Benzene phosphorescence spectra

Vibrational analysis of the benzene phosphorescence bands indicates that the radiative activity is induced predominantly by e2g vibrations [155, 156]. A weak but observable activity of b2g vibrations has also been found [156, 155, 157]. By introducing spin-orbit- and vibronic coupling through second order perturbation theory Albrecht [158] showed that the vibronic interaction within the triplet manifold is responsible for the larger part of the phosphorescence intensity. This also follows from comparison of the vibrational structure in phosphorescence and fluorescence spectra [159]. The benzene phosphorescence spectrum in rigid glasses [155] reveals a dominant vibronic activity of... 

El-Sayed 28> has reported on the phosphorescence spectrum of [2.2]paracyclophane. The emission differs both in wavelength ( 4700 A) and in duration (3.3 s) from that of benzene ( 3400 A, 6 s) hence a favorable intersystem crossing from the lowest singlet to the emitting triplet state was inferred. The emission spectrum also indicates that interactions take place between the two aromatic nuclei in the triplet state. 

Phosphorescence Spectra.—Luminescence from a low-lying triplet state of water368 has been reported. It has been shown that two long-lived emission systems in the biacetyl crystal described previously by Sidman and McClure (see ref. 368) are in fact due to impurities, and a complete analysis is presented of the true 3AU xAg phosphorescence. The zero-zero band in emission is found at 20 327 cm-1.135 A satisfactory account of the six characteristic bands in the phosphorescence spectrum of benzene has been given on the basis of pseudo-Jahn-Teller vibronic interactions between the lower 3Blu and 3Elu states in which two active vibrations in the pseudo-cylindrical approximation are considered.369 The phosphorescence spectra of anthracene,3700 coronene,8706 benzophenone in aqueous solution,371 pyrimidine derivatives,372 porphyrins,298 873 and crystalline charge-transfer complexes 374 have been reported. 

The vibrational species and ei are observed in the phosphorescence spectrum of benzene. The possibilities for the spatial symmetry of the benzene triplet stales are Biu, and Bi . To what extent does the observation of both bt, and ei, vibrations aid the assignment of the lowest triplet level ... 

CF,)jC,H4, 0-, m-, and p-FC,H4Me, o- and m-FC,HiCF PhF, - CeFe, and C,F6H has been studied, and trends in the radiative and non-radiative decay rates of the excited molecules are discussed. - c/s-But-2-ene is effective in increasing the fluorescent and triplet yields of pentafluorobenzene. The decay rate of triplet benzophenone, produced by laser flash photolysis, is ca. ten times faster in hexafluorobenzene than in benzene, and reversible addition of the triplet species to the aromatic ring may play an important role. The phosphorescence spectrum of decafluoro-benzophenone has been studied, and the triplet state of this ketone (see also p. 380) in inert solvents e.g. perfluoromethylcyclohexane) has a lifetime of 20 ps. U.v. irradiation of decafluorobenzophenone in isopropyl alcohol... 

The thermolysis of peroxyacetate [28] and substituted peroxybenzoates [29] gives both direct and indirect chemiluminescence. Thermolysis of peroxyacetate [28] in benzene solution at 100° gives very weak direct chemiluminescence. The emission is so weak that an emission spectrum could not be obtained. When biacetyl, which has a considerably higher quantum efficiency for phosphorescence than acetophenone (Backstrom and Sandros, 1958), is used... 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

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