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H. LINSCHITZ, C. STEEL,,4ND J. A. BELL
Neither o-OH-acetophenone nor Z-acetonaphthone photoreact with alcohols as do other ketones such as acetophenone, etc. From the fact that 2acetonaphthone photosensitizes the decomposition of 1-iodonaphthalene, we can conclude that the triplet state of 2-acetonaphthone is present in solution. The explanation for its unreactive nature surely is, therefore, that it has a lower T-T* and not a lower n-z* triplet state.5 This supports the recent work of Hammond and Leermakers on compounds of this type.26 KO conclusions can be arrived a t concerning the triplet state of o-hydroxyacetophenone. The reasons for the photochemical behavior of compounds containing the o-hydroxybenzoyl group have been discussed elsewhere and the reader is referred to these discussions. 9s20 Experimental It is intended to publish a more detailed account of much of this work and full experimental details therefore will be given elsewhere.’* The materials used were of the highest purit commercially available and often were subjected to furtEer purifkation by standard methods. As solvents, B.D.H. isopropyl alcohol (special for spectroscopy), further purified “Analar” (26) G. S. Hammond and P. A. Leermakers, J . A m . Chem. Soc., 84, 207 (1962).
T’ol. 66
benzene, and doubly distilled water were used throughout. The degassing procedure was identical with that used in ref. 8. Absorption measurements were made using a Perkin-Elmer 4000A recording spectrophotometer. Anthraquinol, acetone,2? and iodine were estimated spectrophotometrically. Oxygen uptake was estimated by measuring the reduction in oxygen pressure above solutions due to irradiation. These solutions were shaken continuously during irradiation. A high pressure mercuryJamp enclosed in a Chance OX 1 filter was used for 3660-A. irradiation. For 3130-k. irradiation, filter solutions were used as recommended by Bowen.*a Quantum yields were measured using potassium ferrioxalate as actinometer in the recommended way.29
Acknowledgments.-It is a pleasure to acknowledge the help given by Dr. E. J. Bowen, who has made supplies of materials and equipment readily available to the author. The author wishes to thank Dr. D. Seaman for making some preliminary measurements on anthraquinones. He also is grateful to Professors G. Porter and J. N. Pitts, Jr., for interesting discussions on various aspects of this subject. (27) See J. N. Pitts, Jr.. R. L. Letsinger, R. P. Taylor, J. M. Patterson, G. Reoktenwald, and R. B. Martin, ibid., 81, 1068 (1959). (28) E. J. Bowen, “The Chemical Aspects of Light,” Oxford University Press, 1946, p. 278. (29) C. G. Hatohard and C. A. Parker, Proc. Roy. Soc. (London), A236,518 (1956).
THE UNCATALYZED DECAY OF AKTHRACENE AND PORPHYRIN TRIPLETS I N FLUID SOLVENTS’ BYHENRY LINSCHITZ, COLINSTEEL,AND JERRYA. BELL Department of Chemistry, Brandeis Universitv, Waltham, Mass. Receiued M a y 91,1968
The decay kinetics of anthracene and porphyrin triplets have been measured, using flash-excitation technique. In purified and thoroughly deoxygenated pyridine or hexane, the first-order rate constant is less than 150 sec-1. In tetrahydrofuran, chemically deoxygenated by liquid NaK alloy, the first-order decay constant is less than 40 sec.-I. Correspondingly low values are found also for porphyrins. The anomalously large viscosity dependence of this rate constant is assigned to pseudo firsborder processes rather than to an intrinsic effect. The bearing of this on radiationless energy degradation in excited molecules is discussed.
