Energy transfer phosphorescence

Dissociation of the molecule, usually into radicals Intermolecular energy transfer giving another electronically exited species, which may undergo reaction Luminescenee ineluding phosphorescence and fluorescence... 

When D and A are similar molecules emission-reabsorption cannot be very important due to the usually small overlap of the emission and absorption spectra. Also, this mechanism should not be important for triplet-triplet energy transfer because of (a) low phosphorescence quantum yields in fluid solutions and (b) the low oscillator strengths for singlet-triplet absorption. 

The intramolecular triplet energy transfer studies described so far generally have used phosphorescence emission measurements to follow the... 

Filipescu and co-workers(84-88) have also examined intramolecular singlet and triplet energy transfer using fluorescence and phosphorescence as a gauge for energy transfer. Lamola<87) has repeated one of the studies... 

The original stabilizer (HBC) was modified as the rapid radiationless deactivation of the stabilizer is (at least partly) due to the intramolecular hydrogen bond, the H-atom was substituted by a methyl group (MBC). This "probe molecule" showed fluorescence and phosphorescence and enabled us to demonstrate the energy transfer to the stabilizer, simply by studying its sensitized luminescence. 

In order to corroborate the existence of energy transfer we studied the phosphorescence polarization. While the directly excited phosphorescence of MBC in... 

C, excitation at 289 nm, emission observed at 525 nm, degassed films. (l2) Measured total phosphorescence intensity of MBC (arbitrary units) ("I, X y) component of the MBC phosphorescence due to direct absorption of MBC molecules (ltenJ component of the MBC phosphorescence due to energy transfer. 

While we used the probe molecule to investigate the energy transfer by sensitized phosphorescence we now turn to the stabilizer itself (e.g. TIN with an intramolecular hydrogen bond) to study its deactivation in the excited states. 

If neither mode of energy transfer is acceptable, a different explanation of the apparent quenching of the DMT phosphorescence must be put forth. It must be recalled that both DMT and 4,4 -BPDC absorb 298 nm light, which introduces the argument that competitive absorption causes the apparent quenching effect. 

The observed luminescence properties of the copolymer yarns can be easily explained if an energy transfer mechanism is assumed to be operating (Figure 7). Triplet-triplet energy transfer from the terephthalate units to the 4,4 -biphenyl -dicarboxyl ate units explains both the dual fluorescent/phospho-rescent emissions from the 4,4 -biphenyldicarboxyl ate units as well as the quenched phosphorescence from the terephthalate units. 

Phosphorescence as a probe of exciton formation and energy transfer in organic light emitting diodes M. Baldo, M. Segal Physica Status Solid A Applied Research, 201 1205-1214... 

The above strategy, using three primary colors as phosphorescent dopants, has a big problem in controlling the efficiency of the energy transfer, which eventualy will result in... 

As an extension of the fluorescent sensitizer concept, Forrest et al. have applied this approach to phosphorescent OLEDs, in which the sensitizer is a phosphorescent molecule such as Ir(ppy)3 [342]. In their system, CBP was used as the host, the green phosphor Ir(ppy)3 as the sensitizer, and the red fluorescent dye DCM2 as the acceptor. Due to the triplet and the singlet state energy transfer processes, the efficiency of such devices is three times higher than that of fluorescent sensitizer-only doped device. The energy transfer processes are shown in Figure 3.21. 

In their follow-up paper, they also demonstrated 100% efficient energy transfer of both singlet and triplet excited states. The device exhibits peak external efficiency and power efficiency of 25 cd/A and 17 lm/W at 0.01 mA/cm2, respectively [343]. Liu demonstrated a high-efficiency red OLED employing DCJTB as a fluorescent dye doped in TPBI with a green phosphorescent Ir(ppy)3 as a sensitizer. A maximum brightness and luminescent efficiency of... 

FIGURE 3.21 (a) Energy transfer mechanisms of phosphorescent dye as a sensitizer and (b) the EL external efficiency of the DCM2 doped devices. (From Baldo, M.A., Thompson, M.E., and Forrest, S.R., Nature, 403, 750, 2000. With permission.)... 

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