Stimuli-responsive coordination cages allow reversible control over guest binding and release, relevant for adaptive receptors, carriers, catalysts, and complex systems. Light serves as an advantageous stimulus, as it can be applied with precise spatial and temporal resolution without producing chemical waste products. We report the first Pd-mediated coordination cage based on ligands embedding a diazocine photoswitch. While the thermodynamically more stable cis -photoisomer sloppily assembles to a mixture of species with general formula [Pd n cis -L 2 n ], the less stable trans -isomer yields a defined [Pd 2 trans -L 4 ] cage that reversibly converts back to the cis -system by irradiation at 530 nm or thermal relaxation. The [Pd n cis -L 2 n ] species do not bind a given guest; however, [Pd 2 trans -L 4 ] is able to encapsulate a bis-sulfonate as long as it is kept assembled, requiring continuous irradiation at 385 nm. In the absence of UV light, thermal relaxation results in back-switching and guest release. Assembly and properties of the system were characterized by a combination of NMR, ion mobility ESI-MS, single-crystal X-ray diffraction, and UV–vis absorption studies.

Photoswitchable neurotransmitters of ionotropic kainate receptors were synthesized by tethering a glutamate moiety to disubstituted C2-bridged azobenzenes, which were prepared through a novel methodology that allows access to diazocines with higher yields and versatility. Because of the singular properties of these photochromes, photoisomerizable compounds were obtained with larger thermal stability for their inert cis isomer than for their biologically activity trans state. This enabled selective neuronal firing upon irradiation without background activity in the dark.

Seven symmetrically 3,3’-substituted diazocines were synthesized. Functional groups include alcohol, azide, amine and vinyl groups, which are suitable for polymer synthesis. Upon irradiation at 385 and 530 nm the diazocines perform a reversible, pincer-type movement switching the 3,3’-distance between 6.1 Å (cis, stable isomer) and 8.2 Å (trans, metastable isomer). Key reactions in the synthesis are an oxidative C–C coupling of 2-nitrotoluenes (75–82% yield) and a reductive ring closure to form the diazocines (56–60% yield). The cyclization of the dinitro compound to the azo compound was improved in yield and reproducibility, by over-reduction to the hydrazine and reoxidation to the azo unit. In contrast to 3,3’- and 4,4’-diaminodiazocine, which have been implemented in macromolecules for conformation switching, our compounds exhibit improved photophysical properties (photostationary states, separation of absorption bands in the cis and trans configuration). Hence they are promising candidates as molecular switches in photo and mechanoresponsive macromolecules and other smart materials.

A convenient two-step synthesis of diazocine starting from 2-nitrotoluene is described. The first step, the oxidative dimerization of 2-nitrotoluene, is improved to 95% yield. The second step, the reductive azo cyclization, is performed as a solvent-free reaction with lead powder in a ball mill (51% yield). As a reference, the previously described azo cyclization with Zn/Ba(OH)2 is investigated in detail. The results explain why in previous experiments the yields are low and extremely dependent on the reaction conditions. In view of potential applications in photopharmacology, we checked the stability under reducing conditions. Diazocine does not react with glutathione, indicating intracellular stability.

In this work, we demonstrate the first synthesis of photochromic polymer thin films via initiated chemical vapor deposition (iCVD) using bifunctional, crystalline styrenediazocine as a photochromic unit. Since it represents a solid compound, an innovative custom-made sublimation unit is introduced, which allows the sublimation of the chromophore as well as its transport inside the reactor chamber, maintaining the functionality of the photoswitch. Styrenediazocine was co-polymerized with 1,3,5trivinyl-1,3,5-trimethylcyclotrisiloxane (V 3 D 3 ), a heterocycle with triple vinyl functionality, in a radical polymerization reaction from the gas phase. The structural as well as functional properties of the resulting films were characterized by various methods, including X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR) spectroscopy, and UV−vis spectroscopy. It could be confirmed that, via iCVD, photoswitchable polymer thin films have been obtained. The polymer is based on a copolymer structure where all photoswitchable molecules are covalently bonded in the main polymer chain. Our approach could pave the way for a completely new class of high-quality, photochromic thin films that are basically applicable on almost every kind of substrate for novel, interdisciplinary application fields, where particularly classical wet chemistry methods cannot be applied.