The nature of radiationless transitions in complex molecules still remains one of the major problems in photochemistry. Interactions with the medium evidently must be involved, if only to remove vibrational energy. However, more specific properties of the solvent also may play a role, as for example, the viscosity. In the case of tripletsinglet transitions, much discussion has been devoted to the seeming paradox of a strong viscosity dependence for the radiationless T + S (triplet to ground state) transition, compared with a weak viscosity dependence of the analogous S’ + T (excited singlet to triplet) tran~ition.2-~Thus, the appearance of phosphorescence in passing from a fluid to a rigid medium requires that the S’--c T transition rate remain high to compete with S’ S
-
(1) This work was assisted by a grant from the U. 9. Atomic Energy Commission to Brandeis University (No. AT (30-11-2003). (2) G . N. Lewis, D. Lipkin, and T. T. Magel, J . A m . Chem. Soc.. 63, 3005 (1941). (3) G. Porter and M. R. Wright, Discussions Faraday Soc., 27, 18 (1959). (4) G. Porter and M. R. Wright, ibid., 2’7, 94 (1959).
transitions, whereas the radiationless T -+ S transition rate must greatly decrease, to permit the longlived emission to occur. However, it has been appreciated for some time that it is extremely difficult to measure the “true” first-order rate constant for the radiationless T --c S transition in fluid solvents because of competing pseudo first-order quenching reactions of the triplet with traces of impurity, particularly oxygen, and perhaps solvent peroxide~.~-? The extremely long radiative lifetime of the triplet makes it especially susceptible to such quenching, and the apparent strong viscosity dependence of the L‘unimolecular”T + S transition may, in fact, be most reasonably interpreted in this In this paper we present further direct measurements of the decay kinetics of anthracene and porphyrin triplets in fluids solvents which set new lower limits to the intrinsic first-order decay rate. (5) H. Linschits and L. Pekkarinen, J . A m . Chem. Soc., 82, 2411 (1960). (6) G. Jackson, R. Livingston, and A. Pugh, Trans. Faraday Soc., 66, 1635 (1960). (7) G. Jackson and R. Livingston, J . Chem. Phys., 35, 2182 (1961).
Dee., 1962
UKCATALYZED DECAY OF ANTHRACENE AND PORPHYRIN TRIPLETS
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Experimental Materials.-Reagent . rade pyridine was twice purified over KOH and distilled tarough an efficient column. Freshlyopened “spectro-grade” hexane was used directly, after distillation was found to have no effect on the decay rate of anthracene in this solvent. Fisher reagent tetrahydrofuran (THF) was distilled from lithium aluminum hydride and then treated with liquid NaK eutectic (see below). Anthracene was either E.K. “fluorescent grade” or a highly purified scintillation grade sample from Pilot Chemical Co. (Waltham). Both materiala gave the same results, in pyridine or hexane, yithin experimental error. Measurements in T H F were confined to the Pilot sample. Porphyrins were purified as described previously.8 Degassing Procedure.-In previous studies,6 solvent degassing was done by repeated trap-to-trap distillation in order to carry all materials through the vapor phase. This is a rather slow operation, subject to unavoidable leaks in the vacuum system and possible re-adsorption of oxygen on successive layers of frozen solvent during the transfer, while the tra s are closed off from the pump. I n this work, pyridine anthexane were degassed by subjectin the solutions to repeated cycles of freezing, pumping, andgthawing, with uigorous agitation of the liquid by an enclosed magnetic stirrer. Solutions were sealed off only after at least three consecutive cycles gave “stick-vacuum.” I n sealing off, care was taken to allow gas reletlsed from the heated glass to be pumped away before completing the closure. Glassware was thoroughly cleaned and rinsed to remove possible traces of heavy-metal contamination. After initial drying and distillation, tetrahydrofuran was degassed by freezing, pumping, and thawing, and then further deoxygenrtted chemically, b allowing the solvent to stand for several week over 1iquidYNaK alloy in a Pyrex ampoule fitted with a greaseless Hoke Hi-vacuum valve, with occasional agitation by an enclosed magnetic stirrer. The completion of the “getter” action is visibly indicated by the appearance of a blue color characteristic of alkali metal solutions in ethers.gJ0 This blue color generally appeared shortly after contact of the T H F with NaK, and deepened somewhat after prolonged standing. In preparing solutions for study, the cleaned flash-cell and side arm assembly was pumped and flamed, removed from the line, solute added, the cell immediately re-evacuated on the line, purified THF quickly distilled in from its storage bulb, and the sample finally sealed off. To study concentration effects, various test solutions were prepared from a single sealed-off reparation, by adjusting: the solute distribution between tge flashcell and side arm. Concentrations were determined on a Gary Model 14 spectrophotometer. Apparatus and Measurement Procedure.