Conjugated polymers, ladder-type

U. Scherf, in Handbook of Conducting Polymers, 2nd edn, (Eds. T. Skolhcim, J.R. Reynolds), Marcel Dekkcr, New York 1997, Chapter 14 (Conjugated Ladder-Type Structures), p. 363. 

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.

To date, the stepwise, kinetically controlled, classical synthesis is the most effective approach to highly annelated chiral Jt-systems. With significant improvements in asymmetric annelation methodologies, multi-step syntheses are likely to remain the main tool in the exploration of novel chiral structures. However, the development of novel synthetic methods will be essential for the preparation of polymers with extended helical-type, ladder-type connectivity of the Jt-systems. Important criteria are to minimize the density of defects in the ladder connectivity and to provide conjugation pathways circumventing at least some of the defects. 

From electronic back to optical properties, in the final Chapter 15, Rajca and Miyasaka provide an overview of the synthesis of molecules with highly anne-lated, chiral, 3t-conjugated systems, primarily of the helicene-type. Tremendous progress has been made in the synthesis of helicenes however, the corresponding helical, ladder-type polymers remain a significant and attractive challenge. In addition to the synthesis, the configurational stability (barriers for racemization) and chirooptical properties are described and discussed in depth in this chapter. In this context, perspectives for isotropic materials with molecule-based chirooptical properties are outlined. 

Oligomer and polymer structures with two-dimensional, so-called ladder-type -conjugation have been shown to be of great relevance in material sciences since, apart from their inherent chemical stability, they seem to possess lower band gaps than their linear analogues. Attractive examples are the [n]acenes... 

The number of organic materials is of great interest in microelectronic and optoelectronic devices [1], In particular, the conjugated polymers, such as ladder-type methyl substituted poly-para-phenylene (Me-LPPP) and poly[2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinilene] (MEH-PPV) and their nanocomposites, have been widely investigated due to their perspectives in light-emitting diodes and solar cells [2—4]. 

FIGURE 1.2. Molecular structure of widely used it-conjugated and other polymers (a) poly(para-phenylene vinylene) (PPV) (b) a (solid line along backbone) and it ( clouds above and below the a line) electron probability densities in PPV (c) poly(2-methoxy-5-(2 -ethyl)-hexoxy-l,4-phenylene vinylene) (MEH-PPV) (d) polyaniline (PANI) (d.l) leucoemeraldine base (LEB), (d.2) emeraldine base (EB), (d.3) pernigraniline base (PNB) (e) poly(3,4-ethylene dioxy-2,4-thiophene)-polystyrene sulfonate (PEDOT-PSS) (f) poly(IV-vinyl carbazole) (PVK) (g) poly(methyl methacrylate) (PMMA) (h) methyl-bridged ladder-type poly(jf-phenylene) (m-LPPP) (i) poly(3-alkyl thiophenes) (P3ATs) (j) polyfluorenes (PFOs) (k) diphenyl-substituted frares -polyacetylenes (f-(CH)x) or poly (diphenyl acetylene) (PDPA). 

The reaction of optically active, helicene derivative 121 first with o-phenylenediamine and then with Ni(OAc)2 led to a helical polymer (Mn 7000) (122) having a unique ladder-type structure with Schiff base moieties immersed in the main chain (Figure 15) 206 -phg polymer showed red-shifted absorptions with respect to nickel salophene, the parent compound for the polymer, supporting the formation of a long conjugation system. Intense CD bands were reported for the polymer. 

Moreover, semi-ladder polymers based on DIC units were obtained in much more classical ways. 2,11- and 3,10-dichloro DIC derivatives were polymerized through a nickel(0)-mediated Yamamoto polymerization reaction [95]. The resulting polymers are soluble in ODCB and partially soluble in CB. The absorption maxima of the BIC-based polymers clearly indicate a planar structure. The absorption maxima of the polymeric precursors show a primary maximum around 350 nm compared to the ladder-type polymers where the maximum is centered around 470 nm [50]. This significant red shift clearly indicates the higher degree of conjugation in the planar rigid polymers. The fluorescence spectra of those polymers also show the same trend. 

---