J. Phys. Chem. 1982, 86, 4029-4033
64
points (due to 0; and excitation, 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
4030
The Journal of Physical Chemisfty, Vol. 86, No. 20, 1982
Bumgarner et ai.
the biacetyl pressures ranged between 1.0 and 3.0 mtorr. Oxygen pressures were within the range 5.0 and 30 mtorr. Kinetic Model. The following mechanism, which is essentially the same as that found to be operative in previous work,*12 is consistent with our experimental results and will be discussed only briefly here:
A
+ hu
'A
A
-
-
'A
I
I
5
10
I
15
1
1
20
25
'A
( 0 2 ) i n mtorr
Flgure 1. Plots of I-' vs. pressure of 02.The top and bottom curves are for 1,2,4,5-tetramethyIbenzene and benzene, respectively.
and decreases with increasing energy of triplet 7r,7r* aromatic molecule^.^ In contrast, values of 12, for some triplet-state aromatic amines and ketones in benzene solution, which are larger than the values in ref 5, increase with increasing tripletstate energy.6 The higher values have been attributed to efficient coupling of the excited-state complex to low-lying triplet charge-transfer states.6 The only systematic study of 3n,7r* states in the vapor phase reported that for ketones and aldehydes k, increases with increase in the quantity l/(I-ETJ2,where I and ET, are the ionization potential and the triplet-state energy, respectively. This trend is consistent with coupling of the excited-state complex to a charge-transfer state. Quenching by O2 of alkylbenzenes has not yet been studied systematically. Furthermore, it is not known which molecular factors-such as nature of the excited state (37r,7r* or 3n,7r*), nature of the chromophore, or phase dependence-are responsible for the existence of the two quenching mechanisms for 02.In order to better understand these molecular factors and in view of the importance of alkylbenzenes in photophysics, we have used a flash-sensitized biacetyl phosphorescence technique, developed by Parmenter and Ring! to determine k, for 37r,7r* alkylbenzenes.
Experimental Section Chemicals. C6D6was obtained from Merck and Co. C6H6and the alkylbenzenes were obtained from Matheson Coleman and Bell or Aldrich Chemical, and, with the exception of 1,2,3-trimethylbenzene,all chemicals had stated purities of 97 % or higher and were used without further purification after three or four deoxygenations. These chemical purities and preparations of samples have previously been found to be adequate.%12 Research-grade oxygen was obtained from Airco Gases. Equipment and Methods. The equipment was the same as that described previously? and the only change in method was the use of different gas pressures. Alkylbenzene pressures ranged between 0.3 and 3.0 torr, and (6) Garner, A,; Wilkinson, F. Chem. Phys. Lett. 1977, 45, 432. (7) Cebul, F. A.; Kirk, K. A.; Lupo, D. W.; Pittenger, L. M.; Schuh, M. D.; Williams, I. R.; Winston, G. C. J. Am. Chem. SOC. 1980,102, 5656. (8) Parmenter, C. S.;Ring, B. L. J . Chem. Phys. 1967,46, 1998. (9) Holtzclaw, K. W.; Schuh, M. D. Chem. Phys. 1981,56, 219. (IO) Schuh, M.D. J.Phys. Chem. 1978,82, 1861. (11) Burke, J. M.; Schuh, M. D.; Sporborg, H. M.; J. Chem. Phys. 1975, 63, 3567. (12) Schuh, M.D.;Sporborg, H. M.; Williams, K. I. Chem. Phys. Lett. 1974, 26,541.
