Benzothiadiazole vs. <i>iso</i>-Benzothiadiazole: Synthesis, Electrochemical and Optical Properties of D–A–D Conjugated Molecules Based on Them

• Main Authors: Nikita S. Gudim, Ekaterina A. Knyazeva, Ludmila V. Mikhalchenko, Ivan S. Golovanov, Vadim V. Popov, Natalia V. Obruchnikova, Oleg A. Rakitin
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Molecules
• Subjects: benzo[<i>d</i>][1,2,3]thiadiazole; benzo[<i>c</i>][1,2,5]thiadiazole; π-spacer–acceptor–π-spacer type structures; cross-coupling reactions; optical properties; electrochemical properties
• Online Access: https://www.mdpi.com/1420-3049/26/16/4931

This paper presents an improved synthesis of 4,7-dibromobenzo[<i>d</i>][1,2,3]thiadiazole from commercially available reagents. According to quantum-mechanical calculations, benzo[<i>d</i>][1,2,3]thiadiazole (isoBTD) has higher values of E_LUMO and energy band gap (E_g), 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[<i>c</i>][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[<i>d</i>][1,2,3]thiadiazole as an internal acceptor can lead to promising photovoltaic materials.