Fluorescence delayed

The long-lived delayed emission as phosphorescence has spectral characteristics very different from fluorescence. But there are delayed emissions whose spectra coincide exactly with the prompt fluorescence from the lowest singlet state, the only difference being in their lifetimes. These processes are known as delayed fluorescence. Two most important types of delayed fluorescence are (A) E-type delayed fluorescence and (B) P-type delayed fluorescence. 

If rate constants are competitive, we expect the emission of prompt fluorescence, phosphorescence and delayed fluorescence of energy quanta /7v/, h jp and //vn) respectively. Although //v/ is equal to ftvED, the lifetime of delayed fluorescence will match the lifetime of triplet decay. The rate constant ArEn for E-type delayed fluorescence is temperature dependent and can be expressed as 

The ratio of quantum yields 4p to kd of phosphorescence and E-type delayed emissions respectively can be deduced as follows. 

On thermal excitation to the S, state, fluorescence will occur with usual fluorescence efficiency f f. Therefore, the efficiency of delayed fluorescence faa is given as 

The ratio of the intensities of the two bands in delayed emission spectrum should thus be independent of the efficiency of triplet formation and of all triplet quenching processes. It should also be independent of the intensity of absorption Ia. The rate of emission is then given as Aed[T1 where Aed is temperature dependent. 

The value of the phosphorescence quantum yield can be determined by measuring the total luminescence spectrum under steady irradiation. If the fluorescence quantum yield is known then the phosphorescence quantum yield may be found by comparing the relative areas under the two corrected spectra. 

In certain compounds a weak emission has been observed with the same spectral characteristics (wavelengths and relative intensities) as fluorescence, but with a lifetime more characteristic of phosphorescence. Two mechanisms are used to account for delayed fluorescence. 

The fluorescence emission discussed so far is produced by direct excitation of a molecule M to one of its excited singlet states that, after IC to Sx when an upper singlet state was initially populated, emits prompt fluorescence from Si with a lifetime on the order of nanoseconds. In addition, several processes can be envisaged that permit repopulation of Sx following ISC to Tx, which can give rise to emission that has the spectral characteristics of fluorescence, but a lifetime much longer than that of prompt fluorescence, that is, to delayed fluorescence. These processes can be sub-classified as P-type and 4E-type delayed fluorescence,32 We do not consider artificial delayed fluorescence due to impurities, or due to ionization followed by recombination with the formation of 1M.  

The classical example of E-type delayed fluorescence is that of eosin (4, 5 -dibromo 2/,7/-dinitrofluorescein disodium salt) in degassed solvents. The name E-type refers to cosin, which has a high quantum yield of ISC and a small singlet triplet energy gap, AEst = 18 kJ mol Thermally activated repopulation of the Sx state by reverse ISC 

An unusual example of delayed fluorescence exemplifying El Sayed s rules (Section 2.1.6) was recently reported for the triplet sensitizer xanthone,127 which undergoes ultrafast ISC within 1 ps. Delayed fluorescence with a lifetime of 700 ps was observed in aqueous solution. Temperature-dependent steady-state and time-resolved fluorescence experiments indicate that the T2(n,it ) state, which is primarily accessed by ISC from Si(ji,ji ), is nearly isoenergetic with the Sj state. The delayed fluorescence is attributed to reverse ISC from T2(n,it ), in competition with internal conversion to Tl(7I,7l ). 

Because two triplets 3M are required for triplet triplet annihilation, the intensity of P-type delayed fluorescence is proportional to the square of the radiant power at moderate intensity of the light source. The decay of P-type delayed fluorescence does not follow a simple rate law. Its decay kinetics are related to those of the triplet state, of mixed first and second order. 

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The impurities may capture this migrating exciton and lose its excess energy. The mutual annihilation of two or more triplet excitons occurs in the same polymer chain and delayed fluorescence is observed. 

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... 

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... 

For two solutions containing the same acceptor but different donor molecules the ratio of the delayed fluorescence intensities is given by... 

If conditions are such that energy transfer from both donors occurs with 100% efficiency, the relative Ol80 values can be obtained by comparing the intensities of the delayed fluorescence of the acceptor. If one OlBO is known, the other can be directly determined. 

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]

By measuring the intensities and lifetimes of delayed fluorescence of both solutions in the same apparatus [(/ ) = (Ia)2] we obtain... 

Since Oet for all donors is essentially the same (generally assumed to be unity), the ratio of delayed fluorescence intensities is simply... 

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 ... 

Marriott, G., Clegg, R. M., Arndt-Jovin, D. J. and Jovin, T. M. (1991). Time resolved imaging microscopy. Phosphorescence and delayed fluorescence imaging. Biophys. J. 60, 1374—87. 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

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 ... 

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