Synthesis of polythiophene derivative bearing methoxy and pyridine groups at the β-position and formation of an intramolecular hydrogen bonding through the polymer reaction

Abstract

Two 4-pyridylthiophene monomers M1 (2,5-dibromo derivative) and M2 (5-bromo-2-iodo derivative) having the methoxy group at the 3-position were synthesized. The Grignard metathesis reaction of M1 using i-PrMgCl·LiCl demonstrated the highest monomer conversion (87%) and regioselectivity (GM1/GM1′ = 76/24). The following Kumada coupling polymerization by Ni(dppe)Cl2 gave P1-OMe with the number-averaged molecular weight ranging from 3000 to 4600. Likewise, P2-OMe was obtained from M2. On the basis of 1H-NMR spectra, the regiorandom structure of P1-OMe and P2-OMe were suggested. The transformation from the methoxy group to the hydroxy one was then carried out using P1-OMe by treating with BBr3 to afford P1-OH. By measuring the UV-vis absorption spectra of polymers in CHCl3 and film along with the DFT calculation of model compounds, it was found that 1) the formation of an intramolecular hydrogen bonding between the pyridine and hydroxy groups brings about the significant red-shift of the absorption maxima from 476 nm (P1-OMe) to 662 nm (P1-OH) and 2) P1-OH has the less planar conformation resulting in the ineffective π-π stacking of polymer chains in the solid state. The proton doping experiments and CV measurements revealed the hydrogen bonding characteristic in P1-OH.