Crown Ether‐Bearing Dihydroazulens [1] : Synthesis and Photochromism of a Benzo[15]crown‐5 Derivative

Photosensitive crown ether 1 composed of 1,8a‐dihydroazulene as a photodynamic unit and a benzo[15]crown‐5 ether subgroup has been synthesized from C‐7‐substituted cycloheptatriene 8 , which has been obtained by nucleophilic addition of 4‐acetylbenzo[15]crown‐5 ( 6 ) to tropenylium tetrafluoroborate ( 7 ). Dehydrogenation of dicyanovinyl derivative 9 leads to heptafulvene 2 which thermally rearranges to dihydroazulene 1 . Attempts to synthesize 1 according to the „[8 + 2]‐cycloaddition route” have been unsuccessful. Crown ether 1 shows photochromic bistability with a photochemically driven forward and a thermal back reaction. The spectroscopic properties of 1 and the photochemical isomerization 1 → 2 have been shown to be almost insensitive to sodium iodide addition. The rate of the thermal back reaction 2 → 1 slightly is affected by sodium iodide addition.