Benzene fluorescence yields

The absolute fluorescence yields for QHj derive with one exception from excitation at 2536 A. Two of these are based on calibration against the phosphorescence yield of biacetyl,which has been a gas phase standard for many years. The first reports a yield of about 0.23 0.05 at 11 torr, but this was subsequently remeasured by Poole as 0.17 in further evolution of that technique in the same laboratory. A later measurement independent of the biacetyl standard is reported by Noyes, Mulac, and Harter. They observe (in their Method B) that f = 0.18 0.04 at 11.5 torr. Further support for this value comes from its use in calibration of singlet relaxation quantum yields in fluorobenzene and toluene in which the sum of observed radiative and nonradiative yields is unity within experimental error. A higher benzene fluorescence yield would push that sum paradoxically above unity. 

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... 

The fluorescence yield of benzene (7 = 0.34) is independent of pressure at sufficiently low pressures.56-59 After excitation to the first singlet, in the low-pressure region, part of the benzene behaves chemically as if it were excited in a triplet state.50... 

From 2530 to 2590 A the sum of the fluorescent yield and the triplet-state yield for benzene is about 0.92 with an experimental error hard to estimate with precision. 

The effect of solvent on fluorescence yields is still imperfectly understood. Mulliken-type interactions of the excited state with the solvent are believed to reduce yields (8). It has been stated in the literature that anthracene fluorescence falls through the solvent series benzene, toluene, xylene, and mesitylene, but recent measurements show that the effect, if any, is insignificant (42,59,64). For the two solvents, ligroine and chloroform, anthracene is much less fluorescent in the latter, but for 9,10 dichloranthracene the order is reversed (14). The fluorescence of anthracene in a number of solvents has been examined by Bowen and West (18). 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

Such chemical evidence as exists, plus a strong wavelength dependence of fluorescence yield from benzene and the low yield of triplet state formation at 2400 A. all point to a competing process whose importance at 2537 A. cannot for the moment be estimated. One must state, therefore, the evidence for the effect of colhsions on the rate of crossover of benzene from excited singlet to excited triplet states is conflicting. In solid matrix, vibrations definitely play an important role in intersystem crossover for benzene. ... 

TABLE R. B. Cundall, et al. 8. Monomer fluorescence yields for benzene in solution. 

TABLE 10. Fluorescence yields for benzene in the solid phase. 

Emission lifetimes for benzene with vapor phase are subject to similar variations with pressure and excitation wavelength as are fluorescence yields. Data are collected in Table 6 and can be seen to show considerable variation with source and experimental technique. Recent measurements, using a single-proton counting technique (114) have shown the emission lifetimes of high pressure CgHg and CgDg to be 77 and 92 ns, respectively, at 25°C. The temperature dependence of the fluorescence lifetime is also shown in Fig. 7. 

Direct observation of fluorescence from higher singlet states of benzene and some methyl derivatives has recently been achieved by Hirayama, Gregory, and Lipsky (247). Using apparatus capable of detecting fluorescence yields as low as 10 they recorded the emission spectra from oxygenated solutions of pure benzene and other aromatics excited at 184.9 nm. Subtraction of the tail of the residual S, emission gives a fluorescence spectrum with approximately 235 nm and = 8 x 10 for... 

The experimental difficulties Involved in measurement of fluorescence yields and lifetimes for benzene, and other systems also, as a function of concentration make it certain that the various thermodynamic quantities have not yet been firmly established. 

A re-investigation of the fluorescence yields and fluorescence lifetimes of benzene in polar and non-polar solvents and an analysis of the quantum yields of benzvalene formation under a variety of conditions has led to the conclusions that benzvalene is formed from the vibrationally excited state of benzene and that the population of non-totally symmetric vibrational levels plays a key role in benzvalene formation.3-8 The quenching of benzene fluorescence by dissolved... 

