are at least four possible explanations for these results, but each one has its complications. (1) The simplest explanation is that the dynamics are straightforward, and indeed the bond strengths are -950 cm-' for argon and -1700 cm-' for xenon. Although simplicity is appealing, such strong van der Waals bonds would be quite unusual in light of data for other similar systems.lW1* (2) It is possible that the dissociations occur with the release of substantial amounts of translational energy. This would make up the energy difference between the actual bond strengths and the amount of relaxation indicated by the triplet-lifetime measurements. Again, previous data1b13argue against such translational energy release, and it is unlikely that xenon would carry away more energy than argon. (13) Ewing, G . E. Faraday Discuss. Chem. SOC.1982, 73, 325
Triplet States of Pyrazine and Pyrimidine (3) The fact that pyrazine is an intermediate-case molecule could explain some of the discrepancy because the triplet states populated in the monomer contain a higher degree of singlet character due to strong spin-orbit coupling in these systems. However when complexes dissociate, the resulting "triplets" are no longer isoenergetic to a singlet state with a resulting decrease in singlet character. Thus part of the lifetime lengthening upon complex dissociation could be due to the change in T -,SOdecay which results from a loss of singlet character in the "triplet" states (because states with more singlet character would couple better to the So manifold). However, one would expect the triplets formed from the two rare-gas complexes to be more similar in lifetime if that were the only effect. (4) Finally, one can use the more realistic bond strengths for the complexes and then assume the longer than expected lifetimes are due to populating the lower portion of the triplet, which exhibits a different excess energy dependence, as assumed for the pyrimidine complexes and the pyrazine dimer. The question then remains, why do the origin and 6aA band results for argon and xenon parallel those of the monomer? If the excess energy dependence is now so much steeper, the line connecting the complex results should have a much larger slope than the monomer results. It is possible that in the relaxation of the triplet certain T -,So nonradiative decay channels, involving efficient promoting modes, have become energetically inaccessible, causing the marked decrease in rate. However, the 583-cm-' change between exciting the origin and pumping the 6aA band must not lead to the opening of any additional efficient decay channels. Thus all complex dissociation leads to an initial decrease in triplet decay rate, but further relaxation, within 583 cm-', exhibits a similar trend in decay behavior to the higher energy region of the triplet. In spite of this puzzling behavior the results for the pyrazine-rare-gas complexes indicate that much less change in the lifetime is taking place in the region of the triplet accessed by the dissociation than is the case in pyrimidine. If we assume a pyrazine-xenon bond strength of 800 cm-', we can put an upper limit on the change of triplet state behavior a t 3300 cm-'. (Excess energy in T, at the S1origin minus the pyrazine-Xe bond strength: 4053 cm-' - 800 cm-' = 3256 cm-I).
Conclusion Looking at the data for both molecules some general conclusions can be made, although we are hampered by the lack of van der
The Journal of Physical Chemistry, Vol. 89, No. 6, 1985 951 Waals bond strengths and knowledge of the details of the dissociation process. Of prime importance is the determination that the region of the triplet where a change in the decay behavior takes place is between roughly 2000- and 3300-cm-' excess energy. As mentioned briefly this is the region in which IVR would be expected to turn on and suggests its possible role in the behavior of the T -,So nonradiative decay. Smalley et a1.6 have made model calculations which show that IVR alone cannot be the explanation of the triplet decay rates dependence on excess vibrational energy. However, theoretical calculations by a number of p e ~ p l e ' ~are . ' ~ continuing on this subject and will hopefully adequately describe the decay behavior of the entire triplet manifold. Finally, the unpredictable lifetimes resulting from a number of the complex dissociations suggests that either the lower levels of the triplet are somehow insensitive to a certain degree of excitation or, more likely, the dissociations are resulting in lower triplet states being formed by relaxation of one quantum of a high-frequency mode. This would explain why pyrimidineargon and pyridine-krypton complexes excited to the origin of S,dissociate in the triplet by the same amount, with the energy in excess of the bond strength being taken away as translational energy of the recoil. While explaining these results this hypothesis has been proven not to be the case in fluorescence studies of complexes of excited vibrational S1states.'*'2 In these studies large frequency vibrations do cause the dissociation, but the energy difference between these vibrations and the complex bond energy appears as population of lower frequency vibrations in S1,not translational recoil energy. The problem then shifts to why the dissociations in the triplet are behaving differently. It is hoped that future results on the dissociation of van der Waals complexes and on the theory of radiationless transitions, aided by these experiments, will enable a better understanding of the behavior of triplet decay and van der Waals dissociation.
Acknowledgment. We thank the National Science Foundation for support of this work. Registry No. Pyrimidine, 289-95-2; pyrazine, 290-37-9; argon, 7440-37-1; krypton, 7439-90-9. (14) Heller, E. J.; Brown, R. C . J . Chem. Phys. 1983, 79, 3336. (15) Hornburger, H.; Schroder, H.; Brand, J. J . Chem. Phys. 1984,80, 3197.
