Fluorescence, delayed, £-type

A similar method for determining intersystem crossing efficiencies has been developed by Parker and Joyce 7> using acceptor delayed fluorescence (P-type, see Section 5.2a). The processes involved in this method are... 

The processes III and IV termed as E-type and P-type delayed emissions have emission spectra identical with that of the normal fluorescence but with longer radiative lifetime. The long life is due to the involvement of the triplet state as an intermediate. Hence the short-lived direct fluorescence emission from the Sx state is referred to as prompt fluorescence. E-type delayed fluorescence was called a-phosphorescence by Lewis in his early works. 

The photophysical investigation of the exciplex formed between 4,4, 4"-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD) in a 50 50 blended film showed that the mechanism behind extra singlet production was consistent with the photoluminescence being enhanced via thermally activated delayed fluorescence (E-type nature). Measurements of the emission intensity change with temperature were used to estimate the exciplex singlet-triplet energy splitting to be around 5 meV. 

If states Si and Ti are energetically close, the molecule can escape from the Ti state and return to the Si state. It is obvious that the Ti molecule must gain some energy to reach the level where the energies of states Ti and Si overlap. The required excess energy can be generated by intermolecular collisions with the surrounded molecules or by triplet-triplet annihilation, which requires interaction of two excited species. In both cases, delayed emission from Si occurs. The first mechanism is called delayed fluorescence of type E (because eosin is an important molecule which exhibits this type of delayed fluorescence) and the second mechanism is referred as type P (according to pyrene). In both cases, it is a slow radiative... 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

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... 

The subject of delayed fluorescence was discussed in Section 5.2a. It was seen that there are two common types of delayed fluorescence, that arising from thermally activated return from the triplet state to the lowest excited singlet (E-type delayed fluorescence) and that arising from collision of two excited triplet molecules resulting in a singlet excited molecule and a ground state molecule (P-type delayed fluorescence). The P-type delayed fluorescence can be used as a convenient tool for the determination of intersystem crossing efficiencies[Pg.125]

A linear plot indicates that the luminescence decay is exponential. The slope of the line gives kt, and rt can be calculated as above. The lifetime obtained by measuring the decay of P-type delayed fluorescence is equal to one-half the lifetime of the triplet state (see Section 5.2). Since in fluid solution at room temperature phosphorescence is generally much weaker than delayed fluorescence, the measurement of delayed fluorescence decay offers a convenient method for determining the lifetime of triplets at room temperature. 

P-type delayed fluorescence, 210-212 determination of 0jsc, 232, 236-237 energy of triplet state, 210-212 Pyrazine, 269, 270, 271 Pyrocalciferol, 411... 

For compounds that are very weakly phosphorescent or that phosphoresce at wavelengths out of the normal range of sensitivity of the spectrometer this method of triplet energy determination cannot be applied. For these compounds triplet energies can sometimes be determined by measuring their E-type or P-type delayed fluorescence. 

Let us now return to the question of how E-type and P-type delayed fluorescence may be used to determine the triplet energy level. The efficiency of E-type delayed fluorescence is given by the following equation ... 

Also there seems to be a certain time delay between photoreaction and complete recovery of the nematic phase. This problem is relevant to molecular mobility in liquid crystals as a function of temperature, rubbing condition, external electric field and most importantly, the type of liquid crystal. Research is now being undertaken on direct determination of molecular mobility by fluorescence technique. 

P-type Delayed Fluorescence (Triplet-Triplet Annihilation)... 

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 ... 

E-type delayed fluorescence is so called because it was first observed in eosin. 

Figure 4.12 Jablonski diagram for the deactivation of a molecule by emission of E-type delayed fluorescence...

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... 

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