Se ha medido la velocidad de desactivación de tripletes carbonilicos (biacetilo, bencilo, benzofenona y 4-metil-2-pentanona) por una serie d,e derivados del benceno. Los resultados pueden ser interpretados por un mecanismo de transferencia de carga. Los compuestos con hidrógenos lábiles (i.e.cumeno) muestran una mayor reactividad. La abstracción de hidrógeno ocurre desde el complejo de transferencia de carga. La desactivación del triplete de la 4-metil-2-pentanona fue medida entre - 5 y 70°C. Las energias de activación obtenidas variaron entre 8.1 kcal (para el cumeno) y 10 kcal (para el benceno)

Encina, M. V.

Lissi, E. A.

Olea, F. A.

Soto, H.

Contribuciones Científicas y Tecnológicas

Contribuciones Científicas y Tecnológicas NO 51 - Area Química XVlll -Noviembre de 1981. * ** *** Quenching of excited carbonyl triplets by benzene derivatives* M. V. Encina**, H. Soto**, E.A. Lissi** and F.A. Olea*** RESUMEN: Se ha medido la velocidad de desactivación de tri-pletes carbonilicos (biacetilo, bencilo, benzofenona y 4-metil-2-pentanona) por una serie d,e derivados del benceno. Los re-sultados pueden ser interpretados por un mecanismo de trans-f erencia de carga. Los compuestos con hidrógenos lábiles (i.e. cumeno) muestran una mayor reactividad. La abstracción de hidrógeno ocurre desde el complejo de transferencia de carga. La desactivación del triplete de la 4-metil-2-pentanona fue medida entre - 5 y 70°C. Las energias de activación obte-nidas variaron entre 8.1 kcal (para el cumeno) y JO kcal (para el benceno). Manuscrito revisado y aprobado en forma definitiva en Octubre de 1981. Departamento de Química, Facultad de Ciencia, Universidad de Santiago de Chile, (ex-Universidad Técnica del Estado). Departamento de Ciencias Formativas, Facultad de Ciencias Básicas y Farmacéuticas, Universidad de Chile. 17 18 S UMMAR Y: The quenching o[ 4-methy/-2-pentanone, biacetyl, benzil and benzophenone triplets by severa/ benzenes deriva-tives have been measured. The results obtained are compatible with a charge transfer mechanism in which the aromatic com-pound /eads charge to the excited carbony/. When the aromat-ic compounds has Jabí/e hydrogen atoms, the intramo/ecular hydrogen abstraction /eads toa [aster rate constan!. This proc-ess takes place, at /east [or 4-methy/-2-pentanone, [rom the excited complex. Quenching o[ 4-methy/-2-pentanone triplets are mea-sured as a function o[ temperature (- 5 to 70°C}. The activa-tion energies obtained range from JO kca/ (benzene as quencher) to 8.1 kca/ (for cumene). INTRODUCTION Benzene and benzene derivatives are well known quenchers of excited carbonyl com-pounds. Quenching rate constants for the desactivation process and/or the lifetime of the carbonyl compounds when an aromatic hydrocarbon (usually benzene) is employed as solvent have been reported for acetone1 ' 2 ' 3 , aliphatic ketones4 ' 5 , a-tri-fluoroceto-phenones6' 7 ' 8 , benzophenone3 ' 9 ' 1 0 ' 1 1 and acetophenone and benu.ldehyde8 ' 10 • Most of the results obtained have been interpreted in terms of a charge transfer mechanism with the aromatic ring acting asan electron donor towards the excited triplet9 ' 1 2 . Never-theless, when the aromatic compound has labile hydrogen atoms, contribution to the quenching rat e by a hydrogen transfer process can not be disregarded7 ' 8 • In spite of the bulk of data obtained in systems employing excited carbonyls and aromatic molecules, only in a work the measurements ha ve been carried out at different temperatures1 0 • In this work, the activation energies and the preexponential A factors varied in a rather erra tic way. In particular it was observed