Singlet states with biacetyl

The most extensively studied diketone has been biacetyl,24 12g) a detailed mechanistic study of its reactions with a number of olefins has appeared 128). It was suggested that a triplet exciplex is the precursor for formation of 161. The occurrence of electron transfer, presumably subsequent to exciplex formation, has been demonstrated 157,l58) in the reaction of biacetyl with tetramethyl-l,3-dioxole (167). Esr-spectroscopy of irradiated mixtures indicated the presence of biacetyl radical anion and dioxole radical cation. This reaction, which produced a complex mixture of products, was suggested to involve the excited singlet state of biacetyl. 

The application of the method of sensitized emission to detect the presence of triplet-state benzene molecules is particularly definite. The ratio of intensity from (70) (emission from the triplet state of biacetyl) to that from (67) (emission from the singlet state of biacetyl) is more than 170 1 and may be infinity. This contrasts with 60 1 after absorption in pure biacetyl. 

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 enolic form has been found to be formed from the second singlet state in dilute solutions of biacetyl in water, methanol, and n-heptane.77 In aqueous solutions, the enol is stabilized and its quantum yield of formation is 0.10 0.02 and varies slightly with the wavelengths. 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

Comparison of Measured Lifetimes of the First Excited Singlet States of Various Donors in Aerated Solutions, td, with Those Estimated from Quenching and Sensitization Constants, Kt and K Obtained for Various Donor-Biacetyl Systems... 

Plots of Qs vs. pressure of biacetyl pass through maxima both with benzene and with fluorobenzene. With benzene, this maximum occurs at about 0.1 mm. pressure of biacetyl. Moreover, the ratio QsVQs , i-e., the ratio of sensitized emission efficiency from the triplet state to that of the singlet state, is greater than 170. These facts indicate that emis-... 

At 3650 A, the efficiency of luminescence increases with increasing biacetyl pressure . This is just the opposite of what would be expected on the basis of the Stern-Volmer relation. Moreover, it was established that the primary decomposition quantum yield decreases with increasing pressure (the plot of Ijcf) versus biacetyl concentration gives a straight line). The results were explained by the assumption that the molecules absorbing radiation of 3650 A are excited to high vibrational levels (of the upper singlet state) from which dissociation can occur, but luminescence cannot. Luminescence can only occur if vibrational excitation is removed by collision. 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

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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