Benzo[1,2,3]thiadiazole as an Attractive Heteroaromatic Platform for Two-Photon Absorbing (TPA) Fluorophores: TPA Enhancement in Quasi-Quadrupolar S,N-Heteroarene-Cored Dyes via Skeletal Editing and Regioisomeric Control

Highly emissive aryl­amine-end-capped fluorophores featuring a D-π-A-π-D architecture with a previously overlooked isobenzo­thia­diazole (iBTD) core were synthesized through skeletal editing of 4,7-diiodo-2-chloro­benzo­thiazole (prepared from readily available 2-chloro­benzo­thiazole by direct iodination), followed by Pd-catalyzed cross-coupling reactions with various N-donor-functionalized phenyl­acetylenes or corresponding aryl­vinyl­boronic acids under Sonogashira- or Suzuki-type conditions. The target iBTD-cored dyes display large two-photon absorption (TPA) cross-sections in the near-infrared region (up to 1633 GM at 820 nm), outperforming congeners based on other widely used electron-accepting S,N-heteroarenes, such as benzo­thiazole and benzo[2,1,3]­thia­diazole. Quantum-chemical calculations rationalize the counter­intuitive superiority of the iBTD scaffold, despite its weaker electron-acceptor strength relative to the regio­isomeric benzo­thia­diazole. Simulations also reveal the limitations of the widely used long-range-corrected hybrid functionals (such as CAM-B3LYP) in reliably predicting TPA cross-sections. In contrast, the explicit electron correlation in approximate coupled-cluster (RI-CC2) theory, combined with the incorporation of experimental spectral linewidths of individual dyes, recovers the experimentally observed TPA trend.