Pyrene delayed fluorescence

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

When D and A are identical (3D + 3D —> 3D + 3D ), triplet-triplet annihilation leads to a delayed fluorescence, called P-type delayed fluorescence because it was first observed with pyrene. Part of the energy resulting from annihilation allows one of the two partners to return to the singlet state from which fluorescence is... 

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. 11. Luminescence of impurities in ethanol at 20°C. sensitized by 10 Mf phenanthrene. Rate of light absorption was 0.7 X 10- einstein liter-1 sec.-1 at 341 to 362 mji. (a) normal fluorescence (b) and (c) delayed fluorescence at 1000 times greater sensitivity (d) fluorescence of dilute pyrene solution. Photodecomposition of (i>) to give (c) was produced by irradiation for 30 min. with rate of light absorption equal to 10 - einstein liter-1 sec.-1.

Fig. 19. Delayed fluorescence of pyrene in ethanol.42 (1) 3 X 10 W, (2) 10 lM, (3) 3 X 10 W, (4) 2 X 10-6M. The instrumental sensitivity settings were approximately 1000 times greater than those for the corresponding curves in Figure 18. The short wavelength ends of the spectra in the more concentrated solutions are distorted by self-absorption.

Concentration of pyrene, c Rate of light absorption,11 einstein liter- sec.-1, Ia Efficiency of delayed fluorescence of monomer, 0M Lifetime of triplet,11 sec., rt ... 

Concentration of pyrene, c Lifetime of delayed fluorescence, msec. ... 

The mechanism proposed for pyrene provides such a coherent and satisfying explanation of the delayed fluorescence that one feels impelled to apply the same mechanism to the other hydrocarbons also. It is proposed therefore that with anthracene (and with phenanthrene), the... 

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. 

Hutton and Stevens49 have observed that while naphthacene vapor alone produces no delayed fluorescence, pyrene vapor can sensitize the delayed fluorescence of naphthacene vapor. It is true that they explained their results in terms of a long-lived excited dirtier but in a recent private communication Stevens has agreed that delayed fluorescence in the vapor state is probably also produced by triplet-triplet quenching and hks suggested that the results with mixtures of pyrene and naphthacene vapor provide evidence for the occurrence of the mixed triplet mechanism. 

The author has recently carried out some measurements in solution using naphthacene as acceptor.63 As donor, anthracene was chosen rather than pyrene, so as to minimize overlap of the delayed fluorescence of donor and acceptor. A typical delayed emission spectrum from a solution containing 5 X 10 W anthracene and 4 X lO M naphthacene... 

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. 

B) P-type delayed fluorescence is so called because it was first observed in pyrene and phenanthrene solutions. In aromatic hydrocarbons singlet-triplet splitting is large and therefore thermal activation to excited singlet state at room temperature is not possible. The mechanism was first formulated by Parker and Hatchard based on the observation that the intensity of emission of the delayed fluorescence Ipd was proportional to the square of the intensity of absorption of the exciting light Ia. 

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.,... 

Fig. 6-9. Magnetic field dependence of the pyrene triplet yield (<1>t(-B)/ Ot (0)) in acetonitrile as derived from delayed fluorescence measurements ...

If M is fluorescent, triplet-triplet annihilation produces delayed fluorescence, reflecting the long lifetime of M (Parker, 1964). This phenomenon was first studied for pyrene, and was therefore dubbed P-type delayed fluorescence in contrast to the E-type delayed fluorescence discussed in Section 5.1.1. 

The luminescence of doped polymers still continues to attract interest. Dimethylterephthalate quenches the delayed fluorescence of polyvinylcarbazole containing pyrene and... 

Nickel, B., Delayed Fluorescence from Upper Excited Singlet States S (n > 1) of the Aromatic Hydrocarbons 1,2 Benzanthracene, Fluoranthene, Pyrene, and Chrysene in Methylcyclohexane, Helv. Chim. Acta 1978, 61, 198 222. 

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