Biacetyl, intersystem crossing

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... 

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... 

The triplet state of biacetyl must undergo a nonradiative process other than decomposition. Strong arguments have been made that this process is 3aI —a intersystem crossing (168a)t although a reversible a intersystem crossing... 

Further low-pressure studies have shown, as discussed previously, that a purely kinetic treatment of the "reversible" intersystem crossing is inadequate for describing the time evolution of the excited state, since biacetyl is an "intermediate-case" molecule. Nevertheless, a kinetic treatment is useful for tracing the energy flow which leads to photodecomposition in a molecule. 

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... 

Biacetyl diffused onto the pre-wetted fiber initially. Then, as monomer vapors were introduced and the surface was irradiated with ultraviolet-visible light, the excited biacetyl dissociated to yield acetyl radicals or decayed to its more stable triple state through intersystem crossing. These radical species abstract accessible hydrogens from the substrate or near the surface of the fiber substrate, which in turn react with monomer in the proximity or possibly within the solvent wetting the substrate. Growing pol)rmer chains were terminated in a manner whereby only limited initiation of homopol3nnerization occurred. 

Intersystem crossing efficiencies (Si — Tf) have been calculated for biacetyl 7,10,122,126 2,3-pentanedione i26>, 2,3-heptanedione i26>, 2,3-octanedione 126>, benzil 80,115), and 9,10-phenanthrenequinone 30>. In all cases the values were close to unity supporting the assumption that the triplet state is the reactive excited state in dione photochemistry. 

The lifetime of incarcerated biacetyl is unaffected by the solvent as long as it doesn t contain heavy atoms. Interestingly, halogenated solvents enhance intersystem crossing via a remote external heavy atom effect. This was demonstrated by Romanova et al. for hemicarceplex 120biacetyl (Figure 9.35). ... 

While the factors resulting here in a given distortion are intramolecular, the conclusions we can draw may be applied to cases where an isolated molecule is distorted by its environment. We would like to examine this approach as starting point for treating the effect of the 11-cw retinal-opsin interaction on the photoisomerization quantum yield of 11-cw to aW-trans retinal. This quantum yield increases from 0.2 for the free chromophore [9] to 0.66 in rhodopsin [10]. This value is close to that of the biacetyl sensitized reaction of the free chromophore, 0.75 [11], and indicates that the triplet state is involved to a large extent in the photoisomerization of ll-cis retinal and that intersystem crossing is inefficient. 

Sensitization offers also a way for increasing the yield of the desired reaction, when the limit is a low value of the intersystem crossing efficiency. This happens, for example, for Co(CN)6, whose reactive lowest triplet is populated with efficiency i/ics = 0.4. Sensitization via triplet biacetyl (that has //jcs = 1) leads to an increase in the quantum yield up to a factor 2.5 times [149] (Fig. 4.10). 

Due to a fast and efficient intersystem crossing process (cf. Table 3.2) most ketones perform mainly triplet photochemistry [25,73], In Table 3.6, the photoreactivity of triplet-excited acetone, biacetyl, and benzophenone is compared with the triplet-excited azoaUcane DBH-T. The data for both chromophores follow similar trends. Namely, dienes and amines quench with quite high rate constants, while ethers and aromatic compounds react rather inefficiently. 

The quantum yield of this reaction has been shown to be independent of oxygen pressure (Brunet and Noyes, 1958) and, therefore, occurs prior to intersystem crossing to a triplet state. In agreement with this conclusion is the fact that sensitized phosphorescence from biacetyl is accompanied by a decrease in the quantum yield of Reaction (46). 

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