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 <i>N</i>-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 <i>S</i>,<i>N</i>-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.