Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-fluoro)thiophenes.

Zhuping Fei,Pierre Boufflet,Sebastian Wood,Martin Heeney,Jessica Wade,Erin L Ratcliff,Eliot Gann,Ji-Seon Kim,Christopher R Mcneill,Henning Sirringhaus,John Moriarty

doi:10.1021/jacs.5b02785

Abstract

We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.

Highlights

Poly(3-alkyl)thiophenes (P3ATs), in particular poly(3-hexyl)thiophene (P3HT), are some of the most widely investigated conjugated polymers yet reported.[1]
In the case of field-effect transistors (FETs), the small ionization potential of P3AT, as a result of its electron rich backbone, can lead to issues with ambient stability, due to expeditious doping by atmospheric oxygen and/or water.[10]
We decided to focus upon the Grignard metathesis (GRIM) route to synthesize the fluorinated polymers, because of the well-known robustness and good control that this route offers for P3HT.1b The critical building block was the synthesis of 2,5-dibromo-4-fluoro-3-alkythiophene

Summary

■ INTRODUCTION

Poly(3-alkyl)thiophenes (P3ATs), in particular poly(3-hexyl)thiophene (P3HT), are some of the most widely investigated conjugated polymers yet reported.[1]. In this case we find a gradual reduction in the relative intensity of the C−C mode and a shift to lower frequency at high temperatures, which is consistent with reduced molecular planarity at elevated temperature as a result of thermally induced backbone twisting, similar to that observed in other thiophene-based polymers.[41]. We observe a pronounced shift in the position of the C C peak from 1445 to 1469 cm−1 occurring between 260 and 270 °C, which reverses more gradually upon cooling (270−240 °C) This transition is indicative of a significant conformational change and may represent the temperature at which the energetic rotational barrier for the inter-thiophene bond is fully overcome to allow full rotational freedom of the backbone. These results further confirm recent studies suggesting that backbone rigidity and co-planarity are important design features for high-performance transistor materials.[43]

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES