Synthesis of Fluorene Based New Ter-Copolymers: Electrochemical and Optical Properties

Abstract

This manuscript reports the synthesis of three new fluorene based ter-copolymers namely, P1 (thiophene/fluorene/phenylene), P2 (thiophene/fluorene/ pyridine) and P3 (pyridine/fluorene/phenylene). The structures of the copolymers were elucidated by element analysis, FT-IR, 1H-NMR and 13C-NMR, fluorescence spectroscopy. The polymers are amorphous in nature with decomposition temperature above 300°C. Absorption and fluorescence spectra were employed to investigate their optical properties both in solution and film state. The maximum absorption peaks of P1 to P3 appeared at 350–406 nm in THF solution and 400–405 nm in solid thin film. The photoluminescence (PL) emission spectra of P1–P3 show a maximum peak at 405–490 nm in THF solution and at 445–503 nm as the solid film state. The HOMO and LUMO energy levels have been estimated from their cyclic voltammograms. The HOMO levels of P1, P2, and P3 were 5.52 (P1), 5.14 (P2), 5.44 (P3) eV, respectively. Whereas, their LUMO levels were P1LUMO=3.57 eV, P2LUMO=3.43 eV, P3LUMO=2.54 eV. In summary, the oxidation potentials for this series of polymers lie between 0.98 to 2.54 V. Reductive n-doping studies of these polymers were examined in the both their neutral and potential range 0.6 to −2.5 V. Charge transfer processes in copolymers film electrodes bathed with Bu4NClO4 (0.10 M) in acetonitrile solution were studied by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The impedance and cyclic voltammetric measurements revealed that there is a rapid charge transport within the polymers and the electrolyte. The charge-transfer resistance was first shown to decrease upon increasing the potential. The combination of CV and EIS elucidated the nature of the different low-frequency behavior of the impedance spectra.