The development of molecular machines relies on molecules capable of changing their properties in a controlled and reversible way in response to external stimuli. Chiroptical molecules are a particular type of photoswitches that change chirality in response to light, with potential applications in molecular information processing or sensing. Most of the photoswitches reported to date show a strong relationship between the UV–visible absorption and the circular dichroism (CD) spectra: upon switching, the sign of the CD signal may invert, but would not disappear unless there is also no absorbance. Now, writing in Journal of the American Chemical Society, Christian Petermayer and Henry Dube report a new type of hemiindigo (HI) switches that feature axial chiral substituents and that are characterized by an unusual decoupling of the absorption and the circular dichroism (CD) spectra.
The photoresponse of chiroptical switches bearing a stereogenic centre has been largely investigated. For example, Dube has previously led a number of studies on hemithioindigo photoswitches featuring a stable stereocentre at the oxidized sulfur atom. In the present work, Dube and Petermayer investigated the photoresponse of a different kind of chirality — axial chirality. They added an o-tolyl or naphthyl group to the N atom of indoxyl fragment of HI, resulting in the formation of chiral axes along the N ─ C bond.
The isomerization of these HI derivatives is photoinducible and photoreversible, but thermally inhibited. Therefore, it is possible to promote rotations along the C ═ C double bond by using specific wavelengths in the visible spectrum, resulting in the formation of Z or E isomers. Such photoisomerization, however, does not induce any rotation along the chiral axes, which leads to an unusual decoupling of the absorption and CD spectra. Although both Z and E isomers are characterized by strong absorption bands, their CD signal can be switched on or off, owing to the differences in molecular twisting. Z isomers are highly twisted and characterized by intense absorption and a strong CD signal. On the contrary, the less-twisted structure of the E isomers leads to suppression of the CD signal, while preserving intense absorption.
Rotations around the chiral axes are only thermally activated, but overall slow. Dube and Petermayer were able to isolate all the isomers and atropisomers of the different HI-derivatives and measured the energy barriers associated with all the thermal atropisomerizations. They found that rotations around the chiral axes are less hindered in the Z isomers than in the E isomers, despite the higher sterical hindrance in the former. “This unexpected finding provides us valuable information for the design and development of next generation HI chiroptical photoswitches and future molecular machines in general,” remarks Dube. “In a way, the chiral axis serves as sensor for the folding state of the molecule, the effects of different substituents and the internal dynamics of an axially chiral molecular structure.”
“In a way, the chiral axis serves as sensor for the folding state of the molecule, the effects of different substituents and the internal dynamics of an axially chiral molecular structure.”
Dube and Petermayer have shown that it is possible to use the same molecule to obtain different kinds of photoresponses. “This extra-information layer could be used, for example, in multi-level photonic information processing, in which we could detect different responses to polarized and nonpolarized light,” envisages Dube, concluding, “the next step would see the implementation of these chiroptical photoswitches into materials, polymers, nano-devices, or use them for surface functionalization.”
Petermayer, C. & Dube, H. Circular dichroism photoswitching with a twist: axially chiral hemiindigo. J. Am. Chem. Soc. https://doi.org/10.1021/jacs.8b07839 (2018)
Wiedbrauk, S. et al. Twisted hemithioindigo photoswitches: solvent polarity determines the type of light-induced rotations. J. Am. Chem. Soc. 138, 12219–12227 (2016)