Benzene fluorescence efficiency

Transfer of singlet electronic energy from cyclohexane to benzene and eventually to tetramethylphenylene diamine is a sequential process which has been studied in detaiP°. An important detail reported in this work is that an upward revision of the benzene fluorescence efficiency in cyclohexane to 0.26 0.02 appears to be required. 

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]. 

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... 

If one of the substances has a known fluorescence efficiency, the value of the other is then simply obtained. Convenient standard solutions are rhodamine B in ethanol with fluorescence in the yellow and efficiency 0.69, quinine bisulfate in 0.1 N sulfuric acid with fluorescence in the blue and efficiency 0.55. anthracene in ethanol with fluorescence in the violet and efficiency 0.27 in the ultraviolet region, naphthalene ( = 0.19), phenol (0 = 0.19), or benzene (0 = 0.042) can be used. With the last four compounds the solution must be deaerated by passing a current of nitrogen before measurement. To minimize the effect of errors in the spectral sensitivity curve it is desirable to use as the standard a solution... 

The fluorescence of anthracene in benzene is efficiently quenched by N,N-dimethylaniline and a strong exciplex emission appears in a longer wavelength than the emission of anthracene [382-384], However, the addition product was not obtained at all, except the (4 + 4) anthracene dimer (Scheme 114). In contrast, the addition product and reductive dimerization product of dimethylaniline to the anthracene ring are produced via photoinduced electron transfer, which was first reported by Pac and Davidson [385-387], In the case of V-mcthylaniline, some addition products are obtained both in nonpolar and polar solvents [386-389],... 

Fio. 7 Correlation of the initial chemiluminescence intensity in the decomposition of [21] in benzene at 24.5°C, corrected for fluorescence efficiency and photomultiplier tube and monochromator response, with the oxidation potential of amine and hydrocarbon activators... 

The apparent bimolecular rate parameters > STS characteristic of the external heavy-atom quenching of the molecular singlet and triplet states of anthracene (A) (solvent = cyclohexane, quenchers = bromobenzene and ethyl iodide) and 9,10-dibromoanthracene (DBA) (solvent = ethanol, quencher = KI) have been determined and used to describe the sensitivity of the external heavy-atom effect acting on the non-radiative Sx - Tx and T -> S0 processes.141 142 The interesting observation that ethyl iodide and benzene increase the fluorescence efficiency from excited dibromoanthracene has been explained in terms of the formation of photoassociation product E (exciplex ).141b This species then undergoes its own characteristic photophysics. The dominant processes for the ethyl iodide case are ... 

The gas-phase fluorescence efficiency and spectral structure of benzene are invariant from 5 x 10 atm to 10 atm. 

The fluorescence efficiency of benzene solutions containing 5 1 mixtures of l-C(hloroanthracene) and P(erylene) was studied for values of [P] < 0.006 M [E. T. Bowen and B. Brockelhurst, Trans. Faraday Soc. 49, 1131 (1953)]. The fluorescence efficiency of P increases as solute concentration increases. 

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. 

Since the efficiency of fluorescence quenching of the sensitizer paralleled the oxidizability of the arene in a series of substituted alkyl benzenes, the reaction was thought to proceed through electron transfer followed by protonation and trapping of the radical by oxygen. 

Chrysene. Since selenium and selenium compounds are toxic this dehydrogenation and the associated work-up procedure must be carried out in an efficient fume cupboard. Mix 3.5 g (0.015 mol) of hexahydrochrysene with 16 g (0.2 mol) of selenium in a boiling tube and heat in a fusible metal bath at 300 °C for 20 hours (fume cupboard). From time to time, melt the crystalline sublimate which gradually forms so that it runs back into the reaction mixture. Remove the cooled product and grind it in a mortar to a fine powder. [CAUTION (3).] Extract by boiling under reflux for 30 minutes with 200 ml of benzene, filter and reflux the filtered extract over a little clean sodium wire (or thin narrow slices of sodium metal) this treatment removes traces of selenium. Evaporate the benzene solution using a rotary evaporator and crystallise the residue from toluene (about 20 ml per 1 g) (4). Colourless plates with a bluish fluorescence, m.p. 254 °C, are obtained. The yield of chrysene is about 2 g (59%). 

Typical intramolecular (2 + 2) photocycloaddition of electron-rich alkenes to the 1-cyanonaphthalene ring via an exciplex has been reported by McCullough and Gilbert, independently. McCullough reported intramolecular photocycloaddition of bichromophoric molecules 255,258, and 259, in which the chromophores are linked by a three-atom ether chain [286,287] (Scheme 71). The fluorescence of the 1-cyanonaphthalene chromophore of 255,258, and 259 is efficiently quenched by alkenes and the typical intramolecular exciplex emissions are observed. Photocycloaddition of these 1-cyanonaphthalenes in benzene occurs at 1,2-, 3,4-, or 5,6-positions of the naphthalene ring, respectively. 

Dorr, Lewis, and co-workers found evidence through quenching experiments and flash spectroscopy for a triplex in the system trans-stilbene — amine — benzene — [105]. They quenched singlet excited trans-stilbene with various mono- and diamines and found a steric effect on the quenching constant The a, co-diamines (dabco, diaminoethane, -propane and -butane) quenched the stilbene fluorescence more efficiently than the monoamines, depending on the chain length between the amino groups. This was ascribed to the formation of cyclic radical cations, with a N-N three electron a-bond. In this case, an exciplex between diamine and stilbene is formed. 

Thermolysis of peroxide [29c] in benzene solution generates a chemiluminescent emission whose spectrum is identical to the fluorescence spectrum of photoexcited p-dimethylaminobenzoic acid under similar conditions. Thus the direct chemiluminescence is attributed to the formation of the singlet excited acid. The yield of directly generated excited acid is reported to be 0.24% (Dixon and Schuster, 1981). Since none of the other peroxybenzoates generate detectable direct chemiluminescence it was not possible to compare this yield to the other peroxides. However, by extrapolation it was concluded that the dimethylamino-substituted peroxide generates excited singlet products at least one thousand times more efficiently than does the peroxyacetate or any of the other peroxybenzoates examined. 

The fluorescence spectra of 2,5-diarylpyrazines have been studied the presence of electron-donating substituents on each aryl group, as in 2,5-bis(p-methoxyphenyl)pyrazine (246), strengthened fluorescence on photoexcitation the fluorescence of 2,5-di(naphthalen-2-yl)pyrazine (247) proved stronger than that of the isomeric 2,5-di(naphthalen-l-yl)pyrazine due to reduced planarity in the latter structure.1288 p-Bis[2-(pyrazin-2-yl)vinyl]benzene (248) proved to be an efficient blue laser dye (emission A-max 438 nm in Me2SO solution) on excitation by a nitrogen laser at 337 nm.1484... 

Nanosecond laser flash photolysis showed that pyrrole-2-carboxyaldehyde does not exhibit fluorescence emission, but undergoes intersystem crossing to the triplet state with an efficiency of 0.80 in benzene <2003PPS418>. 3,5-Dimethyl-2-(2 -pyridyl)pyrrole forms weak 1 1 H-bonded complexes with methanol and /r-ry-butanol in the ground state, while 3,5-di-fet7-butyl-2-(2 -pyridyl)pyrrole forms both 1 1 and 1 2 complexes with the same alcohols, but no excited state proton transfer appears to occur in such complexes <2004MI3948>. 

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