PPPs ladder-type

A copolymer 84 (Scheme 38) containing dialkyl PPP, ladder-type PPP, and electron-transporting diaryloxadiazole segments (m w p = 4 3 3) has been prepared by a similar copolymerisation with 2,5-bis(4-bromophenyl)-l,3>4-oxadiazole added as a comonomer [160]. It emits blue light (A.max = 410, 480 nm) with an efficiency of 0.4% when aluminium cathodes are used [161]. This is twice the efficiency obtained for similar devices using the copolymers 83 without the oxadiazole units. 

Figure 9-1. Materials overview a few sclcclcd conjugated polymers and Ihcir properties have been compiled and ihe following abbreviations arc used DO-PPP...Poly(2-decyloxy-l,4-phcnylcnc), EHO-PPP...Poly(2-(2 -elhylliexyloxy)-l,4-phcnylenc), CN-PPP... Poly(2-(6 -cyano-6 -incthyl-licplyloxy)-l,4-phcnylene), m-LPPP... methyl-substituted ladder-type Poly( 1,4-phenylcne), and PLQY=phololuinincs-ecncc quanluni yield.

This idea was realized impressively in 1991 with the first synthesis of a soluble, conjugated ladder polymer of the PPP-type [41]. This PPP ladder polymer, LPPP 26, was prepared according to a so-called classical route, in which an open-chain, single-stranded precursor polymer was closed to give a double-stranded ladder polymer. The synthetic potential of the so-called classical multi-step sequence has been in doubt for a long time in the 1980s synchronous routes were strongly favoured as preparative method for ladder polymers. 

As the next step in this way, Mullen and coworkers [608] have reported PPP-type polymers 512, containing planar pentaphenylene blocks. As expected, the emission maximum (Apl = 445 nm) of 512 was found between those of indenofluorene 510 (432-434 nm) and completely planar ladder-type PPP (450 nm) (see Section 2.5.2) [608]. Single-layer PLEDs ITO/PEDOT/512b/Ca/Al showed stable pure-blue emission with brightness in excess of 200cd/m2 (at 7 V) (Chart 2.121). 

Another absorbing species present both in photo excited and electrically excited para-phenylenes are polarons. In Fig. 8.9, we have summarized the absorption and emission spectra encountered in para-hexaphenyl (1) the triplet absorption, (2) the stimulated emission observed in time-resolved experiments with a 200-fs resolution, (3) the polaron absorption, and (4) the continuous-wave photoluminescence emission. A clearer picture for the polaronic state can be derived from experiments on the ladder-type PPPs. 

FIGURE 8.11. Femtosecond field-induced pump/probe spectra of ladder-type PPP at different pump/probe delays (rd). Within a few hundred picoseconds, the electric field leads to the dissociation of excitons which can be seen by (1) the decrease in stimulated emission at 2.4 and 2.55 eY and (2) the absorption of the created polarons at 1.9 and 2.1 eV. (Reproduced from Ref. 166.)... 

Again taking the effective masses to be equal to the electron mass, using 300 K for room temperature, and using 2.7 eV for the energy gap of the ladder-type PPPs, we obtain an intrinsic carrier concentration of 10 4 cm 3. This value will rise to 41 cm-3 upon increasing the temperature to 400 K. These values have to be compared to common inorganic semiconductors at room temperature 2.7... 

This approach has been realized by Scherf and Mullen in the synthesis of ladder-type PPP (LPPP) (see Refs. 26, 64, and 65). In the following, an overview over the performance of polymer LEDs based on LPPP-type polymers is presented. 

Scheme 51 Proposed emissive defect in ladder-type PPPs...

An excellent example of the use of Suzuki polycondensation is the synthesis of ladder-type PPPs (67) (see Scheme 6.16) [84]. A precursor polymer 79 is prepared by AA-BB coupling and then converted to the ladder polymers by polymer analogous reactions. Reduction followed by ring closure with boron trifluoride produces a polymer (67a) with bridgehead hydrogens, while addition of methyl lithium instead of reduction leads to Me-LPPP (67b) with methyls at the bridgeheads. 

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