Series of Viologen-Based Metal-Organic Polyhedra with Photo/Electrochromic Behavior for Inkless Printing and UV Detection

The fabrication of molecular crystalline materials with fast, multistimuli-responsive behavior and the construction of the corresponding structure-activity relationship are of extraordinary significance for the development of smart materials. In this context, three multistimuli-responsive functional metal-organic polyhedra (MOP), {[Dy2(bcbp)3(NO3)1.5(H2O)7]·Cl4.2·(NO3)0.3·H2O}n (1), {[Dy2(bcbp)4(H2O)8]Cl6}n (2), and {[Eu2(bcbp)4(H2O)10]Cl6·H2O}n (3; bcbp = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium), were successfully prepared and characterized. All of the compounds exhibit rapid and reversible photochromic and electrochromic dual-responsive behaviors. Furthermore, benefiting from the well-defined crystal structure and different responsive behaviors, the photoinduced electron transfer (PIET) process and structure-activity relationship were explored. In addition, considering the excellent photochromic performance, function filter paper and smart organic glass were successfully prepared and used for ink-free printing and UV light detection.

One-Dimensional Chain Viologen-Based Lanthanide Multistimulus-Responsive Materials with Photochromism, Photoluminescence, Photomagnetism, and Ammonia/Amine Vapor Sensing

In this work, a series of novel multistimulus-responsive lanthanide coordination polymers {[LnL(H₂O)₄]Cl₃·3H₂O}_n (Ln = Dy, Tb, Eu) constructed using a dicarboxylic acid viologen derivative L (L = N,N'-4,4'-bipyridiniodipropionate) and LnCl₃·6H₂O were prepared. All materials showed positive responses to UV light, and the photochromic phenomena accompanied by significant photoquenching of photoluminescence could be observed through a photoelectron transfer mechanism. Strikingly, the Dy analogue displayed photomagnetic behavior, as well as responded positively to small molecules of inorganic ammonia/organic amines. Furthermore, the good photoresponsive and ammonia/amine vapor-responsive properties of the Dy-based material were further fulfilled in dual-function papers involving erasable inkless printing and visual amine detection applications. This work aims to advance the development of multistimulus-responsive multifunctional materials incorporating viologen derivates and versatile lanthanide ions and further enriches the research in this field.

Decorating Metal Nitrate with a Coplanar Bipyridine Moiety: A Simple and General Method for Fabricating Photochromic Complexes

As a significant class of photochromic materials, crystalline hybrid photochromic materials (CHPMs) have attracted widespread attention of researchers because of their possibilities for generating other photoresponsive properties and advantages in understanding the underlying relationship between structure and photoresponsive performance. The predesign of suitable ligands plays a major role in generating desirable CHPMs. Hitherto, most CHPMs have been built from photodeformable or photoresponsive tectons. However, the synthesis of these ligands is usually time-consuming and expensive, and this greatly restricts their large-scale preparation and practical application. Therefore, it is necessary to explore new families of CHPMs besides the existing CHPMs. Herein, a simple and general method for constructing CHPMs by decorating metal nitrate with a coplanar bipyridine moiety, namely 1,10-phenanthroline (phen), is reported. The resulting products exhibit photocoloration in response to Xe-lamp irradiation. The electron transfer (ET) from the coplanar NO₃ ^- species (as π-electron donors, π-EDs) to coplanar phen moieties (as π-electron acceptors, π-EAs) is responsible for the resulting photochromism. The influence of the coordination environment and central metal ion on the photochromism was also studied. This work demonstrates that the introduction of coplanar organic tectons as π-EAs to metal nitrates as π-EDs with the collaboration of ET and coordination-assembly strategies is a simple and general method to manufacture CHPMs.

Two Photochromic Complexes Assembled by a Nonphotochromic Ligand: Photogenerated Radical Enhanced Room-Temperature Phosphorescence

Stimulating tunable room-temperature phosphorescence (RTP) is still a challenge in photochromic systems, which is vital for multifunctional coordination materials. Herein, we synthesized two new photochromic chain complexes through self-assembly of the nonphotochromic 1,3,5-tris(4-pyridyl)benzene ligand, diphosphonate, and Ln(III) ions (1 for Ln(III) = Dy and 2 for Ln(III) = Gd). Both compounds showed fast photoresponses with the color turning from yellow to dark gray with a reversible decoloration by heating or storage in the dark. The electron transfer photochromic behavior with the generated stable radicals was further confirmed by the room-temperature UV-vis and electron paramagnetic resonance spectra. Furthermore, via tuning the generation and disappearance of stable radicals, reversible room-temperature fluorescence and phosphorescence for both compounds were switched by light irradiation and a thermal treatment, with an enhanced intensity for RTP and a decrease in fluorescence during the duration of Xe-lamp light irradiation. This work provides a new strategy that photogenerated radicals could promote and enhance RTP properties in functional materials.

Metal-dependent photochromic performance in two isostructural supramolecular chains

The introduction of nonphotochromic coplanar 1,10-phenanthroline into a metal carboxylate system produces two isostructural supramolecular chains with metal-dependent photochromic performance.