Regioselective halogen–magnesium exchange reaction of a bithiophene derivative bearing methoxy and pyridine groups at the β-position and Kumada coupling polymerization

Abstract

We have previously reported the synthesis of a polythiophene derivative (P2′) bearing pyridine and methoxy groups at the β-position, but the halogen–magnesium exchange reaction of the monomer did not proceed in a regioselective manner and resulted in a twisted polymer conformation. In the present paper, the halogen–magnesium exchange reaction of 4-(5″-hexylpyridine-2″-yl)-3-methoxy-2-(5′-bromothiophene-2′-yl)-5-bromothiophene (M3) and its Kumada coupling polymerization and optoelectronic characterization were investigated. The reaction of M3 with i-PrMgCl·LiCl gave a Grignard monomer (GM3a) in 79% yield, with the halogen–magnesium exchange reaction occurring exclusively at the bromine atom neighboring the pyridine group. Reflux temperature was required for the Kumada coupling polymerization of GM3a using Ni(dppp)Cl2 to proceed smoothly (72% conversion after 24 h) due to the sterically hindered monomer structure. On the other hand, the conversion of GM3a remained at 15% without the addition of LiCl. The number-average molecular weight of the tetrahydrofuran-soluble fraction of the regiocontrolled oligo(bithiophene) (P3′) was 2900 because of its poor solubility. Ultraviolet–visible and cyclic voltammogram measurements indicated that compared with P2′, P3′ has a more planar conformation, an increased highest occupied molecular orbital energy level and a narrower bandgap energy. A non-covalent S···O interaction was assumed to cause the planar conformation, which was supported by theoretical density functional theory calculations. The regioselective halogen–magnesium exchange reaction of 4-(5″-hexylpyridine-2″-yl)-3-methoxy-2-(5′-bromothiophene-2′-yl)-5-bromothiophene (M3) with i-PrMgCl·LiCl, Kumada coupling polymerization using Ni(dppp)Cl2 at the refluxing temperature and optoelectronic characterization of regiocontrolled oligo(bithiophene) (P3′) were investigated. The ultraviolet–visible and cyclic voltammogram measurements indicated that P3′ has the more planar conformation, increased highest occupied molecular orbital energy level and narrower bandgap energy as compared with a polythiophene derivative without the methoxy group. The non-covalent S···O interaction was supposed to be a reason for the planar conformation, which was supported by the theoretical density functional theory calculation.