The fluorescence decay is purely exponential the absence of any long-lived emission has been evidenced in the case of benzene by the absence of pressure effects on the fluorescence yield in the 10 -10 torr pressure range (Kistiakovsky and Parmenter, 1965). The fluorescence decay time is shorter... 

The first criterion is most easily examined. In pure benzene vapor excited at 2536 A or longer wave lengths, both the quantum yield and the observed singlet state lifetime show only a very shallow dependence on benzene pressure above 10 torr. Addition of foreign gases such as hydrocarbons to these experiments also has little (but sometimes finite) effect on fluorescence yields " or lifetimes. Even in the extreme collisional limit of condensed phase at 77°K, the fluorescence yield of 0.2 matches that of the vapor. - (Condensed phase yields drop to about 0.05 at 300°K, but this is probably a special thermal effect somewhat apart from a collision-induced electronic decay. See Section IVC.)... 

The yields, lifetimes, and rate constants for relaxation from the thermal vibrational levels in 10-20 torr of benzene vapor near 300°K are given in Table III, Both fluorescence yields and lifetimes are available from several... 

Fig. 13. Qualitalive fluorescence yields of Parmenter and Schuyler from single vibronic levels of benzene vapor. The solid lines indicate the levels from which moderate to strong fluorescence has been observed when they are individually excited with pressures low enough (ca. 0.2 torr) to preclude significant collisional deactivation prior to electronic decay. The dashed lines indicate levels from which emission was too weak to be observed. The notation on the right identifies the vibrational level. For example, (2 X 6) + 1 indicates emission from the vibrational level (2v + of the state.

Quantum yields are published for only three levels, of which the highest is about 1500 cm Accurate absolute values of fluorescence yields are notoriously difficult to obtain, and it is with these measurements that crucial relationship of the high-pressure data to the isolated molecule data first becomes apparent. Each yield in Table VII derives from comparison of isolated molecule fluorescence intensity with that from benzene with the same excitation but taken to high pressures by addition of inert gas. The isolated molecule yields are then calculated from Noyes conclusion that (1) the high-pressure yields are independent of the state initially excited at these wave lengths and (2) the high-pressure yields equal 0.18. We have confidence in the value 0.18 because of the numerous checks described in Section V, and confidence in (1) derives from direct measurements of the relative isolated molecule yields in other experiments. The relative yields match those in Table VII. 

Parmenter, C.S. and Schuyler, M.W (1970) Single vibronic level fluorescence. III. Fluorescence yields from three vibronic levels in the Bj state of benzene. Chem. Phys. Lett., 6, 339. 

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. 

Purification of anthracene. Dissolve 0-3 g. of crude anthracene (usually yellowish in colour) in 160-200 ml. of hexane, and pass the solution through a column of activated alumina (1 5-2 X 8-10 cm.). Develop the chromatogram with 100 ml. of hexane. Examine the column in the hght of an ultra-violet lamp. A narrow, deep blue fluorescent zone (due to carbazole, m.p. 238°) will be seen near the top of the column. Immediately below this there is a yellow, non-fluorescent zone, due to naphthacene (m.p. 337°). The anthracene forms a broad, blue-violet fluorescent zone in the lower part of the column. Continue the development with hexane until fluorescent material commences to pass into the filtrate. Reject the first runnings which contain soluble impurities and yield a paraffin-hke substance upon evaporation. Now elute the column with hexane-benzene (1 1) until the yellow zone reaches the bottom region of the column. Upon concentration of the filtrate, pure anthracene, m.p. 215-216°, which is fluorescent in dayhght, is obtained. The experiment may be repeated several times in order to obtain a moderate quantity of material. 

Table 7.11 Fluorescence quantum yield

This is a fluorescent benzyl ether used for 2 -protection in nucleotide synthesis. It is introduced using 1 -pyrenylmethyl chloride (KOH, benzene, dioxane, reflux, 2 h, >65% yield). Most methods used for benzyl ether cleavage should be applicable to this ether. 

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