Tuning of the HOMO-LUMO gap of donor-substituted symmetrical and unsymmetrical benzothiadiazoles

This article reports the design and synthesis of donor-substituted symmetrical and unsymmetrical benzo-thiadiazoles (BTDs) 5-12 of type D-pi-A-D, D-1-pi-A-D-2, D-1-A(1)-A(2)-D-2, D-A(1)-A(2)-D and D-A(1)-A(2)-A(1)-D by Ullmann, Suzuki and cycloaddition-retroelectrocyclization reactions. The photophysical, electrochemical and computational properties were studied and show substantial donor-acceptor interaction. Their single photon absorption show strong charge transfer bands in the near-infrared (NIR) region and the electrochemical reduction show multiple reduction waves. The optical HOMO-LUMO gap of BTDs 5-12 was found to be a function of the number and nature of the acceptors. Computational studies reveal that strong cyano-based acceptors, dicyanoquinodimethane (DCNQ) and tetracyanobutadiene (TCBD) lower the LUMO level in BTDs 7-12, which results in a low HOMO-LUMO gap compared to acetylene linked BTDs 5 and 6. The BTDs with carbazole and single TCBD and DCNQ acceptors show better thermal stability.