Synthesis, optical properties, and alkylation of a methylene-bridged donor–acceptor compound: 10,11-dihydrodithieno[3′,2′:2,3;2″,3″:6,7]-as-indaceno[4,5-c][1,2,5]thiadiazole

We report the design, synthesis, and photophysical characterization of a novel donor–acceptor–donor (D–A–D)-type π-conjugated molecule, 10,11-dihydro-dithieno[3′,2′:2,3;2″,3″:6,7]-as-indaceno[4,5-c][1,2,5]thiadiazole (A). This bridging strategy enabled the construction of a highly planar structure while maintaining structural flexibility at the central benzylic position, which facilitated further molecular modifications. Density functional theory calculations revealed that the methylene-bridged architecture significantly increased the HOMO energy level without substantially affecting the LUMO level, thereby leading to a narrow HOMO–LUMO gap and red-shifted absorption. Photophysical studies indicated that compound A displays a broad intramolecular charge transfer (ICT)-derived absorption band at 486 nm and a strong red emission at 638 nm with a relatively high quantum yield (Φ_f = 0.46), which is particularly notable for red-emitting materials. Despite its increased rigidity, compound A displayed an unexpectedly large Stokes shift, which is attributed to the stabilization of the ICT state and enhanced molecular planarity. To improve the solubility of A, the sterically hindered methylene bridge was alkylated, obtaining the hexylated product with enhanced solubility in a moderate yield. This methylene-bridging approach offers a versatile platform for tuning the electronic and optical properties of π-conjugated systems and provides valuable insights for the design of next-generation functional organic materials.