Hemiindigo: Highly Bistable Photoswitching at the Biooptical Window

Hemiindigo
is a long known chromophore that absorbs in the blue
part of the spectrum but has almost completely been ignored as potential
photoswitch. Herein we show how the absorption of hemiindigo is shifted
to the red part of the visible spectrum and how nearly perfect photoswitching
can be achieved using blue or green and red light. Five derivatives
were investigated giving very high isomeric yields in both switching
directions, i.e. >90% E isomer after irradiation
with 470 to 530 nm light and 99% Z isomer with 590
up to 680 nm light. At the same time the thermal bistability is extraordinarily
high leading to half-lives of the pure isomeric states of up to 83
years at 25 °C. The herein developed photoswitches show photochromism
in the visible enabling the two isomeric states to be distinguished
by the naked eye. Substituted hemiindigos therefore constitute extremely
promising new photoswitches with excellent properties for applications
in biology, chemistry, or material sciences.