Synthesis, photophysical, and theoretical studies on π-conjugated copolymers based on benzothiadiazole and cyanopyridine acceptor moieties along with other π-bridge units

A set of pi-conjugated copolymers were synthesized via Pd-catalyzed cross-coupling based on benzothiadiazole and cyanopyridine moieties along with other pi-bridge units (thiophene and/or benzene). Their chemical structures were confirmed by various analytical techniques (elemental, IR, H-1, and C-13 NMR analyses). All copolymers showed good solubility in most common organic solvents. Thermogravimetric analysis (TGA) analysis showed their good thermal stability with no significant weight loss up to similar to 260 degrees C. The estimated optical band gaps for thin films were found to be in the range of 2.25-2.62 eV. It was found that the pi-bridge units (benzene and/or thiophene) significantly affect the molecular architecture and optoelectronic properties of the copolymers. They exhibited good fluorescence in diluted solution where the fluorescence emission spectra were obtained by irradiative excitation at the wavelength in the range of 526.0-532.0 nm. X-ray diffraction (XRD) revealed that all copolymers exhibited a symmetric halo peak at 2 theta = 22.5-23.8 degrees corresponding to the presence of alkyl side chains in addition to a small intensity peak at 2 theta of similar to 43 degrees indicates the existence of some degree of remnant some ranges of crystallinity in the films. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to assess planarity, electronic structure, and optical absorption properties.