N,N′-Disubstituted Indigos as Readily Available Red-Light Photoswitches with Tunable Thermal Half-Lives

Abstract

Some rare indigo derivatives have been known for a long time to be photochromic upon irradiation with red light, which should be advantageous for many applications. However, the absence of strategies to tune their thermal half-lives by modular molecular design as well as the lack of proper synthetic methods to prepare a variety of such molecules from the parent indigo dye have so far precluded their use. In this work, several synthetic protocols for N-functionalization have been developed, and a variety of N-alkyl and N-aryl indigo derivatives have been prepared. By installation of electron-withdrawing substituents on the N-aryl moieties, the thermal stability of the Z-isomers could be enhanced while maintaining the advantageous photoswitching properties upon irradiation with red light (660 nm LED). Both experimental data and computational results suggest that the ability to tune thermal stability without affecting the dyes' absorption maxima originates from the twisted geometry of the N-aryl groups. The new indigo photoswitches reported are expected to have a large impact on the development of optical methods and applications in both life and material sciences.

Reversible Spatial Control in Aqueous Media by Visible Light: A Thioindigo Photoswitch that is Soluble and Operates Efficiently in Water

“…Unlike most other trans ↔ cis isomerizing photochromic compounds, the blueshift in the cis isomer of thioindigos is not caused by sterically induced distortion from planarity and impaired conjugation through the central double bond, but much rather by non‐bonding interaction between the sulfur and oxygen atoms . These features which thioindigos share with only a few other photochromic compounds (particularly N‐functionalized indigos and certain hemithioindigos) make for an excellent molecular machinery for spatial control: depending on the flexibility of the linkage, attached molecular payload can be allowed or even forced to come close or separate on rotation around the central double bond of the chromophore (Scheme b). As a result of trans → cis isomerization, some substituents in opposite hemispheres (such as 4 and 7′) depart from each other and get separated by the central dye atoms, whereas others (especially 4 and 4′) approach and get to face each other.…”

Photoisomerization of Mono‐Arylated Indigo and Water‐Induced Acceleration of Thermal cis‐to‐trans Isomerization

“…This very fast process prevents a deexcitation by trans to cis photoisomerization, which can be observed in other chromophores bearing substituted double bonds such as azobenzene, stilbene or thioindigoid derivatives. However if both nitrogen atoms of indigo are substituted by alkyl, acetyl, tert ‐butyloxycarbonyl, or as recently been found by the Hecht group by aryl residues, the proton transfer mechanism can be excluded and trans to cis isomerization occurs. In case of indigo derivatives bearing only one substituted nitrogen atom (whilst the other nitrogen atom is still connected to hydrogen) a trans to cis photoisomerization was not observed so far.…”

“…In the research field of photoresponsive materials, indigoid chromophores, gained more and more importance in the last years due to their outstanding redshifted absorption. Hemithioindigo based peptide‐switches, protein inhibitors or molecular devices such as molecular calculators, motors, and receptors are interesting examples for shifting addressability at the molecular scale into the region of less energetic light.…”

“…Hemithioindigo based peptide‐switches, protein inhibitors or molecular devices such as molecular calculators, motors, and receptors are interesting examples for shifting addressability at the molecular scale into the region of less energetic light. Very recently it was reported that di‐arylated indigos can be efficiently photoisomerized while possessing high barriers for thermal cis to trans isomerization . Especially twofold arylated indigos bearing electron deficient groups at the N‐aryl substituents show remarkable thermal stabilities of their cis isomers.…”

Light‐Gated Rotation in a Molecular Motor Functionalized with a Dithienylethene Switch

“…These advantages have stimulated the application of light‐driven molecular switches14 and motors in functional materials8, 15, 16, 17, 18, 19, 20 and biological systems 21, 22, 23, 24. However, potentially harmful UV light is typically used for their operation while, for practical applications, the use of visible light is often desired 25, 26, 27, 28, 29, 30, 31…”