Thermally-activated delayed fluorescence

E-type Delayed Fluorescence (Thermally-activated Delayed Fluorescence)... 

The Jablonski diagram for thermally-activated delayed fluorescence is shown in Figure 4.12. 

Thermally activated delayed fluorescence Reverse intersystem crossing Ti — Si can... 

Thermally activated delayed fluorescence See delayed fluorescence. 

Thermally activated delayed fluorescence Reverse intersystem crossing Ti —> Si can occur when the energy difference between Si and Ti is small and when the lifetime of Ti is long enough. This results in emission with the same spectral distribution as normal fluorescence but with a much longer decay time constant because the molecules stay in the triplet state before emitting from Sp This fluorescence emission is thermally activated consequently, its efficiency increases with increasing tempera-... 

However, one of the most exciting recent developments in LEDs has been the development of other routes, notably thermally activated delayed fluorescence (TADF), to use the triplet energy in electroluminescent devices. This will be discussed in the next section. 

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. 

The studies on copper complexes demonstrated that the phanephos ligand (see Fig. 2) can be successfully applied to engineer highly luminescent Cu(i) complexes. The rigid [Cu(dmp)(phanephos)] complex displays a high luminescence quantum yield of 0.8 at ambient temperature. In contrast to the long-lived phosphorescence of 240 ps at low temperature, the ambient-temperature emission represents a thermally activated delayed fluorescence with a decay time of 14 ps. ... 

Due to the thermally activated delayed fluorescence, they are able to harvest both singlet and triplet excitons in electroluminescent devices, making them ideal candidates for OLED emitters. ... 

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]

Absorption 2 Vibrational relaxation 3 Intersystem crossing 4 Vibrational relaxation 5 Thermal activation 6 Delayed fluorescence... 

The intensity of the delayed fluorescence emission from eosin decreases as the temperature is lowered and this indicates that an energy barrier is involved. Since the delayed fluorescence is spectrally identical to normal fluorescence, emission must occur from the lowest vibrational level of Si. However, the fact that the lifetime is characteristic of phosphorescence implies that the excitation originates from T,. The explanation of this requires a small Si-Ti energy gap, where T, is initially populated by intersystem crossing from Si. Ti to Si intersystem crossing then occurs by thermal activation. 

In the delayed emission spectrum of eosin in glycerol or ethanol two bands are present, the relative intensities of which are strongly temperature-dependent (see Fig. 12). The visible band at 1.8 has a contour identical with that of the fluorescence band. It no doubt corresponds to the visible phosphorescence observed by Boudin.26 To interpret the results it was assumed that this band of delayed fluorescence was produced by thermal activation of the eosin triplet to the upper singlet level followed by radiative transition from there to the ground state. The far red band was assumed to correspond to the direct transition from the triplet level to the ground state and was therefore called phosphorescence. To determine the relationship between the intensities of the two bands we write the equations for the formation and consumption of triplet molecules as follows ... 

The delayed fluorescence produced by triplet-triplet quenching is to be sharply differentiated from that observed with eosin or proflavine hydrochloride. The latter type has the same lifetime as the triplet and its intensity is proportional to the first power of the rate of light absorption. It is produced by thermal activation of molecules from the triplet level to the excited singlet level and can occur with any substance for which... 

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. 

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... 

All experiments to be discussed herein were conducted on films at 77K. This limitation was imposed for two reasons (1) the photophysics of polymer films (including P2VN) at 77K has been relatively well characterized, and (2) thermally activated processes (including E-type delayed fluorescence) are minimized. 

Delayed fluorescence T-T annihilation (P-type) and thermally activated (E-type)... 

E-type delayed fluorescence. As defined by the lUPAC Gold book [2], this is the process in which the first excited singlet state becomes populated by a thermally activated radiationless transition from the first excited triplet state. Since in this case the populations of the singlet and triplet states are in thermal equilibrium, the lifetimes of delayed fluorescence and the concomitant phosphorescence are equal. This process takes its name from eosin and is typically observed with dyes, where the Si-Ti gap is small. 

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