that quenching of triplet benzaldehyde by benzene, in spite of a modera te quenching rate constant of 2 x 107 M- 1 s- 1 at 25°C, the activation energy obtained was negative. From the above considerations, and given the importance that as solvents ha ve the aromatic hydrocarbons, the present work was carried o ut with three aims: i) To extend the presently available data regarding the quenching of carbonyl triplets by aromatics to other carbonyl co mpounds, ii) To measure the rate for severa! alkyl benzenes with different degrees of a substitution in arder to evaluate the contribution of the intramolecular hydrogen abstraction, and iü) To obtain the Arrhenius parameters for severa! processcs that occur through the formation of charge transfer complexes in arder to evaluate if low, and even negative, energies of activation axe a general feat ure of these processes. EXPERIMENTAL The experimental method employed when 4-methyl-2-pentanone was employed as donor was similar to that described previously13 . The temperature was varied from - 5 to 70°C. Quenching experiments employing benzophenone, biacetyl and benzil as donors were carried out by measuring the phosphorescence yield as a function of the added hydrocarbon concentration. The measurements were carried out at room temperature (20 ± 2°C) in a 204-S Perkin Elmer Spectrofluorimeter. RESULTS The results obtained are summarized in Table 1 and 2. Values of ko at room tempera-ture were obtained from the slopes of the Stern-Volmer plots and the known lifetimes of the sensitizers under the conditions employed 1 3 •14 . The change in ko with temperature was deterrnined by measuring at each temperature, the effect of the quencher considered and 19 that of cis-1, 3-pentadiene. From the relative values of the slopes of the Stern-Volmer plots, and taking for the quenching by the diene kQ = 5 x 1010 exp(-1.3/RT)1 5 , values of kQ for the quencher considered at each temperature can be obtained. By plotting these values as a function of the inverse t emperature, the values of Ea and log A given in Table 1 were ob-tained . TABLEI Rate constants and Arrhenius parameters for the quenching of 4-methyl-2-pentanone triplets by substitutes benzenesa. Quencher (kQ) 20°C E a log A X 10- 6 benzene 1.1 9.1 12.8 toluene 3.6 9.1 13.3 ethylbenzene 5.3 8.2 12.8 isoproylbenzene 4.1 7.4 12.1 tert-butylbenzene 2 .7 7.9 12.3 cyclohexene 4.3 9.0 13.3 a) n·hexane as solvent. TABLE2 Quenching of carbonyl excited triplets by substitutes benzenes'l Quencher toluene p-chlorotoluene tert -b utylbenzene p-xylene p-methylanisole isopro pylbenzene IP 8.8 8.69 8.68 8.4 8.1 Donor Biacetyl Benzil Benzophenone 0.2 0.1 6.0 3.8 3.9 16.0 0.01 0.04 0.22 0.34 0.63 1.10 50 23 34 310 2450 370 a) Rate constants given in 104 M- ' s- 1 . Benzene as solvent. DISCUSSION For a quenching process involving a charge transfer complex with partía! charge transference from the acceptor to the excited triplet the quenching rate constant increases when the Ionization Potential of the quencher decreases. This type of dependence is shown by all the data given in Table 1 and 2 . Furthermore , Figure 1 shows that as expected9 , there 20 exist a nearly linear relationship between log kQ and the quencher IP. Nevertheless, it is relevant to consider that for all the systems considered in the present work the cumene quenches the donors at a faster rate than that expected from its ionization potential. This can be explained by a contribution of the hydrogen transfer to thc quenching process. This photoreduction can take place from the charge transfer complex or by a parallel and complete! y independent reaction