Cu(I)-Glutathione Assembly Supported on ZIF-8 as Robust and Efficient Catalyst for Mild CO2 Conversions.

The Cu-glutathione (GSH) redox system, essential in biology, is designed here as a supramacromolecular assembly in which the tetrahedral 18e Cu(I) center loses a thiol ligand upon adsorption onto ZIF-8, as shown by EXAFS and DFT calculation, to generate a very robust 16e planar trigonal single-atom Cu(I) catalyst. Synergy between Cu(I) and ZIF-8, revealed by catalytic experiments and DFT, affords CO2 conversion into high-value-added chemicals with a wide scope of substrates by reaction with terminal alkynes or propargyl amines in excellent yields under mild conditions and reuse at least 10 times without significant decrease in catalytic efficiency.

Photochromic rotaxanes and pseudorotaxanes.

Among stimulus-responsive molecular ring-on-thread rotaxanes and pseudorotaxanes, variants incorporating photochromic sub-units are attracting considerable attention as their properties and structure can be remotely and precisely controlled, additionally without producing chemical waste. The focus herein is on photoswitching-driven assembly/disassembly and modulation of properties resulting from light-activated isomerization or changes in electronic properties.

Photoreversible stretching of a BAPTA chelator marshalling Ca2+-binding in aqueous media.

Free calcium ion concentration is known to govern numerous biological processes and indeed calcium acts as an important biological secondary messenger for muscle contraction, neurotransmitter release, ion-channel gating, and exocytosis. As such, the development of molecules with the ability to instantaneously increase or diminish free calcium concentrations potentially allows greater control over certain biological functions. In order to permit remote regulation of Ca2+, a selective BAPTA-type synthetic receptor / host was integrated with a photoswitchable azobenzene motif, which upon photoirradiation would enhance (or diminish) the capacity to bind calcium upon acting on the conformation of the adjacent binding site, rendering it a stronger or weaker binder. Photoswitching was studied in pseudo-physiological conditions (pH 7.2, [KCl] = 100 mM) and dissociation constants for azobenzene cis- and trans-isomers have been determined (0.230 µM and 0.102 µM, respectively). Reversible photoliberation/uptake leading to a variation of free calcium concentration in solution was detected using a fluorescent Ca2+ chemosensor.

Synthetic water soluble di-/tritopic molecular receptors exhibiting Ca2+/Mg2+ exchange.

Structural integration of two synthetic water soluble receptors for Ca2+ and Mg2+, namely 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and o-aminophenol-N,N,O-triacetic acid (APTRA), respectively, gave novel di- and tritopic ionophores (1 and 2). As Mg2+ and Ca2+ cannot be simultaneously complexed by the receptors, allosteric control of complexation results. Potentiometric measurements established stepwise protonation constants and showed high affinity for Ca2+ (log K = 6.08 and 8.70 for 1 and 2, respectively) and an excellent selectivity over Mg2+ (log K = 3.70 and 5.60 for 1 and 2, respectively), which is compatible with magnesium-calcium ion exchange. While ion-exchange of a single Mg2+ for a single Ca2+ is possible in both 1 and 2, the simultaneous binding of two Mg2+ by 2 appears prohibitive for replacement of these two ions by a single Ca2+. Ion-binding and exchange was further rationalized by DFT calculations.

Harnessing Reversible Electronic Energy Transfer: From Molecular Dyads to Molecular Machines.

Reversible electronic energy transfer (REET) may be instilled in bi-/multichromophoric molecule-based systems, following photoexcitation, upon judicious structural integration of matched chromophores. This leads to a new set of photophysical properties for the ensemble, which can be fully characterized by steady-state and time-resolved spectroscopic methods. Herein, we take a comprehensive look at progress in the development of this type of supermolecule in the last five years, which has seen systems evolve from covalently tethered dyads to synthetic molecular machines, exemplified by two different pseudorotaxanes. Indeed, REET holds promise in the control of movement in molecular machines, their assembly/disassembly, as well as in charge separation.

Second-Order Nonlinear Optical Properties of a Dithienylethene-Indolinooxazolidine Hybrid: A Joint Experimental and Theoretical Investigation.

