Synthesis and structure-property relationship of new donor-acceptor-type conjugated monomers and polymers on the basis of thiophene and benzothiadiazole

We have synthesized three new donor-acceptor-type monomers to achieve soluble and processable low-band gap polymers, 4,7-bis(4-octyl-2-thienyl)-2,1,3-benzothiadiazole (B4TB), 4,7-bis(3-octyl-2-thienyl)-2,1,3-benzothiadiazole (B3TB), and 4-(3-octyl-2-thienyl)-7-(4-octyl-2-thienyl)-2,1,3-benzothiadiazole (B34TB), by the Suzuki coupling reaction. Using B4TB and B3TB, two soluble high molecular weight regioregular head-to-head and tail-to-tail polymers poly[4,7-bis(4-octyl-2-thienyl)-2,1,3- benzothiadiazole] (PB4TB) and poly[4,7-bis(3-octyl-2-thienyl)-2,1,3-benzothiadiazole] (PB3TB) were prepared via iron(III) chloride-mediated oxidative polymerization. The structures of the polymers were confirmed by 1H and 13C NMR, and the molecular weights were determined by size exclusion chromatography. The optical properties (absorbance and fluorescence) of the monomers and polymers were studied and compared with unsubstituted analogues. The monomers and polymers bearing octyl substituents on the thiophene rings pointing away from the benzothiadiazole units (B4TB and PB4TB) possess a more planar structure, and their optical spectra appear redshifted as compared with those having the octyl chain nearer to the benzothiadiazole (B3TB and PB3TB). The optical band gaps of PB3BT (Eg = 2.01 eV) and PB4BT (Eg = 1.96 eV), however, are at much higher energy levels than that of the unsubstituted electrochemically polymerized PBTB material (Eg = 1.1-1.2 eV) as a result of steric effects of the octyl chains. The electrochemical properties of the monomers and polymers were examined using cyclic voltammetry and reflect the effect of alkyl substitution. B4TB and PB4TB were oxidized at a lower potential than B3TB and PB3TB, whereas their reduction potentials were less negative. The electrochemical band gap calculated from the onset of the reduction and oxidation process agreed with the optical band gap calculated from the absorption edges