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.
Keywords
Spiropyran - Merocyanine - Photochromism - Substituent-effects - Rate constant - Dipole
moment