Effect of indoline substitution on ring opening in 6-nitro BIPS spiropyran derivatives

Spiropyrans constitute an important class of photoswitchable compound that have been studied in a variety of applications, including nanomaterials and biological sensing. However, many reported spiropyrans are impractical for these contexts due to a low quantum yield of switching, particularly in the ring opening direction. In this work, we report a series of spiropyrans substituted at the 5-indoline position of 6-NO2-BIPS, as a means to understand the electronic effects of substitution at this position on photochromism. Analysis of 1H NMR spectra reveals that switching is greatest for the spiropyran with the most electron donating group at the 5-position. These findings inform the design of new spiropyran derivatives with improved switching to facilitate their use as functional switches.