Experimental Synthesis and <scp>DFT</scp> Analysis of Two D–A Copolymers Based on Dithienopyrrole‐Benzothiadiazole Dicarboxylic Imides: Electrochemical and Nonlinear Optical Properties

ABSTRACT Two new donor‐acceptor (D–A) interchanging copolymers PDTPDTBDI‐8 and PDTPDTBDI‐DMO with N ‐alkyldithienopyrrole (DTP) as the donor unit, benzothiadiazole dicarboxylic imide (BDI) with varying substituents as acceptor repeat units, and two thiophene units as the spacer have been synthesized using Stille coupling polymerization. The effects of various alkyl side chains connected to the BDI unit's imide functionality on the copolymers' optical, electrochemical, thermal, and structural characteristics were studied. Both polymers exhibit low optical band gaps (1.2 eV), high thermal stability with decomposition temperatures above 400°C, and HOMO/LUMO energy levels of around −5.00 and −3.60 eV, respectively. Complementarily, DFT calculations reveal efficient ICT and narrow electronic band gaps, along with significant nonlinear optical (NLO) responses. PDTPDTBDI‐DMO displays the highest third‐order polarizability (1212 × 10 −36 esu) and linear isotropic polarizability (109.8 × 10 −24 esu), attributed to its lower transition energy (1.7487 eV). The calculated absorption maximum at ~709 nm agrees closely with the experimental value of 695 nm, confirming the reliability of the simulations. Photovoltaic analysis demonstrates high light‐harvesting efficiency (0.862), favorable electron‐injection potential (−1.439 eV), open‐circuit voltage (0.995 eV), and effective dye regeneration energy (0.230 eV), supporting their suitability for solar cell applications. Our results highlight the dual functionality of the studied polymers as efficient NLO‐active chromophores and high‐performance candidates for dye‐sensitized solar cells.