Lichtemittierende Polymere

1. Introduction

1,4-Diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) and some of its derivatives are commercialized as high performance pigments with exceptional light, weather and heat stability [1]. Due to the excellent photostability and high quantum yield of fluorescence, they are potential materials for luminescent devices. In order to utilize the DPP chromophore in luminescent and other electronic devices, it is necessary to prepare soluble and film forming DPP compounds such as oligomers [2] or polymers [3-9]. Previous studies have shown that DPP-containing polymers can be prepared upon radical polymerization [3], polycondensation [4], polymer modification [5], or Pd-catalyzed cross-coupling reactions [6-10]. The latter approach is especially interesting, because highly photoluminescent conjugated polymers containing DPP chromophores in the main chain can be obtained [7-10]. In our research group new DPP-containing conjugated polymers are prepared using Suzuki-, Stille- and Heck-coupling. The polymers are characterized with regard to their optical, photophysical and electrochemical properties. In the following, some of the polymers are described in detail.

2. Polymers prepared upon Suzuki polycondensation

Our first attempts to prepare conjugated DPP-containing polymers were based on Suzuki polycondensation of the dibromo-DPP-derivative 1 and various arene diboronic acid or diboronic ester derivatives using tetrakis(triphenylphosphine)palladium(0) as catalyst. A typical procedure is outlined in Scheme 1:

Monomer 1 was obtained from dibromo-DPP upon subsequent alkylation of the lactam units using 1-bromohexane and potassium carbonate in dimethylformamide at 120 ⁰C. In Tab. 1, several polymers are listed, which were synthesized according to Scheme 1. Some of their typical properties are also indicated. The polymers were obtained as bright red powders, which could be easily dissolved in common organic solvents and processed into thin film upon solution casting (or spin coating). Molecular weights are between 6 and 24•10³ Da. Optical absorption and fluorescence maxima in chloroform depend on the comonomer unit Ar, the electron-rich Ar units causing a-red shift of the absorbance and fluorescence and a decrease of the band gap. All polymers exhibit large Stokes-shifts between 60 and 100 nm and low LUMO-levels of – 3,3 to – 3,57 eV. For some polymers, high photoluminescence quantum yields up to 85 % were found. Typical absorption and emission colours of monomer and polymer solutions are shown in Fig. 1. Cyclovoltammetric studies indicate a quasireversible oxidation behaviour, whereas the reductive cycles are irreversible. The polymer with Ar being 2,5-dihexoxy-1,4-phenylene was also studied on its electroluminescent properties. Using a multilayer device of ITO/DPP-polymer/OXD7/Ca/Mg:Al:Zn, the polymer is electroluminescent with maximum emission at 640 nm, the turn-on voltage is 3 V [10,11].

3. Polymers prepared upon Stille and Heck coupling

In a recent study, new conjugated DPP-polymers prepared upon Stille and Heck coupling were reported [9]. DPP-thienylene copolymers were prepared from 1 and distannyl derivatives of thiophenes and bithiophene using tetrakistriphenylphosphinepalladium(0) as catalyst (Stille coupling). Poly(DPP-phenylenevinylene)s were synthesized upon Heck coupling of 1 and divinylbenzene. The catalyst was palladium(II)acetate/tris(o-tolyl)phosphine. The polymerization reactions are outlined in Scheme 2:

Characteristic properties of the polymers are listed in Tab. 2. All polymers are well soluble in organic solvents and exhibit a bright purple fluorescence with maxima of 600 - 635 nm in chloroform and 685 to 723 nm in the solid state. The fluorescence quantum yields are lower than for the polymers prepared upon Suzuki coupling. One reason might be a quenching caused by traces of tin, which could hardly be removed from the Stille products. As indicated in the Table, the molecular weights of the poly-DPP-thienylenes are between 7 and 12•10³ Da, whereas for the Heck product a much higher molecular weight is found. Typical absorption and emission colours of monomer and polymer solutions are shown in Fig. 2. Cyclic voltammetric studies indicate a reversible oxidation behaviour for the poly-DPP-polythienylenes.

4. Summary

Our studies demonstrate that DPP-containing conjugated polymers can be prepared from dialkylated dibromo-DPP 1 and various comonomers using palladium-catalyzed polycondensation reactions (such as Suzuki-, Stille- and Heck-coupling) or electrochemical polymerization. The polymers are readily soluble in common organic solvents. The solutions and films exhibit purple red colours and a purple luminescence with maxima up to 635 nm (in solution) and 723 nm (in film). The quantum yield of photoluminescence is up to 85 %. Preliminary studies indicate that the polymers are electroluminescent. Cyclic voltammetric studies show HOMO levels of the polymers between – 5.7 and – 5.1 eV and LUMO levels between – 3.7 and - 3.3 eV depending on the comonomer units. Some of the copolymers exhibit quasireversible oxidation and/or reduction behaviour. In future work, other coupling reactions will be studied and polymers containing other chromophores than DPP will be investigated.