-The apparatus and general measuring procedure have been described previously.5-7*11 A slight modification of technique was made to improve the precision of measuring small absorbance changes (less than 0.2). Monochromator slit width and photometer gain were adjusted so that the initial light transmission of t,he sample gave full-scale deflection on the oscilloscope screen. The oscilloscope gain then was increased by a known factor, and the vertical control re-adjusted to bring the deflected sweep back on scale. The relatively small change in absorption due to the flash then could be measured a t higher gain. The change in optical density, A D , due to the flash, then is given by A D = log [j~~/(fr~ A r ) ] , where r0 is the transmission of the original solution, in oscilloscope scale units, Ar is t$e observed (algebraic) change in reading due to the flash, and f is the ratio by which the gain is increased. The signal to noise level of the photometer and the stability of the measuring light source permitted “f‘’ values as high as 4 or 5 to be used. This technique is most helpful, since the points a t low A D are especially significant in the extrapolation by which the first-order component of the decay curve is cibtairied. The precision of the experimental
+
(8) L. Pekkarinen and H. Linschitz, J . A m . Chem. Soc., 82, 2407 (1960). (9) J. L. Downs, J . Lewis, B. Moore, and G. Rilkinson, J . Chem. SOC.,3767 (1959). (10) We wish t o tliauk RIr. R. Pelton for assistance in this yreparation. (11) H. Linschitz ,and li. Sarkanen, J . Am. Chem. SOC.,80, 4828
(1958).
HEXANE
/ 2.0m
0 X
1.5
-
E!
-0”
I.0-
0 1 ;
0.5 -
1 04
1
08
I .I2
I 16
I
I
20
AD.
Fig. 2.-Decay kinetics of anthracene in pyGdine (6 X l o r 6 M )and hexane (4 X 10-bM): slit = 20 A., X = 4240 A. data may be judged from a typical oscillogram, shown in Fig. 1 . The data were treated as in our earlier work,6J1taking the rate law t o be
dt
in which 121 is the total (radiative and radiationless) unimolecular rate constant and k0, ks,and k4 correspond, respectively, to quenching by other triplets, ground state molecules, and foreign substances. A plot of d/dt [In ( A D o / A D ) ]us. AD has an intercept A given by
A = kl
+ k2CO + C(k4)1(w i
iii which Cois the total solute concentration.5
Results and Discussion 1. Kinetics in Hexane and Pyridine.-Figure
2 presents typical decay data for anthracene in hexane and pyridine, from which the uncertainty in the extrapolation may be judged. For pyridine, the least squares plot (shown in Fig. 2) had an intercept of 55 and standard error of A 6 2 see.-’. In hexane, a typical result was 56 st 76 set.-'. illthough it is difficult to evaluate systematic errors, a conservative upper limit for A , in both solvents, is about 150 see.-’. Accordingly, the value for A
H. LINSCHITZ, C. STEEL,AND J. A. BELL
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1I
Vol. 66
Jackson and Livingston have attempted to elimiDECAY K I N E T I C S OF A N T H R A C E N E T R PLET ITETRAHYDROFURANI
A
0 nate the bimolecular viscosity-dependent contri-
butions to the first-order decay by plotting A against lc2 for a given solution a t a series of temperatures, and finding the limiting value of A corre31 sponding to k2 = 0. This limit then is taken to be -: 1 the intrinsic k,, on the assumption that k , is temperature independent. The values thus obtained 0 for anthracene are 110 sec.-’in T H F and 160 sec.-l in hexane. This THE’ rate is higher than the directly measured value reported here. The difference in kl’s in hexane seems to be within the ex21 perimental error of both methods. 0 290 I Measurements also were carried out on zinc tetraphenylporphine in T H F (2.75 X lo-’ X). ‘ Data taken a t two wave lengths ccp-esponding, respectively, to dye bleaching (4220 A.) or triplet 005 0 10 0 I5 0 20 absorption (4500 fell on lines, with intercept 50 A D (.k=42408). set.-' and estimated error =t50 sec.-l, again conFig. 3.