Molecular photoswitches have attracted much attention in biological and materials contexts. Despite the fact that existing classes of these highly interesting functional molecules have been heavily investigated and optimized, distinct obstacles and inherent limitations remain. Considerable synthetic efforts and complex structure-property relationships render the development and exploitation of new photoswitch families difficult. Here, we focus our attention on acylhydrazones: a novel, yet underexploited class of photochromic molecules based on the imine structural motif. We optimized the synthesis of these potent photoswitches and prepared a library of over 40 compounds, bearing different substituents in all four crucial positions of the backbone fragment, and conducted a systematic study of their photochromic properties as a function of structural variation. This modular family of organic photoswitches offers a unique combination of properties and the compounds are easily prepared on large scales within hours, through an atom-economic synthesis, from commercially available starting materials. During our thorough spectroscopic investigations, we identified photoswitches covering a wide range of thermal half-lives of their (Z)-isomers, from short-lived T-type to thermally stable P-type derivatives. By proper substitution, excellent band separation between the absorbance maxima of (E)- and (Z)-isomers in the UV or visible region could be achieved. Our library furthermore includes notable examples of rare negative photochromic systems, and we show that acylhydrazones are highly fatigue resistant and exhibit good quantum yields.

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.

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Adjusting the length, composition, and microstructure of a polymer during the process of its formation in principle allows achieving the desired properties, thereby enabling custom-design of the thus generated polymer for its targeted function.