Photoswitching of chiral supramolecular environments and photoinduced lower critical solution temperature transitions in aqueous media following a supramolecular approach

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films triggered by UV radiation

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA300-co-PAHA), are triggered by UV radiation. Both rapid kinetic processes are probed by in situ neutron reflectivity (NR). The transition temperatures (TTs) of P(OEGMA300-co-PAHA) are 53.0 (ambient conditions) and 52.5 °C (UV radiation, λ = 365 nm). Thin P(OEGMA300-co-PAHA) films show a random distribution of OEGMA300 and PAHA segments. They swell in a D2O vapor atmosphere at 23 °C (below TT) to a swelling ratio d/das-prep of 1.61 ± 0.01 and exhibit a D2O volume fraction φ(D2O) of 39.3 ± 0.5%. After being exposed to UV radiation for only 60 s, d/das-prep and φ(D2O) significantly decrease to 1.00 ± 0.01 and 13.4 ± 0.5%, respectively. Although the UV-induced trans-cis isomerization of the azobenzene in PAHA induces increased hydrophilicity, the configuration change causes a breaking of the intermolecular hydrogen bonds between OEGMA300 and D2O molecules and unexpected film shrinkage. As compared to thermal stimulus-induced dehydration, the present dehydration rate is 100 times faster. Removal of the UV radiation causes immediate rehydration. After 200 s, d/das-prep and φ(D2O) recover to their hydrated states, which is also 30 times faster than the initial hydration. At 60 °C (above TT), thin P(OEGMA300-co-PAHA) films switch to their collapsed state and are insensitive to UV radiation. Thus, the UV-induced fast dehydration and rehydration depend on the existence of hydrogen bonds.

Non-Oxido-Vanadium(IV) Metalloradical Complexes with Bidentate 1,2-Dithienylethene Ligands: Observation of Reversible Cyclization of the Ligand Scaffold in Solution

Derivatives of 1,2‐dithienylethene (DTE) have superb photochromic properties due to an efficient reversible photocyclization reaction of their hexatriene structure and, thus, have application potential in materials for optoelectronics and (multi‐responsive) molecular switches. Transition‐metal complexes bearing switchable DTE motifs commonly incorporate their coordination site rather distant from the hexatriene system. In this work the redox active ligand 1,2‐bis(2,5‐dimethylthiophen‐3‐yl)ethane‐1,2‐dione is described, which reacts with [V(TMEDA)₂Cl₂] to give a rare non‐oxido vanadium(IV) species 3(M,M/P,P). This blue complex has two bidentate en‐diolato ligands which chelate the V^IV center and give rise to two five‐membered metallacycles with the adjacent hexatriene DTE backbone bearing axial chirality. Upon irradiation with UVA light or prolonged heating in solution, the blue compound 3(M,M/P,P) converts into the purple atropisomer 4(para,M/para,P). Both complexes were isolated and structurally characterized by single‐crystal X‐ray diffraction analysis (using lab source and synchrotron radiation). The antiparallel configuration (M or P helicity) present in both 3(M,M/P,P) and 4(para,M/para,P) is a prerequisite for (reversible) 6π cyclization reactions. A CW EPR spectroscopic study reveals the metalloradical character for 3(M,M/P,P) and 4(para,M/para,P) and indicates dynamic reversible cyclization of the DTE backbone in complex 3(M,M/P,P) at ambient temperature in solution.

Photochromism of Amphiphilic Dithienylethenes as Langmuir-Schaefer Films

Surface pressure-area isotherms were recorded under different irradiation conditions for single-component Langmuir films of three photochromic amphiphilic dithienylethenes. Nonirradiated films of these photochromic amphiphiles were mechanically stable. In addition, a shift of the isotherms to larger mean molecular areas was observed for films prepared from UV-light-irradiated dithienylethenes. Unexpectedly, a significant expansion was observed for a film prepared from visible-light-irradiated dithienylethene incorporating large branched alkyl chains. Upon further study, atomic force microscopy and transmission electron microscopy images of Langmuir-Schaefer films revealed that this pronged dialkyl derivative undergoes a photoinduced change in morphology, as circular aggregates coalesce into larger continuous aggregated structures. Nevertheless, its photoisomerization was completely reversible as single-component multilayer thin films upon direct UV or visible light irradiation.

Efficiently Photocontrollable or Not? Biological Activity of Photoisomerizable Diarylethenes

Diarylethene derivatives, the biological activity of which can be reversibly changed by irradiation with light of different wavelengths, have shown promise as scientific tools and as candidates for photocontrollable drugs. However, examples demonstrating efficient photocontrol of their biological activity are still relatively rare. This concept article discusses the possible reasons for this situation and presents a critical analysis of existing data and hypotheses in this field, in order to extract the design principles enabling the construction of efficient photocontrollable diarylethene-based molecules. Papers addressing biologically relevant interactions between diarylethenes and biomolecules are analyzed; however, in most published cases, the efficiency of photocontrol in living systems remains to be demonstrated. We hope that this article will encourage further discussion of design principles, primarily among pharmacologists, synthetic and medicinal chemists.