Alternating Copolymers Based on 2,1,3-Benzothiadiazole and Hexylthiophene: Positioning Effect of Hexyl Chains on the Photophysical and Electrochemical Properties

A series of donor-acceptor alternating pi-conjugated copolymers based on 2,1,3-benzothiadiazole and hexylthiophene units has been synthesized by the palladium-catalyzed Stille cross-coupling method. Various precursory monomers possessing dibromo and bis(tributylstannyl) functionalities were readily prepared in high yields. Microwave-assisted polymerization proved efficient for the production of high-molecular-weight copolymers, ranging from 13550 to 52490 g mol(-1). All copolymers exhibited excellent solubilities in most common organic solvents. The thermal properties of these copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry, and the polymers showed high thermal stabilities. Incorporation of the benzothiadiazole unit into polyhexylthiophene chains affected the photo-physical and electrochemical properties. The thin-film absorption spectra of all polymers are significantly red-shifted relative to the corresponding absorption bands in solution and exhibit broader absorption bands. The optical band gaps were estimated to be in the range of 2.02-1.74 eV. The highest occupied and lowest unoccupied molecular orbital energy levels are in the ranges of -5.37 to -5.66 eV and -3.33 to -3.44 eV, respectively. In the X-ray diffraction analysis of the deposited film of the copolymer P3, strong diffraction peaks were observed at 2 theta = 5.72 degrees (15.43 angstrom) and 23.12 degrees (3.84 angstrom). These values relate to the distances between chains with interdigitated hexyl chains and to pi-pi stacking between the conjugated chains.