Synthesis and characterization of low band gap thiadiazolobenzo-imidazole chromophores

Thiadiazolobenzoimidazole (TBI) acceptor was designed in order to increase the electron-withdrawing ability of 2,1,3-benzothiadiazole (BT). The key feature of TBI is the three electron deficient imine groups, rendering it a stronger acceptor than BT. Three TBI-containing compounds TBI-a to TBI-c were synthesized by reaction between 5,6-diamino-2,1,3-benzothiadiazole analogue and the corresponding anhydride and exhibit maximum absorption at 600 nm. The band gaps of TBI-a to TBI-c are in between that of BT-containing compounds and benzo(1,2-c:4,5-c′)bis[1,2,5]thiadiazole-containing compounds, ranging from 1.60 to 1.88 eV, which make them potentially useful for organic solar cell application. Moreover, all the compounds are electrochromic and become absorbing at the telecommunication wavelengths upon electrochemical switching.