Triplet state of benzene

In contrast to borazine, the three corresponding excited singlet states of benzene have a much wider spread of absorbing wavelengths and exhibit easily distinguished vibrational fine structure. Many photolysis experiments have been performed using laser lines tuned to selective excite a particular vibrational level of a particular excited state of benzene. Such experiments are more difficult with borazine. The triplet states of benzene have been located experimentally and quantum yields for fluorescence and phosphorescence at various wavelengths and pressure conditions have been determined. 

Molecular orbital calculations indicate that the lowest triplet state of benzene and its derivatives should be appreciably more basic than the ground states of the molecules.211 Furthermore, it is predicted that the preferential sites of protonation may be different in the excited states than they are in the ground states. 

Triplet states for naphthalene, anthracene, and other aromatic compounds had been identified by absorption spectroscopy mainly with the aid of flash photolysis by G. Porter and his co-workers.22 Although a triplet state of benzene had been identified in a glassy matrix and had been associated with a long-lived emission of 10 sec or more duration,5 no evidence for the existence of this state by spectroscopic means had been produced until recently.23 Thus it has been known for some time that benzene in a glassy matrix when irradiated at wavelengths around 2500 A produces molecules which cross over to a triplet state with a relatively high probability. 

The formation of the triplet state of benzene in the gas phase was proven only recently, but this has now been accomplished by three independent methods.24-27 These methods will be discussed shortly in more detail, but it should be mentioned in passing that the mean life of the triplet state in the gas phase of triplet benzene is far shorter than it is in a glassy matrix and is indeed between 10-5 and 10-4 sec.26d,2S... 

While the general applicability of the Cundall method to all situations may be questioned even when the proper energy relationships exist,34 there seem to be no reasonable doubts about its applicability to benzene.29,35 Neither of the two butenes quenches the singlet emission from benzene in the 1B2u state, and the triplet states of benzene which might be of importance all lie above the triplet state of the 2-butene (which should be nearly the same for both the cis and trans forms). 

As already mentioned, the triplet state of benzene was identified by the cis-trans isomerization of the 2-butenes. Nevertheless, Wilzbach and Kaplan in the liquid phase, following work of Srinivasan,42 find a photochemical adduct of benzene to olefins. This adduct they find to be an adduct of benzvalene and not of benzene itself.414 There is also evidence of formation of a small amount of this adduct even in the vapor phase.35 Benzene exposed to ultraviolet radiation in a flowing system diluted with nitrogen and condensed shows compound formation with trifluoroethanol, CF3CH2OH. Again the adduct is one of benzvalene.410... 

The lifetime of the triplet state (probably 3B2u) of benzene has recently been determined in two different laboratories26,28 with agreement within a factor of about two. Thus, barring improbable effects of impurity, the mean lifetime of the triplet state of benzene in the gaseous... 

The excitations of lower triplet states of benzene in these two approximations have been calculated in Ref. 133. A similar study of excitation cross sections for triplet states of a nitrogen molecule has been done in Ref. 134. 

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

The use of biacetyl to identify the triplet state of benzene has been described in a recent publication so that we will give only a brief summary of results and conclusions and then contrast these with results obtained in other ways. 

There undoubtedly are other triplet states of benzene whose locations are at present unknown. They may lie below the first excited singlet state. We are unable to state what role they might play. 

This is an interesting situation since the triplet emission of biacetyl is sensitized by the triplet state of benzene and yet slope/intercept is the same for (77) as for (75), and the latter is the equation for the singlet state of benzene. 

Attempts to directly detect the triplet state of benzene under these or similar conditions by transient spectroscopy techniques have so far been unsuccessful. Thomas and Mani (244) found a transitory species with an absorption maximum and a decay time of 112 ns in benzene and benzene cyclohexane mixtures. The results from addition of naphthalene, blacetyl, oxygen, and plperylene suggest that this is a product of reaction between triplet and ground state benzenes. Build-in times for this species, which are 3 ns for pure benzene and 20 ns for 10 percent benzene/cyclohexane mixture are temperature independent. Laser... 

The lifetime of the triplet state of benzene in fluid solutions (at room temperature and above) is determined by the, seemingly efficient, interaction of that state with ground state molecules. This is not inconsistent with the low temperature behavior from which it has been deduced (51-55), that a thermally... 

Ishikawa, H and W. A. Noyes, Jr. Photosensitization by Benzene Vapor Biacetyl. The Triplet State of Benzene. J. Chem. Phys. 37, 583 (1962). 

V-Trimethylborazine quenches the singlet state of benzene excited at 267 nm, but it does not affect the triplet state of benzene. There is no evidence for chemical reaction between i -trimethylborazine and benzene or biacetyl. 

Finally, the values of = 0.7 compare well with that obtained by examination of sensitized phosphorescence in biaeetyl. This technique is mechanistically more complicated than the butene-2 method. Its principal complication arises from the fact that energy transfer occurs from the singlet state as well as from the triplet state of benzene, and a rather subtle correction is required. Such corrections have not been applied to the original determination of l is method and (j>p = 0.63 derived... 

Fig. 1.15 Electronic energy levels of singlet and triplet states of benzene, with absolute values relative to the vacuum level (the Fermi limit) obtained from photoelectron spectra and relative values (with reference to the ground state) obtained experimentally from UV/Vis absorption and photoluminescence spectra. The absolute values are based on Koopmans theorem [41], that the energy of the highest occupied molecular orbital (HOMO) is the first vertical ionisation energy of a molecule. The energy of the HOMO level is obtained from the vertical ionisation energy of benzene in Ref. [42] and the energies of the excited states are fl om Ref. [43]. Assignment of the symmetry of the S2 ( Bju) state is from Ref. [44]...

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