*Article* **Photophysical, Thermal and Structural Properties of Thiophene and Benzodithiophene-Based Copolymers Synthesized by Direct Arylation Polycondensation Method**

**Newayemedhin A. Tegegne 1,\* , Zelalem Abdissa <sup>2</sup> and Wendimagegn Mammo <sup>2</sup>**


**\*** Correspondence: newaye.medhin@aau.edu.et

**Abstract:** Three low-band-gap copolymers based on isoindigo acceptor units were designed and successfully synthesized by direct arylation polycondensation method. Two of them were benzodithiophene (BDT)-isoindigo copolymers (**PBDTI-OD** and **PBDTI-DT**) with 2-octlydodecyl (OD) and 2-decyltetradecyl (DT) substituted isoindigo units, respectively. Thiophene donor and DTsubstituted isoindigo acceptor units were copolymerized to synthesize **PTI-DT**. The copolymers have a broad absorption range that extends to over 760 nm with a band gap ≈1.5 eV. The photophysical property studies showed that the BDT-based copolymers have non-polar ground states. Their emission exhibited the population of the intramolecular charge transfer (ICT) state in polar solvents and tightly bound excitonic state in non-polar solvents due to self-aggregation. On the contrary, the emission from the thiophene-based copolymers was only from the tightly bound excitonic state. The thermal decomposition temperature of the copolymers was above 380 °C. The X-ray diffraction pattern of the three copolymers showed a halo due to *π* − *π* stacking. A second, sharper peak was observed in the BDT-based copolymer with a longer side chain on the isoindigo unit (**PBDTI-DT**), and the thiophene-based copolymers with **PTI-DT**, exhibiting a better structural order.

**Keywords:** intramolecular charge transfer; copolymers; *π* − *π* stacking; direct arylation polycondensation; and excitonic state
