D?A?D low band gap molecule containing triphenylamine and benzoxadiazole/benzothiadiazole units: Synthesis and photophysical properties

Two D–A–D-type low band gap organic dyes based on triphenylamine and benzoxadiazole/benzothiadiazole, 4,7-Bis{5-{4-{2-[4-(N,N-diphenylamino)phenyl]-1-nitrilethenyl}phenyl}-2-thienyl}-2,1,3-benzoxadiazole (BDNTBX) and 4,7-Bis{5-{4-{2-[4-(N,N-diphenylamino)phenyl]-1-nitrilethenyl}phenyl}-2-thienyl}-2,1,3-benzothiadiazole (BDNTBT) were successfully synthesized. The properties of two compounds were investigated by density functional theory (DFT) calculations, UV–vis absorption spectroscopy, cyclic voltammetry and fluorescence quenching experiment. The calculated ground-state geometries demonstrate intramolecular charge transfer (ICT) occurs in both molecules during the procedure of charge excitation from HOMO to LUMO. From the data in electrochemistry and fluorescence quenching experiments, the molecules reveal lower HOMO energy levels compared with that of P3HT and proper LUMO energy levels to obtain efficient charge separation with PCBM. Two synthesized compounds exhibit broad absorption range covering the whole visible spectral region. These photophysical and electrochemical properties call attention to that our materials are prospective candidates as donor materials for solution-processable organic photovoltaic cells.