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.

Dynamic covalent chemistry relies on the formation of reversible covalent bonds under thermodynamic control to generate dynamic combinatorial libraries. It provides access to numerous types of complex functional architectures, and thereby targets several technologically relevant applications, such as in drug discovery, (bio)sensing and dynamic materials. In liquid media it was proved that by taking advantage of the reversible nature of the bond formation it is possible to combine the error-correction capacity of supramolecular chemistry with the robustness of covalent bonding to generate adaptive systems. Here we show that double imine formation between 4-(hexadecyloxy)benzaldehyde and different α,ω-diamines as well as reversible bistransimination reactions can be achieved at the solid/liquid interface, as monitored on the submolecular scale by in situ scanning tunnelling microscopy imaging. Our modular approach enables the structurally controlled reversible incorporation of various molecular components to form sophisticated covalent architectures, which opens up perspectives towards responsive multicomponent two-dimensional materials and devices.

Various aldehyde-containing photoswitches have been developed whose reactivity toward amines can be controlled externally. A thermally stable bifunctional diarylethene, which in its ring-closed form exhibits imine formation accelerated by one order of magnitude, was used as a photoswitchable crosslinker and mixed with a commercially available amino-functionalized polysiloxane to yield a rubbery material with viscoelastic and self-healing properties that can be reversibly tuned by irradiation.

The formation and exchange processes of imines of salicylaldehyde, pyridine-2-carboxaldehyde, and benzaldehyde have been studied, showing that the former has features of particular interest for dynamic covalent chemistry, displaying high efficiency and fast rates. The monoimines formed with aliphatic α,ω-diamines display an internal exchange process of self-transimination type, inducing a local motion of either "stepping-in-place" or "single-step" type by bond interchange, whose rate decreases rapidly with the distance of the terminal amino groups. Control of the speed of the process over a wide range may be achieved by substituents, solvent composition, and temperature. These monoimines also undergo intermolecular exchange, thus merging motional and constitutional covalent behavior within the same molecule. With polyamines, the monoimines formed execute internal motions that have been characterized by extensive one-dimensional, two-dimensional, and EXSY proton NMR studies. In particular, with linear polyamines, nondirectional displacement occurs by shifting of the aldehyde residue along the polyamine chain serving as molecular track. Imines thus behave as simple prototypes of systems displaying relative motions of molecular moieties, a subject of high current interest in the investigation of synthetic and biological molecular motors. The motional processes described are of dynamic covalent nature and take place without change in molecular constitution. They thus represent a category of dynamic covalent motions, resulting from reversible covalent bond formation and dissociation. They extend dynamic covalent chemistry into the area of molecular motions. A major further step will be to achieve control of directionality. The results reported here for imines open wide perspectives, together with other chemical groups, for the implementation of such features in multifunctional molecules toward the design of molecular devices presenting a complex combination of motional and constitutional dynamic behaviors.