Ladder PPP

Consequently, by combining the synthetic procedure of Yamamoto [26] with the introduction of more extended solubilizing substituents, advantageous results were expected. Accordingly, alkyl-substituted dihydrophenanthrenes or tetrahy-dropyrenes offered themselves as starting monomers for the preparation of soluble step-ladder PPPs of this type. 2,7-Dibromo-4,9-dialkyl-4,5,9,10-tetrahydro-pyrenes (10) represent suitable starting monomers for the realization of this synthetic route. These difunctionalized tetrahydropyrene monomers were first prepared by Mullen et al. and reacted in a Yamamoto coupling [27]. 

A block of substituted phenyl sequences connected with double bonds has allowed for the preparation of copolymer structures that can be used as a thin active layer in an LED, with emission in the 460 nm region and internal quantum efficiencies from 2% to 4% [126]. The Mullen and Scherf approach [127,128] to ladder PPP (L-PPP) provided a good structural improvement for the preparation of stable and tuneable light emitting polymers. 

Photophysics of Methyl-Substituted PPP-Type Ladder Polymers... 

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.

In Figure 8-1 we show the chemical structure of m-LPPP. The increase in conjugation and the reduction of geometrical defects was the main motivation to incorporate a poly(/ -phenylene)(PPP) backbone into a ladder polymer structure [21]. Due to the side groups attached to the PPP main chain excellent solubility in nonpolar solvents is achieved. This is the prerequisite for producing polymer films of high optical quality. A detailed presentation of the synthesis, sample preparation,... 

T. Pauck, R. Hennig, M. Pcnter, U. Lemmer, U. Siegncr, R. F. Malm, U. Scherf, K. Mullen, H. Bassler, E.O. Gobel, Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-typc ladder polymer Client. Phys. Lett. 1995, 244, 171. 

As described for the corresponding polymers, a powerful strategy for arriving at soluble oligomers with maximum conjugative interaction is incorporation of the PPP backbone into a stepladder - or ladder (see Sect. 2.2.) - framework, in... 

The logical continuation of the stepladder strategy outlined above for minimizing the mutual distorsion of adjacent main chain phenylene units was the incorporation of the complete PPP-parent chromophore into the network of a completely planar ladder polymer. The complete flattening of the conjugated 7T-system by bridging of all the subunits should then lead to maximum conjuga-tive interaction. As with the PTHP 11 systems, alkyl or alkoxy side chains should lead to solubilization of the polymers. 

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. 

J Huber, K Mullen, J Salbeck, H Schenk, U Scherf, T Stehlin, and R Stern, Blue light-emitting diodes based on ladder polymers of the PPP type, Acta Polym., 45 244-247, 1994. 

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). 

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