4‐Heteryl‐Vinyl‐Butenolides as Visible‐Light T‐Type Photoswitches Quantified by Light‐Emitting Diode Nuclear Magnetic Resonance Spectroscopy

We introduce 4‐heteryl‐vinyl‐butenolides as a new and readily accessible family of T‐type molecular photoswitches that combine visible‐light addressability with high photostationary‐state (PSS) selectivity and tunable thermal stability of the metastable isomer. A series of derivatives was synthesized and evaluated by UV‐Vis spectroscopy and in situ light‐emitting diode nuclear magnetic resonance spectroscopy (LED‐NMR), enabling direct quantification of E/Z ratios and thermal Z → E back‐isomerization kinetics in solution. All compounds undergo efficient E →  Z photoisomerization under LED irradiation in the 365–470 nm range, reaching high Z enrichment at PSS (up to 95%–96%) for the best‐performing derivatives. The thermal lifetimes of the Z ‐isomer are readily tuned by substituent electronics and solvent, spanning t _1/2  ≈ 1–40 h (with rate constants down to ~10 ⁻⁶ s ⁻¹ ), thus providing a practical balance between photoaddressability and kinetic stability. Importantly, repeated switching is reproducible under ambient conditions and proceeds with minimal side reactions for the majority of the series. These results establish 4‐heteryl‐vinyl‐butenolides as a versatile, designable photoswitch platform for photochemical control in functional materials and molecular systems.