4026
J. Phys. Chem. 1982, 86, 4026-4029
Pyrazine and Pyrlmidine Triplet Decay in a Supersonic Beam T. G. Dletz,+ M. A. Duncan,$A. C. Pulu, and R. E. Smalley' Rice Quantum Instkute and Department of Chemlstty, Rice University, Houston, Texas 77251 (Received: May 13, 1982; In Final Form: June 16, 1982)
Photoionization has been used to measure the triplet lifetimes of collision-freepyrazine and pyrimidine in a supersonic beam as a function of excitation energy, E,. The decay rates for pyrazine were found to vary smoothly from 3.4 X lo6 s-' at E, = 4056 cm-' (corresponding to excitation at the S1 So origin) to 3 X lo7 s-' at E, = 10 774 cm-'. For pyrazine, IzT varied smoothly between -1 X lo6 s-' near E, = 2543 cm-' to 3 X lo7 s-l at E, = 9054 cm-'. These fast but weakly E,-dependent triplet decay rates contrast sharply with the slow but abruptly increasing decay rates known to apply to triplet states of these molecules under thermal gas-phase conditions.
-
Introduction With photoionization techniques it is now relatively straightforward to sensitively monitor the collision-free formation and decay of organic triplet states in a molecular beam. Thus far only two molecules have been examined by this technique (benzene' and toluene2 ), but the results the triplet have already been rather surprising. First of d, decay rates, kT, are found to be exceedingly fast. For the vibrationally hot triplet, Tt, formed by intersystem crossing from the origin of S1in toluene, for example, the decay rate is found to be 3.5 X lo5 s-', which is over 6 orders of magnitude faster than the known decay rate of cold toluene T1phosphorescence in a crystal. More remarkable was the fact that this fast decay rate showed very little tendency to speed up with furtehr increase in vibrational energy in T'. In toluene the triplet decay rate apparently increases by over 6 orders of magnitude as the vibrational energy, E,, ranges from 0 to 8600 cm-', but then only increases by a factor of 1.5 over the next thousand cm-'. Unfortunately, for both benzene and toluene higher excitation energies could not be attained due to laser limitations; but in the energy range that was attainable, the triplet decay rate dependence on vibrational energy (kT vs. E,) was dramatically not the simple exponential function predicted by elementary This paper extends the photoionization measurements of triplet decay rates to two other molecules which are similar to the benzenes in that they are also monocyclic aromatics with high triplet quantum yield^,^ but which differ markedly from the benzenes in the strength and mechanism of intersystem crossing, the size of the S1-Tl splitting, and the energy range over which triplet decay rates may be measured.* The results of these measurements indicate that, in spite of these substantial differences in the S1 T1 So processes between the benzenes and the azines, remarkably similar (and highly nonexponential) behavior is found in the energy dependence of the triplet decay rate.
benzene' and toluene2 and the reader is referred to these papers for more detailed description of the apparatus and technique. A pulsed supersonic beam was prepared by passing helium over crystals of the azine and expanding through a pulsed nozzle6 (reservoir conditions: Po = 4.5 atm, To= 300 K; orifice diameter: (0.1 cm). For a wide variety of molecules these expansion conditions are known to be sufficient to attain rotational cooling to Trot< 1 K and vibrational cooling such that less than 0.1% of the molecules in the beam have any residual vibrational excitation.68 The pulsed supersonic free jet was skimmed and the collimated beam thereby produced was passed through a differential pumping chamber into the detection chamber of a flow-through time-of-flight mass spectrometer (TOFMS) which was maintained at a vacuum of torr by a 15-cm diameter diffusion pump and liquid-nitrogen-cooled cryobaffle. Two lasers were used to produce photoion signal. The first (the pump laser) was a Nd:YAG-pumped dye laser doubled into the ultraviolet and tuned to resonance with So absorption the desired vibronic band of the S1 spectrum of the azine. This pump laser beam was directed down the molecular beam so as to produce a fairly uniform excitation of S1states all along the full 1.2-m length of the supersonic beam apparatus. The second laser (the probe) was an ArF excimer laser (1930 A) which crossed the supersonic beam at a right angle in the ionization region of the TOFMS. The photon energy of this excimer laser corresponds to 6.4 eV and therefore any electronically excited state of the jet-cooled azine lying within 6.4 eV of the ionization potential was efficiently ionized by the probe. For pyrazinegand pyrimidinelothe first ionization potentials lie at 9.22 and 9.32 eV, respectively, measured from the ground electronic state, So. The lowest excited triplet state of pyrazine, T1, lies at 3.33 eV" above So, and the lowest excited singlet, S1,lies at 3.83 eV.12 Both S1
Experimental Section Photoionization measurements of the triplet lifetimes were performed as described in earlier studies with
(7) Dietz, T. G.; Duncan, M. A.; Liverman, M. G.; Smalley, R.E. J. Chem. Phys. 1980, 73, 4816. (8) Beck, S. M.; Liverman, M. G.; Monts, D. L.; Smalley, R. E. J. Chem. Phys. 1979, 70, 1062. (9) Fridh, C.; Asbrink, L.; Jonsson, B. 0.;Lindholm, E. Znt. J. Mass Spectrom. Ion Phys. 1972,8, 101. (10) &brink, L.; Fridh, C.; Jonason, B. 0.;Lindholm, E. Int. J. Mass Spectrom. Ion Phys. 1972,8, 215. (11) Tumer, R.E.; Vaida, V.; Molini, C. A.; Berg, J. 0.;Parker, D. H. Chem. Phys. 1977,28,47. (12) Innes, K. K.; Byme, J. P.; Ross,I. G. J.Mol. Spectrosc. 1967,22, 125. (13) Hochstrasser, R.M.; Marzzacco, C. 3. J. Mol. Spectrosc. 1972,42, 75. (14) Hopkins, J. B.; Powers, D. E.; Smalley, R.E. J.Phys. Chem. 1981, 85, 3739. (15) Dietz, T. G.; Duncan, M. A.; Smalley, R.E., unpublished results.
