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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Tuning COPs (Tuning both the photoluminescence and conductive properties of new COP materials)

Teaser

Summary

Since energy will be a key issue in the 21st century, we have to save energy and one possible way is the savings on lighting. For this purpose the use of organic light emitting diodes (OLEDs) could be a perfect choice, being far more energy efficient than incandescent lamps or fluorescent tubes. Most of the modern OLEDs have two important parts: an emissive layer and a conductive layer, which is often a so-called conductive organic polymer (COP). While most of the organic polymers have low electric conductivity and are insulators, COPs have unique electronic properties: they are conductors. Recent project focused on the synthesis of new conductive organic polymers (COPs), which are one of the most important part of the modern organic light emitting diodes (OLEDs). The main goal was to prepare COPs with tuneable properties, which could be achieved with application of tuneable doping agents. Cobaltabisdicarbollide [Co(C2B9H11)2]-, [COSANE]- anion is an excellent platform, since their redoxpotential can be tuned by halogen substitutions. The project focused on two kind of COP materials, namely the polyethylene dioxythiophene (PEDOT) and poli(p-phenylene vinylene) (PPV).

Work performed

The main objective of the first work package of the project (WP-1) was the investigation of the tuneability of conductive and the optical properties of PEDOT (polyethylene dioxythiophene) via the redox potential of the different anionic boron cluster containing doping agents. According to this theoretical data, the halogen substitution decrease the energy level both the HOMO and the LUMO, which was in good agreement with the earlier calculations. According these calculations, the non-halogenated parent compound is the easiest to oxidize, which has the more positive HOMO energy. The halogen substitution stabilized the LUMO as well, thus the halogenated species are easier to reduce. These data were in good agreement with the experimentally observed E1/2 values and these data show the redox potential of the doping anion is tuneable by halogen substitution as previously experimentally reported in the host group. Parallel with the theoretical calculations several halogenated cobaltabisdicarbollide [Co(C2B9HnX11-n)2]-, [COSAN]- derivatives were synthesized. The purity of the compounds was checked by analytical methods (e.g. NMR, MS, etc.). As it can be expected the properties (morphology, conductive properties, etc.) of the PEDOT polymer can be tuned with the different doping anions, but unfortunately the photophysical properties not. In all investigated cases the polymers were black. In case of the parent [Co(C2B9H11)2]- system we have investigated the role of the counter ion (H+, Li+, Na+, K+, Cs+) and its influence on the stoichiometry and physical chemical properties of resulting PEDOT polymer. Although, the [Co(C2B9H11)2]- anion can be considered as a non-coordinating anion, it binds to the accompanying cations with a distinct strength. This negatively charged unit interacts with the solvated cations distinctly. If the interaction is strong they form a relatively stable ion pair whereas if it is weak, the ions are free to move. Indeed, the application of different cations result in different polymers with different properties (electrochemical response, electronic conductivity, ionic conductivity and capacitance). The different cations can be classified into two class. In case of the Cs+ and H+ the ratio EDOT:[Co(C2B9H11)2]- is around 2:1 while in case of Li+, Na+, K+ the ratio is around 3:1. These results indicate that, for the [COSAN]- Cs+ and H+ salt the interaction with the doping anion is stronger, which was supported by DFT calculations as well.

The second work package aimed the synthesis of self-doped poli(p-phenylene vinylene) (PPV). In this work package, we had faced several synthetic problems. First we planned to link the monomer unit to the [Co(C2B9H11)2]- through its boron atom at the 8 position via a -(CH2)2-O-(CH2) 2-O- linker (starting from COSAN-dioxanate). The last step in the synthesis of functionalized synthon is a bromomethylation, which requires strong acidic media (glacial acetic acid, HBr). Under this condition, the B-O bond cleaved in the -CH2-O-B(cluster) branch of the COSAN derivative unit. Furthermore, the other B-H vertices are brominated and probably bromomethylated as well according the results of the mass spectra. To solve the B-O bond cleavage other alternative route was designed. The linker was planned to bounded to the [Co(C2B9H11)2]- unit through a -(CH2)4- linker (B-C bond), but in this case the cluster brominated as well, and the different products are not isolable. As a final attempt, the monomer unit was bound to the carbon atoms of the cluster (C-C(cluster) bond), but similar problems, due to the presence of different different brominated products occurred.

Other results which were triggered by the preliminary DFT studies of Work Package 1
The original proposal did not contain this work package, but the preliminary DFT calculations helped to understand of the properties of some related systems, which were investigated in the research group. Investigating the Kohn-Sahm orbitals of

Final results

The results contributed significantly to the technology of the new COP materials. We have highlighted the importance and the role of the accompanying cations on the properties (morphology, conductivity, etc.) of the PEDOT materials, which were not emphasized earlier. The properties can be further tuned by changing the redox properties of the anions. The investigation of the photophysical properties and the reactivity of different carborane containing systems expand the possible application of these unique cluster compounds.