The nonlinear optical (NLO) properties of a double photochrome molecular switch are reported for the first time by considering the four trans forms of a dithienylethene-indolinooxazolidine hybrid. The four forms are characterized by means of hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations. Experimental measurements provide evidence that the pH- and light-triggered transformations between the different forms of the hybrid are accompanied by large variations of the first hyperpolarizability, which makes this compound an effective multistate NLO switch. Quantum chemical calculations conducted at the time-dependent Hartree-Fock and time-dependent DFT levels agree with the experimental data and allow a complete rationalization of the NLO responses of the different forms. The HRS signal of the forms with an open indolinooxazolidine moiety are more than one order of magnitude larger than that measured for the other forms, whereas the open/closed status of the dithienylethene subunit barely influences the dynamic NLO properties. However, extrapolation of the NLO responses to the static limit leads to univocally distinguishable intrinsic responses for three of the various forms. This hybrid system thus acts as a highly efficient multistate NLO switch for eventual exploitation in optical memory systems with multiple storage and nondestructive readout capacity.

A simple molecule-based octastate switch.

Dithienylethene oxazolidine hybrid system connected through an isomerizable double bond exists under eight molecular states on demand. Combinations of electrocyclization of dithienylethene, Z/E isomerization and acid-base oxazolidine change cause selective addressabilities. Two intricate gated photochromic performances allow the execution of an 8-step molecular switch, which renders this molecular system the most complex known up to date.

Design and characterization of molecular nonlinear optical switches.

Nanoscale structures, including molecules, supramolecules, polymers, functionalized surfaces, and crystalline/amorphous solids, can commute between two or more forms, displaying contrasts in their nonlinear optical (NLO) properties. Because of this property, they have high potential for applications in data storage, signal processing, and sensing. As potential candidates for integration into responsive materials, scientists have been intensely studying organic and organometallic molecules with switchable first hyperpolarizability over the past two decades. As a result of this, researchers have been able to synthesize and characterize several families of molecular NLO switches that differ by the stimulus used to trigger the commutation. These stimuli can include light irradiation, pH variation, redox reaction, and ion recognition, among others. The design of multistate (including several switchable units) and multifunctional (triggered with different stimuli) systems has also motivated a large amount of work, aiming at the improvement of the storage capacity of optical memories or the diversification of the addressability of the devices. In complement to the synthesis of the compounds and the characterization of their NLO responses by means of hyper-Rayleigh scattering, quantum chemical calculations play a key role in the design of molecular switches with high first hyperpolarizability contrasts. Through the latter, we can gain a fundamental understanding of the various factors governing the efficiency of the switches. These are not easily accessible experimentally, and include donor/acceptor contributions, frequency dispersion, and solvent effects. In this Account, we illustrate the similarities of the experimental and theoretical tools to design and characterize highly efficient NLO switches but also the difficulties in comparing them. After providing a critical overview of the different theoretical approaches used for evaluating the first hyperpolarizabilities, we report two case studies in which theoretical simulations have provided guidelines to design NLO switches with improved efficiencies. The first example presents the joint theoretical/experimental characterization of a new family of multi-addressable NLO switches based on benzazolo-oxazolidine derivatives. The second focuses on the photoinduced commutation in merocyanine-spiropyran systems, where the significant NLO contrast could be exploited for metal cation identification in a new generation of multiusage sensing devices. Finally, we illustrate the impact of environment on the NLO switching properties, with examples based on the keto-enol equilibrium in anil derivatives. Through these representative examples, we demonstrate that the rational design of molecular NLO switches, which combines experimental and theoretical approaches, has reached maturity. Future challenges consist in extending the investigated objects to supramolecular architectures involving several NLO-responsive units, in order to exploit their cooperative effects for enhancing the NLO responses and contrasts.

Synthesis of symmetrical and nonsymmetrical bisthienylcyclopentenes.

