Eosin delayed fluorescence

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

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... 

E-Type Delayed Fluorescence eosin, proflavine hydrochloride. 

Another less-common form of unimolecular decay is also temperature dependent and results in the phenomenon known as is-type delayed fluorescence.183 For example, the lowest excited singlet and triplet states of eosin are quite close together in energy, so that excited... 

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... 

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. 

Thermal repopulation, where the triplet level is close enough to the singlet level that the singlet state can be thermally populated from the triplet. Sometimes called E-type delayed fluorescence, after the compound eosin (4.4) which exhibits this behaviour. The lifetime of E-type delayed fluorescence is equal to the triplet lifetime. Other materials which exhibit E-type delayed fluorescence are palladium porphyrins (5.4) and thiocarbonyls (11.21). 

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... 

Fig. 12. Eosin in glycerol (7 X 10 Af) and eosin in ethanol (1.5 X 10 W). (a) Fluorescence emission spectrum at +30°C. (6) delayed emission spectrum (DES) at +69°C. (c) DES at +48°C. (d) DES at + 18°C. (e) DES at -40°C. Delayed emission spectra at a sensitivity 600 times greater than that for the fluorescence emission spectrum. (/) Fluorescence emission spectrum at -J-22°C. (g) delayed emission spectrum (DES) at +71 °C. (h) DES at +43°C. (j) DES at +22°C. (Z) DES at — 7°C. (m) DES at —58°C. (s) Sensitivity of 1)558 photomultiplier with quartz monochromator (unite of quanta and frequency). Delayed emission spectra at a sensitivity 3000 times greater than that for the fluorescence emission spectrum.

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