The Journal of
Physical Chemistry ~~~
0 Copyright, 1991, by the American Chemical Society
VOLUME 95, NUMBER 19 SEPTEMBER 19, 1991
LETTERS Quenching of Triplet Benzophenone by 1,rl-Diazabicycio[ 2.2.2loctane in Acetonitrile Revislted Edwin Haselbach,* Patrice Jacques? Denis Pilloud, Paul Suppan, and Eric Vauthey Institute of Physical Chemistry, University of Fribourg, CH- 1700 Fribourg, Switzerland (Received: May 3, 1991; In Final Form: July 27, 1991)
In conditions of laser flash photolysis, the kinetics of decay of the absorption of the benzophenone radical anion show that free, solvated ions are formed after electron transfer between the title compounds in neat, dry acetonitrile. Furthermore, it is shown that the opposite conclusion claimed by Devadoss and Fessenden (J. Phys. Chem., 1990, 94,4540), Le., no ion pair dissociation, results from a misinterpretation of the transient decay rate.
Introduction In many applications of intermolecular electron-transfer (ET) processes, the separation of the product radical ions is the essential step toward irreversible chemical changes. For this reason, the determination of free ion yields has become an active area of research in recent years, and some general rules are becoming established. The rate constant of separation of the product molecules k has seldom heen measured directly, but its calculated value from%ffusion theory is in agreement with recent experimental data; it depends on the viscosity and on the dielectronic constant of the solvent. The other factor which determines the free ion yield is the rate constant of charge recombination kke within the geminate ion pair, the free ion yield then being
free energy of the charge recombination. Of special relevance to the present work is the former observation that very high ion yields are obtained when BP or anthraquinone triplets are quenched by DABCO in CH3CN.' This report has, however, been challenged in a recent paper by Devadoss and Fessenden (DF),2 who conclude that charge separation does not take place in the BP/DABCO system in neat CH3CN. This assertion is based on the kinetics of decay of the BP radical anion (BP-), which according to DF follows a firstorder law. Should this be true, it would raise a number of questions of far-reaching consequences: 1. The high yield of free ions for the BP/DABCO/CH3CN system has been confirmed in different laboratories by transient photocurrent measurements. Clearly, this experimental method which is being increasingly used in photoinduced ET work would then be unreliable or would be subject to serious artefact^.^ (1) Haselbach, E.; Vauthey, E.; Suppan, P. Tetrahedron 1988,44,7335.
if a0is the quantum yield of the primary ET process, e.g., the quenching of an excited molecule by a ground-state ET partner. The values of kbc show very large variations which are related in particular to the spin state of the geminate ion pair and to the 0022-3654/91/2095-711 SSO2.50/0
( 2 ) Dcvadose, C.; Fessendcn, R. W. J. Phys. Chem. 1990, 94,4540. ( 3 ) (a) Zhu, Q.Q.;Schnabel, W.; Jacques, P. J . Chem. Soc., Faraday Tram. 1991.87, 1531. (b) Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N. Bull. Chem.Soc. Jpn. 1990,63,3385. (c) Miyasaka. H.; Matap, N. Bull. Chem. Soc. Jpn. 1990,63,131. (d) Kuhlmann, R.; Schnabel, W. J . Phorothem. 1977, 7 , 287.
0 1991 American Chemical Society
7116 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Letters
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Figure 2. Apparent half-life of transient decay measured at 710 nm at various starting ion concentrations. Abscissa, reciprocal of initial a b sorbance of B P - at 710 nm.
The nanosecond laser YAG (with an excitation wavelength at
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Figure 1. Transient absorptions in the BP/DABCO/CH$N system. (A) Decay at 7 10 nm over 5 0 0 ps. Insert: first part of this decay, over 16 ps. (e) Decay at 710 nm over 16 ps, with the baseline set at OD = 0. Inserts: first- and second-order linear correlations. (C)Same as (B) but with a raised baseline corresponding to the residual absorbance at 13 LCS. Inserts: first- and second-order linear correlations.
