Direct Iodination of Electron-Deficient Benzothiazoles: Rapid Access to Two-Photon Absorbing Fluorophores with Quadrupolar D‑π-A-π‑D Architecture and Tunable Heteroaromatic Core

Direct iodination of benzothiazoles under strong oxidative/acidic conditions leads to a mixture of iodinated heteroarenes with 1–2 major components, which are easily separable and which structures depend on the I2 equivalents used. Among the unexpected but dominant products were identified 4,7-diiodobenzothiazoles with a rare substitution pattern for SEAr reactions of this scaffold. These were employed in the synthesis of 4,7-bis­(triarylamine-ethynyl)­benzothiazoles  a new class of highly efficient quasi-quadrupolar fluorophores displaying large two-photon absorption cross sections (540–1374 GM) in the near-infrared region.