2174
The Journal of Physical Chemistry, Vol. 83,No. 16, 1979
G. Kittel, G. Kohler and N. Getoff
Sensitized Ketonization of Ethyl Acetoacetate. A Method for the Determination of Triplet Quantum Yields G. Kittel, G. Kohler, and N. Getoff" Institut fur Theoretische Chemie und Strahlenchemie der Universitdt Wien, 1090 Wien, Austria (Received October 16, 1978; Revised Manuscript Received January 22, 1979) Publication costs assisted by the University of Vienna
A new method for the determination of triplet quantum yields, QT, based on the ketonization of ethyl acetoacetate (EAA) sensitized by triplet energy transfer, is described. Steady state and fluorescence measurements were carried out, using benzene, toluene, and p-xylene as energy donors. The QT values obtained by this simple method are in very good agreement with data obtained by other methods. A probable mechanism for photoketonization is discussed.
Introduction Ethyl acetoacetate (EAA) exists in solution in a thermal equilibrium of both tautomeric forms, the chelated enol and the diketone (eq I) .l The enol form was shown to
chelated cis-enol
di ketone
undergo facile phototautomerization to the diketone under UV i r r a d i a t i ~ n This . ~ ~ ~reversible photoketonization was clearly demonstrated by the decrease of the intensity of the characteristic HK* band of the enolic form of EAA around 245 nm during irradiation in this wavelength range. Thereby the IR absorption band of the enol a t 1650 cm-l decreases, whereas the band of the ketone a t 1730 cm-l shows a corresponding increase. A rather long-lived transient, blue shifted compared to the chelated enol and converting quantitatively to the diketone, was reported to occur during this p r o c e s ~ . This ~ intermediate was tentatively assigned to a nonchelated isomer originating from a rotation around the C4-C5 double bond or the c 5 4 6 single bond of the cis-enol; this view was also supported by CNDO/S calculation^.^ As carbonyl compounds of this type generally undergo efficient intersystem crossing,6it seems reasonable that the photoketonization proceeds from the triplet states. Moreover, energy transfer from excited molecules of polar and nonpolar aliphatic solvents, causing also ketonization of EAA, was reported p r e v i ~ u s l y .This ~ focused our interest on the question, whether the tautomerization could be sensitized by energy transfer from aromatic triplet states. Since in this case EAA is directly excited in its triplet state, this study should give a deeper insight into the mechanisms involved in the ketonization process from excited states. Moreover, the outlook that this could lead to a method for the determination of triplet quantum yield of the aromatic donor is of importance. Some preliminary results in this respect were reported previously.8 The aim of the present work was a detailed investigation of the sensitized ketonization by some benzene derivatives as donors and, furthermore, to elucidate the possibility of the use of this system as a simple way to determine triplet state quantum yields. The photoketonization mechanism will be discussed according to the present results. Experimental Section and Results Preparation of Solutions. EAA (p.A., Merck, Darmstadt) was dried by Na2S04and distilled under vacuum. 0022-3654/79/2083-2174$01 .OO/O
Benzene (p.A., Merck) and p-xylene (puriss, Fluka) were purified by zone refining, toluene (p.A., Merck) was purified by distillation under vacuum, and cyclohexane (p.A., Merck) by column chromatography (basic A1203). All solutions were degassed and saturated with oxygen-free argon (Oxygena, Vienna). In equilibriated cyclohexane solution EAA exists to 56% (in infinitely diluted solutions) in the enol form.g This is true for concentrations up to mol dm-3, which were used throughout the experiments. The minimal time for establishing tautomeric equilibrium was about 40 h. Absorption and Fluorescence Measurements. Absorption measurements were performed on a UV-visible spectrophotometer, Coleman 575 (Perkin-Elmer). The fluorimeter used in this work was described earlier.l0 The absorption spectrum of EAA shows a very weak nn* transition of the keto form (Amm = 275 nm, emax = 100 dm3 molw1cm-lll), which is hidden by the very strong mr* transition of the chelated enol (A,, = 244 nm, = (12000 f 400) dm3 mol-l cm-I). This t value is normalized to the concentration of the pure enol. Photochemical Experiments. As the excitation source, a low pressure mercury lamp (Osram, Type HNS 10 W, ozone free) was used, which emits light only at X = 254 nm. The windows of the irradiation cell were made of suprasil quartz and the optical depth was 5 cm. Actinometry was carried out by using an aqueous solution of chloroacetic acid ( c = 1 mol dm-3, saturated with air, Q(Cl-) = 0.36, T = 30 0C)12~13 yielding an intensity of Io = 1.82 X 10l6 hv mL min-l. During irradiation all solutions were stirred and the temperature was kept constant to a t least f0.5 "C. Irradiation of a EAA solution in cyclohexane with 254-nm UV light results in a decrease of the absorption @E,) a t 244 nm. This effect depends linearly on the irradiation time and is independent of the solute concentration. The diketone quantum yield (QKE), which corresponds to the disappearance of the enol absorption, was derived from eq 1. For oxygen free solutions a t 24 EE(6.02 X 10'') QKE
