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

Abstract

This article reports the design and synthesis of donor-substituted symmetrical and unsymmetrical benzothiadiazoles (BTDs) 5–12 of type D–π–A–D, D₁–π–A–D₂, D₁–A₁–A₂–D₂, D–A₁–A₂–D and D–A₁–A₂–A₁–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.