Cyclization magic: theoretical design of externally controlled organic materials with intensified mid infrared absorption beyond 3000 nm

This study presents the rational design of a series of oligo-<italic>α</italic>-BTD-TTF molecules, combining electron-accepting 2,1,3-benzothiadiazole (BTD) and redox-active tetrathiafulvalene (TTF) units, as novel materials with widely tunable optical absorption from the UV-Vis to the infrared (IR) region. Theoretical investigations reveal that the absorption range can be effectively tuned by varying the molecular charge state. This is achieved through redox reactions that planarize the TTF unit, thereby dramatically narrowing the HOMO-LUMO gap. A key finding is that molecules with an even number of positive charges, particularly the doubly charged species (e.g., <italic>cis</italic>-2<italic>α</italic>-BT²⁺), exhibit the narrowest gaps. <italic>cis</italic>-2<italic>α</italic>-BT²⁺ demonstrates absorption covering the entire solar spectrum, making it a promising candidate for solar cell applications. Furthermore, cyclization of the oligomers, such as forming a six-membered BT ring, significantly enhances the absorption intensity in the mid-IR region due to increased molecular symmetry. This work demonstrates that redox state and cyclization are powerful, externally tunable strategies for manipulating the orbital energies and absorption properties of these materials, offering a new design pathway for advanced optical and electronic applications, including transparent photovoltaics and IR materials.