3982
R. B. CUNDALL,G. B. EVANS, AND E. J. LAND
Rate Constants for Quenching of Biacetyl Triplets in Benzene by R. B. Cundall, G. B. Evans, Department of Chemistry, The University, Nottingham, England
and E. J. Land Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester, England
(Received April 17,1969)
The quenching of the biacetyl triplet by a number of molecules including a stable triplet, free radicals, a paramagnetic complex, and some diamagnetic species has been studied. There is only a small variation in the rate constants for the efficientquenchers (5.5-2.4 X lo9 M-l sec-l). The role of spin statistical factors and complex formation in reducing the rate below diffusion control is discussed. Excited triplet states may be deactivated by bimolecular processes which involve energy transfer or other mechanisms which are not yet clearly defined. It is usually stated that if the energy-transfer process is exothermic, the rate of transfer is diffusion controlled. Although this seems true for solvents of high viscosity, it may not apply to solutions of low viscosity (7 < 3 cP).' The fact that most published rate constants for triplet quenching are about 5 X log M-l sec-I whereas calculated diffusion rates are 3 to 5 times higher than this does not support this generalization in the less viscous solvents.2 On the other hand, the experimental finding that all measured triplet quenching rate constants have almost the same value within the limits of experimental accuracy has been used to support unit efficiency for the deactivation process. Contrary evidence has been obtained by Hammond and c o ~ o r k e r s who , ~ show that some triplet deactivation processes are subject to steric hindrance and SO cannot be truly diffusion limited. Reversal of energy transfer is possible if the excitation energies of the donor and acceptor triplets are similar as shown by S a n d r ~ s . Recently ~ Nordin and Strong5 have also indicated that if the energy difference AE is not too large, energy transfer efficiencies should differ from diffusion control to an extent which depends upon the actual lifetimes of the acceptor and donor triplet states. We wish here to report some results obtained on the quenching of biacetyl phosphorescence by a number of species including free radicals and bis-galvinoxyl which exists in its ground state as a tripletn6 Pulse radiolysis has been used to populate the excited states of biacetyl, largely by energy transfer from the solvent benzene, and to follow the subsequent decay of the phosphorescing triplet. Quenching solutes were present in low concentration and direct excitation of quencher was avoided.
Experimental Section and Results Pure crystallizable benzene (B.D.H.) was shaken several times with concentrated sulfuric acid, washed, The Journal of Physical Chemistry
dried (with Na2C03)and then fractionally crystallized three times. It was dried once more using PzOb or silica gel and was fractionally distilled. All other materials were distilled or recrystallized samples. The pulse radiolysis apparatus has been described by Keene.' In these experiments the biacetyl phosphorescence was viewed under strictly comparable conditions for the different solutions. The solutions were reproducibly deoxygenated by bubbling with argon and all experiments were carried out a t room temperature (25'). The decay of the biacetyl triplet is a function of radiation dosage.6 Using 2 psec pulses of 4-MeV electrons at doses below 1000 rads the lifetime of the biacetyl triplet was sufficiently long for its decay to be studied. It proved convenient to use pulses of 60 rads in the quenching experiments since under these conditions, interference from secondary processes, such as radiolytic product quenching, seemed negligible, or at least independent of dose. The excellence of the observed first-order decay behavior and low noise level inherent in emission measurements allowed the determination of precise first-order rate constants. There was no detectable formation of biacetyl triplets after the pulse. The observed spectrum for biacetyl phosphorescence6 agreed very closely with the long-lived biacetyl emission spectra published by Backstrom and Sandrosg and Dubois and Wilkinson.'O The form of the spectrum (1) P. J. Wagner and I. Kochevar, J. Amer. Chem. SOC.,90, 2232 (1968). (2) e.g., J. T. Dubois and R. L. van Hamert, J . Chem. Phys., 40, 923 (1964). (3) G. S. Hammond and R. P. Foss, J . Phys. Chem., 68,3739 (1964); W. G. Herkstroeter, L. B. Jones, and G . S. Hammond, J. Amer. Chem. Soc., 88,4777 (1966). (4) K. Sandros, Acta Chem. Scand., 18,2355 (1964). (6) 8.Nordin and R.L. Strong, Chem. Phys. Lett., 2,429 (1968). (6) E. A. Chandross, J . Amer. Chem. SOC.,86, 1263 (1964). (7) J. P. Keene, J. Sci. Instr., 41,493 (1964). (8) R. B. Cundall, G. B. Evans, P. A. Griffiths, and J. P. Keene, J . Phys. Chem., 72,3871 (1968). (9) H. L. J. BackstrBm and K. Sandros, Acta Chem. Scand., 14, 48 (1960).
