3 years ago

Shedding Light on the Photoisomerization Pathway of Donor–Acceptor Stenhouse Adducts

Shedding Light on the Photoisomerization Pathway of Donor–Acceptor Stenhouse Adducts
Denis Jacquemin, Adèle D. Laurent, Wiktor Szymański, Wybren Jan Buma, Miroslav Medved’, Michael M. Lerch, Alessandro Iagatti, Ben L. Feringa, Laura Bussotti, Svante P. Ihrig, Mariangela Di Donato, Paolo Foggi, Andrea Lapini
Donor–acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump–probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z–E photoisomerization of C2–C3 is followed by a rotation around C3–C4 and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b09081

DOI: 10.1021/jacs.7b09081

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