Biacetyl, triplet state energy

Sandros, K., and H. L, J. Bdckstrom Transfer of triplet state energy in fluid solutions. II. Further studies of the quenching of biacetyl phos-phorscence in solution. Acta chem. scand. 16, 958 (1962). 

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... 

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... 

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... 

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... 

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. 

Since the triplet state of biacetyl dissociates with an appreciable activation energy, the value of ft , and hence of any other ft, will be very temperature dependent. Thus, biacetyl is mainly useful at room temperature and below. 

Another fact must be considered for fluorobenzene, viz., if the Almy and Gillette value for the emission yield from the triplet state of biacetyl is 0.15, there is some discrepancy in the energy balance. However, recent work seems to have removed this difficulty. ... 

These observations show that energy transfer may occur from the triplet acetone to the aldehyde molecule, and that triplet biacetyl can be formed as a result of energy transfer from the excited aldehyde. However, the data available at present leave unsettled the queastion as to whether reaction (5), or (5 ) followed by (5 ), is responsible for the formation of the triplet state in the photolysis of -butyr-aldehyde. 

Ausloos and Rebbert carried out similar experiments with 2-pentanone at 3130 A. They also observed the sensitized emission and the quenching of primary process II in the presence of biacetyl. Moreover, they have demonstrated that the efficiency of the fluorescence, emitted by 2-pentanone, changed by no more than 2 % when biacetyl was added. This observation proves that, under the experimental conditions studied, only triplet-triplet energy transfer occurred, and that the triplet state of the ketone was the precursor, completely or at least partly, of primary process II. 

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