=
E10
(1)
"C, QKE = 0.21 f 0.01 and, a t 30 "C, QKE = 0.22 f 0.01 were obtained, in very good agreement with the value of 0.12: giving 0.215 when normalized for the pure enol form. Practically no influence of oxygen on QKE was observed, since, in solutions saturated with oxygen and irradiated a t 24 "C, QKE = 0.22 f 0.01 was found. The initial equilibrium was completely restored after 300 h, when kept in the dark a t room temperature. 0 1979 American Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 16, 1979 2175
Sensitized Ketonization of Ethyl Acetoacetate I
I
TABLE 11: QT Values, Determined by the EAA Method at 24 O C , in Comparison with Results from Other Methodsa QT values
EAA (this work)
:: donor
benzene toluene p-xylene
0.02 0
0.5
1.o
C [mol 1.5dm-31
2 .o .1o-L
Figure 1. Q as a function of the concentration of ethyl acetoacetate for 5.7 X 10' mol dm-3 toluene in oxygen-free cyclohexane solution, excited with A = 253.7 nm, at 30 'C.
TABLE I: Rate Parameters for the Quenching of Donor Fluorescence by EAA at 20 f 2 C in Cyclohexane Solvent
Ksv w-')
donor benzene toluene p-xylene
2 1 5 2 25 246i: 12 180 i: 8
-"
:'?; 7 . 1 * 1.5 7.2 * 0.8 6.0 * 0.7
It was further established that the ketonization of EAA can be sensitized by benzene (c = 0.081 mol dm-3),toluene (c = 0.075 mol dm-3), and p-xylene (c = 0.081 mol dm-3) used as energy donors. The concentrations were chosen avoiding dimer and excimer formation of the aromatic donors, but ensuring that the absorption of the acceptor EAA a t 254 nm can be neglected. For each of these solutions the quantum yield of the sensitized ketonization (QKE') was determined as a function of the concentration of EAA, C(EAA). After irradiation A& was determined against a reference solution of the same donor concentration. No change in the absorption of the donor was observed during the irradiation. QKES was calculated following eq 1. The results for toluene as donor are given in Figure 1. The dependence of QKE5 on c(EAA) is rather the same for all three donors. It increases linearly with c(EAA) a t low acceptor concentrations reaching a plateau value above 1.2 X mol dm-3 of EAA (Figure 1). This was the lower limit for the EAA concentration used in the following experiments. In the course of these investigations the quenching of the donor fluorescence by EAA was measured. The obtained Stern-Volmer constants (&") and the corresponding rate constants are given in Table I. The singlet lifetime values were taken from ref 14. These data show that for the applied acceptor concentrations singlet quenching can be neglected. Evaluation of the Results. Assuming that sensitized ketonization of EAA follows energy transfer from the donor triplet state, the donor triplet quantum yield (QT) can be calculated from eq 2. QAT represents the quantum yield (2) for energy transfer from the donor triplet to the acceptor (EAA) and QmT for the efficiency of ketonization from the EAA triplet state. QAT is given by QKE'
= QTQATQKE~
A simple combination of eq 2 and 3 leads to 1 1 1 I +--- 1 -=(4) QKE' QTQKE~ Q T Q K E ~ T T ~ A T[enol] A plot of I/QKEs vs. l/[enol] gives, therefore, the rate
A
B
C
0.252&ph 0.25 i- 0.02' 0.24 f 0.02 0.23e 0.50 i 0.03 0 . 4 5 3 ' 1 ~ 0.530b,ai 0.51 * 0.03' 0.61 * 0.03 0,566d1g 0.63bJ 0.64 f 0.03'
a The values are obtained in cyclohexane as solvent except where noted. The other methods used were (A) sensitized isomerization of simple olefins; (B) sensitized phosphorescence of biacetyl; (C) method A in connection with heavy atom quenching experiments. 20 'C, cyclohexane solvent. ' 24 6, methylcyclohexane solvent. 22 O C , methylcyclohexane solvent, e Reference 16. f Reference 17. g Reference 18. Reference 15. Reference 19. Reference 15. Reference 14. J
'
parameters for the triplet quenching, e.g., for toluene ~ / ( T T hAT) = (6.3 f 1.0) x M. With TT = 3.3 x lo4 S,15 AT = (4.8 f 0.9) X lo9 M-l s-l was obtained. For EAA concentrations were the plateau value is reached, it is supposed that the donor triplet state decays only by an energy transfer mechanism. This means that k~T[enol]>> l / r T and therefore QAT 3 1. For further calculations QmT was assumed to equal Qm for direct photoketonization. On these premises the values of Q T for the three donors were calculated and they are presented together with results obtained by other methods in Table 11. Furthermore, it should be mentioned, that under our experimental conditions QT can be simply determined from changes in the absorptions during irradiation: QT = aEE(with donor)/A&(without donor) (5)