'A
(5)
3A
(6)
A
(7)
2A
A
O2
3A
3A
A
B
3A
-+
A
'BII
(9)
O2
(10)
2A
02*
A
O2
(8)
3B
A
B
3B
3B
+ hvf
B
3A 3A
(4)
A
'A
I
'A
+ + - + + - + + + + + - + -+ + - + 'A
L
-
hv,
3B
B
O2
B
02*
(11) (12)
(13) (14) (15) (16)
(17)
B and A represent biacetyl and alkylbenzene, respectively. Superscripts denote spin multiplicites, and the II subscript on B denotes the second excited singlet state. k has been argued in our previous work,*12 vibrational relaxation within the triplet-state manifolds of biacetyl and alkylbenzenes is complete at the pressures of our experiments. The small number of 'BII molecules formed at the low biacetyl pressure appear to decay photochemically and to have no effect on the reaction scheme.13 The formation and the decay of singlet-state alkylbenzenes closely parallel the flash intensity vs. time profile and lead to maximum triplet-state concentration shortly after the flash has expired. The shape of the phosphorescence vs. time profile arises from a competition between formation of triplet-state biacetyl, which dominates at short times and leads to increasing intensity, and decay of triplet-state biacetyl, which occurs exponentially at long times with a lifetime 7p. T, y d t,, the time when phosphorescence reaches its maximum, are the two experimental parameters. The rate equations for 'A, 3A, and 3B, which are expressed in terms of eq 18 and 19, are solved analytically T-'
= k1l 7p-1
+ kiz(B) + k13(A) + k,,(Oz) =
k15
+
k16
+ k17(02)
(18) (19)
-'
for the concentration of 3B. The experimental value of T and an arbitrary value for 7-l are substituted into t i e integrated rate equation for (3B),which is plotted vs. time to yield a value oft,. The use of the same value for 7p-1 and other arbitrary values for T-' in the same equation which are plotted vs. T-'. The produces more values oft,, same procedure, but with different values for T;', is used to generate a family of t , vs. T-' curves, one for each value of 7;'. The average of five experimental values for t,, per run is used with this family of curves to evaluate 7-l (13) Ishikawa, H.; Noyes, W. A., Jr. J. Chem. Phys. 1962, 37, 583.
The Journal of Physical Chemistry, Vol. 86, No. 20, 1982 4031
Quenching of Triplet-State Alkylbenzenes
TABLE I compd benzene
10-lOk,,
ET,:
I,f
M-1 s-l
cm-'
eV
1.2a 1.2 * 0.1 benzene-d, 1.5 f 0.1 methylbenzene 1.5 i 0.1 n-propylbenzene 1.5 i: 0.1 tert-butylbenzene 1.3 i: 0.1 1,3-dimethylbenzene 2.0 i: 0.2 1,4-dimethylbenzene 1.8 i 0.2 1,2,4-trimethylbenzene 2.5 i: 0.2 1,3,5-trimethylbenzene 2.2 i 0.2 1,2,3,4-tetramethyl2.9 * 0 . 3 benzene 1,2,4,5-tetramethyl2.9 f 0.3 benzene anthracene 0.20,b 0.80c naphthalene 0 . 1 0 3 d 0.21b
29500 9.25 9.25 29430 9.25 2 8 9 2 0 8.81 2 8 9 0 0 8.72 2 8 9 0 0 8.68 28705 8.59 28135 8.44 2 8 0 0 0 8.27 2 8 0 1 0 8.39 27 900
32
ref 23 24 24 25 26 27 23 23 26 23
24
k, (M-' sec-')
16
2 8 6 0 0 8.03 23 1 4 9 2 8 7.38 27 21 300 8.12 23
a This value was reported in ref 22. This value was reported in ref 14. This value was reported in ref 15a. This value w a s reported in ref 15b. e These triplet-state energies are those observed for the 0,O transitions in phosphorescence spectra obtained in solid matrices at 77 K. f Ionization potential.
as a function of 0, pressure. kl4 is determined from the slope of a plot of 7-l vs. 0, pressure, with pressures of A and B held constant.
