Keto defects

The polarized emission experiments on partially photooxidized aligned PF films indicate that the emission from the keto defects exhibits a somewhat smaller polarization ratio than the blue emission from the defect-free chains [263]. This observation was explained with the support of quantum mechanical calculations, which showed that the polarization of the fluorenone emission is influenced by local disorder [263]. 

E.J.W. List, R. Guentner, P.S. de Freitas, and U. Scherf, The effect of keto defect sites on the emission properties of polyfluorene-type materials, Adv. Mater., 14 374-378, 2002. 

Keywords ASE Charge pairs Exciton dynamics Fluorescence lifetime Keto defects Phosphorescence Polyfluorene beta-phase... 

In thin films a similar pattern is observed at short wavelengths, see Fig. 5. The decay is composed of a sum of three exponentials, see Table 2. However, at very long wavelengths, the decay is almost completely dominated by a long component of around 3 ns, attributed to the presence of photo oxidized, keto defects or other emissive defects, which are very easily populated by efficient energy migration. This is described in Sect. 1.6. The fast component appears as a rise time of the emission intensity and a component with 349 ps indicates the PFO lifetime. 

From the chemist s point of view, the keto defect sites can be formed during polymer synthesis as a consequence of incomplete monomer alkylation, as well as a result of photo-, electro-, or thermooxidative degradation processes occurring after polymer synthesis. Acting as low-energy trapping sites... 

In addition to this experimental evidence for the existence of keto defects, a significant impact on the photophysical properties of the corresponding polymers could be identified (see Fig. 3). While the monoalkylated polymers exhibited a strong contribution of the green emission band in solution and, in particular, in the solid state after polymer synthesis, the corresponding greenish emission was not found in the pristine dialkylated polyfluorene. In the latter case, the low-energy emission band was only observed after photo-, electro-, or thermooxidative stress. 

Fig. 2 a Infrared spectra of monoalkylated (dotted line) and dialkylated polyfluorenes (solid line) show that in the case of monoalkylated polymers keto defects are already present after polymer synthesis, b Besides polymer synthesis, keto defects can also be generated by, e.g., photooxidative degradation. Here, the corresponding infrared spectra after photooxidative degradation of dialkylated polyfluorenes are shown (modified from [16,17])... 

Scheme 3 Proposed mechanism for the generation of keto defect sites during the synthesis of polyfluorenes (modified from [16,17,40])...

From the above results it can be concluded that keto defect sites are preferably formed during polymer synthesis when non- or monoalkylated fluorene species are present in the reaction mixture. This points to the necessity of avoiding even small amounts of these components in order to provide polyfluorenes and polyfluorene-based materials without such centers of degradation and to realize high molecular weight polymers. This prerequisite for polyfluorenes with increased stability can also be transferred to other blue emitter materials as shown, e.g., by Romaner et al. [43] for ladder-type polyparaphenylenes. In this study, again, full alkylation was identified as an important parameter for highly stable materials as it was derived from com-... 

Having discussed different aspects of the chemical origin of keto defects and its general impact on photophysical properties from the synthesis point of view, the keto defect and its impact on the solid state physics and device performance will now be considered. 

A common mode of device degradation in polyfluorene-type PLEDs is the formation of keto defects during device operation and the related change of the emission spectrum with a broad peak emerging around 2.3 eV (Fig. 7). 

In this work we have reviewed the latest scientific results, putting special focus on chemical defects (keto defects, fluorenoles, etc.) as potential explanation for the low energy emission band of polyfluorenes. Along this line,... 

Keywords (3 Phase Keto defect n Conjugation Single molecules Polyfluorene... 

However, the work of List et al. [48], based on PL and IR spectroscopy, questioned this picture demonstrating a correlation between the green emission and the presence of keto defects [82] on the PF chain. These can be statistically introduced during the synthetic procedure or arise as the result of photo-oxidation. 

On the other hand, the red-shifted emission from conjugated polymers can originate also from chemically different moieties such as keto defects [53] created via accidental photochemical degradation or deliberate exciplex formation at the internal interface between conjugated polymers [54, 55, 56],... 

It was suspected that the low energy emission band results from keto defects that were introduced either during synthesis or by photo-oxidation during service. Experiments with poly(9,9-dioctylfluorene-co-fluo-renone) with 1% fluorenone as a model compound demonstrated that flu-orenone defects are generated by photo-oxidation and by thermal-oxidation. " Moreover, the formation of these defects is catalyzed by the metals with a low work function that are used as cathode materials in light-emitting diodes. 

SCHEME 5.5 Synthesis of para-phenylene ladder polymers after Scherf and Mullen (MCP 1991) [64] (R, and R2 are typically n-alkyls with Cg-Cio, R3 is preferably a methyl group resulting in MeLPPP, R3 = H leads to LPPP ladder polymers which are sensitive to oxidative degradation-formation of keto defects [ 16,17,70,71 ]). 

The overall quantum yield of the tr —tr emission is finally reduced in all polymers. The highest reduction of the quantum yield is found for PF 111/12 and the lowest for sp-PF-2. In all polymers, an absolute increase of the keto emission, not only a relative increase with respect to the PF emission, is observed for moderate heating times (not more than 1 h at 200°C). For the spiro-type polymers this true increase is observed for excitation within the PF backbone absorption peak (370 nm), while it was only found for direct excitation of the keto defects with excitation energies lower than the optical bandgap of... 

Note that the results presented in this chapter, in particular the different stability with respect to the formation of keto defects, might not only be influenced by the particular chemical structure of the polymers, but also by the particular chemical purity of the used polymers. 

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