Synthesis and optoelectronic and charge storage characterizations of conducting polymers based on tetraphenylethylene and thienothiophenes

Syntheses of four monomers (Z-TTpTPE, A-TTpTPE, Z-Th2TTpTPE and A-Th2TTpTPE), possessing thienothiophene (TT) and tetraphenylethylene (TPE) units, were achieved by Suzuki coupling reaction, starting from two isomers of TT, namely, thieno[3,2-b]thiophene (ZTT) and thieno[2,3-b]thiophene (ATT). This is the first report that TPE has been attached on to two analogues of TT, i.e. cross conjugated and conjugated, through phenyl moiety and the properties of the resultant electropolymers were investigated. While the corresponding polymer films, i.e. P[Z-TTpTPE], P[Z-Th2TTpTPE] and P[A-Th2TTpTPE] were obtained, the monomer A-TTpTPE did not form any polymer on the electrode surface. Capacitive and electrochromic properties of the resultant polymers were investigated by electrochemical and in-situ-spectrophotometric measurements. Electrochemical polymerization mechanism was investigated at DFT level, the results of which suggested that although A-Th2TTpTPE and Z-Th2TTpTPE had enough spin density on their peripheral α-carbons, Z-TTpTPE and A-TTpTPE did not. Optical band gaps of P[Z-Th2TTpTPE] and P[A-Th2TTpTPE] were determined to be 1.89 and 2.23 eV. Electrochromic behaviors of the polymers suggested that P[Z-Th2TTpTPE] has promising electrochromic properties with high coloration efficiency of 309 cm² C⁻¹. Low frequency capacitance (C_LF) values at different E_DC potentials were calculated. The shape of the capacitance-potential graph was found to be in a good agreement with CV of the polymer films, and the highest capacitance values were obtained at their peak potentials. Electrical equivalent circuits were applied to explain the parameters of each element obtained from the experimental electrochemical impedance spectroscopy (EIS). Energy and power densities were calculated from the galvanostatic charge-discharge (GCD) curves. P[Z-Th2TTpTPE] showed a higher energy density compared to P[A-Th2TTpTPE], possibly due to its reversible redox behavior, porous structure, and good roughness, which was supported by CV and AFM measurements. All results suggested that electronic and optical properties of TT were improved by inclusion of TPE, and further enhancement was achieved by attachment of two thiophene rings to the peripherals of the monomers. All the polymers demonstrated promising properties for electrochromic and charge storage applications.