Discussion The lowest excited singlet state of EAA in its enolic form is of nT* character and it is not observed by absorption spectroscopy in dilute solution (a CNDO/S calculation gave 3.382 eV, corresponding to 365 nm for its vertical energy5). In comparison to similar a,@-unsaturated ketones, the 3 n ~ and * %7r* states can be expected a t energies below 3 eV (410 nm), nearly degenerated with one another.20 Benzene and its simple derivatives are therefore efficient triplet energy donors for EAA, since the energy of their relaxed triplet in hydrocarbon solution lies above 3.4 eV.21 In such a process only the EAA triplet state is excited and its role in the ketonization can therefore be elucidated. The results given in Figure 1 and Table I1 show that energy transfer from the aromatic triplet to the EAA triplet causes ketonization and the obtained values for the quantum yield of triplet formation of the donor molecules, QT, are very close to those from the literature.14-19 Therefore, the efficiency of the processes competing with relaxation from the initially excited TT* state should be very low. Hence, the photoketonization proceeds predominantly from the relaxed triplet. For the explanation of this process, s-cis-s-trans isomerization by rotation around the C5-Cs single bond could be assumed since the H bond is broken in the excited state. Because of the rather high activation energy associated with such an isomerization process, it appears unlikely to occur.2o On the other hand, twisting of the ethylenic double bond is indeed a stabilization process and this is also an important mode for loss of quanta in comparable molecules.20The orthogonal triplet so formed is nearly degenerated with the ground state and efficient
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The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
intersystem crossing occurs. This perpendicular state was also found by laser flash photolysis recently.22 Since the energy of such an orthogonal triplet state should be very close to or even lower than that of the orthogonal diradical ground state, quenching of this state should be inefficient. This explains the lack of O2 quenching of the photoketonization. In this perpendicular configuration the proton of the OH group can easily migrate to the unoccupied orbital on C5, resulting in the formation of the diketone. However, the trans form can be also formed and this is most probably the intermediate found p r e v i ~ u s l y .The ~ formation of the diketone from trans-EAA is a relaxation process and it should favorably occur rather than rotation around the C& double bond.23 The values for QT determined by the method described above are somewhat higher than these obtained by sensitized isomerization of simple olefins.16-18 A combination of the last method with heavy atom quenching14 leads, however, to good agreement with our data, as well as with those determined by sensitized phosphorescence of biacetyl.l5>l9 Following eq 5, the rate parameters kAT for triplet quenching by this electron resonance mechanism was calculated and the obtained value, kAT = 4.8 X lo9 M-I s-l, is near the limit of diffusion control. It is comparable to the kAT value for biacetyl as a triplet quencher (kAT = 5 X lo9 M-l s-l15),but considerably higher than that for the dienes (e.g., for trans-2-octene, hAT = 0.74 X lo9 M-l s-l181. This could be due to a possibly lower triplet energy of EAA than that of the olefins. On the other hand, the kQMvalues, the rate constants for singlet quenching by EAA (Table I), are comparable to those for dienes (e.g., for toluene quenched by cis-bibut are somewhat perylene, kQM = 5.9 X lo9 M-l lower compared to those for biacetyl (for toluene, kQM= 15 x 109 M I s-115 ). Therefore, it is evident that EAA exhibits properties of a very useful triplet quencher. While the biacetyl method15J9 requires a complicated evaluation of the kinetics because of the delicate corrections for singlet quenching, calculations for the EAA method are very simple. There is only one product formed, which does not react further, and its spectrophotometric determination is simple and reliable. The results show that the triplet quantum yields of appropriate donor molecules can easily be determined by
Communications to the Editor
sensitized ketonization of EAA. Molecules with triplet energies above 3 eV are efficient energy donors. As already outlined above, EAA exhibits good acceptor properties. In general, it is worthwhile to check the reliability of QT values by comparing the results of various methods in order t o exclude methodical errors. For Q T determinations from T-T absorption spectra, an indirect method will still be necessary for the determination of the triplet absorption coefficients. Acknowledgment. The authors thank the Fonds zur Forderung der wissenschaftlichen Forschung in Austria and the Ludwig Boltzmann Gesellschaft for generous financial support.
