Synthesis and characterization of cyclopentadithiophene (CPDT)-naphthalene (NDI) push-pull ABA-type oligomers and copolymers

Abstract

This thesis presents the synthesis, characterization, and device performance of a series of cyclopentadithiophene (CPDT)-naphthalene (NDI) donor-acceptor (D-A) ABA-type oligomers and copolymers. These oligomers composed of CPDT-NDI-CPDT unit with various alkyl chains are successfully synthesized via direct arylation using palladium complex catalyst. The corresponding copolymers are synthesized by oxidative polymerization using FeCl3. All of oligomers and copolymers are systematically characterized and analyzed by gel permeation chromatography (GPC), 1H NMR and UV-vis-NIR absorption spectroscopies, cyclic voltammetry (CV), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). GPC measurement shows that these polymers are of relatively high molecular weight, Mn = 21800-76000. These copolymers show deep-red absorption including near-infrared region (up to 1100 nm) due to their quite narrow bandgap. Impressively, the electrochemical property of the resulting copolymers exhibits lowest unoccupied molecular orbital (LUMO) at about - 3.7 eV, which has been considered as the favorable level as n-type materials for use in organic photovoltaic devices (OPVs). These copolymers exhibit only n-type property giving the highest electron mobility of 3.7 x 10-4 cm2 V-1 s-1 in organic field-effect transistor because of its strong intermolecular interaction. On the other hand, the copolymer with highly soluble branched alkyl chain shows the highest power conversion efficiency of 0.25% in organic photovoltaic device. To investigate the versatile reactivity of C-H direct arylation, the copolymers composed of above unit with benzothiadiazole, thiophene, or bithiophene are also II synthesized by direct arylation polymerization. All polymers have higher molecular weight (Mn = 18000-52000) than the alternative polymers of dibromo-NDI and CPDT obtained from similar reaction condition. The optical and electrochemical properties of these polymers are measured by UV-vis-NIR spectra, 1H-NMR, GPC and CVs.