Thermally-activated delayed

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. ... 

Heuristic Fxplanation As we can see from Fig. 22-31, the DEP response of real (as opposed to perfect insulator) particles with frequency can be rather complicated. We use a simple illustration to account for such a response. The force is proportional to the difference between the dielectric permittivities of the particle and the surrounding medium. Since a part of the polarization in real systems is thermally activated, there is a delayed response which shows as a phase lag between D, the dielectric displacement, and E, the electric-field intensity. To take this into account we may replace the simple (absolute) dielectric constant by the complex (absolute) dielectric... 

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 ratio of the intensities of the two delayed emission bands should thus be completely independent of 4>t and of all triplet quenching processes. Since ke represents a thermally activated process,... 

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. 

This idea was probed by irradiating two samples of CN-DHA with the same concentration and volume in a one pump pulse and a two pump pulses experiment [7] In the one pulse experiment the photoconversion to CN-VHF-trans was triggered at 340 nm under similar conditions as in the two color time resolved measurements. The irradiation time t was chosen such that a significant amount of CN-VHF-trans was converted. In the two pulses experiment an additional pulse at 530 nm which was delayed by 25 ps from the first pulse excited the transient CN-VHF-cis for the same time t. After the irradiation much less CN-VHF-trans is found in the two pulse experiment than in the one pulse experiment (Fig. 4). We conclude that a significant amount of CN-DHA was regenerated from the transient species by the second pulse. Photodegradation and thermal activation by the 530 nm pulses were eliminated as possible reasons for the observed effect. 

A particularly useful application is the oxidation stability test in which the thermal activity of a specimen is followed in oxygen until a sudden large exotherm occurs. If the sample contains an oxidation inhibitor the onset of the exotherm will be delayed, compared to that of an uninhibited sample. The amount of this delay is a measure of the amount of oxidation inhibitor remaining. 

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