2. The many independent reports of charge separation in similar systems in CH3CN would have to be recon~idered.~ 3. Much of the knowledge gained in recent years concerning the rate constants of charge separation would have to be reconsidered as well. According to DF, the geminate ion pair (BPDABCO+) recombines through first-order kinetics with a lifetime of a few microseconds. If the free ion yield is less than 176, the rate constant of ion separation would be under lo4 s-I, against the generally accepted value of over IO* s-l, an amazing discrepancy which would be a revolution in the theories of diffusional processes.5 For these fundamental reasons, as well as for the controversy between our conclusions and those of DF concerning charge separation in the BP/DABCO/CH3CN system, we have reexamined in great detail the evidence for and against the high free ion yield, and the results are reported here. Experimental Section Benzophenone (IO mM), BP (Fluka), was twice recrystallized from aqueous ethanol and sublimed under vacuum. 1,CDiazabicyclo[2.2.2]octane (20 mM), DABCO (Fluka), was recrystallized from a 1:1 benzene-hexane mixture and then twice sublimed under vacuum. Acetonitrile (CH3CN) was purchased from Rathburn. Water contents were determined by using a Mettler DL18 Karl Fischer Titrator with Hydranal Composite 5 as titrant. H.;Mataga, N. Ace. Chem. Res. 1981, 34, 312. ( 5 ) Eigen, M. Z. f h y s . Chem. (Frankfurr) 1954, I , 176. (4) Masuhara,
Results 1. Kinetics of Transient Absorption and Photocurrent of B P . While our earlier measurements of the kinetics of absorption of BP-were made at 630 nm, we have now extended these to 710 nm which corresponds to the absorption maximum-in this respect we are in agreement with DF. Wefind again that the decay is a text-book example of second-order kinetics, identical at both wavelengths. As discussed in detail further on, the water contents of CH$N were always kept to less than 50 ppm. Figure 1 shows a representative kinetic trace of the absorption of B P at 710 nm, together with the first- and second-order linear correlations. Three points are worthy of attention: i. The trace comes back to the starting baseline over a very long time, of the order of 100 ps. This is characteristic of second-order kinetics, the rate becoming very slow at long times. The fact that the baseline remains unchanged after prolonged irradiation or after many laser pulses is in agreement with the observation that the BP/DABCO/CH3CN system is remarkably photostable, showing that the free ions eventually recombine to re-form the ground-state neutrals without any irreversible change. ii. When the starting concentrationof free ions is varied through the intensity of the laser pulse, the apparent half-life changes as expected for a second-order process. Figure 2 shows the apparent half-lives obtained for different laser pulse energies, the energy being decreased by means of liquid filters. The actual energies El were measured by a joulemeter to ensure that they followed the filter transmissions. The initial optical density (OD) of the transient is directly proportional to E l . The apparent half-lives tllz increase as t l l z a (OD)-I within experimental error, and this is the best criterion for distinguishing between first- and second-order kinetics.' iii. The second-order decay is substantially slowed down (by a factor of 1.8 in the apparent half-life) by the addition of an electrolyte (0.05 M tetrabutylammonium perchlorate) as expected for a diffusion-controlled reaction between oppasitely charged ions. Finally there is little room for doubt in the existence of free ions since intense photocurrents are observed. Moreooer the transient photocurrent follows precisely the kinetics of transient absorption, confirming the fact that both techniques monitor the same species, that is, the free ions solvated in neat, dry CH3CN. 2. Analysis of tbe Kinetic Data of DF. Now the question does arises, how could DF fit such kinetic data to a first-order law? The answer lies in their treatment of the decay exemplified by (6) Guerry-Butty, E.;Hasclbach, E.;Pasquier, C.; Suppan, P.; Phillips. D. Helu. Chim. Acta 1985, 68, 912. (7) Eyring, H.; Lin, S. H.; Lin, S . M. Basic Chemical Kinerfcs; Wiley: New York, 1980 p 2.
7117
J. Phys. Chem. 1991,95, 71 17-7 118
Figure 3. Changes of transient absorption spectra in the BP/DABCO/CHICN system with additions of water: (A) 1 vol % H,O;(B) 10 VOI % H20.
