Studies of novel trifluoroacetylated diaryl hydrazone molecular photoswitches in solution and in the solid state

Photoisomerization of trifluoroacetylated diaryl hydrazones induces a change in color due to the formation of a new hydrogen bond in solution and in the solid state.

Fabrication of Photoresponsive Crystalline Artificial Muscles Based on PEGylated Covalent Organic Framework Membranes

Seeking new photoresponsive materials with high energy conversion efficiency, good mechanical properties, as well as well-defined photoactuation mechanisms is of paramount significance. To address these challenges, we first introduced crystalline covalent organic frameworks (COFs) into the photoactuator field and created a facile fabrication strategy to directly install photoresponsive functional groups (i.e., acylhydrazone) on the skeletons of COFs. Herein, an approach to use polyethylene glycol (PEG) cross-linked dimers as the building blocks of the COF-42 platform was developed and afforded a series of uniform and freestanding membranes (PEG-COF-42) with outstanding mechanical properties (e.g., high flexibility and mechanical strength). Notably, these membranes possessed a fast mechanical response (e.g., bending) to UV light and good reversibility upon blue light or heating. After an in-depth investigation of the photoactuation mechanism via various techniques, we proposed a mechanism for the photoresponsive performance of PEG-COF-42: configurational change of acylhydrazone (i.e., E ↔ Z isomerization) accompanied by an excited-state intramolecular proton transfer (ESIPT) process intramolecularly transferring hydrogens from hydrogen donors (N-H) to hydrogen acceptors (oxygen in PEG). Moreover, attributed to the PEG moieties, PEG-COF-42 also demonstrated a vapor-responsive performance. This study not only broadens the application scopes of COFs but also provides new opportunities for the construction of multi-stimuli-responsive materials.

Deciphering the grounds of the suitability of acylhydrazones as efficient photoswitches

Many efforts are currently being devoted to designing molecular photoswitches with specific properties. In this sense, a recent publication [D. J. van Dijken et al., J. Am. Chem. Soc., 2015, 137, 14982-14991] has synthesized and analyzed the photochromic properties of a large set of acylhydrazones (ACHs), a relatively unexploited class of potential photoswitches with two stable E and Z isomers. This study has revealed a very diverse and complex pattern of the absorption/emission properties of ACHs depending on the substituents attached to the ACH motif. In this work, high level theoretical calculations are performed on a representative set of the experimentally studied ACHs in order to analyze, at the molecular level, the reasons behind the different photochemistries experimentally observed. This systematic study allows for the classification of the full set of ACHs into just four categories. The two more common groups display a small, either positive or negative, shift of the maximum wavelength of absorption between the E and Z isomers. Less common, but far more interesting from a practical point of view, are the compounds that show a large (>100 nm) Stokes shift. This behavior may arise from two different situations. The most common one implies the possibility of an intramolecular proton transfer in the excited electronic state of the less stable Z isomer. The less likely scenario would also involve a loss of the azidic proton through an intermolecular proton transfer that would take place with the aid of the solvent.

A molecular roundabout: triple cyclically arranged hydrogen bonds in light of experiment and theory

This paper dwells on the synthesis and diverse studies of cyclically arranged hydrogen bridges in tris-hydroxy aryl Schiff bases. Experimental (IINS, IR, Raman and X-ray) and theoretical (CPMD, DFTP and DFT) studies of tris-hydroxy aryl Schiff bases have been performed in the solid state.