Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H′-octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB), into the pores of the well-known metal-organic frameworks MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF-AZB_0.34@MIL-53(Al), a structural model based on high-resolution synchrotron powder diffraction data was developed and the host-guest and guest-guest interactions were elucidated from this model. These interactions of O−H⋅⋅⋅F and π⋅⋅⋅π type were confirmed by significant shifts of the O−H frequencies in loaded and unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably, all of the synthesized F-AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo-isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.     

Quantitative Photoswitching in Bis(dithiazole)ethene Enables Modulation of Light for Encoding Optical Signals

Using one ray of light to encode another ray of light is highly desirable because information in optical format can be directly transferred from one beam to another without converting back to the electronic format. One key medium to accomplish such an amazing task is photoswitchable molecules. Using bis(dithiazole)ethene that can be photoswitched between its ring‐open and ring‐closed states quantitatively with excellent fatigue resistance and high thermal stability, it is shown that quantitative photoreversibility allowed the photoswitching light to control other light travelling through the photoswitchable medium, a phenomenon of transferring information encoded in one light ray to others, thus imparting photo‐optical modulation on the orthogonal light beam.     

Photoswitching Behavior of a Cyclohexene‐Bridged versus a Cyclopentene‐Bridged Dithienylethene System

Photoswitching is an intriguing way of incorporating functionality into molecules or their subunits. Dithienylethene switches are particularly promising, but have so far mostly been studied with five‐membered ring (cyclopentenyl) backbones. We aim at comparing the switching properties of backbones with five and six carbon atoms in the ring. A major advantage is that cyclohexenyl rings offer new options for chiral functionalization. A slight change in the reaction conditions of a McMurry ring closure reaction leads to the formation of dithienyl derivatives with a cyclohexene backbone in reasonable yield. Density functional theory calculations were carried out, demonstrating the similarity of both compounds. Experimental results confirm the theoretical outcomes.     

Vaporisation of a Dicationic Ionic Liquid Revisited

The vaporization of a dicationic ionic liquid at moderate temperatures and under reduced pressures —recently studied by line‐of‐sight mass spectrometry—was further analyzed using an ion‐cyclotron resonance mass spectroscopy technique that allows the monitoring of the different species present in the gas phase through the implementation of controlled ion–molecule reactions. The results support the view that the vapour phase of an aprotic dicationic ionic liquid is composed of neutral ion triplets (one dication attached to two anions). Molecular dynamics simulations were also performed in order to explain the magnitude of the vaporization enthalpies of dicationic ionic liquids vis‐à‐vis their monocationic counterparts.     

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors have been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc. The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.     

Reversible Photoswitching of a Spin‐Crossover Molecular Complex in the Solid State at Room Temperature

Spin‐crossover metal complexes are highly promising magnetic molecular switches for prospective molecule‐based devices. The spin‐crossover molecular photoswitches developed so far operate either at very low temperatures or in the liquid phase, which hinders practical applications. Herein, we present a molecular spin‐crossover iron(II) complex that can be switched between paramagnetic high‐spin and diamagnetic low‐spin states with light at room temperature in the solid state. The reversible photoswitching is induced by alternating irradiation with ultraviolet and visible light and proceeds at the molecular level.     

Profiling and imaging of tissues by imaging ion mobility-mass spectrometry

Molecular profiling and imaging mass spectrometry (IMS) of tissues can often result in complex spectra that are difficult to interpret without additional information about specific signals. This report describes increasing data dimensionality in IMS by combining two-dimensional separations at each spatial location on the basis of imaging ion mobility-mass spectrometry (IM-MS). Analyte ions are separated on the basis of both ion-neutral collision cross section and m/z, which provides rapid separation of isobaric, but structurally distinct ions. The advantages of imaging using ion mobility prior to MS analysis are demonstrated for profiling of human glioma and selective lipid imaging from rat brain.     

Modulation of Electronic Mobility of a One‐Dimensional Coordination Polymeric Molecular Wire with Light

Metal ions often influence the photoswitching efficiency of a photochromic system. This article reports a one‐dimensional polymer having cyclic azobenzenes coordinated to silver ions that are bridged by nitrates. The coordination polymer (CP‐ 2 ) displays a photoresponsive behavior. The switching ability in the polymer form was faster compared to the parent azobenzene ligand without the metal ions. Azobenzenes are reported to be poorly conducting. Here, although the azobenzene ligand does not show significant electronic mobility, the coordination polymer (CP‐ 2 ) displays a modest conductivity. The conductance in the cis form of the polymer is significantly higher compared to the trans form. Upon exposure to visible light, the cis form undergoes photoisomerization to the trans form with a drastic drop in the electronic mobility. The trans form can be reverted to the cis form thermally or by using UV light. Thus, this system offers a reversible control of the conductivity using light.     

Switching Diarylethenes Reliably in Both Directions with Visible Light

A diarylethene photoswitch was covalently connected to two small triplet sensitizer moieties in a conjugated and nonconjugated fashion and the photochromic performance of the resulting compounds was investigated. In comparison with the parent diarylethene (without sensitizers) and one featuring saturated linkages, the conjugated photoswitch offers superior fatigue resistance upon visible‐light excitation due to effective triplet energy transfer from the biacetyl termini to the diarylethene core. Our design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion based on extending π‐conjugation and by‐product‐free ring‐closure via the triplet manifold.     

