Photoisomerization and Fluorescence Properties of Hemiindigo Compounds Having Intramolecular Hydrogen Bonding

Intramolecularly hydrogen bonded hemiindigo compounds 2-4 were prepared and their photochemical behavior was investigated. Z-2 did not undergo isomerization, while 3 underwent isomerization mutually between the Z-isomer and E-isomer in solution to exhibit a color change between greenish yellow and reddish orange in benzene. Introduction of a formyl group at the 2-position of the pyrrole ring of 3 brought about the increase in the fluorescence quantum yield and fluorescence lifetime by a factor of 10, and the decrease of the quantum yield of isomerization of 4 in benzene. In addition, introduction of the formyl group affects the thermal stability. The intramolecular hydrogen bonding can be used to construct a novel photochromic molecule.