Fluorescence of benzene

The addition of benzene to 2,3-dihydropyran was examined in the presence of cyclopropyl bromide [177] in an attempt to determine the nature of the excited state of benzene responsible for the formation of the ortho adduct [11,12], The absorption of the bromide at 254 nm was taken into account (90% of the radiation was absorbed by benzene) and a decrease of the rate of formation of 50% in the heavy-atom solvent was observed. Cyclopropyl bromide also quenched the fluorescence of benzene (kq = 3.82 X 10s L/mol 1 /s ). These data are interpreted as enhanced intersystem crossing of Si benzene and the necessary involvement of the Si state in formation of the ortho cycloadduct. 

Triethylamine not only quenches the fluorescence of the exciplex and the formation of product but it also quenches the fluorescence of benzene. In dioxane,... 

Benzene is photoreduced by primary, secondary and tertiary amines (Bellas, et al., 1977) and the various products have been identified. The reaction with tertiary amines is accelerated by the addition of small amounts of protic solvents and use of CH3OD leads to deuterium incorporation. These findings are strongly indicative of the participation of radical ions. Primary and secondary amines quench the fluorescence of benzene but whether or not this leads to radical ions is not known. Product studies have been made of the... 

Butene-2 even at high pressures does not quench the fluorescence of benzene vapor. 

TABLE 7. Rate constants for fluorescence of benzene-h6 and benzene-d5, as a function of temperature assuming a non-quenchable internal conversion process. 

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

Table 7.11 Fluorescence quantum yield

Nevertheless, 1,4-difluorobenzene has a rich two-photon fluorescence excitation spectrum, shown in Figure 9.29. The position of the forbidden Og (labelled 0-0) band is shown. All the vibronic transitions observed in the band system are induced by non-totally symmetric vibrations, rather like the one-photon case of benzene discussed in Section 7.3.4.2(b). The two-photon transition moment may become non-zero when certain vibrations are excited. 

Triton X-100 ethoxyquin (antioxidant in spices) > 200-fold, stabilization > 15 h spray solution, 33% in benzene the fluorescence of aflatoxin Bi is reduced by 10 to 15% [292]. 

Fig. 14. Quenching of fluorescence of PP solution in benzene. Quenchers (Q) (1) tetracyanoethylene, (2) chioranil, (3) trinitrobenzene / fluorescence in relative units in the absence of a quencher, / in the presence of a quencher...

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

If the refractive indices of the solvents used for the sample and the fluorescence standard are not the same, a further correction must be made. For example, quinine sulfate in 0.1 N H2S04 (Or = 0.5) is commonly used as a fluorescence standard. If the fluorescence of the sample whose relative quantum yield is desired is determined in benzene, a correction factor of 27% must be applied in determining the relative areas under the fluorescence bands. If ethanol is used, this correction is only 5.5%. 

Schwarz, F.P., Wasik, S.P. (1976) Fluorescence measurements of benzene, naphthalene, anthracene, pyrene, fluoranthene, and benzo[a]pyrene in water. Anal. Chem. 48, 524—528. 

Similarly, the affixment of conjugate substituents onto aromatic systems extends the conjugation of the latter and causes fluorescence maxima of the substituted derivatives to lie at wavelengths longer than those of the parent compound. Hence, the fluorescence of aniline lies at 340 nm while that of the parent hydrocarbon benzene lies at 262 nm. 

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. 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

The yield determined in a certain type of experiment usually strongly depends on the assumptions made about the formation mechanism. In the older literature, the excited molecules were often assumed to be produced solely in neutral excitations [127,139-143] and energy-transfer experiments with Stern-Volmer-type extrapolation (linear concentration dependence) were used to derive G(5 i). For instance, by sensitization of benzene fiuorescence, Baxendale and Mayer established G(5 i) = 0.3 for cyclohexane [141]. Later Busi [140] corrected this value to G(5 i) = 0.51 on the basis that in the transfer, in addition to the fiuorescing benzene state S, the S2 and S3 states also form and the 82- 81 and 83 81 conversion efficiencies are smaller than 1. Johnson and Lipsky [144] reported an efficiency factor of 0.26 0.02 per encounter for sensitization of benzene fluorescence via energy transfer from cyclohexane. Using this efficiency factor the corrected yield is G(5 i) = 1.15. Based on energy-transfer measurements Beck and Thomas estimated G(5 i) = 1 for cyclohexane [145]. Relatively small G(5 i) values were determined in energy-transfer experiments for some other alkanes as well -hexane 1.4, -heptane 1.1 [140], cyclopentane 0.07 [142] and 0.12 [140], cyclooctane 0.07 [142] and 1.46 [140], methylcyclohexane 0.95, cifi-decalin 0.26 [140], and cis/trans-decalin mixture 0.15 [142]. 

The fluorescence of liquid alkanes is supposed to originate entirely from the relaxed Si state. Walter and Lipsky [154], by measuring the fluorescence yields of alkane solutions irradiated with 165 nm photons or Kr beta particles ( niax = 0-67 MeV) relative to benzene fluorescence, determined the following yields 2.3-dimethylbutane G Si) < 1.3, cyclohexane 1.4-1.7, methylcyclohexane 1.9-2.2, dodecane 3.3-3.9, hexadecane 3.3-3.9, d5-decalin 3.4, and bicyclohexyl 3.5. After reinvestigating the intrinsic quantum yield of cyclohexane fluorescence, Choi et al. published G(5 i) = 1.45 for this alkane in Ref. 155. For tra 5-decalin a G Si) value of 2.8-3.1 has been accepted [65,128,132]. The uncertainties in the values reflect the uncertainties in the intrinsic fluorescence quantum yields. 

Carbon disulfide quenches the fluorescence of anthracene quite efficiently,145,149 but seems to have little effect on its triplet lifetime.147 Diphenylanthracene in benzene fluoresces with a quantum yield of 0.8 and shows a high sensitivity to the oxygen concentration in photooxygenation reactions. With about 1 vol% of CS2 present, AC>2 is practically independent of [02] (> 10"5 mole/liter). In jjoth cases, where carbon disulfide was either used as solvent or was added to an otherwise strongly fluorescent solution, the quantum yields of photooxygenation followed... 

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