Redox-Responsive Colloidal Particles Based on Coordination Polymers Incorporating Viologen Units

Harnessing Radicals: Advances in Self-Assembly and Molecular Machinery

Radicals, with their unpaired electrons, exhibit unique chemical and physical properties that have long intrigued chemists. Despite early skepticism about their stability, the discovery of persistent radicals has opened new possibilities for molecular interactions. This review examines the mechanisms and applications of radically driven self-assembly, focusing on key motifs such as naphthalene diimides, tetrathiafulvalenes, and viologens, which serve as models for radical assembly. The potential of radical interactions in the development of artificial molecular machines (AMMs) are also discussed. These AMMs, powered by radical-radical interactions, represent significant advancements in non-equilibrium chemistry, mimicking the functionalities of biological systems. From molecular switches to ratchets and pumps, the versatility and unique properties of radically powered AMMs are highlighted. Additionally, the applications of radical assembly in materials science are explored, particularly in creating smart materials with redox-responsive properties. The review concludes by comparing AMMs to biological molecular machines, offering insights into future directions. This overview underscores the impact of radical chemistry on molecular assembly and its promising applications in both synthetic and biological systems.

Hydrogen-Bonded Organic Framework-Polyoxometalate-Based System for Electrochromic Devices

A porous hydrogen-bonded organic framework (HOF) structure was explored for the first time in the design of high-performance electrochromic devices (ECDs) using polyoxometalate (POM)-based charge-balancing layers as counter electrodes (CEs). The novelty of this work lies in the facile construction of films using small molecule-based EC materials to form a porous HOF structure. A full-cell model of an ECD was constructed by utilizing a POM-based CE to optimize the voltage distribution on the HOF-coated working electrode (WE). The addition of PW12O403- (PW12) on CE significantly enhanced the voltage distribution on EC electrodes and decreased the overvoltage on the WE, further preventing the formation of non-EC species and resulting in a 3.3-fold increase in the lifetime of the ECD. The optical contrast was enhanced from 47% (TiO2 only) to 68%, and the coloration efficiency was enhanced from 185 (TiO2 only) to 373 cm2 C-1. The optimized voltage distribution on the WE, leading to the fast response time and high optical EC contrast, could be explained by the charge-balancing effect. Overall, this new finding provides a robust framework for designing high-performance ECDs, taking advantage of the porous morphology and potential matching of the HOF and PW12.

Photochromic Uranyl-Based Coordination Polymer for Quantitative and On-Site Detection of UV Radiation Dose

A highly sensitive detection of ultraviolet (UV) radiation is required in a broad range of scientific research, chemical industries, and health-related applications. Traditional UV photodetectors fabricated by direct wide-band-gap inorganic semiconductors often suffer from several disadvantages such as complicated manufacturing procedures, requiring multiple operations and high-cost instruments to obtain a readout. Searching for new materials or simple strategies to develop UV dosimeters for quantitative, accurate, and on-site detection of UV radiation dose is still highly desirable. Herein, a photochromic uranyl-based coordination polymer [(UO2)(PBPCA)·DMF]·DMF (PBPCA = pyridine-3,5-bis(phenyl-4-carboxylate), DMF = N,N'-dimethylformamide, denoted as SXU-1) with highly radiolytic and chemical stabilities was successfully synthesized via the solvothermal method at 100 °C. Surprisingly, the fresh samples of SXU-1 underwent an ultra-fast UV-induced (365 nm, 2 mW) color variation from yellow to orange in less than 1 s, and then the color changed further from orange to brick red after the subsequent irradiation, inspiring us to develop a colorimetric dosimeter based on red-green-blue (RGB) parameters. The mechanism of radical-induced photochromism was intensively investigated by UV-vis absorption spectra, EPR analysis, and SC-XRD data. Furthermore, SXU-1 was incorporated into an optoelectronic device to fabricate a novel dosimeter for convenient, quantitative, and on-site detection of UV radiation dose.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.