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J . Phys. Chem. 1991, 95, 2022-2026
Kinetics of the Photoenollzatlon of 5,8-DImethyCl-tetralone. Hydrogen-Transfer Tunnel Effects in the Exclted Trlplet State W. AI-Soufi, A. Eychmiiller,' and K. H. Grellmann* Max-Planck- Institut f a r Biophysikalische Chemie. Abteilung Spektroskopie, Am Fassberg, 0-3400 Cdttingen, Germany (Received: July 25, 1990; I n Final Form: October 8, 1990}
The kinetics of the photoenolization of 5,8-dimethyl-l-tetralone(DMT) has been studied laser-flash photolytically in a polar solvent at different temperatures between 80 and 290 K. The transient absorption spectra of the tautomeric keto and enol triplet states of DMT were determined between 320 and 490 nm by analyzing absorbance decay traces kinetically. It could be shown that the hydrogen-transferreaction in the excited triplet states of DMT is largely governed by tunnel effects. Nonlinear Arrhenius plots and large isotope effects provide convincing evidence for these tunnel contributions to the rate of the keto-enol tautomerization in the excited triplet state. The hydrogen and deuterium transfer rate data of DMT are very similar to those of 2-(2'-hydroxyphenyl)benzoxazole, where tunnel effects in the excited triplet states also play an important role.
CH3 0
1. Introduction
Methyl-substituted ketones such as 5,g-dimethyl- 1-tetralone (DMT) are photochemically very stable compounds because photoexcitation leads to enolization and the enol form (E) reverts thermally to the keto form (K)? This process involves the excited triplet states (3K* and 3E*) of the two tautomers, and the enol triplet state relaxes by intersystem crossing into its singlet ground state (3E* 1E).3*4 We have shown some time ago that the thermally activated hydrogen transfer from the enol to the keto singlet round state ('E 'K)is governed by hydrogen tunnel effects. Evidence for such effects comes from nonlinear Arrhenius plots and large isotope effects. The latter are observed when the 8-methyl group of DMT is deuterated (DMTD).
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f
c,H2
H'O
CH3
K
E
DMT
We believe that in many hydrogen- and proton-transfer reactions tunnel effects play an important role, and the number of examples is increasing where such effects have been demonstrated experimentally. Very little is known, however, about hydrogen tunneling in electronically excited states. DMT is a promising candidate to study such a reaction because a hydrogen atom is transferred from the 8-methyl group of DMT to its carbonyl group during the photoenolization, ,K* 3E*, in the triplet manifold. To find out whether tunnel effects contribute significantly to the rate of this reaction, the rate constant k, in the reaction cycle (1)
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k k h 1K 2 IK* + 3K* A 3E* 2 1E
(1)
has to be determined as a function of the temperature for DMT and its isotopomer DMTD. This turned out to be a difficult task because the absorption spectra of the three transients 3K*, 3E*, and 'E overlap and the rate constants k l and k2 become very similar under certain conditions. In principle, one could also investigate 2'-methylacetophenone (MAP) because its photophysical properties are very similar to those of DMT.'" However, ( I ) P r c m t address: Hahn-Meitner-lnstitut, Abt. Strahlenchemie, D-1000 Berlin 39, Germany. (2) Yang, N.C.; Rivas, C. J . Am. Chcm. Soc. 1%1,83, 2213. (3) Haag, R.; Wirz, J.; We ner, P. J. Hclv. Chim. Acra 1977, 60, 2595. (4) Scaiano, J. C. Chcm. Pfys. krr. 1980, 73, 319 (5) Grellmann, K. H.; Weller, H.; T a w , E. Chcm. bhys. krr. 1980, 95,
195. (6) Baron, U.; Bartelt, G.; EychmlIller, A.; Grellmann, K. H.; Schmitt, U.; T a w , E.; Weller, H. J . Phorochcm. 1985, 28, 187.
