Effects of Polar Substituents and Media on the Performance of N,N′‐Di‐tert‐Butoxycarbonyl‐Indigos as Photoswitches

Abstract

AbstractThe N,N′‐di‐tert‐butoxycarbonylindigo parent compound and four symmetrically substituted derivatives alternatively containing methoxy and nitro substituents in positions 5 and 6 were synthesized. The photochromic behavior was studied in solvents ranging in polarity from toluene to acetonitrile/water mixtures and characterized in terms of trans and cis isomer spectra, conversion quantum yields and thermal half‐lives of the metastable cis isomers. The half‐lives at 21 °C varied from a few seconds (5‐nitro in polar solvents) to several days (6‐methoxy in apolar media) and can be described by the substituents’ Hammett constants. Spectroscopic and photochemical parameters, in contrast, are governed largely by the substituents’ resonance effects according to a scheme well known from thioindigos and these are termed here favorable and unfavorable resonance effects (FRE and URE, respectively). The title photoswitches show little propensity to quenching by protons and may be operated in aqueous media.