Diarylethenes possess unique structural properties, which enabled them to find widespread applications in the field of photochromism. Nowadays, bisthienylcyclopentenes (BTCs) present the most popular subfamily of these compounds, which are widely used as P-type chromophores. This minireview summarises the main strategies for the synthesis of symmetrical and nonsymmetrical BTCs. In addition, attention is drawn to desymmetrisations achieved by monosubstitutions, which is not frequently utilised, although it can be highly advantageous. This is supported with some of the authors' latest results.

Facile MOF Support Improvement in Synergy with Light Acceleration for Efficient Nanoalloy-Catalyzed H2 Production from Formic Acid.

Hydrogen (H2) generation and storage are actively investigated to provide a green source of energy, and formic acid (HCOOH), a major product obtained from the biomass, is regarded as a productive source of H2. Therefore, improvements in heterogeneous catalysts are called for. Here, a novel type of catalyst support is proposed involving simple addition of the mixture of metal ion precursors to core-shell ZIF-8@ZIF-67, followed by reduction with NaBH4, with performances surpassing those obtained using nanocatalysts in ZIF-8 or ZIF-67. The nanocatalysts PdxAg were optimized with ZIF-8@Pd2Ag1@ZIF-67 under visible-light illumination for selective HCOOH dehydrogenation involving a turnover frequency value of 430 h-1 under light irradiation at 353 K. These results also reveal the crucial roles of the Pd sites electronically promoted in the presence of visible light by the Ag plasmon resonance and the advantageous core-shell MOF structure. In order to examine the potential of extending this catalyst improvement principle to other catalytic reactions, 4-nitrophenol reduction, a benchmarking model of catalytic reaction, was tested, and the results also confirmed the superiority of the performance of ZIF-8@Pd2Ag1@ZIF-67 over Pd2Ag1@ZIF-8 and Pd2Ag1@ZIF-67, confirming the interest in the novel catalyst design.

Nonlinear optical molecular switches as selective cation sensors.

This work demonstrates that the recognition of cations by molecular switches can give rise to large contrasts of the second-order nonlinear optical (NLO) properties, which can therefore be used as a powerful and multi-usage detection tool. The proof of concept is given by evidencing, by means of ab initio calculations, the ability of spiropyran/merocyanine systems to selectively detect alkali, alkaline earth, and transition-metal cations.

Synthesis of Oxazino[4,3-a]indoles and biological applications.

This review brings together the various pathways to the oxazino[4,3-a]indole motif over the last decades. Representative examples showing the scope of these processes will illustrate the synthetic pathways and the biological activity of the synthesized oxazinoindoles will be mentioned wherever possible.

Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane.

We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic "reservoir" chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of cation binding. Modulation of REET processes represents an unexplored mechanism in luminescent molecular sensor development.

Photochromic performance of a dithienylethene-indolinooxazolidine hybrid.

The synthesis and the photochromic performance of a single-molecule system based on the covalent combination of dithienylperfluorocyclopentene and indolino[2,1-b]oxazolidine are described. The photoinduced interconversion between four states was shown to be deeply dependent on solvent and can be selectively controlled by adjusting the irradiation wavelength in chlorobenzene solutions. The color can be switched between colorless, yellow, pink and green on demand. This biphotochromic hybrid compound, despite poor fatigue resistance properties of the most conjugated colored isomer, displays large contrasts in structure, switching between zwitterionic and neutral character, independent chromophores and highly conjugated pi systems.

Visible-Light Acceleration of H2 Evolution from Aqueous Solutions of Inorganic Hydrides Catalyzed by Gold-Transition-Metal Nanoalloys.

