Fluorenone defects

The Problem of Pure Blue Emission in Polyfluorenes Excimer and Aggregate Formation or Fluorenone Defects ... 

X. Gong, P.K. Iyer, D. Moses, G.C. Bazan, A.J. Heeger, and S.S. Xiao, Stabilized blue emission from polyfluorene-based light-emitting diodes elimination of fluorenone defects, Adv. Funct. Mater., 13 325-329, 2003. 

X Gong, D Moses, AJ Heeger, and S Xiao, White light electrophosphorescence from polyfluorene-based light-emitting diodes utilization of fluorenone defects, J. Phys. Chem. B, 108 8601-8605,2004. 

The blue emission from PDAFs is unstable, with the appearance of a strong emission band around 530 nm after annealing or upon running an EL device [14,15]. Initially this long wavelength emission band was believed to be due to emission from excimers [63,64], but subsequent work has shown that it is due to emission from fluorenone defects within the PDAF chain [15]. 

Fig. 10 PL from single PF molecules containing on-chain fluorenone defects, a Chemical structure of the dioctylfluorene-fluorenone copolymer investigated, b Room-temperature single molecule fluorescence wide-field images of the copolymer dispersed in a Zeonex matrix. Note the PL intensity encoded in a negative scale with respect to Fig. 2b. Excitation was performed at 400 nm in the tail of the backbone absorption. The PL was recorded in 5-s exposure windows. Spectral selection was performed by means of blue (left column) and green (centre column) band-pass filters centred at 460 and 550 nm, respectively. Adapted from [26]...

The results presented above show that the low-energy broad PL band at 480 nm in the OMBE-grown films is seen only in the time-delayed emission spectra and it stems from some defects in the material and its relative intensity depends on the sample morphology. This defect band can in principle be either due to structural defects in epitaxially-grown nanocrystallites or related to some sort of chemical defects, as fluorenone defects possibly created by oxidation of PSP molecules. 

Efficient Excitation Energy Transfer from PFO to the Fluorenone Defect... 

Thus, for PFO-F(l %), nearly all the incident photons are absorbed by the PFO. These data indicate efficient Forster energy transfer from PFO to the fluorenone defects. 

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of fluorenone are shown in Fig. 4.28. Consistent with charge trapping, the fluorenone defects function as both a hole trap and an electron trap the HOMO and LUMO of fluorenone fall within the Jt-it gap of PFO [47]. In addition, the hole (electron) can be injected from the PEDOT PSS (Ca) electrode directly into the HOMO (LUMO) of fluorenone because of the small energy barrier between PEDOT PSS and the HOMO (or between Ca and the LUMO) of fluorenone. 

Therefore, the more pronounced green emission from PFO containing fluorenone defects results from a combination of efficient energy transfer, charge carrier trapping and relatively easy injection (from the electrodes) of carriers into the fluorenone traps. 

X. Gong, D. Moses, A. J. Heeger, and S. Xiao. Excitation energy transfer from polyfluorene to fluorenone defects. Synth. Met, 141(1-2) 17-20, March 2004. 

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-fluorenone) with 1% fluorenone as a model compound demonstrated that fluorenone defects are generated by photo-oxidation and by thermal-oxidation [104]. 

As depicted in Figure 5.10b the incorporation of the 9-fluorenone defect sites in MA-PF dramatically changes the emission properties in photoluminescence of the polymer backbone as compared with the photoluminescence of defect-free pristine DA-PF. 

The better spectral stability of sp-PF-2 as compared with sp-PF-1 maybe attributed to a less effective exciton migration to the fluorenone defects. As shown in the inserts of Figure 5.12b and Figure 5.12c the amount of defects created in the two spiro-type materials is similar, if not even more defects are created in sp-PF-2. Nevertheless, sp-PF-1 shows an almost twofold relative increase in keto emission after... 

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