RATECONSTANTS FOR QUENCHING OF BIACETYL TRIPLETS IN BENZENE was independent of biacetyl concentration although the intensity increased to a limiting value a t about 3 X 10-2 M biacetyl. I n the experiments reported M here the biacetyl concentration was fixed at and added quencher concentration varied between 5 X lo-' and M. The rate of decay of biacetyl phosphorescence was dependent on the purity of the solvent and in particular the amount of residual oxygen as well as the radiation dose. Most batches of benzene gave observed lifetimes (l/e values) of about 230 psec. This is a value less than that achieved under photochemical conditions,8 and may be due to excited state quenching by radicals. Repeated pulsing of a solution sometimes resulted in small increases of biacetyl triplet lifetime, presumably due to the consumption of small amounts of O2 by radiolytic intermediates. This test was applied to all the observations. The method for measuring the rate constant for quenching of the biacetyl phosphorescence was as follows. ro,the lifetime of the biacetyl triplet state in the absence of quencher, and 7 , the lifetime in the presence of the quencher at concentration [&I, were measured from the oscilloscope traces. From plots
Table I : Quenching R a t e Constants (k4) for Biacetyl Phosphorescence in Benzene-Biacetyl Solutions Added quencher Diphenylpicrylhydrazyl ( D P P H )
k,, M-' see-1 X 10-9 5.5 f0.5
bis-Galvinoxyl (g.s. triplet)
5 . 5 rt 1 . 0
trans-Stilbene Ferric Acetylacetonate Galvinoxyl (free radical)
4.0
f0.2
3.1
&
2.9
f 1.0
0.5
p n
3983
of 7-' vs. [&I, IC, the quenching rate constant could be determined from 1/7 =
1/70
+ hq[QI
The results are shown in Table I.
Discussion The results for the various rate constants are consistent with many data in the literature. The highest quenching rate constaats for DPPH and bis-galvinoxyl do not exceed other values which have been reported for this type of process. They are less than values expected if the most efficient quenching process in benzene was completely diffusion controlled. The modified Debye equation used by Osborne and Porter," ViZ.
givesavalueof 1.55 X 10'OM-'sec-lforbenzeneat 25". It has been indicated that more complex expressions, which should presumably be more accurate, increase the predicted value. Spectrofluorimetric studies which we have made give a value of 4.0 X 1O'O M - I sec-' for the quenching of the first excited singlet state of benzene by biacetyl. This supports the evidence of Dubois and van Hemert? that the Debye equation yields values close to those of experiment in singletquenching experiments, even when dipole-dipole transfer effects are expected to be negligible and allowance made for the possibility of energy migration through the liquid. The value for trans-stilbene corresponds closely with 4.4 X 109 M-I sec-' obtained by Backstrom and Sandros. Ferric acetylacetonate, which has five unpaired electrons, gives a value in satisfactory agreement with the results of Hammond12 using a number of different sensitizers, and also Linschitz,13 using benzophenone. The k , value for ferrocene is about three times smaller than the high value of 7.0 X lo9 M-' sec-' measured by Hammond for the quenching of the benail triplet. However, the values obtained by Hammond and his coworkers12vary with the triplet being quenched and so there may be no other significance in the discrepancy. There is no obvious correlation with the magnetic moment or number of unpaired electrons of the quenching molecule as first shown for the triplets of naphthalene and anthracene by Porter and Wright.14 The
'
0
4
Asulene Ferrocene Cy clohexene Biacetyl
2 . 4 =t0.2 2 . 4 f 0.2 (1.18 =t0.10)io-4 ( 5 f 1)10-5
(10) J. T. Dubois and F. Wilkinson, J . Chem. Phys., 39, 377 (1963). (11) A. D. Osborne and G. Porter, Proc. Roy. Soc. A284, 9 (1965). (12) A. J. Fry, R. S. H. Liu, and G. S. Hammond, J . Amei. Chem. Soe., 88,4781 (1966). (13) J. A. Bell and H. Linsohitz, ibid., 85,528 (1963). (14) G. Porter and M. R. Wright, J . Chim. Phys., 5 5 , 705 (1958); Discussions Faraday SOC., 27, 18 (1959). Volume 78, Number 11 November 1969
