Naphthalene delayed fluorescence

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))...

Fig. 9. Decay of luminescence with time. Ordinate In (luminescence intensity) one division = 0.25. Abscissa time one division = 0.0003 sec. for curves (a) and (6) 0.0005 sec. for curve (c) 1.0 sec. for curve (d) and 0.1 sec. for curve (e). (a) and (6) Delayed fluorescence of pyrene monomer and dimer in ethanol at +23°C. (c) Delayed fluorescence of naphthalene in ethanol at —23°C. (d) Triplet-singlet phosphorescence of 10-W phenanthrene in EPA at 77°K. (e) Delayed fluorescence of 10-lAf phenanthrene in EPA at 77°K.

Fig. 21. Emission spectra of 3 X 10-lAf naphthalene in ethanol.46 Excitation at 313 ran with frontal illumination, (a) Normal fluorescence at — 105°C. at sensitivity IX (6) delayed fluorescence at — 105°C. at sensitivity 60X (c) delayed fluorescence at -f 22°C. at sensitivity 140X.

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. 

Fig. 25. Sensitized anti-Stokes delayed fluorescence from naphthalene.60 (1, 2) Delayed fluorescence from 10 "s3/ phenanthrene, and from mixture of phenanthrene (10"W) and naphthalene (3 X 10-3M), in ethanol. (3) Normal fluorescence from solutions 1 and 2 at 260 times less sensitivity. (4) Spectral distribution of exciting light for curves 1, 2, and 3, 0.7 X IQ-8 einstein liter"1 sec.-1 absorbed. (5, 6) Normal and delayed fluorescence from 3 X 10 lM naphthalene in ethanol excited by 313 m/i, 2 X 10-4 einstein liter-1 sec.-1 absorbed. Curve (5) at a sensitivity 100 times less than curve (6). Temperature for all curves was — 72° 3°C.

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

P-Type Delayed Fluorescence anthracene, phenanthrene, naphthalene, pyrene, acenaphthene, fluoranthene, and 3 4-benzpyrene. 

Sensitized P-Type Delayed Fluorescence phenanthrene/anthracene, anthracene/naphthacene, phenanthrene/naphthalene, proflavine hydro-chloride/anthracene. Undoubtedly, many more examples will soon be found. 

This phenomenon is possible in molecules like naphthalene, anthracene and Pyrene, each of which happens to have the lowest singlet energy level about twice hat of the respective triplet. For the generation of these triplets in high concentrations (T — T) type energy transfer from a suitable donor is necessary. Delayed fluorescence in naphthalene has been sensitized by phenanthrene. according to the following scheme ... 

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

It has been known for some time that bimolecular collisions between triplet states in solution leads to quenching of the triplet state by a diffusion controlled process.71 Recently Parker and Hatchard have shown that delayed fluorescence from solutions of compounds such as pyrene, naphthalene, and anthracene, is due to triplet-triplet annihilation, i.e.,... 

Parker and Hatchard have shown that sensitized fluorescence may be observed following triplet-triplet energy transfer.63 With the donor-acceptor pair phenanthrene-naphthalene in ethanol solution, sensitized anti-Stokes delayed fluorescence was obtained, the most energetic quanta emitted being 5 kcal.mole-1 greater than the exciting light.60... 

Yokoyama et al. (1976, 1977) measured photo generation efficiencies of N-vinylcarbazole (VCZ) and VCZ-1-vinylnaphthalene copolymers. Both the photocurrent and the delayed fluorescence were enhanced in the copolymer. A quadratic dependence of the photocurient on light intensity was observed at high fields. From the photocurrent and emission spectra, it was concluded that photogeneration occurs via energy transfer from the caibazole (Cz) triplet state to the naphthalene (Nap) as... 

Time and temperature dependences of the delayed fluorescence in isotopi-cally mixed naphthalene crystals have been presented for various concentrations of traps. Coherent two-photon processes in naphthalene in the strong exciton-photon counting regime have also been investigated. Excited-state spectra of 1,5-naphthyridine in several solvents support those calculated using INDO molecular orbital formalism and show the lowest excited singlet state to... 

An important difference in photophysical behavior between the naphthalenic and carbazole polymers is the lifetime of the mobile exciton compared with that of the triplet excimer. For PVCA the mobile exciton is much shorter lived than the excimeric species but for PIVN just the reverse is true. In both polymers the primary mode of delayed fluorescence production involves a hetero-annihilation of the type... 

The triplet state photophysics of naphthalene and a,u-diphenylpolyenes photogenerated in trapped heavy cation exchanged zeolites have been characterized. 0 The application of the modified zeolite environment provides a powerful new method for induction and observation of external heavy atom effects. Triplet excitation energy transport kinetics in vapour deposited naphthalene has been followed by an analysis of the kinetics of phosphorescence and delayed fluorescence. The assorted decays... 

Furthermore, interrupting the sequence of naphthalene units, for example, by copolymerization with methyl methacrylate, results in dimunition of the delayed fluorescence, which is observed only as long as significant blocks of naphthyl groups still remain. It is for similar reasons that delayed fluorescence, for example, in poly(2-vinylnaphthalene), is dependent on molecular weight, that is, on chain length. ... 

Triplet energy migration can be probed by addition of free quenchers to the polymer, and observing which kinetics, Stern-Volmer or Perrin (see Section II above), pertain. In the case of the quenching of naphthalene phosphorescence or delayed fluorescence in a variety of co-polymers by the dloleflns plperylene and cycloocta-1,3-diene at low temperature where material diffusion... 

FIGURE 20. Delayed-emission spectra at 77 K for (a) mixtures of poly(styrene) and poly(l-vinyl-naphthalene), (b) the corresponding co-polymer within each case 1 mo1e% vinyl naphthalene. (1) is poly(l-vinylnaphthalene) delayed fluorescence, (2) is poly(styrene) phosphorescence, and (3) is poly(l-vinylnaphthalene) phosphorescence [after figure in Macromolecules, 2, 187 (1969)]. 

One such method is sensitised fluorescence (see additional remarks on this subject in Chap. 6). Here, the detection limit for fluorescing compounds lies at less than 10" molecules/host molecule or ca. 10 impurity molecules per cm. Fig. 3.6 shows as an example of such a measurement the detection of -methyl-naphthalene in naphthalene. Still more sensitive is the method of sensitised delayed fluorescence (Sect. 6.9). Here, the detection limit is at ca. 10 molecules/host molecule or 10 impurity molecules per cm [5,6]. 

Fig. 6.22 Sensitised delayed fluorescence, schematically, a The mixed crystal H + C is excited into the triplet band T] of the host (naphthalene green light). The delayed fluorescence contains a relatively large proportion of guest light (anthracene violet light) and little host light (naphthalene ultraviolet), b The ratio of guest intensity Qc to host intensity Qh iti the delayed fluorescence is plotted against the guest...

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