New Insight into the Photochromism of Spiropyrans: Substituent Effects on Equilibrium, Rate Constants, and Dipole Moments

Abstract Spiropyrans (SPs) and merocyanines (MCs) are among the most widely used photochromic systems today. Besides solvent polarity and temperature, the electronic properties of the substituents have a significant influence on photochromic behavior. In this work, a comprehensive study on the influences of electron-donating and -withdrawing substituents at the benzopyran as well as the indole moiety of the photochromic system are elucidated. For this purpose, changes in rate constants and the shift of the SP/MC equilibrium are investigated utilizing UV/vis and NMR spectroscopy. Based on DFT calculations, a relationship between these properties and the dipole moments of the molecules is established.