3984 influence of electronic spin statistical factors is not clear and would depend on the nature of the collision complex. It is possible that the failure of the triplet quenching processes to have rate constants which are effectively diffusion controlled when other factors seem favorable may partly be due to this effect. Porter and Wright interpreted some of the variation in quenching efficiency in just this way. If the spin and unchanged in the number of the quencher is deactivation process, then the efficiency should be reduced l/3. In addition, the probability of quenching should be further affected by variation in stability of the triplet-quencher complex. For the situation under discussion this second factor should have a value between l and ’/&which decreases with the extent of interaction in the quenching process. If formation of a well-defined complex as distinct from a very short lived encounter is essential to transfer then the effectiveness of an encounter may be reduced even further by steric and solvation effects, as seems to be the case for the first transition and lanthanide series of ions in aqueous ~ o l u t i o n . ~ ~ ~ ~ ~ Quenching of biacetyl triplet by bis-galvinoxyl is analogous to the mixed triplet-triplet interactions which lead to delayed (P-type) fluorescence studied by Parker.’C The overall efficiency of triplet quenching is not high, especially if long range dipole-dipole transfer is possible as has been suggested for this type of process in the polyacenes. Parker15 has shown that the efficiency of the triplet-triplet interaction varies between 0.56 and values which, in the case of two dyes, may be as low as when compared with the diffusion rate calculated from the Debye equation. Spin statistical factors may operate here as indicated by Birlrs16 in a somewhat different context. The results quoted are consistent with the view that some chemical complex formation involving steric
The Journal of Physical Chemistry
R. B. CUNDALL, G. B. EVANS, AND E. J. LAND and probably other restrictions is involved in the bimolecular deactivation of the triplet state.3 In all the cases where the quenching is efficient electronic excitation-energy transfer could be a more significant mechanism than the catalyzed T --t S intersystem crossing without electronic change in the quencher when both are possible. This has been demonstrated by Binet, Goldberg, and Forster” for quenching of benzil phosphorescence by chromium complexes. It should be possible to study this type of process in detail by pulse radiolysis. In none of the investigated cases is reverse energy transfer expected to lead to an apparent decrease in efficiency of quenching since acceptor-donor energy differences are all likely to be greater than at least 5 kcal (AE for biacetyl-trans-stilbene). l8 Also in the case of stilbene the lifetime of the triplet is probably too short in the fluid state for significant reverse transfer, as required by the scheme of Nordin and Strong.5 The quenching by cyclohexene is almost certainly a chemical effect as suggested by Backstrom and S a n d r o ~ . ~The quenching by ground-state biacetyl is more marked than found by these authors (460 M-l) and is probably due to a product formed by the reaction of a radiolytic intermediate with biacetyl. These experiments, which we hope to extend, show how pulse radiolysis can be used with advantage to study the behavior of excited states when practical difficulties would make similar photochemical experiments difficult. (15) e.g., C. A. Parker, “Fast Reactions and Primary Processes in Chemical Kinetics,” S. Claesson, Ed., Interscience Publishers, New York. N. Y., 1967, p 317. (16) J. E. Birks, Phys. Lett., 24A,479 (1967). (17) D. J. Binet, E. L. Goldberg, and L. S. Forster, J. Phys. Chem., 72,3017 (1968). (18) W. G. Herkstroeter and G. S. Hammond, J . Arne?. Chem. Soc. 90, 4769 (1966).