-Decay kinetics of anthrac$ne in T H F at yarious con- siderably lower than our previous measurement~.~ centrations: slit = 35 A,, X = 1240 A. The bimolecular triplet quenching constant, k2, for ZnTPP was 5.5 X lo9ilk1set.-'. is taken to be about ’75 + 75 sec.-l. This upper 3. Remarks on Radiationless Transitions.limit of A , and therefore IC,, is appreciably less than that found in our previous work (380 see.-’ in In the light of this and other work5-’ there can be hexane) or in other direct measurements (340 no doubt that the apparent (and puzzling) large To check that the differences found in difference in viscosity dependence between the our work are due to modifications of technique, S’ -+ T and T -+ S radiationless transitions is an first-order decay rates were redetermined for tetra- artifact. At least for the cases considered, the phenylporphine and zinc tetraphenylporphine in “intrinsic” T 4 S transition is evidently much T process, even in fluid solpyridine, a new value of A of 150 set.-' being ob- slower than the S’ tained instead of i00-800 sec.-1.8 The decrease is vents. Thus, the triplet-singlet transitions fall in comparable to that observed for anthracene. The with the familiar generalization that radiationless second-order slopes (Fig. 2 ) agree closely with our transitions among the upper excited states are much faster than the final transition from the lowest exprevious results.8 2. Kinetics in Tetrahydrofuran.-Since the cited state to the ground state. This finds its chemical deoxygenation of T H F appeared to be simplest explanation in the Franck-Condon Prineffective, particularly detailed studies were made in ciple13 and in the intuitively helpful notion of an this solvent. The anthracene data are summarized intramolecular sensitized transition. l4 In conin Fig. 3, in which only a few typical runs can be sidering the pattern of energy degradation in elecshown. The straight line is a least-squares plot, tronically excited molecules, the intercombination using 150 points in the AD range from 0.01 to 0.10, transitions may be treated similarly to their spinderived from several oscillograms taken at each of coriserving counterparts, except of course for the 9 different concentrations. S o drift in A , nor any special spin restriction. For molecules with inirreversibility, was detected throughout the entire herently high triplet yields, as for example, those series of measurements. The points a t AD’S with low-lying n - r levels, bimolecular quenching higher than 0.10 scattered somewhat, possibly due processes, particularly by ubiquitous oxygen, must play a major role in energy degradation. The to tailing of the flash, and were not used. The intercept is actually very slightly negative mechanism of reversible quenching thus assumes (-6 sec.-l). Xinety per cent of the points lay new importance for our understanding of such disbelow a line drawn parallel to that of Fig. 3 and sipative processes. The question is still open whether viscosity can 11-ith intercept 50 sec.-l. The estimated error of +40 set.-' is taken as an upper limit for k1 in indeed affect the rate of a “unimolecular” process THF. Kithiii our error, this is evidently indis- in solution,16l7 or whether temperature alone is tinguishable from the radiative rate constant of the essential rate-determining variable for radiaanthracene (between 10 and 100 sec.-l)l2 or the tionless decay processes in thermally equilibrated excited states. The trend of “A” over the past limiting rate in highly viscous media (2’7 sec.-1),7 years suggests that the latter is indeed the case, exIt is noteworthy that, over the concentration range studied (2.2 X to 29 X X ) no cept perhapq in the situation where slow torsional trend toward increasing intercept with conceutra- vibrations of large molecular segments are coupled tion was discernible. Thus, k3Co < 40 sec.-l and k g to the electronic transitions. must be less than about 106 1W-l see.-’. Values (13) G W Robinson and R. E’. Frosch, International Symposium of X.3 near 10‘ J l - I see.-’ reported for chlorophyll on Reversible Photochemical Processes, Durham, iS C. April, 1962, id references therein triplets may possibly be iii error due to impuritici (14) J Franok and H Sponer, J Chem Pliys 26, 172 (1956) introduced ivith the solute at high concentrations.ll (15) E Clement1 and M Kasha, J Molec. Spectry., 2, 297 (1958). I
i;.: t
x.)
-+
(12) F. LIcGlynn, 11 Padllye and AI. Iiasha, J . Chem Pli7,s 593 (1055).
23,
N Leuis and 12 Callin Chem Rev , 26, 273 (1939) (17) J 1-rancii and R Lilingston, J Cliem Ehgs., 9, 184 (1941).
(16) G