References and Notes C. Reichard, "Losungsmittel-Effekte in der organischen Chemie", Verlag Chemie, Weinheim/Bergstrasse, 1969. P. Markov, L. Shishkova, and Z. Zdravkova, Tetrahedron Lett., 39, 4017 (1972). P. Markov, L. Shishkova, and A. Radushev, Tetrahedron, 29, 3203 (1973). D. Veierov, T. Bercovici, E. Fischer, Y. Mazur, and A. Ycgev, J. Am. Chem. SOC.,99, 2723 (1977). P. Markov and F. Fratev, C . R . Acad. Bulgar. Sci., 28, 771 (1975). M. A. El-Sayed, J . Chem. Phys., 38, 2834 (1963). N. Getoff and F. Fratev, 2.Phys. Chem. (Frankfurfam Main), 104, 131 (1977). G. Kittel, G. Kohler, and N. Getoff, Proceedings of the IXth International Conference on Photochemistry,Cambridge, England, Aug 7-9, 1978: J. Photochem., 9, 257 (1978). H. Mauser and B. Nickel, Chem. Ber., 97, 1745 (1964). G. Kohler and N. Getoff, Chem. Phys. Lett., 26, 525 (1974). A. S. N. Murthy, A. Babsubramian, and C. N. R. Rao, Can. J. Chem., 40. 2267 (1962). R. N. Smith, P. A. Leighton, and G. W. Leighton, J. Am. Chem. SOC., 61, 2299 (1939). M. Neumann-Spallart and N. Getoff, Monatsh. Chem., 106, 1359 (1975). F. A. Carol1 and F. H. Quina, J . Am. Chem. SOC.,98, 6 (1976). K. Sandros, Acta Chim. Scand., 23, 2815 (1969). R. B. Cundall and D. A. Robinson, Trans. Faraday SOC.,68, 1145 (1972). R. B. Cundall, L. C. Pereira, and D. A. Robinson, Trans. Faraday Soc., 69, 701 (1973). R. R. Hentz and R. M. Thibault, J . Phys. Chem., 77, 1105 (1973). K. Sandros, Acta Chim. Scand., 25, 3651 (1971). A. Devaquet, J. Am. Chem. SOC.,94, 5160 (1972). J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, London, 1970. J. Joussot-Dubien, R. Bonneau, P. Fornier, and D. Violet in "Excited States in Organic Chemistry and Biochemistry", P. Pullman and N. Goldblum, Ed., Reidel Publishing Co., Dordrecht, 1977, p 271. M. Dewar and M. Shanshal, J . Chem. SOC.A , 25 (1971). P. M. Froehlich and H. Morrison, J. Am. Chem. SOC.,96, 332 (1974).
COMMUNICATIONS TO THE EDITOR Gas Phase UV-Visible Spectra of Reacting Hydroaromatics Systems Publication costs assisted by Brookhaven National Laboratory
Sir: Work a t this laboratory on the reaction kinetics of the hydroaromatic system tetralin (T)-l,Z-dihydronaphthalene (D)-naphthalene (N) at 400 "C utilized UV-visible spectral data to follow the individual species concentration/time profi1es.l This communication is a companion paper detailing the analytical method and establishing the gas-phase spectra and extinction coeffi0022-3654/79/2083-2176$01 .00/0
cients ( E ) . The optical cells were acid cleaned, rinsed, dried, and vacuum flamed. The vessels, 10-cm optical pathlength/ 1.0-cm diameter quartz cells with Suprasil 1 windows, were filled with degassed aromatic (purity determined by gas chromatography) and sealed. The E determinations involve the following sequence: recording spectra a t 120 "C, recording spectra a t 400 "C, recording spectra at 120 "C, cooling (77 K), adding hexane while cell warms to room temperature, recording solution spectra, and determining mass of hexane added. The second 120 "C spectra assured absence of reaction. The solution spectra yielded the aromatic concentration. Temperature C 1979 American Chemical Society