path 7 ,s . In order to decide bctween this two possibilities it is useful to compare the behaviour of the n- 1r* excited triplet to that of the tert-butoxy radical , that is a good model for simple hydrogen abstractions. The relative rates (cumene/ cyclohexane) are ~ 10 for the excited 4-methyl-2-pentanone (from this work and ref. 4), ~ 1.6 for benzophenone triplets1 6 and ~ 0.38 for tert-butoxy radicals at 40°C17 . The dif-ference between both set of values would favour a hydrogen transfer contribution from the exciplex and not as a completely unrelated process. Furthermore, the values for ethyl-benzene and cumene relative to toluene are 1.47 and 1.13 for 4-methyl-2-pentanone and 4.5 and 3.8 for tert-butoxy radicals1 8 . The small selectivity shown by 4-methyl-2-pentanone triplets is also compatible with a hydrogen abstraction process that take place from the excited complex. The mechanism to be considered then for the aliphatic ketones (and probably also for the other carbonyl compounds) can be represented, as in the quenching by amines, by the following set of reactions 3K + A k¡ [Kó- Aó•J k2 k3 k4 This mechanism leads to 3[Kó- Aó•] 3K + K + KH + A A A(- H) ( 1) (2) (3) (4) (5) The value of kQ would depend then upon the lability of the hydrogen atoms and the " selectivity" would depend not only of the relative values of k4 but also upon the relative values of k4 and (k2 + k3). The data given in Table 1 show that the activation energies and the pre-exponential A factors are relatively high for all the systems considered. The experimentally deterrnined activation energies, defined by d In kQ Eexp = - R __ d_(_l ..,...ln....:::... _ _ and the pre-exponentlal A factor , defined by log A = log kQ + Eexp/RT will be related to the Arrhenius parameters of k 1 , k2, k 3 and k4 in a rather complex way (see Eqn. 5). A simple relationship will be obtained only in the lirnit where k2 ~ (k3 + k4 ). Nevertheless, in the present case and due to the low binding energy of the complex, the back dissociation is a fast process and cannot be disregarded1 . It is then difficult to assign to 21 the values given in Table 1 a meaning in terms of the parameters of the i.;1dividual steps. Nevertheless, it is relevant to point out that the values of E and A obtained in the present work are substantially different to those reported other systems where the quenching is also assumed to take place through the formation of charge transfer complexes1 0 •1 9 . Never-theless, in all these studies aromatic carbonyls were employed as donors, and the plot of Fig. 2 would indicat~ that the complex behaviour r.ould be significantly different when dif-ferent types of carbonyls are employed. For example, the fact that the reactivity of 4-methyl-2-pentanone seems to be larger that expected from its (E¡- + E(A - /A)) value (see figure 2) would imply that (k3 + k4 ) / k2 can be particular! y large in these types of com-pounds. These differences could lead to significantly different "Arrhenius parameters" for the experimentally deterrnined quenching rate constants. The data given in Table 1 and 2, as well as the plotsof log kQ v /s IP are rather similar for all the carbonyls considered in spite of the fact that the absolute rates differ by more than two orders of magnitude. This lack of relationship between "selectivity" and "reactivi.ty" is a general feature of processes that take place through charge transfer complexes. A possible reason why these processes do not obey the Hammond's rule has been given elsewhere20 . For a given quencher it has been proposed9 that the values of kQ could be related to the triplet energy of the