Results Typical plots of 7-l vs. (0,)are shown in Figure 1. All such plots were linear, and the values of k,, obtained from the slopes, are presented in Table I. Also included in Table I are quenching constants reported in the literature. As a test of a proposed mechanism involving coupling of the initial complex to a charge-transfer state, a plot of k , vs. l / ( E T 1 - ECT), is shown in Figure 2. ET, is the energy of the Ti alkylbenzene, and ECTis the energy of the charge-transfer state. Photochemical quenching by 0, was not observed, as the shape and intensity of sensitized biacetyl phosphoresence remained unchanged after exposure of samples to 100 flashes. Discussion Edge-on approach of an 0, molecule to the benzene ring with its bond axis in the plane of the benzene ring and parallel to a C-C bond, which would allow maximum (14) Porter, G.; West, P. Proc. R. SOC. London, Ser. A 1964,279,302. (15) (a) Ashpole, C. W.; Formosinho, S. J.; West, M. A. J. Chem. Soc., Faraday Trans. 2 1976, 71,615. (b) Hippler, H.; Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1978,68, 5103. (16) Hunter, T. F.; Stock, M. G. Chem. Phys. Lett 1973, 22, 368. (17) Birks, J.; Pantos, E.; Hamilton, T. D. S. Chem. Phys. Lett. 1972, 20, 544. See references in this article for determinations of EA. (18) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J. Chem. Phys. 1967, 46, 1842. (19) Brown, R. G.; Phillips, D. J. Chem. SOC.,Faraday Trans. 2 1974, 70, 630. (20) Kearns, D. R. Chem. Reu. 1971, 71, 395. (21) Watkins, A. R. Chem. Phys. Lett. 1979, 65, 380. (22) Morikawa, A,; Cvetanovic, R. J . J. Chem. Phys. 1970,52,3237. (23) Watanabe, K.; Nakayama, T.; Mottl, J. 'Final Report on Ionization Potential of Molecules by a Photoionization Method", US.Army Report ORD-TB2-0001-00R-1624, 1959. (24) El-Sayed, M. F. A,; Kaaha, M.; Tanaka, Y. J. Chem. Phys. 1961, 34, 334. (25) Vilesov, F. I. Zh.Fiz. Khim. 1961, 35, 2010. (26) Price, W. C.; Bralsford, R.; Harris, P. V.; Ridley, R. G. Spectrochim. Acta 1959,14,45. (27) Bagdaaaryan, Kh.; Sinitspa, Z. A.; Muromtsev, V. Dokl. Akad. Nauk SSSR 1963,153,374.
Figure 2. Plot of k , vs. l/(,ET,- E,)* in eV-'. anthracene and naphthalene, respectively.
4 and + refer to
overlap between pn orbitals on adjacent C atoms and rz* and rY*orbitals of 02, is hindered by methyl groups in 1,3,5-trimethylbenzeneand 1,2,4,5-tetramethylbenzenebut not in the 1,2,4-trimethyl and 1,2,3,4-tetramethyl derivatives. Yet, as is seen in Table I, k, generally increases with increasing number of alkyl suustituents and decreasing triplet-state energy and is about the same for both trimethylbenzenes and for both tetramethylbenzenes. If edge-on approach were essential for quenching, then k , should be expected to be less for those alkylbenzenes with adjacent methylated C atoms. Our contradictory results suggest that the preferred approach is onto the face of the benzene ring. Although the trend of increasing k , with decreasing triplet-state energy is similar to that observed for polycyclic aromatic hydrocarbons in solution,6 it cannot be due to an exchange mechanism, dominated by Franck-Condon factors, as was proposed in ref 5. It is seen in Table I that the vapor-phase quenching constants are about 10 times larger for the alkylbenzenes than for naphthalenei4 and ar~thracene.'~However, the triplebatate energies are higher for alkylbenzenes. Therefore, if Franck-Condon factors are important in all vapor-phase aromatic hydrocarbons, the values of k should be larger for naphthalene and anthracene than !or the alkylbenzenes. Furthermore, Table I shows that there is no deuterium effect in benzene, which, owing to its large T1S, energy gap, should have the largest deuterium effect of the compounds in Table I. These facts prompt consideration of an alternate quenching mechanism for alkylbenzenes. Since k , decreases with increase in ET$for anthracene and naphthalene, it is possible that a simple exchange mechanism is operative in the quenching of these molecules. However, more polycyclic aromatic hydrocarbons must be studied to substantiate this possibility. In accordance with eq 1 and 2, k , can be expressed as
The absence of a solvent cage in the vapor phase makes reasonable the asumption Jtisc, ket
