Photo-induced control of bubble domains in chiral–nematic liquid crystal by a violet laser beam

The stable bubble domains generated by mixing 10% of chiral molecules into an azobenzene liquid crystal (LC)-doped nematic host can be optically controlled by a violet laser beam (415 nm). The photon-induced reversible trans–cis photo-isomerisation of azobenzene changes the helical twisting power (HTP) of LC mixtures in which the HTP of cis-azobenzene LC is lower than trans-azobenzene LC. Under the irradiation of an optical field (>20 mW cm^???2), the helical pitch distance, which is inverted proportional to the HTP, increases and the bubble domains disappear. Immediate obstruction of laser light irradiation initiates cholesteric nucleation, merging of domains and the subsequent generation of stably dispersed bubble domains.     

The role of crosslinking and polarity in the dark relaxation of azobenzene-based,polymer-stabilised cholesteric liquid crystals

Prior examinations have reported that polymer stabilisation of azobenzene-based cholesteric liquid crystal (CLC) mixtures can reduce the time necessary for complete colour restoration in the dark from three days to as few as five minutes. This work extends upon these prior examinations by exploring and elucidating the role of crosslinker concentration and monomer polarity on the colour restoration of a representative CLC mixture composed of a high HTP bis(azo) binapthanyl chiral dopant (QL76) mixed into the cyanobiphenyl nematic liquid crystal host MDA-00-1444. The impact of these variables was unexpectedly convoluted. In all the formulations examined here, polymer stabilisation dramatically reduces the time for complete colour restoration of the starting reflection notch. In mixtures based on nonpolar liquid crystal monomers, increasing the crosslinker concentration reduces the time necessary for complete colour restoration. However, the dependence on crosslinker concentration reverses in mixtures composed from polar liquid crystal monomers in which increasing the crosslinker concentration serves to increase the time necessary for complete colour restoration.     

Synthesis of New Glucosylated Porphyrins Bearing an α‐d‐Linkage

This paper presents the synthesis of two new glucosyl tritolylporphyrins in which the carbohydrate moiety is connected through a carboxymethyl glycosidic α‐D‐linkage. These compounds have been obtained by reaction between porphyrins bearing an amino function with a lactone prepared from the available disaccharide isomaltulose. The photocytotoxicity of these compounds against K562 human chronic myelogenous leukemia cells has been evaluated in comparison to Photofrin II.     

Redox‐Switchable Ring‐Closing Metathesis: Catalyst Design,Synthesis, and Study

High yielding syntheses of 1‐(ferrocenylmethyl)‐3‐mesitylimidazolium iodide ( 1 ) and 1‐(ferrocenylmethyl)‐3‐mesitylimidazol‐2‐ylidene ( 2 ) were developed. Complexation of 2 to [{Ir(cod)Cl}₂] (cod=cis,cis‐1,5‐cyclooctadiene) or [Ru(PCy₃)Cl₂(?CH‐o‐O‐iPrC₆H₄)] (Cy=cyclohexyl) afforded 3 ([Ir( 2 )(cod)Cl]) and 5 ([Ru( 2 )Cl₂(?CH‐o‐O‐iPrC₆H₄)]), respectively. Complex 4 ([Ir( 2 )(CO)₂Cl]) was obtained by bubbling carbon monoxide through a solution of 3 in CH₂Cl₂. Spectroelectrochemical IR analysis of 4 revealed that the oxidation of the ferrocene moiety in 2 significantly reduced the electron‐donating ability of the N‐heterocyclic carbene ligand (ΔTEP=9 cm^?1; TEP=Tolman electronic parameter). The oxidation of 5 with [Fe(η⁵‐C₅H₄COMe)Cp][BF₄] as well as the subsequent reduction of the corresponding product [ 5 ][BF₄] with decamethylferrocene (Fc*) each proceeded in greater than 95 % yield. Mössbauer, UV/Vis and EPR spectroscopy analysis confirmed that [ 5 ][BF₄] contained a ferrocenium species, indicating that the iron center was selectively oxidized over the ruthenium center. Complexes 5 and [ 5 ][BF₄] were found to catalyze the ring‐closing metathesis (RCM) of diethyl diallylmalonate with observed pseudo‐first‐order rate constants (k_obs) of 3.1×10^?4 and 1.2×10^?5 s^?1, respectively. By adding suitable oxidants or reductants over the course of a RCM reaction, complex 5 was switched between different states of catalytic activity. A second‐generation N‐heterocyclic carbene that featured a 1′,2′,3′,4′,5′‐ pentamethylferrocenyl moiety ( 10 ) was also prepared and metal complexes containing this ligand were found to undergo iron‐centered oxidations at lower potentials than analogous complexes supported by 2 (0.30–0.36 V vs. 0.56–0.62 V, respectively). Redox switching experiments using [Ru( 10 )Cl₂(?CH‐o‐O‐iPrC₆H₄)] revealed that greater than 94 % of the initial catalytic activity was restored after an oxidation–reduction cycle.     