0022-3654/91/2095-2022$02.50/0
b " C H 3
MAP
CH2 OH W
C
MAP
H
CH2 CH3 3
- EZ
@OH
MAP
- EE
the analysis of transient decay curves would be even more complicated because photoenolization leads to two isomers, MAP-EZ and MAP-EE, which in the ground state have practically identical absorption spectra but drastically different lifetimes. 2. Experimental Technique
I. Solvents and Compounds. The solvent mixture EPA (diethyl ether-isopentane-ethanol 52:s by volume) was prepared from diethyl ether (Uvasol, 98%, 2% ethanol, Merck), isopentane purum (Fluka) purified on a silica (Woelm 100-200 active)-aluminum oxide (Woelm B, Super 1, W 200) column, and ethanol (LiChrosolv, 99.576, Merck). S&Dimethyltetralone (DMT) was prepared from pxylene and succinic acid according to Khalaf et al.;' 5,8-dimethyltetralone-d8 (DMTD) was prepared analogously from p-xylene-dlo (Aldrich, 99% D). Deuteration of the methyl groups and the two aromatic hydrogens was 298% (checked by NMR). It should be mentioned, as a warning, that a synthesis described by Katritzky et aL8 leads to a mixture of dimethyltetralones because of CH3group migration during the reaction. Under various conditions it was not possible to fully deuterate the 8-methyl group of DMT photochemically. The highest degree of deuteration we could obtain was 60%. Similar difficulties were reported by Haag et al.3 Immediately before use DMT and DMTD were purified gas chromatographically. The samples were degassed on a greaseless vacuum line by the usual freeze-pump-thaw technique and sealed-off thereafter. 2. Flash Apparatus. Laser-flash experiments were carried out with an exciplex laser (Lambda Physik EMG 500). For excitation the 248-nm KrF line was used throughout. Details of the experimental setup and the computer analysis have been reported re~ently.~ 3. Results In section 3.1 we will give a qualitative description of transient decay curves leading to a rough picture of the transient spectra. For this purpose, the reaction cycle (1) is simplified and approximate lifetimes are assumed for the relevant transients. In section 3.2 the kinetic analysis of the decay curves is described. It yields more accurate transient absorption spectra, which are (7) Khalaf, A. A.; Abdel-Wahab, A. A.; El-Khawaga, A. M.; El-Zohry,
M.F. Bull. Soc. Chim. Fr. 1984, 285. (8) Katritzky, A. R.; Marson, C. M.;Thind, S. S.; Ellison, J. J . Chcm.
SOC.,Pcrkin T r a m I 1983, 487. (9) Grellmann, K. H.; Mordzinski, A.; Heinrich, A. Chcm. Phys. 1989, 136, 201.
Ca 1991 American Chemical Society
Hydrogen-Transfer Tunnel Effects
D m
0.11
1 Y/Dh
H' s
o 0.4
0 11
0 I
f
06
~
"
"
" ~ 350
"
'
and therefore one observes on a 1-ps time scale (Figure lAl) after the instantaneous rise of the 3K*absorption a further absorbance increase within about 1 ps, due to the formation of 3E*, which then decays within about 30 ps (Figure l A , and ]Az) to the "constant" absorbance level of 'E. At 370 nm, the extinction coefficients of the three transients are about equal (Figure lBl ) c(~E*) and 1B2)and a t 430 nm, the order is e('E) > E ( ~ K *> (Figure l C I and lCz). At 465 nm (Figure IE), the absorption spectra of 3E* and 'E have an isosbestic point (e('E*) = €('E)) and the decay of 3K*can be seen without overlap of growing-in or decay processes. Below and above that wavelength the extinction coefficientsdiffer (e.g., 450 nm, Figure ID, e('E) > C(~E*); 480 nm, Figure lF, e('E) C c(~E*)),with the result that the transient absorbance decay pattern is more complicated than in Figure 1E. This qualitative interpretation has to be, of course, consistent with the transient decay kinetics (see section 3.2) at each wavelength of this spectral region. 2. Transient Decay Kinetics. According to Figure 1 a simple monoexponential decay of the absorbance is not to be expected at any experimentally accessible monitoring wavelength, because each of the three transients 3K*, 3E*, and 'E absorbs at all wavelengths between 320 and 490 nm. For a reaction sequence A B C the concentrations a, b, and c of the transients A and B and of the final product C change with time according to eqs 3-5 under the condition a(t=O) = a.
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t '€77 -
0
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 2023
" 400
X/nm
"
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'
'
~ 450
I
Figure 1. Top: Absorbance decay traces observed after exciting a 1 X IO-' M solution of DMT in EPA at 150 K with the 248-nm KrF line of an exciplex laser. The monitoring wavelengths are indicated in the figure. Note that the absorbance scales range from 0.1 1 to 0.70. The optical path length of the flash cell was d = 2 cm. Bottom: Absorption spectra of the transients 3K*( O ) ,3E* (+), and 'E (A) obtained in 5-10-nm steps (bandwidth 1-3 nm) at 150 K by fitting eq 7 to traces like those shown in the upper panel.
