Vilsmeier–Haack Reagent‐Catalyzed C₄(sp²)─H Formylation in Thiophene Derivatives: Insights into Unexpected Fluorescence Properties

A simple and efficient method was developed to prepare the novel fluorescent tetra‐substituted thiophene derivatives. This method involves using the Gewald's conditions, followed by the Vilsmeier–Haack–Arnold (VHA) formylation. By judiciously controlling the use of the VHA reagent, we have selectively introduced a formyl group into position 5 of the thiophene. This had a direct impact on its fluorescence properties. The resulting compounds have showed an intense fluorescence in the emission wavelength range of 420–500 nm with a 10 ⁻⁶   M concentration. All new products have been structurally confirmed by spectroscopic data. The structure of compound 2g was confirmed by single‐crystal X‐ray diffraction.