-- --
(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) (a) Freed, K. F. Adu. Chem. Phys. 1980,42, 207. (b) Freed, K. F. Chem. Phys. Lett. 1976,42,600. (c) Freed, K. F. Top. AppZ. Phys. 1976, 15, 24. (d) Prais, M. C.; Heller, D. F.; Freed, K. F. Chem. Phys. Lett. 1974, 6, 331. (4) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77, 793. (5) Knight, A. E. W.; Parmenter, C. S. Chem. Phys. 1976, 15, 85. 0022-365418212086-4026$0 1.25101
-
(6) Liverman, M. G.; Beck, S. M.; Smalley, R. E. J. Chem. Phys. 1979,
70, 192.
0 1982 American Chemical Society
The Journal of Physical Chemisry, Voi. 86, No. 20, 1982 4027
Pyrazine and PyrlmMine Triplet Decay
and T1states of pyrazine are therefore easily photoionized by absorption of a single 6.4-eV ArF excimer laser photon, while ionization of So would require sequential absorption of two 6.4-eV photons passing through a rapidly predissociating intermediate level. The same situation applies to pyrimidine where the S1and T1states lie at 3.85 eV12 and near 3.55 eV,24respectively. For both azines the ArF photoionization laser efficiently probes the S1+ T1population in the supersonic beam. In order to obtain decay curves for the S1+ Tl population, we scanned the delay between the S1 So pump laser and the ionization probe laser under computer control using an Evans Associates Model 4145-2 digital delay generator.l Photoion signal from both azines was observed only in the C4N2H4+parent ion mass channel and this signal was recorded as the pump-probe delay was repeatedly scanned over the relevant time interval for S1 T1 So decay. As had been noted previously in our work on benzene1J4 and toluene? it was essential to minimize the content of van der Waals dimers and higher clusters in the supersonic azine beam. Photoions are not observed from Somonomer species because the ionization process must step on a rapidly prediasociating state at 6.4 eV above So. However, this interfor a van der Waals dimer such as (C4N2H4)2 mediate state can now undergo an efficient predissociation to form two monomers, one of which remains in an excited but stabilized electronic state. This excited monomer then can absorb another 6.4-eV ArF laser photon to generate a C4N21-&+photoion. Unfortunately for this study, van der Waals dimers of aromatic molecules are very efficiently formed in a pulsed supersonic expan~ion.'~In order to prevent this, we had to use very low concentrations of the azine in the helium carrier gas (0.3) when excited at the 0 : band: Corresponding to a very substantial singlet character in the typical mixed eigenstate. This extra radiative contribution to the overall excited-state decay accounts for the f i t two
-f
*
-
--
J. Phys. Chem. 1982, 86, 4029-4033
points (due to 0; and 64excitation, respectively) falling above the smooth monotonic curve connecting the other decay time measurements in Figure 4. For both pyrazine and pyrimidine, then, the smooth, monotonic line drawn in the figures is a fairly good measure of the triplet decay So$process varying rate, kT, corresponding to the Tf as a function of energy, E,, in the triplet manifold. Also plotted in Figures 1 and 2 are the known triplet decay rates for the azines in cryogenic crystals23and from room-temperture static gas-phase measurements by phosphore~cence'~~,2~ and biacetyl s e n s i t i z a t i ~ n . Note ~~ how a smooth curve connecting all results for kT vs. E, shows the same deviation from a simple exponential dependence observed previously for toluene.2 Apparently in these azines as well as the alkylbenzenes, the triplet decay rate climbs drastically within the first few thousand cm-l of vibrational excitation in the triplet manifold and then begins to level out. This sharp negative curvature of log kT vs. E, plots is quite unexpected by conventional radiationless transition theory. The initial rapid rise of kT with E, has long been known from bulb but never fully ex-
--
(25) Aizawa, K.; Igarashi, H.; Kaya, K. Chem. Phys. 1977,23, 273.
