A photoswitching scaffold called Donor-Acceptor Stenhouse Adducts (DASAs) has shown exceptional versatility and could expand the functionality and use of molecular switches. Scientists from the University of Groningen worked with colleagues from the University of Amsterdam, the University of Twente, and the European Laboratory for Non-Linear Spectroscopy to map the complete switching pathway for DASAs. Solutions of different photoswitches. Courtesy of Michael Lerch. DASA molecules exhibit a profound change of shape when switched. Further, they are triggered by red light, which is more acceptable for medical applications than the potentially damaging UV light that is used in most molecular switches. Scientists have characterized how DASAs are activated by light absorption, but not the thermal steps involved in full switching. To investigate the thermal steps that follow the initial photochemical step, the researchers recorded how the DASA molecules vibrate during switching using rapid-scan Fourier Transform InfraRed spectroscopy. The frequencies of the vibrations provided a fingerprint of the molecular structure, revealing the changes in the molecular shape that occurred after the light-induced switching. The researchers made an IR movie of how the molecule changed its structure after being activated. The movie followed how the spectrum changed over time. However, linking the spectra to specific changes in the molecular structure was not straightforward, because the molecular shape could not be observed directly. So the researchers performed quantum chemical calculations on all possible interconversion pathways. These calculations enabled them to identify spectral features in the movie as specific structural markers. Two types of DASAs dropped into dichloromethane. Courtesy of Dusan Kolarski/Michael Lerch. The researchers identified a number of principles — some of them surprising — that could be used to steer DASAs along multiple switching pathways. For example, the molecule can take different routes to move from the “on” to the “off” position, depending on the solvent. Also, the thermal steps are more important in DASAs than in other light-activated switches. The researchers confirmed that different changes occur in the molecule when different solvents are used. They also found a solvent that prevents the switch from getting stuck in an “in-between” state that can reduce the efficiency of the molecular switch. Although DASAs were introduced just four years ago, examples of applications have already been reported in areas ranging from material sciences to pharmacology. Now that the “instruction manual” for DASAs can be read, this information can be used to direct their operation, better control their performance, and develop novel switches with targeted properties. The research was published in the Journal of the American Chemical Society (https://pubs.acs.org/doi/10.1021/jacs.9b00341).