Abstract
Modification of functional groups attached to conjugated polymer backbones can drastically alter the material properties. Oxidation of electron-donating thioalkyl substituents to electron-withdrawing sulfoxides or sulfones is a particularly effective modification. However, so far, this reaction has not been studied for the modification of conjugated polymers used in organic electronics. Crucial questions regarding selectivity and reaction time waited to be addressed. Here, we show that the reaction is highly selective and complete within just a few minutes when using dimethyldioxirane (DMDO) for the oxidation of thioalkyl substituents attached to the well-investigated conjugated polymers poly(9-(1-octylnonyl)carbazole-alt-4,7-dithienylbenzothiadiazole) (PCDTBT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The selectivity was confirmed by comparison with polymers obtained from pre-oxidized monomers and by control experiments using related polymers without thioalkyl substituents. Using DMDO, the oxidation yields acetone as the only side-product, which reduces the work-up to mere evaporation of solvents and excessive reagent. Our results show that this oxidation is an exciting method for the preparation of electron-deficient conjugated polymers. It may even allow the preparation of electron acceptors for solar cells directly from the electron donors.
Highlights
Conjugated polymers are most frequently used in organic electronics, but the specific applications strongly depend on the properties of the polymer (Swager, 2017; Ibanez et al, 2018)
PCDTBT-based polymer 1a-S (Figure 2, top left) was used for initial investigations focusing on reaction conditions and selectivity, as the carbazole, thiophene, and benzothiadiazole units of the polymer are all potentially prone to oxidation by DMDO (Table 2 gives an overview of the tested reaction conditions)
The polymer precipitated during reaction, which resulted in side reactions indicated by the same 1H NMR peaks that we found later when treating the polymer with a large excess of DMDO for 24 h (Figure 3)
Summary
Conjugated polymers are most frequently used in organic electronics, but the specific applications strongly depend on the properties of the polymer (Swager, 2017; Ibanez et al, 2018). Functional groups can be used to tune these properties (Guo et al, 2013); the desired functional groups can be introduced either by using appropriate monomers for polymerization or by post-polymerization modification. The latter can be achieved either by introducing new functional groups onto the polymer backbone (Crossley et al, 2017) or by modifying or substituting functional groups already present (Schelkle et al, 2014; Creamer et al, 2018). As the main advantage of post-polymerization modification, polymers with varying properties can be obtained from the same starting polymer by using different reagents for modification or by varying the degree or distribution of modification. The modifications can be designed to slightly tune or optimize the optoelectronic properties (e.g., for tuning LUMO/LUMO offsets in solar cells) or to change them drastically (e.g., for turning electron donors into electron acceptors)
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