Biacetyl triplet state

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

The a-diketones show both phosphorescence and fluorescence emission, not only in a glass at 77°K but also in fluid solutions at room temperature, a property which has made these compounds useful in energy transfer studies.25-28 Fluorescence, however, is quite weak with intersystem crossing and decay through the triplet state being the principle mode of decay. The absolute fluorescence yields of biacetyl and benzil in solution are reported to be 0.22% and 0.27%, respectively, while the measured phosphorescence yield... 

Biacetyl (5) is reported to undergo enolization when irradiated in water, methanol, or hexane solution, presumably via a highly strained four-membered transition state instead of the usual six-membered one.40-43 From the wavelength dependence of enol formation, it was concluded that enolization occurred from the second triplet state. 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

Energy transfer from singlet or triplet states of suitable organic molecules can cause excitation of the central metal ion in a coordination compound. Photosensitization of Cr(III) complexes by biacetyl leads to aquation reaction of Cr(NH3)5 (NCS)2 ion. The aquation of (NH3) is hundred times more than that of (NCS) for the sensitized reaction, whereas it is only 66 times on direct excitation of Cr(IlI). This shows that energy... 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

Azoalkanes are very efficient quenchers of both acetone and biacetyl triplets,160 and undergo decomposition from their excited triplet states. This decomposition can be sensitized very efficiently by triplets whose energies are as low as 42 kcal, much below the spectroscopic triplet levels of the azo compounds.461... 

The determination of the triplet-state yield when benzene vapor is irradiated depends not only on the emission yield of biacetyl as determined by Almy and Gillette32 but on the absolute emission efficiency of benzene itself. When all corrections are applied, the biacetyl method gives a triplet-state yield at incident wavelengths 2530-2590 A of about 0.63 with an uncertainty hard to estimate but perhaps 0.1. Below 2530 A the emission efficiency decreases, and by 2400 A the emission efficiency has fallen to zero. 

At pressures more than about 20 torr and at wavelengths from 2530 to about 2660 A the triplet state quantum yield for benzene is about 0.72 as recalculated with the factor 1.02 instead of I.37.89,90 Several other methods give about the same result.91 The biacetyl method and the butene method may be said to be in agreement within a rather large experimental error except at this one wavelength. At wavelengths below 2500 A the triplet-state yields decrease but the error also increases. Probably around 2400 A the triplet-state yield has become zero but this statement is made with some reservations. 

When irradiated at 2420 A or above, the sum of the fluorescent and triplet-state yields of fluorobenzene is unity or slightly greater than unity. Use of the correct branching ratio is impossible because it has not been determined for fluorobenzene.30,31 This has been substantiated recently, using the Cundall technique, to measure triplet-state yields rather than the biacetyl sensitization technique. The... 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

The emission step (117) does not involve a change in multiplicity whereas (122) does. The lifetime of the triplet state of acetone should be longer than that of the singlet state and indeed it is 2 x 10 4 sec as determined by Duncan and Kaskan60. The lifetime of the triplet state of biacetyl (126) is even longer, about 2x 10 3 sec60,62,63. However in both cases as well as for (117) the molecules in question take part in several concurrent reactions and the experimentally determined lifetimes must be shorter than the true radiative lifetimes. 

Since the photochemical quantum yields for acetone (as well as for biacetyl) increase with increase in temperature it is logical to say that the triplet state because of its long lifetime is subject to a thermal dissociation with an activation energy. Due to the complexity of the mechanism an unambiguous determination of this activation energy is difficult. The value for biacetyl is about 16 2 kcal50 and the most recent determination for acetone indicates a probable similar value64. 

Whether the singlet is a (n, jr ) state or an ( , it ) state can be established from solvent effect. A blue shift in polar solvent suggests an (n, 7i ) state, a red shift a (tc, tc ) state. A rough guess can be made from the radiative lifetime for fluorescence which is of the order of 10-5 s for biacetyl. Hence, it appears that the lowest excited singlet is an Sx (n, n ) state. From dilute solution value of f, we find that only 1 % of the initially excited molecules are capable of emission with a rate constant 1 x 105 s-1 == 1/t . Therefore 99% of the molecules must be transferred to the triplet state, assuming S S do not occur. The rate constant for... 

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