Naphthalene fluorescence spectrum

The normal (short-lived) fluorescence spectrum of 3 X 10 2M naphthalene at —105 °C. [Fig. 21, curve (a) ] shows not only the band due to the singlet excited monomer but also the broad dimer emission band, with maximum at 400 m which is similar to that observed by Doller and Forster46 in toluene solutions. The spectrum of the delayed emission at the same temperature [Fig. 21, curve (b)] also shows both bands, but the intensity of the dimer band is relatively much greater. When the concentration is reduced to 3 X 10 W, the intensity of the dimer band at —105 °C. is very small in normal fluorescence but is still quite large in delayed fluorescence.45 The behavior of naphthalene solutions at —105° C. is thus qualitatively similar to that of pyrene at room temperature. At temperatures greater than — 67 °C. (Table XII) the proportion of dimer observed in delayed fluorescence is almost the same as that observed in normal fluorescence, and presumably at these temperatures, establishment of equilibrium between the excited dimer and excited monomer is substantially complete before fluorescence occurs to an appreciable extent. The higher the temperature, the lower is the proportion of dimer observed in either normal or delayed fluorescence because the position of equilibrium shifts in favor of the excited monomer. 

The third group ofpolychromophoric compounds to be discussed are homopolymers in which the pendant rings are separated from the backbone by one or more atoms. The polymers of allyl arenes, which lack only the n = 3 ring spacing of aryl vinyl polymers, have been studied very little. The fluorescence spectrum of poly(l-allyl-naphthalene) in dilute dichloromethane solution has been reported 28). Like 1-ethyl-naphthalene, the maximum intensity was seen at 337 nm, but a weak, broad shoulder was also recorded for the polymer at 410 nm. The fluorescence ratio Iu/IM for poly(l-allylnaphthalene) was only 1/100 th the value for P1VN 28). The excimeric nature of the 410 nm emission in the allyl-based polymer has not been confirmed, since neither the lifetime nor the excitation spectrum of this fluorescence band are known. 

Fig. 6. The fluorescence spectrum of naphthalene vapor reported by Watts...

A fluorescence spectrum is characteristic of a given compound. It is observed as a result of radiative emission of the energy absorbed by the molecule. The observed spectrum does not depend on the wavelength of the exciting light, except that the spectrum will be more intense if irradiation occurs at the absorption maximum. The spectral intensity is called the quantum efficiency and is usually abbreviated as . The quantum yield or quantum efficiency, d>, which is solvent dependent, is the ratio Approximate values of quantum efficiencies are as follows naphthalene, 0.1 anthracene, 0.3 indole, 0.5 and fluorescein, 0.9. 

Figure 10. Enlargement of spectral region where differences exist in the fluorescence spectrum of ethylbenzene (Figure 9) indicating the presence of naphthalene...

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. 

Figure 6. (a) Fluorescence spectrum of a mixture of naphthalene, phenanthrene, anthracene, perylene, ana tetracene (Xga. = 258 nm). (b) Synchronous signal (AX = 3 nm) of the same mixture. 

P>MA Particles. PIB-stabilized PbWA particles were prepared containing naphthalene [N] groups covalently attached to the fTWA chains. This was effected quite simply by adding l naphthylmethyl methacrylate to the NWA polymerization step of the particle synthesis. From reactivity ratios, one knows that the N groups are randomly distributed along the P>MA chains. The particles were purified by repeated centrifugation, replacement of the supernatant serum with fresh solvent (isooctane) and redispersion. A fluorescence spectrum of the dispersion was typical of that of a 1-alkyl-naphthalene. Chemical analysis indicated a particle composition IB/NWA/N of 13/100/10. 

Naphthalene was linked to P-CD through a polyether chain to give 60 [211]. For [60]<10 M, the fluorescence spectrum exhibited only a... 

Fig. 3.6 In the fluorescence spectrum of naphthalene at 4.2 K, the impurity concentration of -methyl-naphthalene can be estimated from the intensity ratio of a vibronic naphthalene line and the 0.0 line of )3-methyl-naphthalene to be 3 10 . After [3].

The fluorescence quenching experiments were carried out according to the following procedures. The stock solution of naphthalene derivative in methanol (1 x 10 M) was injected to the 1 x 10 M aqueous 3-CD solution to adjust [naphthalene derivative] = 1 x 10 M. An appropriate amount of TMA was added and the sample solution was shaken gently before fluorescence spectral measurement. Since the fluorescence intensities of naphthalene derivatives gradually decreased upon aging and no reproducible result was obtained, the fluorescence spectrum of each solution was measured immediately after the preparation of sample by using a Hitachi 650-60 spectrofluorimeter. The... 

When the ethylene glycol moiety of PET is replaced by a flexible polytetramethylene ether glycol chain of molar mass 2 kD, the fluorescence spectrum in concentrated solution appears very similar to that of PET and it was found to be able to thermally crystallize at 50-114 C for days into spherulities with diameters up to 20 pm.When the terephthaloyl group is replaced by naphthalene 2,6-dicarboxyloyl group, it has been possible to get TEM pictures showing crystalline lamellae of ca. 10 nm... 

Figure Bl.1.4. Two-photon fluorescence excitation spectrum of naphthalene. Reprinted from [35], Courtesy, Tata McGraw-Hill Publishing Company Ltd, 7 West Patel Nagar, New Dehli, 110008, India.

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

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. 

For some aromatic hydrocarbons such as naphthalene, anthracene and pery-lene, the absorption and fluorescence spectra exhibit vibrational bands. The energy spacing between the vibrational levels and the Franck-Condon factors (see Chapter 2) that determine the relative intensities of the vibronic bands are similar in So and Si so that the emission spectrum often appears to be symmetrical to the absorption spectrum ( mirror image rule), as illustrated in Figure B3.1. 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

Fig. 22 B. Exeitation spectrum at room temperature showing the intensity of delayed fluorescence of a naphthalene crystal as a function of the wavelength of the exciting light. The ordinate is proportional to the square of the singlet-triplet absorption coefficient. (From Avakian and Abramson, Ref.52))...

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