Amino Acid Conjugated Spiropyrans: Synthesis and Photoisomerization Studies

Abstract Among photo‐switchable compounds, spiropyrans have attracted significant attention owing to their multi‐responsiveness towards a change in the pH, solvent polarity, and temperature, along with light. Upon exposure to the external stimulants, the merocyanine form is obtained due to bond cleavage between spiro carbon and oxygen, which transforms the optically active spiropyrans into an achiral form. The back conversion of the resulting achiral merocyanine form to the spiropyrans brings about a racemic mixture. In this study, D and L enantiomers of alanine and phenylalanine derivatives were incorporated onto a spiropyran bearing hydroxyl group to obtain optically active spiropyrans. Photochemical and photophysical properties of these compounds were studied in methanol, isopropanol, dichloromethane, and hexane. The effect of the benzyl and methyl units on the ring closure rate of the merocyanines was also investigated. For this purpose, ring closure kinetics of different merocyanines containing hydroxyl group, D ‐alanine, and D ‐phenylalanine derivatives were compared. The results reveal that the ring closure rate of the merocyanine with benzyl group attached is faster than the rate of the structures with methyl or hydroxyl groups.