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shown in the lower panel of Figure 1. Finally, in section 3.3 the temperature dependence of the hydrogen transfer (3K* 3E*) and the intersystem crossing reaction (3E* 'E)and the effect of isotopic H-D substitution on the rate constants of these two processes are reported. 1. Tranrient Absorption Spectra. Figure 1, upper panel, shows a selection of transient absorption decay curves of a DMT solution in EPA that was flashed at 150 K. Obviously, the decay pattern depends strongly on the wavelength of the monitoring light because the absorption spectra of the flash-photolytically produced transients overlap. Decay traces were recorded between 320 and 490 nm in 5-IO-nm steps (bandwidth 1-3 nm). To construct transient absorption spectra from decay traces such as those shown in Figure 1, the reaction cycle (1) can be simplified for two reasons. First, the intersystem crossing process 'K* 3K*occurs within picosecond^,'^ and second, the lifetime of the DMT enol singlet state, s('E), is long at 150 K, because the ground-state hydrogen back transfer 'E 'K is strongly temperature dependent in polar solvents such as EPA and therefore slow at this temperature.- On the time scales of Figure 1, the formation of 3K* occurs therefore "instantaneously", the lifetime of 'E is "infinite", and for a kinetic analysis the reaction cycle (1) can be reduced to a sequence of two consecutive reactions:
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At 150 K the approximate lifetimes (for more exact values see below) of the three transients are #K*) FLI 0.5 ps, 7(3E*) 10 ps, and s('E) = 300 ms. With these values in mind one can interpret the decay traces in Figure 1 as follows: at 340 nm, 3E* has the largest extinction coefficient (€()E*) > 4 K * )> c('E)),
a = a. exp(-klr)
(3)
b = aoQ[exp(-klr) - exp(-kzt)]
(4)
c = ao(1 - Q[(kz/ki) exp(-kit) - ex~(-kzt)ll (5) and b(t-0) = c(t=O) = 0, where Q = k , / ( k 2 - kl). The observed time-dependent total absorbance, Aobr is Aobs = (tall + ebb + C,C)d (6) e,, t b , and e, are the extinction coefficients of A, B, and c a t a given monitoring wavelength; d is the optical pathlength. With (3)-(5) the total absorbance decay becomes Aok = A. exp(-k,t) BoQ[exp(-klt) - exp(-k2t)] + Co(l - Q [ ( b / k J exp(-kit) - ex~(-k2t)ll (7)
+
where at a given monitoring wavelength A0 = e,aod Bo = CbUod
co = C,U&
In the present case, e,, Cb, and e, are the extinction coefficients of 3K*, 3E*, and 'E, respectively, and a. is the 3K* concentration at time zero, Le., immediately after flash excitation. With the five adjustable parameters Ao,Bo,Co, k l , and k2 eq 7 is fitted to the absorbance decay traces Aob,observed at different monitoring wavelengths, whereby the absolute value of a. is not known and is set arbitrarily to a. = 1. The results of such fits are, of course, meaningful only if, within experimental error, at a given temperature two of the parameters, namely the rate constants kl and k2,do not depend on the monitoring wavelength. Thus, the requirements for a set of fits are more stringent than might appear at first sight for such a large number of parameters. In Figure 2 four examples for such fits are depicted. Besides Aokr which is the sum of three exponential functions (cf. eq 7), the individual contributions of these three functions to Aob are also shown, labeled 3K*, 3E*, and 'E. At 320 nm (Figure 2a), where 'E absorbs very little, mainly the decay of ,E* is observed, overlapped at the very beginning by the "instantaneous" formation of 3K* and its subsequent decay into ,E*. At 420 nm, on the other hand, where 'E absorbs most strongly, the formation of 'E from 3E* can be best analyzed (Figure 2b). On a compressed time scale (20 ps/division, Figure 2c) one can see at the same wavelength the grow-in of the final IE absorbance, which has an "infinite" lifetime on this time scale. The photophysically most interesting can process, i.e., the hydrogen transfer during the decay of 3K*, be observed most directly around 480 nm (Figure 2d) because in this wavelength region 3K* has the highest extinction coefficient of the three transients. From the fits of eq 7 to the observed absorbance decays, Aob, at different monitoring wavelengths one
2024 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
AI-Soufi et al.
