. J. Chem. 68, 812 (1990).Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows A, , at -335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. On a produit des radicaux diphknylmtthyles en excitant, l'aide d'un laser optrant a 266 nm, de la 1,1,3,3-tktraphknylacttone adsorbte sur du gel de silice ou incluse dans des ztolithes de NaX et de Silicalite et on les a ttudits en faisant appel aux techniques de photolyse <> a l'aide de laser i rtflectance diffuse. Le spectre du radical prksente un A, , a environ 335 nm dans chacun des trois supports et il est semblable a celui observt en solution. Les radicaux se dtcomposent a des vitesses qui varient de quelques centikmes de nanosecondes a des minutes; certaines indications nous laissent croire que certains radicaux pourraient se dkcomposer a des vitesses plus rapides que celles que l'on peut mesurer. L'efficacitt du pikgeage par l'oxygkne augmente lorsqu'on passe de la Silicalite au NaX au gel de silice; cet ordre est en accord avec la plus grande disponibilitk de l'oxygkne sur les pores du gel de silice par comparaison avec les canaux Ctroits qui existent sur la Silicalite. Pour divers supports, on a aussi examink les effets de la dose du laser ainsi que de la quantitC de cttone utiliste. On discute aussi des applications potentielles d'un traitement cinttique des donntes bast sur la cinttique de rkactions dispersives cornrne moyen de rksoudre le problkme de la cinktique de dtcompositions qui se produisent a des vitesses trks diffkrentes.

We have previously reportedS that alkenes linearly quench the photorearrangment of enone 1 to the bicyclic ketone 2 (lumiketone) concomitant with formation of [2 + 21 cycloadducts, implicating a common intermediate on the two reaction pathways.

The excited-state properties of radicals I-V have been examined in solution by using two-photon, two-laser excitation techniques. The radicals were normally generated by photolysis (308 nm) of the corresponding halomethyl precursor. The radicals were then excited by laser irradiation at 337 nm, and the fluorescence spectra, lifetimes, and, whenever possible, transient spectra (i.e., I and IV) for the excited radicals were recorded. For example, for the 2-phenanthrylmethyl radical (I), the excited-state lifetime is 79 ns in toluene at room temperature, where it fluoresces with 0, = 593 nm and its absorption spectrum shows Xmax = 400 nm. The lifetime of excited I shows only a very minor temperature dependence (£a = 0.6 kcal/mol in the 183-340 K range).

The photolysis of diethyl ketone and the photolysis and phosphorescence of diethyl ketone-biacetyl mixtures have been studied at 3130 A and 2537 A. The addition of biacetyl at 3130 A decreases the photodecomposition of diethyl ketone and increases the phosphorescence of biacetyl. An energy transfer from excited ainglot diethyl ketone to biacetyl is proposed and there is evidence that the propionyl radical formed from the singlet state at 3130 A Is sufficiently "hot" to dissociate into an ethyl radical and carbon monoxide. At 2537 A dissociation appears to occur from the initially formed upper singlet state and there is no evidence that the triplet state intervenes. A detailed mechanism is presented for the primary process in diethyl ketone and for the energy transfer between diethyl ketone and b:Vacetyl. IntroductionThe vapor phase photodecomposition of diethyl ketoiv at ° 3-12 3130 A has been extensively studied and a comprehensive ResultsThe results were observed at two incident wavelengths, 3130A and 2537Ä. For the intensity measurements carried out at 3130A, the filter combination depcribed above was replaced by a pyrex plate to obtain a larger variation in incident intensity.The absorbed intensity I a is given in quanta/ml/sec as calculated from the cell volume and the rate of absorption, o 3130A: The rate of formation of the total fraction (CaJU + CaHs) was rensurod. Individual values for CatU and for C 2 H« were r.ot obtained. At This wavelength it is to be expected that the rate of fcoration of C^A and of CaHa will he the'same at room terperature. If one-half of the total Ca fraction is taken to represent the rate of formation of ethylene, then a value of 0.135 in obtained for th* ratio of rate of CaH* forration to rate of CaHs Torration. Thip value corpares favorably 0 1C 1° with i-rovio\;r ^vnlur'tlor.r of t>ir ratio '' ".-6- The variation of biacetyl emission is given in microamps.This value is obtained from the difference in photomultiplier readings when the cell is empty and when the cell is full.The primary quantum yields are obtained by considering the following reaction scheme: Effect of Biacetyl on the Decomposition of DiethylKetone and on the Phosphorescence of Biacetyl -These two effects are shown in Table I and Figure 1, respectively. Table I shows the effect of biacetyl pressure on the,;t> , ( 'f.and,"H_ at 25° and on h " n at 57°. The effect on ö and r.bat 25° are also shown where t* is the primary quantum yield

. J. Chem. 66, 2595Chem. 66, (1988.Laser flash photolysis studies lead to the conclusion that the short-lived triplet states of cyclohexenones are readily quenched by amines. For example, in the case of 2-cyclohexen-1-one (I) its triplet state (TT = 40 ns in acetonitrile) is quenched by triethylamine with a rate constant of (9.0 2 0.8) x lo7 M-' s-I. Cyclohexenone triplets are also quenched efficiently by DABCO and by triphenylamine leading to the formation of the corresponding amine radical cations. The new evidence reported rules out the involvement of long-lived detectable exciplexes. Cohen and Peters and their co-workers have studied this process with benzophenone and related ketones, and have demonstrated convincingly that the quenching process involves electron transfer to give initially amine radical cations and ketone radical anions (2). Schuster and Brisimitzakis demonstrated that reactive 2,5-cyclohexadienone triplets are quenched by amines of appropriate ionization potential to give a set of products not observed in the absence of amines. In all cases, the amine interacts directly with ketone triplet (3).Reactions of photoexcited a,~-unsaturated carbonyl compounds with amines are known to give 1:l adducts, presumably via the enone triplet states (4). Dunn et al. have reported results of nanosecond flash studies on excitation of cvclohexanone I and analogues such as I1 (see Chart I) in the presence of 1,4-diazabicyclo [2.2.2]octane (DABCO) which demonstrate that long-lived DABCO radical cations (X,,, 450 nm) are formed from an intermediate which has the same lifetime (ca. 25 ns) as the transient species (X,,, 280 nm) generated on laser excitation of the enone in the absence of amines (5). This transient has been suggested by Bonneau (6) and by Schuster et al. (7) to be a highly twisted triplet excited state of these cyclohexenones. In the presence of even small amounts of DABCO, long-lived absorption is observed which interferes with analysis of the fast component of the decay curve at wavelengths in the range 270-300 nm; at the DABCO concentrations required for observation of the growth of the radical cation at 450 nm, the fast decay at 280 nm cannot be monitored (5,8).In contrast to the above findings, Pienta and McKimmey reported results of product analyses from excitation of I in the