donor and to its reduction potential through (6) The conditions that must be fulfilled to apply this equation to systems that involves only partial charge transfer makes the validity of this equation only relative2 1 • Nevertheless, when the data obtained in the present and previQus works employing toluene as quencher are plotted according to Eqn. (6), (see figure 2) in fair correlationship is obtained, leading further support to the proposed mechanism. The lack of linear correlation can be partially due to the lirnitations of Eqn. (6) to the different solvents employed and to indetermina-tions in the E(A- / A) values2 2 • Furthermore, it has to be considered that Eqn. (6) refers to the thermodynamics of the exciplex formation and do not ~.:onsider the possible differences in k3 and ~ values for different carbonyl compounds. 22 87 •• Fig. 1. Plot of log k0 carbonyl triplets agamst the 10mzauon potential of the substituted benzene derivatives. Unfilled points corresponds to the values obtained employing cumene as quencher. ~ Triplet benzophenone 8 Triplet benzi l 8-5 f-1 20 / / e/3 7 / / t/s 1 1 1 . 6 1 30 / / / ,.......-; __ _. , -1 40 1 50 Fig. 2. Relation between the quenching rate constant by toluene, k0 , and the triplet energy and reduction potential (ET + E (A-/Al), of the carbonyl donors: 1, benzaldehyde; 2,a- trifluoroacetophenone; 3, benzopheno· ne; 4, 4- methyl- 2- pentanone; 5, acetophenone; 6, biacetyl. ACKNOWLEDGEMENTS Thanks are given to DIPLAN 11 · DICYT for finantial support ofthis work. REFERENCES l. WILSON, T . and HALPER, A. M., J. Am. Chem. So c., 102, 7279, (1980). 2. PORTER, G., DOGRA, S.K., LOUFTY, R.D., SUGAMORI, S.E. and YIP, R.W., J. Chem. Soc., Faraday Trans. 1, 69, 1460, (197 3). 3. LOUFTY, R.D. and YIP, R.W., Canad., J. Chem., 51 , 1881, (1973). 4. ENCINA, M. V. and LISSI, E.A. , J. Photochem., 5, 287, (1976). 5. ABUlN, E., ENCINA, M. V. and LISSI, E.A., J. Photochem., 3, 143 , (1974). 6. WAGNER, P.J. and LAM, H.M.H ., J. Am. Chem. Soc., 102,4167, (1980). 1. WAGNER, P.J. and PUCHALSKI, A.E., J. Am. Chem. Soc., 100, 5948, (1978). 8. WAGNER , P.J. and PUCHALSKI, A.E., J . Am. Chem. Soc., 102, 6178, (1980).' 9. GUTTEMPLAN, J.B. and COHEN, S.G., J. Am. Chem. Soc., 94, 4040, (1972). 10. GIERING, L., BERGER, M. and STEEL, C., J. Am. Chem. Soc., 96, 953, (1974). 11. SCHUSTER, D.l., WEIL, T .M. and HALPERN, A.M., J. Am. C!~em. Soc., 94,8248, (1972). 12. SCHUSTER, D.I., Pure and Applied Chem., 41 , 601, (1975). 13. ENCINA, M.V., NOGALES , A. and LISSI , E.A., J. Photochem., 4, 75, (1975). 14. ENCINA, M. V. and SCAIANO, J .C., J. Am. Chem. Soc., 101 , 7740, (1979) ; INBAR, S., LINS· CHITZ, H. and COHEN , S.G., J. Am. Chem. Soc. , 102,1419, (1980); TURRO, N.J. and ENGEL, R., J. Am. Chem. Soc., 91 ,7113, (1969). 23 15. SCA1ANO, J.C., unpublished results. 16. SCAIANO, J.C., J. Photochem., 2, 81, (1973/74). 17. WALLING, C. and THALER, W., J. A m. Chem. Soc., 83, 3877 , (1961). 18. PAUL, H. , SMALL, R.O. and SCAIANO, J.C. , J. Am. Chem. So c., 100, 4520, (1978) . 19. ENCINA, M. V. and SCAIANO, J.C. , to be published ; MAHARAJ, V. and WINNIK, M.A. , J. Am. Chem. Soc., 103 , 2328, (1981). 20. ENCINA , M. V., LISS1 , E.A. and OLEA, F.A. , J. Photochem., 14 ,233 , (1980). 21. RUBIO , M.A ., ENCINA , M.V . and LISSI , E.A., J. Photochem., in press. 22. GORMAN, A.A., PAREKH, C.T., RODGERS , M.A.J. and SMITH, P.G. , J. Photochem., 9 , 11 , (1978). 24 

Quenching of excited carbonyl triplets by benzene derivatives

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Quenching of excited carbonyl triplets by benzene derivatives

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