4029
plained. We now find that at least in toluene, pyrazine, and pyrimidine this rapid increase in kT does not continue forever but rapidly levels out at a value in the range of 106-108ns-l. Subsequent increments in vibrational energy have very little effect. A common explanation for such behavior has been the onset of facile intramolecular vibrational randomization (IVR)within the triplet manifold? However, model calculations currently in progressn reveal that, while IVR is necessary, it is not sufficient to explain such a sharp curvature in log kT vs. E, plots. The correct explanation lies elsewhere. Further studies are in progress on a wide variety of other molecules including benzoquinone, naphthalene, and pyridine. Initial results show this same curvature to apply in all cases. Whatever the correct explanation, it must apply to an increasingly large and disparate group of organic triplet states.
Acknowledgment. Acknowledgment is also made to the donors of the Petroleum Research Fund, administrated by the American Chemical Society, for partial support of this research, and to the National Science Foundation and The Robert A. Welch Foundation for additional support. (26) Holtzclaw, K.N.;Schuh, M. D. Chem. Phys. 1981,515,219. (27) Morse, M. D.;Puiu, A. C.; Smalley, R. E. J.Chem. Phys., in press.
Mechanism for Quenching of Triplet-State Alkylbenzenes by O2 in the Vapor Phase Laurie S. Bumgarner, Merlyn D. Schuh,' and Mark P. Thomas Ewpartment of Chsmlsw, Davmson College, Davidson, North Carolina 28036 (Received: June 21, 1982)
Rate constants, ranging between 1.2 X 1O'O and 2.9 X 1O1O M-' s-l, for quenching of the lowest 31r,1r* state of benzene, benzene-d6,and alkylbenzenes by oxygen in the vapor phase have been measured with a flash-sensitization technique. In general, the rate constants increase with decreasing difference in energy between the ionization potential and triplebstate energy of the alkylbenzene. This behavior is similar to that of singlet-state alkylbenzene vapors and triplet-state ketones and aldehydes in the vapor phase and is consistent with coupling of the initial complex to a charge-transfer state. Evidence is presented that indicates the existence of different mechanisms for quenching of alkylbenzenea and polycyclic aromatic hydrocarbons. It is proposed that O2interacts best on the face and not the edge of the benzene ring.
Introduction Oxygen may quench triplet states by intermolecular transfer of electronic energy (et) and/or enhancement of intersystem crossing (isc). Both processes may involve the formation of a complex with singlet, triplet, or quintet spin multiplicity. However, spin selection rules disallow relaxation of the quintet-state complex, and processes 1and 2 are expected to be most important. T and So are the
(3)
triplet and ground states of the organic molecule, respectively. In the vapor phase the collision partners in reacl13kc, tions 1-3 form the complex with rates given by and 5/gkc, respectively, where k , is the hard-sphere collisional rate constant, and 1/3,and 6/9 are the statistical
probabilities for formation of a complex with singlet, triplet, and quintet spin multiplicity, respectively. The collision complex dissociates with a rate constant k , to form reactants. Since the energy gap between T and So states is greater than the gap between the T state of the donor molecule and excited electronic state of 02,the Franck-Condon factor is expected to be larger for reaction 1than for reaction 2, provided that the geometries of the '(T.-.O2)* and 3(T-..0z)*complexes are similar, and reaction 1is expected to be dominant. With few exceptions,lt2this expectation is realized in many solution-phase experiments in which the quenching rate constant, k,, is generally less than kd/9 (where kd is the rate constant for diffusional ont tact)^" (1) Saltiel, J. L.; Thomas, B. Chem. Phys. Lett 1976,37, 147. (2) Safarzadeh, A.; Condviston,D. A.; Verrall, R. E.; Steer, R. P. Chem. Phys. Lett. 1981,77,99. (3) Merkel, P.B.;Kearns, D. J. Chem. Phys. 1973,58,398. (4) Morina, V. F.;Sveshnikova, E. B. Opt. Spectrosc. (Engl. Transl.) 1973.34. ~. . ., - - , 359. ( 5 ) Gijzeman, 0. L. J.; Kaufman, F.; Porter, G. J. Chem. Soc., Faraday Trans. 2 1973,69,708.
0022-3654/82/2Q86-4029~Q1.25/Q0 1982 American Chemical Society
