Triplet states with biacetyl

As stated in the first section, there is no doubt that when using 4358 A. light the triplet state of biacetyl reacts directly with oxygen. The same reaction may also occur when using 3130 A. light. 

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. 

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 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... 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

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 determination of the triplet-state yield when benzene vapor is irradiated depends not only on the emission yield of biacetyl as determined by Almy and Gillette32 but on the absolute emission efficiency of benzene itself. When all corrections are applied, the biacetyl method gives a triplet-state yield at incident wavelengths 2530-2590 A of about 0.63 with an uncertainty hard to estimate but perhaps 0.1. Below 2530 A the emission efficiency decreases, and by 2400 A the emission efficiency has fallen to zero. 

At pressures more than about 20 torr and at wavelengths from 2530 to about 2660 A the triplet state quantum yield for benzene is about 0.72 as recalculated with the factor 1.02 instead of I.37.89,90 Several other methods give about the same result.91 The biacetyl method and the butene method may be said to be in agreement within a rather large experimental error except at this one wavelength. At wavelengths below 2500 A the triplet-state yields decrease but the error also increases. Probably around 2400 A the triplet-state yield has become zero but this statement is made with some reservations. 

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. 

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... 

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]. 

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. 

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. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

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