Threading-gated photochromism in [2]pseudorotaxanes

Pillar[5]arene based water-soluble [3]pseudorotaxane with enhanced fluorescence emission for cell imaging and both type I and II photodynamic cancer therapy

Water-soluble [3]pseudorotaxane with enhanced fluorescence emission was successfully constructed and applied in cell imaging and photodynamic cancer therapy.

Stimulus-Responsive Lanthanide MOF Materials Encapsulated with Viologen Derivatives: Characterization, Photophysical Properties and Sensing on Nitrophenols

Viologens (1,1'-disubstituted 4,4'-bipyridyls) possessing electron-deficient properties and redox activity are a class of suitable chromophores to assemble metal-organic hybrid photochromic materials. Thus, viologen-functionalized metal-organic frameworks (MOFs) have attracted much attention for their photochromic properties; however, the syntheses of lanthanide-viologen hybrid crystalline photochromic materials still face many challenges. For example, the structures and properties of the final products are difficult to predict and are limited by molecular configurations. In this work, host-guest composite-material Ln-NH₂ BDC-pbpy MOFs were constructed by encapsulating viologen derivative pbpyCl₂ . The pbpy²⁺ moieties are uniformly embed by their π-π conjugation in the pores of the 3D structure by electrostatic interactions. Due to the encapsulation of the chromophore pbpy²⁺ moieties, Ln-NH₂ BDC-pbpy MOFs have reversible photochromic properties: they can change color after irradiation and can return to the original color after being protected from light or heating. Interestingly, the fluorescence intensity decreases with illumination time and recovers in the dark. As a result, Ln-NH₂ BDC-pbpy MOFs show both photochromic and photomodulated fluorescence. Based on the outstanding fluorescence performance of the Ln-NH₂ BDC-pbpy MOFs, they also show a wonderful effect for detecting nitrophenols, especially TNP.

Monosulfonated dibenzo-24-crown-8 and its properties

In this paper, we describe a method for preparing a monosulfonated dibenzo-24-crown-8 ether, SDB24C8, by direct sulfonation of the parent crown (DB24C8). Since neutral DB24C8 readily interacts with cationic guests, permanently charged SDB24C8 is an advantageous candidate for future supramolecular applications. SDB24C8 can be isolated as a sulfonic acid to be used as it is or converted to a salt of choice. The crystallographic analysis provides the first known host-guest assembly with a DB24C8-based scaffold complexing hydronium and potassium cations. Supramolecular investigations of the interactions of this anionic macrocycle with alkali cations were also performed. According to the expectations, the introduction of the sulfonic group into the DB24C8 scaffold increases the affinities of the receptor. An unusual selectivity of SDB24C8 towards a sodium cation was also observed and further investigated with DFT calculations.

Push-Pull Stiff-Stilbene: Proton-Gated Visible-Light Photoswitching and Acid-Catalyzed Isomerization

Donor‐acceptor substituted stiff‐stilbene is shown to undergo isomerization induced by visible light avoiding the need for harmful UV light. This visible‐light photoswitching is inhibited by protonation of the dimethylamino‐donor unit, disrupting the push‐pull character and thus, gating of the photochromic properties is allowed by acid/base addition. Remarkably, the addition of a mild acid also triggers fast thermal back‐isomerization, which is unprecedented for stiff‐stilbene photoswitches usually having a very high energy barrier for this process. These combined features offer unique orthogonal control over switching behavior by light and protonation, which is investigated in detail by ¹H NMR and UV/Vis spectroscopy. In addition, TD‐DFT calculations are used to gain further insight into the absorption properties. Our results will help elevating the level of control over dynamic behavior in stiff‐stilbene applications.

Direct synthetic routes to functionalised crown ethers

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

Gating the photoactivity of azobenzene-type ligands trapped within a dynamic system of an M4L6 tetrahedral cage, an M2L2 metallocycle and mononuclear MLn complexes

Complexation of metal ions by a ligand bearing a azobenzene linker yielded two types of complexes: an [M2L2]4+ metallocycle and an [M4L6]8+ tetrahedral cage, which can be further converted into libraries of mononuclear [M(L′/L′′)n]2+ species.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a Pd^II₆L₄ coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Configurational Selection in Azobenzene-Based Supramolecular Systems Through Dual-Stimuli Processes

Azobenzene is one of the most studied light-controlled molecular switches and it has been incorporated in a large variety of supramolecular systems to control their structural and functional properties. Given the peculiar isomeric distribution at the photoexcited state (PSS), azobenzene derivatives have been used as photoactive framework to build metastable supramolecular systems that are out of the thermodynamic equilibrium. This could be achieved exploiting the peculiar E/Z photoisomerization process that can lead to isomeric ratios that are unreachable in thermal equilibrium conditions. The challenge in the field is to find molecular architectures that, under given external circumstances, lead to a given isomeric ratio in a reversible and predictable manner, ensuring an ultimate control of the configurational distribution and system composition. By reviewing early and recent works in the field, this review aims at describing photoswitchable systems that, containing an azobenzene dye, display a controlled configurational equilibrium by means of a molecular recognition event. Specifically, examples include programmed photoactive molecular architectures binding cations, anions and H-bonded neutral guests. In these systems the non-covalent molecular recognition adds onto the thermal and light stimuli, equipping the supramolecular architecture with an additional external trigger to select the desired configuration composition.