0.07
0
0
limo
Figure 2. Kinetic analysis of transient absorbancies obtained under the same conditions as in Figure 1 and monitored at 320 nm, time scale 2 @/division (a), 420 nm, time scale 2 @/division (b), 420 nm, 20 @/division (c) and 480 nm, time scale 2 @/division (d). In addition to the (slightly wavy) observed decay traces, the fit of eq 7 to these traces together with the individual contributionsof the three transients (see text) is shown. For the five-parameter fit the following values were used: A, = 0.60,Bo = 0.71, Co = 0.055, k,/s-' = 1.6 X IO6, k2/s-' = 1.5 X IO5 (a); A. = 0.18, Eo = 0.12, Co = 0.45, kl/s-'= 2.0 X 106, k2/s-' = 1.0 X IO' (b,c); A. = 0.066, Bo = 0.029, Co = 0.0057, k,/s-' = 2.0 X lo6, k 2 / & = 1.0 X IO5 (d).
obtains the relative optical densities Ao.Eo, and Coof ,K*,jE*, and 'E,respectively, normalized to the initial concentration, ao, of ,K*.Such values, determined at 150 K, are plotted in the lower panel of Figure 1. Deviations of the laser excitation energy, I,,,, from a standard value (&IO% from shot to shot) were accounted for by adjusting Ao, Bo, and Counder the assumption that the initial concentration a. is proportional to Zac. At low temperatures the spectrum of 'E can also be measured with a conventional spectrophotometer because of the long lifetime of 'E. It agrees very well with that depicted in Figure 1. For each fit in Figure 2 and at the other monitoring wavelengths (cf. Figure 1, lower panel, experimental points) the same values for the two parameters k , = (1.8 f 0.2) X lo6 s-I and k2 = (1.1 f 0.1) X IO5 s-I (cf. Figure 2) were used. Very similar spectra were obtained at temperatures above 150 K. At temperatures 1135 K where the solvent mixture EPA becomes highly viscous, problems arise with the kinetic analysis of the decay traces (see section 3.3). The transient absorption spectra shown in Figure 1 are similar to those published by Haag et al.) for the corresponding transients of 3,3,6,8-tetramethyl- 1-tetralone (TMT). In our analysis of transient decay cuwes we did not consider the possibility that a fraction of the enol tautomer is formed (diabatically or adiabatically) via the excited singlet state, 'K*, of the keto tautomer. Haag et aL3 report that addition of >5 M of piperilene to a solution of T M T does not completely suppress photoenolization although the formation of the enol triplet state is completely quenched. They concluded that about 30% of the enol tautomer is formed in the singlet manifold. If this is also true for DMT, eq 6 becomes A La = (t,a + ebb + ec(c + c,))d (6') where c, is the 'E concentration, which builds up directly and "instantly" from 'K' during flash excitation. A fit of eq 7 to a given decay trace assuming c, = 0 (as in Figure 2) or c, > 0 (by subtracting a certain constant value tcc,d from Aoa in eq 7) will yield the same rate constants, but with c, > 0 the absolute values
of A,,, Bo, and Coand the ratios Ao/Coand Bo/Cowill be smaller than those shown in Figure 1. Their order, however (e&, Co> A. > Bo between 410 and 450 nm) and the positions of the isosbestic points will not change. One can estimate from the decay traces the upper limit of an eventual singlet-pathway contribution. In Figure ICl, for instance, the absorbance of the instantly formed enol singlet ground state could at most have the level of the minimum a t 1 ps, provided the decay of 3K* is practically finished at this point and the extinction coefficient of ,E*is zero. This maximum level amounts to 30% of the 'E end absorption, which develops from ,E*within about 40 ps (cf. Figure IC2). At other wavelengths one could assume a higher percentage of singlet-pathway contribution, if the ratio of enol triplet to enol singlet absorbance, Bo/Co,is larger than at 430 nm (e.g., at 420 nm, cf. Figure 2b,c); but that would, of course, lead to an overcompensation (i.e., a negative extinction coefficient of ,E*) at 430 nm, where the ratio Bo/Cois smallest. 3. Temperature Dependence of the Transient Decay Rates. The temperature dependence of the two rate constants k l (hydrogen transfer jK* jE*) and k2 (intersystem crossing 3E* 'E) was investigated between 80 and 270 K by using the monitoring wavelengths (cf. Figure 2) 480, 420, and 320 nm. The results of these measurements are summarized as Arrhenius plot in Figure 3. At 200 K the hydrogen-transfer reaction (rate constant k , ) is about 2 orders of magnitude faster than the intersystem crossing reaction (rate constant k2). It is therefore very easy to distinguish the two reactions around this temperature. Above about 230 K the decay of the enol singlet ground state into the keto singlet ground state, 'E 'K,has to be taken into account, Le., the lifetime of 'E is no longer "infinite" and the determination of the rate constants is less accurate than below 230 K. Above 250 K the rate constant of the reketonization process 'E 'K becomes even larger than k2,i.e., the intersystem crossing step jE* 'E becomes rate determining. The hydrogen-transfer rate is strongly temperature dependent, in contrast to the intersystem crossing rate 'E* 'E, which is
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The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 2025 4. Discussion
3r
