Synthesis of a Photochromic Fused 2 H ‐Chromene Capable of Generating a Single Coloured Species

A new photochromic fused 2 H ‐chromene that has a dehydropyran bridge between the pyran double bond and the benzene ring was prepared. Unlike standard 2 H ‐chromenes that give rise to two coloured species (one short‐ and one long‐lived) under UV irradiation, flash photolysis studies on this particular 2 H ‐chromene show that the opening of the pyran ring led to the formation of a single, coloured, open species that bleaches to the uncoloured initial form in <1 ms. The design of this fused 2 H ‐chromene avoids the formation of the undesirable long‐lived, transoid–trans , coloured, open form; therefore, the colour fades very quickly, according to monoexponential kinetics, without the persistence of any residual colour commonly observed in 2 H ‐chromenes. Acid treatment of 4‐[(ethoxycarbonyl)methyl]‐2 H ‐chromenes led to the formation of lactones.