Fabrication of Alternating Copolymers Based on Cyclopentadithiophene-Benzothiadiazole Dicarboxylic Imide with Reduced Optical Band Gap: Synthesis, Optical, Electrochemical, Thermal, and Structural Properties

Ary R Murad; Ahmed Iraqi; Shujahadeen B Aziz; Sozan N Abdullah; Mohamad A Brza; Salah R Saeed; Rebar T Abdulwahid

doi:10.3390/polym13010063

Fabrication of Alternating Copolymers Based on Cyclopentadithiophene-Benzothiadiazole Dicarboxylic Imide with Reduced Optical Band Gap: Synthesis, Optical, Electrochemical, Thermal, and Structural Properties

Polymers (Basel). 2020 Dec 26;13(1):63. doi: 10.3390/polym13010063.

Authors

Ary R Murad¹, Ahmed Iraqi², Shujahadeen B Aziz^{3

4}, Sozan N Abdullah⁵, Mohamad A Brza^{3

6}, Salah R Saeed⁷, Rebar T Abdulwahid³

Affiliations

¹ Department of Pharmaceutical Chemistry, College of Medical and Applied Sciences, Charmo University, Chamchamal 46023, Iraq.
² Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.
³ Hameed Majid Advanced Polymeric Materials Research Lab., Department of Physics, College of Science, University of Sulaimani, Sulaimani 46001, Iraq.
⁴ Department of Civil Engineering, College of Engineering, Komar University of Science and Technology, Sulaimani 46001, Iraq.
⁵ Department of Chemistry, College of Science, University of Sulaimani, Sulaimani 46001, Iraq.
⁶ Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University of Malaysia, Kuala Lumpur, Gombak 53100, Malaysia.
⁷ Charmo Research Center, Charmo University, Chamchamal 46023, Iraq.

Abstract

A series of alternating copolymers containing cyclopentadithiophene (CPDT) flanked by thienyl moieties as electron-donor units and benzothiadiazole dicarboxylic imide (BTDI) as electron-acceptor units were designed and synthesized for solar cell applications. Different solubilizing side chains, including 2-ethylhexyl chains and n-octyl chains were attached to CPDT units, whereas 3,7-dimethyloctyl chains and n-octyl chains were anchored to the BTDI moieties. The impact of these substituents on the solubilities, molecular weights, optical and electrochemical properties, and thermal and structural properties of the resulting polymers was investigated. PCPDTDTBTDI-EH, DMO was synthesized via Suzuki polymerization, whereas PCPDTDTBTDI-8, DMO, and PCPDTDTBTDI-EH, 8 were prepared through direct arylation polymerization. PCPDTDTBTDI-8, DMO has the highest number average molecular weight (M_n = 17,400 g mol^-1) among all polymers prepared. The PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8 which have n-octyl substituents on their CPDT units have comparable optical band gaps (E_g ~ 1.3 eV), which are around 0.1 eV lower than PCPDTDTBTDI-EH, DMO analogues that have 2-ethylhexyl substituents on their CPDT units. The polymers have their HOMO levels between -5.10 and -5.22 eV with PCPDTDTBTDI-EH, DMO having the deepest highest occupied molecular orbital (HOMO) energy level. The lowest unoccupied molecular orbital (LUMO) levels of the polymers are between -3.4 and -3.5 eV. All polymers exhibit good thermal stability with decomposition temperatures surpassing 350 °C. Powder X-ray diffraction (XRD) studies have shown that all polymers have the amorphous nature in solid state.

Keywords: UV-vis; XRD study; band gap study; conjugated copolymers; cyclic voltammetry; cyclopentadithiophene; direct arylation polymerization; thermogravimetric analysis (TGA).