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

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