Optimized Rieke zinc synthetic protocols toward polythiophene copolymers for organic photovoltaic applications

This dissertation has investigated side chain functionalized random and block copolythiophenes synthesized through an optimized Rieke polymerization protocol, which uses highly reactive Rieke zinc (Zn*) for the preparation of the organozinc intermediates. The major goal was to optimize the synthetic procedure for the active Rieke zinc metal and later on to employ this protocol for the synthesis of small molecules and polymers, more in particular functionalized thiophene building blocks and poly(3-alkylthiophenes). Rieke zinc is generally prepared by the reduction of zinc chloride with lithium employing a stoichiometric amount of naphthalene. We have found out that the ‘quality’ of the resulting zinc metal particles is strongly dependent upon the naphthalene source. When we started to perform the preparation of Rieke zinc with a new source of naphthalene, the in situ formed zinc particles agglomerated to chunky metal clusters, unlike upon application of the ‘old’ naphthalene source, which gave Rieke zinc as a finely distributed black slurry. Based on these findings, it was postulated that the old naphthalene batch had to contain something that helps to stabilize the zinc particles. ICP-MS analysis of the naphthalene samples and the Rieke zinc powder showed that sulfur was present in both naphthalene sources. Furthermore, HPLC-FLD analysis was conducted, indicating that both samples contained the same contaminant with a signal around 230 nm. However, the amount of the contaminant was found to be nine times higher in the old naphthalene. In addition, GC-MS analysis performed on different naphthalene batches revealed that the main contaminant has a molar mass of 134 g/mol, in accordance with the spectrum of benzothiophene under the same circumstances. Apparently, the old naphthalene was contaminated with benzothiophene to a larger extent, which has tremendous implications on the stabilization of the Rieke zinc in THF solution. It was also found that the amount of benzothiophene is very important, with an optimum of 3 mol% (with respect to zinc chloride), to prevent coagulation of the zinc particles, as confirmed by SEM-EDX analysis of the Rieke metal powder. Therefore, from that moment on, Rieke zinc was readily prepared from zinc chloride by adding 3 mol% of benzothiophene into the lithium naphthalenide solution. Employing the Rieke zinc obtained with this enhanced procedure, several regioregular copolythiophenes were synthesized. First of all, a series of ester functionalized random copolythiophenes was prepared and the appended ester moieties were subsequently converted into different functions by postpolymerization reactions. The polymers were fully characterized and applied in bulk heterojunction organic solar cell devices as donor materials (in polymer:PC61BM active layer blends). Accelerated lifetime measurements showed that, although side chain functionalization might lead to a small decrease (if any) in the initial power conversion efficiency, the lifetimes of the photovoltaic devices were considerably prolonged due to the enhanced thermal stability of the active layer blends. The optimized Rieke zinc protocol was also applied to prepare ester functionalized dibromothiophene monomers with different alkyl side chain lengths from commercially available 3-bromothiophene and α,ω-diols in a straightforward way. Regioregular poly{[3-hexylthiophene]-co-[3-(ω- acetoxylalkyl)thiophene]} polymers were prepared from these monomers and they were applied as donor materials in bulk heterojunction solar cells. The side chain length seems to have a minor effect on the efficiencies of the photovoltaic devices, which have to be evaluated further on in reliability tests. We have also synthesized block copolythiophenes by sequential addition of two organozinc monomers. The formation of diblock structures was confirmed by SEC, NMR and DSC analysis. Further optimization is still needed to obtain higher molar mass polymers. Finally, it was also tried to extend the applicability of the Rieke protocol to different (hetero)aromatic units apart from thiophenes. Accordingly, dibromo and/or bromo-iodo compounds were polymerized through the Rieke method. It was seen that in some cases addition of LiCl was required for complete oxidative addition of Rieke zinc to the carbon-halogen bond. However, this protocol has to be studied in more detail to allow smooth synthesis of (hetero)arylenes, since so far only polymers with low molar masses could be obtained in low yields. The findings from this study add several (important) contributions to the current literature. First, the optimized procedure enables the preparation of Rieke zinc in a convenient and reproducible manner in large quantities without significant batch to batch variations. Secondly, we believe that this new procedure will definitely increase the application of Rieke zinc in both general organozinc chemistry and functionalized conjugated polymer synthesis as organozinc reagents show excellent chemoselectivity and good functional group tolerance and stability compared to the more reactive Grignard reagents. Finally, the most important limitation so far lies in the fact that the Rieke procedure has only been proven its effectiveness toward polythiophenebased (co)polymers. Therefore, further experimental investigations are needed to be able to extend the versatility of the method to other (hetero)arylenes. A reasonable approach to tackle this issue would be to study the kinetics of the coupling reaction for different catalyst/ligand variations.

Publication year:2013

Accessibility:Open