the insert of Figure 3 of ref 2. This resembles greatly our insert in Figure 1, in which the observation is restricted to a time of some IO ps, whereas it was noted above that the full decay back to the starting baseline can be observed only over much longer times. Concerning their kinetic trace, DF state three things which explain the problem. In the first instance, it is written that "the traces depart only slightly from first-order kinetics"; and this is indeed true if the baseline is taken as the signal reached at 12 p rather than the initial baseline (which according to our results is identical with the signal at -100 FS; see Figure 1A). The second point actually follows from this, since DF observe that "there is a small residual absorbance". If this were true, it would imply that their samples were not photostable, so that some light-absorbing photoproducts were formed after each flash. In that case,there would be some impurity in their samples, impurities which would lead to an overall, irreversible reaction which is not observed in our experiments. It seems, however, more probable that the problem arises simply from the misleading truncation of their kinetic traces at around 12 ps and that they would have observed the slow retum to the original baseline had they followed the decays over much longer times. Thirdly, DF note that "there is only a slight increase in the lifetime" when the initial concentration of BP is lowered. This is also related to the truncation of the kinetics at some arbitrary time which gives an artificially raised baseline; the half-life of the decay then varies with the truncation time, and the data obtained in this way are not meaningful. 3. Influence of Water in CH3CN. CH3CN is used as a polar, aprotic solvent in many experiments which involve ET. The question of its purity has been raised repeatedly, in particular with respect to its water content. In spite of the potential problem caused by the well-known hygroscopic properties of CH3CN, hardly any laboratory actually measures the water content of "neat" or "dry" samples.
Spectroscopic grade CH3CN is of course free from light-absorbing or luminescent impurities, but the water content can be measured only by Karl-Fischer titration. Elaborate drying and handling procedures are required to keep the water content below 50 ppm; the best we could achieve was 8 ppm following the procedure described in ref 8. A sample of CH3CN with less than 50 ppm can therefore be considered to be "dry". In view of the cross-accusations of various research groups concerning the role of traces of water in the others' samples, it is to be hoped that the actual measurement of the water contents should become a matter of routine. Excellent automated Karl-Fischer titrators are available at prices that every laboratory can afford. Measurements of the transient spectra were repeated in CH3CN solutions with added water, the results being summarized in Figure = 710 3. The shift of the absorption spectrum of BP- from A, nm in dry CH3CN to A,, = 630 nm in neat water is observed at quite high water concentrations, around 1 vol % and above. Under these conditions the absorption of B F shows a much faster decrease and approaches first-order kinetics, and the absorption at 540 nm shows a slower decay of complex kinetics. This is attributed to the protonation of the radical anion to form the neutral ketyl radical, the acid-base equilibrium being in favor of the neutral species at a pH of around 7. In any case, these results show that traces of water are not involved in the separation of ions in CH3CN, since B P - would then have an absorption maximum at 630 nm if it were hydrogen bonded to water molecules. The spectral shift of A- of BP' from 710 nm in dry CH3CN to 630 nm in water can be understood on the basis of the charge transfer involved in this Do DI transition. In the ground state, the negative charge resides essentially on the carbonyl oxygen and hence the strong hydrogen bonding with protic solvents; the Do D, transition corresponds to a charge transfer from the 0 atom toward the aromatic system, a situation somewhat similar to the So SI transition in anilines which explains the blue shift of the absorption spectrum in protic solvent^.^
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Conclusions The presence of free ions in the system BP/DABCO in dry CH$N in conditions of laser flash photolysis is unambiguously established by the second-order kinetics of transient absorption as well as transient photocurrent measurements. Acknowledgment. This work is part of project No. 2G28842.90 of the "Schweizerischer Nationalfonds zur Fiirderung der wissenschaftlichen Forschung". (8) Pilloud, D.; Suppan, P.; Van Haelst, L. Chem. Phys. Lett. 1987, 137, 130.
(9) Suppan, P. J. Photochem. Photobiol. A 1990, 50, 293.
EPR Spectrum of C,o- Trapped In Molecular Sieve 13Xt P. N. Keizer, J. R. Morton, K. F. Preston,* and A. K. Sugden Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9 (Received: June 3, 1991; In Final Form: August 5, 1991) When C, is adsorbed by activated 13X molecular sieve, an intense EPR spectrum at g = 1.9995 is observed between 4 and 300 K. The spectrum consists of a single line (AH= 3.5 G)plus a pair of satellites 9.7G on either side of it having a combined intensity equal to (5 I)% of the central line. It was concluded that these satellites were either (a) due to hyperfine interactions of 19.4 G (54 MHz) with five equivalent carbon nuclei of the C ,- ion or (b) due to pairs of C,- ions exhibiting a triplet spectrum.
*
The recent confirmation' of the icosahedral structure proposed* for the new allotrope of carbon, C,, has naturally led to an explosion of interest in the chemistry of this extraordinary mol-
mule. The successful entrapment and stabilization of free radicals in zeolites' encouraged us to attempt such an approach with C, (1) Johnson, 112,8983.
'NRCC No. 33240. 0022-3654/91/2095-7117S02.50/0
R. D.; Meijer, G.;Bethune, D. S.J. Am. Chem. Soc. 1990,
Published 1991 by the American Chemical Society
