Biacetyl fluorescence

Fig. 4. Uncorrected experimental traces showing sensitization of biacetyl fluorescence by benzene in aerated hexane. (All solutions contain 0.005AT biacetyl—exciting wavelength 2650 A.)...

Sensitized biacetyl phosphorescence and sensitized decomposition occur in the vapour phase as a result of electronic energy transfer from benzene to biacetyl. Some fluorescence was also observed however, the ratio of phosphorescence to fluorescence was very high. In fluid solutions benzene and some other organic compounds sensitize the biacetyl fluorescence but not the phosphorescence (refs. 316-318), in contrast to the findings for the vapour phase. A satisfactory explanation for this difference is not available at present. 

B2) Tttrro, N. J., and R. Engel Quenching of Biacetyl Fluorescence and Phosphorescence. A New Mechanism for Quenching of Ketone Excited States. Mol. Photochem. 1, 143 (1969). 

FIGURE 8- 16b Simultaneous scalar and velocity map for a nonreacting nozzle flow using biacetyl fluorescence and PIV (Frank et al. 1996). (Reprinted by permission of Frank et al.)... 

Backstrom HLJ, Sandros K (1955) Quenching of biacetyl fluorescence in solution. J Chem Phys 23 2197... 

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 as sensitizer phosphorescence quenched fluorescence not affected, release of NH S No apparent degradation of biacetyl ... 

Figure 12.5—Relationship betweenfluorescence intensity and concentration. As concentration increases, the emission of fluorescence is no longer proportional to concentration. A maximum fluorescence intensity is observed, beyond which fluorescence decreases or tends to roll over. The example given is for biacetyl in CCL,.

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... 

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... 

B ckstrom, H. L. J., and K. Sandros The quenching of the long-lived fluorescence of biacetyl in solution. Acta chem. scand. 12, 823 (1958). 

Chaiken, J., Benson, T., Gumick, M. and McDonald, J.D. (1979). Quantum beats in single rovibronic state fluorescence of biacetyl, Chem. Phys. Lett., 61, 195-198. 

Triplet-triplet absorption by benzene has never been detected and evidence for the presence of triplet benzene in fluids has been obtained only recently using energy transfer measurements. Benzene sensitizes both the fluorescence and the phosphorescence of biacetyl. The concentration dependence of this sensitized luminescence has been shown to be consistent with the presence of triplet benzene in the gas phase43 and in cyclohexane solution,80 though another study using hexane as solvent did not yield any evidence for the presence of triplet benzene,27 presumably because of impurities in the hexane. 

Sometimes the fluorescence and phosphorescence spectra of a compound in solution overlap. They may be separated as follows. If a suitable triplet energy acceptor is added, this will quench the phosphorescence, leaving the fluorescence unaffected, while a suitable triplet donor will sensitize the phosphorescence in the absence of any fluorescence. Back-strom and Sandros have analyzed the total luminescence spectra of biacetyl, benzil, and anisil in this way, using pyrene as the triplet acceptor and benzophenone as triplet donor.3... 

The first quantitative photochemical study of a Rh111 amine was reported by Moggi,8 who noted that both 254 nm (LMCT) and 365 nm (ligand field) excitation of [Rh(NH3)5Cl]2+ caused chloride labilization (equation 131). Other early reports include Basolo s study of the photoinduced stereo-retentive halide aquation from [M(en)2X2]+ (M = Rh, Ir X = Cl, Br, I), and Broomhead s observation of chloride aquation from [RhCl2(phen)2]+.726 While halide labilization dominates upon photolysis of [Rh(NH3)5Cl]2+, both bromo and ammine loss occur upon photolysis of the bromo analog (equation 132)685,707 and ammine is labilized from the iodo analog (equation 133).70 Biacetyl sensitization of the bromo complex quenches the biacetyl phosphorescence, but not the fluorescence,707 consistent with a photoreactive triplet state. 

Circular dichroism provides an additional spectroscopic tool for characterization of excited states 35). Considerable interest has also been extended to esr-spectra of anion radicals of a-diketones 36). Circular polarization of the phosphorescence of camphorquinone has been determined, 51). Biacetyl has been the subject of a CIDNP study 152), of fluorescence quenching by a variety of substrates 153), and of steric effects in quenching of triplet states of alkylbenzenes 154). 

Class B in Table I includes seven olefins, which are characterized by dialkyl substitution at one of the carbons of the olefinic double bond. The emission spectrum produced by reaction of these olefins with ozone is characterized by a narrow band peaking at about 520 nm with broad shoulders at 465 nm and 565 nm. Figure 3 gives the chemiluminescent emission spectrum obtained by reaction of a typical member of Class B, tetramethylethylene, with ozone at a pressure of 0.8 torr. It is similar to the fluorescent emission spectrum of biacetyl [broad emission from 440-495 nm (11)1 combined with the phosphorescent emission of the same compound [narrow peaks at about 512 and 561 nm and a broad... 

Rebbert and Ausloos reported that -butyraldehyde quenches the phosphorescence, but exerts no influence on the fluorescence, of acetone (excited by radiation of 3130 A wavelength). Borkowski and Ausloos reported that strong phosphorescence can be observed on irradiating n-butyraldehyde in the presence of biacetyl. The phosphorescence yield increases with increasing aldehyde concentration, and... 

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