Organocatalytic Transformation of Aldehydes to Thioesters with Visible Light

A metal- and oxidant-free catalytic method for accessing structurally diverse thioesters from readily accessible, widespread aldehydes, is described. A strategy of a simple organic 9,10-phenanthrenequinone-promoted hydrogen atom transfer (HAT) with visible light was successfully implemented to selectively generate acyl radicals without inducing crossover reactivity of thioester products. The preparative power of the method was demonstrated by broad substrate scope and wide functional group tolerance, and enabled the late-stage modification of complex structures, which are difficult to achieve with the existing protocols.     

Size dependence of the folding of multiply charged sodium cationized polylactides revealed by ion mobility mass spectrometry and molecular modelling

Ion mobility spectrometry coupled with mass spectrometry was used to experimentally determine the three-dimensional structure of multiply charged sodium cationized polylactides (PLA). In particular, the experiments were conducted to evaluate the influence of the charge state and the size on the gas-phase conformation of cationized PLA. The measured collision cross sections were then compared to calculated values obtained by computational chemistry methods. The most striking feature was the experimental and theoretical observation of a breaking point in the quasilinear relationship between the average collision cross sections and the number of monomer units for the triply charged cations. This breaking point was theoretically demonstrated, for the doubly and triply charged cations, to be associated with a significant folding of the polymer chains around the cationizing agents. The occurrence of such breaking points could be exploited to correlate the charge state of the most intense ion series observed upon electrospray ionization with the number-average molecular mass of a polymer.     

Catalysis with Palladium Complexes Photoexcited by Visible Light

Palladium catalysis induced by visible light is an emerging field of catalysis. In contrast to classical reactions catalyzed by Pd complexes in the ground state, which mostly proceed through two‐electron redox processes, the mechanisms of these new methods based on photoexcited Pd complexes usually operate through transfer of a single electron. Such processes lead to putative hybrid Pd/radical species, which exhibit both radical and classical Pd‐type reactivity. This Minireview highlights the recent progress in this rapidly growing area.     

Ion mobility‐mass spectrometry for the separation and analysis of procyanidins

Procyanidins are polymeric flavan‐3‐ones occurring in many plants with antioxidant and other beneficial bioactivities. They are composed of catechin and epicatechin monomeric units connected by single carbon‐carbon B‐type linkages or A‐type linkages containing both carbon‐carbon and carbon‐oxygen‐carbon bonds. Their polymeric structure makes analysis of procyanidin mixtures always difficult. Evaluation of procyanidins according to degree of polymerization (DP) using high‐performance liquid chromatography (HPLC) is time‐consuming and at best has resolved polymeric families up to DP‐17. To expedite studies of procyanidins, the utility of positive ion electrospray ion mobility‐mass spectrometry (IM‐MS) was investigated for the rapid separation and characterization of procyanidins in mixtures. Applying IM‐MS to analyse structurally defined standards containing up to five subunits, procyanidins could be resolved in less than 6 ms not only by degree of polymerization but also by linkage type. A‐type procyanidins could be resolved from B‐type and both could be at least partially resolved from mixed‐type procyanidins of the same DP. IM‐MS separated higher order procyanidins with DP of at least 24 from extracts of cranberry. As DP increased, the abundances of multiply‐charged procyanidins also increased. During IM‐MS of ions of similar m/z, the ion drift times decreased inversely with increasing charge state. Therefore, IM‐MS was shown to separate mixtures of procyanidins containing at least 24 interconnected subunits in less than 16 ms, not only according to DP, but also according to linkage type between subunits and charge state.     

Identification of Isomeric N-Glycans by Conformer Distribution Fingerprinting using Ion Mobility Mass Spectrometry

Glycans possess unparalleled structural complexity arising from chemically similar monosaccharide building blocks, configurations of anomeric linkages and different branching patterns, potentially giving rise to many isomers. This level of complexity is one of the main reasons that identification of exact glycan structures in biological samples still lags behind that of other biomolecules. Here, we introduce a methodology to identify isomeric N-glycans by determining gas phase conformer distributions (CDs) by measuring arrival time distributions (ATDs) using drift-tube ion mobility spectrometry-mass spectrometry. Key to the approach is the use of a range of well-defined synthetic glycans that made it possible to investigate conformer distributions in the gas phase of isomeric glycans in a systematic manner. In addition, we have computed CD fingerprints by molecular dynamics (MD) simulation, which compared well with experimentally determined CDs. It supports that ATDs resemble conformational populations in the gas phase and offer the prospect that such an approach can contribute to generating a library of CCS distributions (CCSDs) for structure identification.     

Hydrosulfonylation of Alkenes with Sulfonyl Chlorides under Visible Light Activation

Sulfonyl chlorides are inexpensive reactants extensively explored for functionalization, but never considered for radical hydrosulfonylation of alkenes. Herein, we report that tris(trimethylsilyl)silane is an ideal hydrogen atom donor enabling highly effective photoredox‐catalyzed hydrosulfonylation of electron‐deficient alkenes with sulfonyl chlorides. To increase the generality of this transformation, polarity‐reversal catalysis (PRC) was successfully implemented for alkenes bearing alkyl substituents. This late‐stage functionalization method tolerates a remarkably wide range of functional groups, is operationally simple, scalable, and allows access to building blocks which are important for medicinal chemistry and drug discovery.     

Optical processing with photochromic switches

The absorbance, fluorescence, and refractive index of a photochromic material can be modulated under the influence of optical stimulations. The reversible modification of these macroscopic properties is a result of photoinduced transformations at the molecular level. These processes can be exploited to mediate the interplay of optical signals and offer the opportunity to design and implement photonic devices for optical processing based on molecular components.