Phosphorescence, of biacetyl

Backstrom and Sandros<54-55) found that the phosphorescence of biacetyl in benzene solution at room temperature was quenched at a diffusion-controlled rate by aromatic hydrocarbons when the triplet energy of the hydrocarbon was sufficiently below that of biacetyl. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

In benzene solution, quantum yields for formation of olefin triplets from 2537 A irradiation are only 0.20.235 There is some doubt whether triplet benzene is the actual energy donor in solution, since the quantum yield for sensitized phosphorescence of biacetyl depends on a greater than first order of benzene concentration.238 Dubois has suggested that an excited dimer is responsible for energy transfer.236... 

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... 

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. 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

The cis-trans isomerization of 1,3-pentadiene and sensitized phosphorescence of biacetyl have been used to measure the effect of... 

Figure 12. Relative quantum yield of sensitized phosphorescence of biacetyl (top), and nonradiative decay rate of naphthalene vapor (bottom), as a function of the excess vibrational energy in Sl naphthalene. (From ref. [40] with permission.)...

The phosphorescence of biacetyl (2,3-butadione) can be sensitized by chlorobenzene in the aqueous media and biacetyl Inhibits the photohydrolysis of chlorobenzene (19). Blacetyl in water is hydrated in the proportion of 73.5% ( ) according to the reaction ... 

The phosphorescence of a 5 X lO" M solution of biacetyl in de-aerated 2-propanol at room temperature could be quenched completely by 1 a,d,e (10 8 M) 84). In all three cases, the corresponding photoreduction products 2a,d,e emerge from analogous preparative scale biacetyl sensitized runs. Since 2e is also formed, steric hindrance to hydrogen abstraction from solvent cannot be too effective when a (probably longer-lived) triplet is populated, whereas it might be effective in the direct photolysis ot 1 e 88) where isomerisation competes with reduction probably in the (short-lived) singlet state. 

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... 

In studies of the vapor phase photolysis of biacetyl it was observed that a new product, which quenched both the phosphorescence and primary dissociation of biacetyl, was formed. A strong absorption at 275 nm, which was associated with the quenching activity, was also observed. Since no new product which displayed significant quenching activity could be isolated, it was concluded that the quenching was due to the enol of biacetyl (8).57 This assignment was supported by the disappearance of the absorption at 275 nm when IC1 was added to an irradiated aqueous solution of biacetyl.58... 

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

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... 

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... 

A study of [Rh(phen)3]3+ confirmed that emission comes from a (n-n ) state.819 No emission was observed in fluid solution, but a long-lived excited state occurs in DMF at room temperature, as 290 nm irradiation of [Rh(phen)3]3+ causes the characteristic 780 nm phosphorescence of [Cr(CN)6]3 (equation 146). The Sterm-Volmer constant of 3.0 x 103 M I implies a lower limit for the excited state lifetime of ca. 0.30 ms. Sensitization of biacetyl, and the oxidation of diphenylamine (in acetonitrile) and of 1,3,5-trimethoxybenzene are also observed for [Rh(phen)3]i+, implying that the 3(7t-7t ) state of [Rh(phen)3]3+ is a powerful oxidant. A potential of 2.00 V is calculated for the [Rh(phcn),]3+/[Rh(phen),]3 + couple,819 making the excited state of [Rh(bipy)3]3+ a better oxidant than the Ru , Cr111 or Os11 analogs. 

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... 

Acetaldehyde, irradiated by light of 3130 and 3340 A wavelength, sensitizes the phosphorescence of the biacetyl added. This must be the result of triplet-triplet energy transfer between the excited aldehyde and the biacetyl molecules, as the singlet state probably has a very short life-time. 

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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