Production of hydrogen (H2) upon hydrolysis of inorganic hydrides potentially is a key step in green energy production. We find that visible-light irradiation of aqueous solutions of ammonia-borane (AB) or NaBH4 containing "click"-dendrimer-stabilized alloyed nanocatalysts composed of nanogold and another late transition-metal nanoparticle (LTMNP) highly enhances catalytic activity for H2 generation while also inducing alloy to Au core@M shell nanocatalyst restructuration. In terms of visible-light-induced acceleration of H2 production from both AB and NaBH4, the Au1Ru1 alloy catalysts show the most significant light-boosting effect. Au-Rh and Au-PtNPs are also remarkable with total H2 release time from AB and NaBH4 down to 1.3 min at 25 °C (AuRh), 3 times less than in the dark, and Co is the best earth-abundant metal alloyed with nanogold. This boosting effect is explained by the transfer of plasmon-induced hot electron from the Au atoms to the LTMNP atoms facilitating water O-H oxidative addition on the LTMNP surface, as shown by the large primary kinetic isotope effect kH/kD upon using D2O obtained for both AB and NaBH4. The second simultaneous and progressive effect of visible-light irradiation during these reactions, alloy to Au core@M shell restructuration, enhances the catalytic activity in the recycling, because, in the resulting Au core@M shell, the surface metal (such as Ru) is much more active than the original Au-containing alloy surface in dark reactions. There is no light effect on the rate of hydrogen production for the recycled nanocatalyst because of the absence of Au on the NP surface, but it is still very efficient in hydrogen release during four cycles because of the initial light-induced restructuration, although it is slightly less efficient than the original nanoalloy in the presence of light. The dendritic triazole coordination on each LTMNP surface appears to play a key role in these remarkable light-induced processes.

Two-way molecular switches with large nonlinear optical contrast.

Molecular switches: Highly efficient acido- and photoswitchable frequency doublers (see scheme) based on the indolinooxazolidine core are studied by means of hyper-Rayleigh experiments and quantum-chemical calculations.To optimize the nonlinear optical (NLO) contrast, a series of indolinooxazolidine derivatives with electron-withdrawing substituents in the para position on the indolinic residue have been synthesized. Their linear and nonlinear optical properties have been characterized by UV-visible absorption and hyper-Rayleigh scattering measurements, as well as by ab initio calculations. The two-way photo- or pH-triggered switching mechanism has been demonstrated by comparing the absorption spectra of the zwitterionic and protonated open forms (POF). Hyper-Rayleigh measurements have revealed that the second-order NLO contrast between the closed indolinooxazolidine and the open pi-conjugated colored forms remain very large upon substitution. Theory and measurements show that for the POFs the amplitude of the first hyperpolarizability follows the Hammett parameters of the withdrawing groups. However, because the measurements are performed in resonance, to recover this behavior, elaborate procedures including homogeneous and inhomogeneous broadenings, as well as single-mode vibronic structures are necessary to extrapolate to the static limit.

Shear-induced fractures and three-dimensional motions in an organogel.

The flow behavior of a viscoelastic organogel is investigated using ultrasonic velocimetry combined with rheometry. Our gel presents a decreasing flow curve, i.e., the measured stress decreases as a function of the applied shear rate. Strikingly, we note that the local flow curve calculated from the velocity profiles also exhibits a decreasing part. We attribute this regime to the presence of a fracturing process and three-dimensional motions in the bulk of the sample.

Acido-triggered nonlinear optical switches: benzazolo-oxazolidines.

This work is the continuation of our previous experimental and theoretical studies aiming at designing efficient nonlinear optical (NLO) switches derived from the benzazolo-oxazolidine core. Here, we report the synthesis and the characterization of the linear and nonlinear optical properties of benzothiazolo[2,3-b]oxazolidine acidochromes by means of hyper-Rayleigh scattering as well as quantum chemical calculations. It is shown that these new derivatives incorporating a benzothiazole subunit exhibit very high static first hyperpolarizability values in their acido-generated form. On the basis of previously reported NLO responses of indolino- and benzimidazolo-oxazolidines, structure-properties relationships within the benzazolo-oxazolidine series are proposed.

Acidoswitchable NLO-phores: benzimidazolo[2,3-b]oxazolidines.

This paper presents a series of acidoswitchable NLO-phores combining the 9-methylbenzimidazolo[2,3-b]oxazolidine core with various pi systems such as phenylethenyl, phenylethynyl, and naphthylethenyl. All the prepared derivatives are shown to display acidochromic behavior at ambient temperature. The remarkable contrast in the NLO response along the reversible transformations observed in HRS experiments is rationalized by high level theoretical calculations.