Benzene, fluorescence spectrum

Figure 4.2 Absorption spectrum (continuous line) and fluorescence spectrum (dashed line) of anthracene in benzene...

Fig. 7 Selected 2PA spectra obtained by absolute fluorescence-based methods, a Spectra for fluorescein, rhodamine B, coumarin 307, and , -p-bis(o-methylstyryl)benzene (the solvent is indicated in the legend) obtained by Xu and Webb [78]. In the case of coumarin 307, the ordinate displays the quantity rjSy where rj is the fluorescence quantum yield, b Spectrum for , -p-bis(o-methylstyryl)benzene (this spectrum is obtained from the tabulated values for the band shape reported by Kennedy and Lytle [90] and the cross section at 585 nm reported by Fisher et al. [80], to correct for a typographical error in the 1986 paper). Part (a) reproduced with permission from [78]. 1996, Optical Society of America...

Figure 7. Comparison of fluorescence spectrum of benzene in methanol (32 OMA accumulations) with published fluorescence spectrum of benzene in water...

Mason and Smith (1969) found that for a series of mono- and bicyclic aromatic hydrocarbons the changes in the fluorescence spectrum with acidity reflected the ground state protonation reaction. The p Sj )-values calculated for benzene, toluene, naphthalene, azulene, and indolizine do not correspond to observable processes since the rate of protonation is too slow to compete with deactivation of the Sj state. Photochemical deuterium and tritium exchange experiments in 1 mole dm-3 perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by a factor >10s. 

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. 

Activated chemiluminescence is observed from these secondary peroxy-esters as well. When the thermolysis of peroxyacetate [281 in benzene solution is carried out in the presence of a small amount of an easily oxidized substance the course of the reaction is changed. For example, addition of N,N-dimethyldihydrodibenzol[ac]phenazine (DMAC) to peroxyester [28] in benzene accelerates the rate of reaction and causes the generation of a modest yield of singlet excited DMAC. This is evidenced by the chemiluminescence emission spectrum which is identical to the fluorescence spectrum of DMAC obtained under similar conditions. Spectroscopic measurements indicate that the DMAC is not consumed in its reaction with peroxyester 28 even when the peroxyester is present in thirty-fold excess. The products of the reaction in the presence of DMAC remain acetophenone and acetic acid. These observations indicate that DMAC is a true catalyst for the reaction of peroxyacetate 28. The results of these experiments with DMAC, plotted according to (27) give k2 = 9.73 x 10-2 M-1 s-1. 

Once laid, the polystyrene films were further purified by exhaustive extraction with methanol or n-heptane the progress of extraction was followed by the ultraviolet spectra of the extracts. These preirradiation extractions showed considerable variation in purity among the three polystyrenes in spite of reprecipitation measures. The degree to which solvents can remain with a 20/ film is suggested by the need of seven days of continuous methanol or n-heptane extraction to remove all of the extractable benzene from a film laid from that solvent and dried in vacuum at 65°C. for 24 hours. For film laid from methylene chloride, an optically clean n-heptane extract was obtained from the AIBN-ini-tiated sample within a few hours, but up to 48 hours were required for the benzoyl peroxide-initiated samples. The extracted 20/ polystyrene films were essentially non-absorbing above 285 m/, no absorption attributable to material other than polystyrene could be observed, and only one peak (337 m/ ) was seen in the fluorescence spectrum in methylene chloride. Once the films were purified by extraction, the products and wettability changes resulting from irradiation were the same for all polystyrene samples and were independent of the solvent from which the films were laid. 

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... 

Fig.11.2 Optical spectra of various optical probes, (a) Photo-luminescence spectra of Q-dots (from the work by Michalet et al. [2]), (b) scattering spectra of gold nanoparticles with different aspect ratio (from the work by Jain et al. [3]), and (c) fluorescence spectrum of FITC (black line) and SERS spectrum (red line) of benzene thiol with 514.5 nm photo-excitation (from the work by Jun et al. [4]). Raman band has much narrower bandwidth than the others...

Figure 5.10. Absorption and fluorescence spectrum of perylene in benzene (by permission from Lakowicz, 1983).

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 formation of a benzene dimer from association of a ground state monomer and an excited monomer is a well-established observation of this species was first noted by Dammers de Klerk (270) and Ivanova and co-workers (271) in a study of the effects of concentration on the fluorescence spectrum of benzene. Broad structureless emission on the long wavelength side of monomer emission at room temperature is clearly observed at concentrations... 

Figure 4. Total fluorescence spectrum for P2NMA in benzene at 25 C. Overlaid data (large circles) indicate the contribution of the components with lifetimes Ti, T2 and T3 to the total emission.

The spectrum of emission III, whose maximum intensity is stronger than that of emission II, shows two maxima at 423 and 445 nm. After emission III disappeared, N-a11y1acridone (3b) was detected on t.l.c. and also by spectrophotometry. 3b is suggested to be the light emitter. The fluorescence spectrum of 3b in a mixed solution of benzene and ethanol which is similar to the solution over the slurry in the emission III, showed two maxima at 421 and 433 nm. The observed shifts of maxima... 

It is known that [3.3]paracyclophane, which has the almost highest transannular interaction of the less distorted benzenes (12), has the fluorescence emission at longer wavelength (356 nm) (18) than the excimer of 1,3-diphenylpropane (332 nm). The fluorescence spectrum of the cyclopolymer, poly(St-C3-St), recorded under the same conditions as for [3.3]paracyclophane is illustrated in Figure 1 (20). Both have the fluorescence at the same wavelength, and therefore the polymer is supported to contain [3.3]paracyclophane units as sequence units. The fluorescence emission at 312 nm is ascribed to the residual styryl groups. 

L. M. Logan, I. Buduls, and I. G. Ross, Pressure dependence of the fluorescence spectrum of benzene, in Molecular Luminescence, E. C. Lim, ed., Benjamin, New York,... 

In the case of the dicyanodiphenyltriafulvene (VI), the dipole moment of the compound in the ground state is 7.9D (dioxane solution, 30°C) [14]. Application of the Lippert-Mataga approach, with the assumption of a cavity radius of 4 A, leads to a value of ID for the dipole moment in the excited state. This is of course a very crude estimation. Too much reliance should not be placed on the quantitative meaning of this result [31]. In any event, the blue shift in the absorption spectrum and the red shift in the fluorescence spectrum on going to more polar solvents or from cyclohexane to benzene, support the substantive decline of the dipole moment in the transition from the ground state to the first excited state. 

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. 

Molecular structure can have a profound effect on the position in the spectrum where fluorescence occurs, as well as on its intensity. It can be shown by quantum mechanics that the more extended a conjugated system is, the smaller will be the separation in energy between the ground state and the lowest excited singlet state. This is evident in the fact that benzene, naphthalene, and anthracene, having one, two, and three rings, fluoresce maximally at 262 nm, 320 nm, and 379 nm, respectively. 

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