Synthesis of LPPPs

By using phenyllithium instead of methyl lithium Ph-LPPP (108) can be made [ 124], In this case it has been found that AICI3 needs to be used instead of BF3 as the Lewis acid to achieve complete ring closure. 

While complete ring closure is indicated by mass spectral and NMR analysis, characterization of 5 by X-ray and neutron scattering [133], and by dynamic light scattering experiments [134] suggested this polymer has 

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In a classical multi-step route the main point is to be able to conduct the ring closure quantitatively and regioselectively. In the synthesis of LPPP, the precursor polymer 27 is initially prepared by aryl-aryl coupling of an aromatic diboronic acid and an aromatic dibromoketone. 

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. 

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

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. 

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. 

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 ]). 

Scheme 5 Synthesis of methylene-bridged ladder polymers of the poly(pa/-a-phenylene) type (LPPP).

Scheme 7 Synthesis of stepladder copolymers of the LPPP type.

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,... 

First, we would like to address the question how sample quality influences the observed results. Synthesis and sample treatment influence the electronic properties of conjugated materials in a defined way [23]. We have already shown [31] that the shape and intensity of photoinduced absorption spectra in different representatives of the LPPPs may vary (see Fig. 9-16), indicating at least different trap densities but also different electronic properties of these traps, depending on the synthesis and subsequent treatment of the polymers. However, the electronic properties for this class of polymers can be imderstood in terms of effective conjugation length [23-25] charge transfer by photoexcitation or redox reactions [31] and also photo-oxidation upon intense visible irradiation under the influence of oxygen [23]. Therefore, by optical spectroscopy (absorption, photoluminescence, or photoinduced absorption) we can assess the quality of a sample. 

Since PFs, PIFs, PPP-type ladder polymers (LPPPs), and other bridged PPPs are a very promising class of blue emitters of PLEDs [17,35,36], the synthesis, characterization, and application of such aromatic polymers have been very extensively worked on in the last decade. It seems impossible to review the enormous amount of scientific publications in this field. This review can only give a short overview that... 

That even low levels of defects can produce strong emission is exemplified by the case of Ph-LPPP (71). The synthesis (Scheme 32) is similar to that of Me-LPPP (66), except that complete ring closure of the polyalcohol 71 could not be obtained using boron trifluoride [119]. As a result, other reagents had to be tested and it was found that complete ring closure could be obtained by using aluminium chloride [149]. 

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