Mechanistic Elucidation of the Hula-Twist Photoreaction in Hemithioindigo

The Hula-Twist (HT)photoreaction represents a fundamentalphotochemicalpathway for bond isomerizations and is defined by the coupled motionof a double bond and an adjacent single bond. This photoreaction hasbeen suggested as the defining motion for a plethora of light-responsivechromophores such as retinal within opsins, coumaric acid within photoactiveyellow protein, or vitamin D precursors, and stilbenes in solution.However, due to the fleeting character of HT photoproducts a directexperimental observation of this coupled molecular motion was severelyhampered until recently. To solve this dilemma, the Dube group hasdesigned a molecular framework able to deliver unambiguous experimentalevidence of the HT photoreaction. Using sterically crowded atropisomerichemithioindigo (HTI) the HT photoproducts are rendered thermally stableand can be observed directly after their formation. However, followingthe ultrafast excited state process of the HT photoreaction itselfhas not been achieved so far and thus crucial information for an elementaryunderstanding is still missing. In this work, we present the firstultrafast spectroscopy study of the HT photoreaction in HTI and probethe competition between different excited state processes. Togetherwith extensive excited state calculations a detailed mechanistic pictureis developed explaining the significant solvent effects on the HTphotoreaction and revealing the intricate interplay between productiveisomerizations and unproductive twisted intramolecular charge transfer(TICT) processes. With this study essential insights are thus gainedinto the mechanism of complex multibond rotations in the excited state,which will be of primary importance for further developments in thisfield.

Automated summary

The Hula-Twist (HT) photoreaction, which involves the motion of a double bond and an adjacent single bond, is a fundamental pathway for bond isomerizations. However, the fleeting nature of HT photoproducts has hindered direct experimental observation of this coupled motion. In this study, the Dube group designed a molecular framework using sterically crowded atropisomeric hemithioindigo (HTI) to stabilize the HT photoproducts and enable their direct observation. Furthermore, the researchers investigated the ultrafast excited state processes of the HT photoreaction, providing important insights into the mechanism of complex multibond rotations in the excited state.