Heteroaryl iminothioindoxyl (HA-ITI) photoswitches <i>via</i> regioselective aza-Wittig synthesis: unifying red-shifted absorption, large <i>E</i> / <i>Z</i> band separation, and tunable thermal recovery

Herein we introduce heteroaryl-substituted iminothioindoxyl (HA-ITI) photoswitches featuring <i>N</i>-heterocyclic moieties based on indole or benzimidazole. A regioselective aza-Wittig reaction of thioisatin, previously unexplored for the synthesis of imino-based photoswitches, enabled the obtention of HA-ITI derivatives and provides an alternative and versatile synthetic access to similar analogues. HA-ITIs undergo blue light-induced <i>Z</i> → <i>E</i> photoisomerization, followed by very fast thermal <i>E</i> → <i>Z</i> back isomerization at room temperature (µs to ms timescale; resolved by transient absorption spectroscopy). Irradiation at low temperature (<200 K) provided clear proof for the reversible T-type switching of HA-ITI. This behavior was corroborated by the changes in the UV/vis absorption spectra, with <i>λ</i> _abs values around 470 nm for the thermodynamically more stable <i>Z</i> form and <i>ca.</i> 525-550 nm for the metastable <i>E</i> isomer. Additional compelling evidence for the <i>Z</i> → <i>E</i> photoisomerization was obtained by NMR spectroscopy of samples irradiated <i>in situ</i> at low temperature. The combination of NMR data, single-crystal X-ray structures, and density functional theory calculations allowed the identification of both inter- and intramolecular interactions (chalcogen and hydrogen bonding), which are present in the <i>Z</i> and <i>E</i> isomers. Balancing these interactions dictates the differential performance of the switches, resulting in the significant kinetic stabilization, from µs to ms, of the <i>E</i> isomer in the benzimidazole-containing photoswitch. These findings establish HA-ITIs as a modular photoswitch platform with highly desirable and tunable photochemical features, thereby broadening the synthetic and conceptual landscape of heteroaryl photoactive systems.