Benzothiadiazole vs. -Benzothiadiazole: Synthesis, Electrochemical and Optical Properties of D-A-D Conjugated Molecules Based on Them.

This paper presents an improved synthesis of 4,7-dibromobenzo[][1,2,3]thiadiazole from commercially available reagents. According to quantum-mechanical calculations, benzo[][1,2,3]thiadiazole (isoBTD) has higher values of E and energy band gap (E), which indicates high electron conductivity, occurring due to the high stability of the molecule in the excited state. We studied the cross-coupling reactions of this dibromide and found that the highest yields of π-spacer-acceptor-π-spacer type compounds were obtained by means of the Stille reaction. Therefore, 6 new structures of this type have been synthesized. A detailed study of the optical and electrochemical properties of the obtained π-spacer-acceptor-π-spacer type compounds in comparison with isomeric structures based on benzo[][1,2,5]thiadiazole (BTD) showed a red shift of absorption maxima with lower absorptive and luminescent capacity. However, the addition of the 2,2'-bithiophene fragment as a π-spacer resulted in an unexpected increase of the extinction coefficient in the UV/vis spectra along with a blue shift of both absorption maxima for the isoBTD-based compound as compared to the BTD-based compound. Thus, a thorough selection of components in the designing of appropriate compounds with benzo[][1,2,3]thiadiazole as an internal acceptor can lead to promising photovoltaic materials.