Acid–Base‐Controlled Stereoselective Metalation of Overhanging Carboxylic Acid Porphyrins: Consequences for the Formation of Heterobimetallic Complexes

Overhanging carboxylic acid porphyrins have revealed promising ditopic ligands offering a new entry in the field of supramolecular coordination chemistry of porphyrinoids. Notably, the adjunction of a so‐called hanging‐atop (HAT) Pb^II cation to regular Pb^II porphyrin complexes allowed a stereoselective incorporation of the N‐core bound cation, and an allosterically controlled Newton’s cradle‐like motion of the two Pb^II ions also emerged from such bimetallic complexes. In this contribution, we have extended this work to other ligands and metal ions, aiming at understanding the parameters that control the HAT Pb^II coordination. The nature of the N‐core bound metal ion (Zn^II, Cd^II), the influence of the deprotonation state of the overhanging COOH group and the presence of a neutral ligand on the opposite side (exogenous or intramolecular), have been examined through ¹H NMR spectroscopic experiments with the help of radiocrystallographic structures and DFT calculations. Single and bis‐strap ligands have been considered. They all incorporate a COOH group hung over the N‐core on one side. For the bis‐strap ligands, either an ester or an amide group has been introduced on the other side. In the presence of a base, the mononuclear Zn^II or Cd^II complexes incorporate the carbonyl of the overhanging carboxylate as apical ligand, decreasing its availability for the binding of a HAT Pb^II. An allosteric effector (e.g., 4‐dimethylaminopyridine (DMAP), in the case of a single‐strap ligand) or an intramolecular ligand (e.g., an amide group), strong enough to compete with the carbonyl of the hung COO^?, is required to switch the N‐core bound cation to the opposite side with concomitant release of the COO^?, thereby allowing HAT Pb^II complexation. In the absence of a base, Zn^II or Cd^II binds preferentially the carbonyl of the intramolecular ester or amide groups in apical position rather than that of the COOH. This better preorganization, with the overhanging COOH fully available, is responsible for a stronger binding of the HAT Pb^II. Thus, either allosteric or acid–base control is achieved through stereoselective metalation of Zn^II or Cd^II. In the latter case, according to the deprotonation state of the COOH group, the best electron‐donating ligand is located on one or the other side of the porphyrin (COO^?>CONHR>COOR>COOH): the lower affinity of COOH for Zn^II and Cd^II, the higher for a HAT Pb^II. These insights provide new opportunities for the elaboration of innovative bimetallic molecular switches.     

Peptide conjugated cellulose nanocrystals with sensitive human neutrophil elastase sensor activity

In chronic wounds, elevated human neutrophil elastase (HNE) is a destructive protease that has been proposed as a biomarker. Numerous wound dressing designs have been introduced in an effort to lower HNE levels. The clinical detection of HNE as a point of care biomarker or an in situ colorimetric adjuvant to chronic wound dressings presents potential advantages in the management of chronic wounds. A colorimetric approach to the detection of HNE using peptide conjugated cotton cellulose nanocrystals (CCN) is reported here. For this purpose a HNE tripeptide substrate, n-Succinyl-Alanine–Alanine-Valine-para-nitroanilide (Suc-Ala–Ala-Val-pNA), was covalently attached to glycine esterified CCN and compared with a similar tetrapeptide analog for colorimetric HNE sensor activity. Visible HNE activity was significantly higher on CCN tripeptide conjugates when compared with similar analogs synthesized on paper. Upon enzymatic release of para-nitroaniline (pNA) from the Glycine-CCN conjugate of succinyl-Ala–Ala-Val-pNA, amplification of the colorimetric response from pNA with reactive dyes enhanced visible absorption of the chromogen. Two color amplifying dyes that react with pNA were compared for their ability to enhance the visual sensor response to HNE activity. The colorimetric detection of HNE with CCN tripeptide conjugates was sensitive at HNE levels previously reported in chronic wound fluid (0.05 U/mL HNE). The HNE sensor and the chromogen amplifying dyes were interfaced with 50 and 10 kD dialysis cellulose membranes (DCM) to model filtration of HNE and chromogen (pNA) from a model wound dressing surface before and after sensor reactivity. The detection sensitivity to HNE activity was assessed with the CCN-tripeptide conjugate interfaced at the DCM surface distal and proximal to a dressing surface. The HNE sensor interfaced proximal to the dressing surface was most efficient with 10 kD membrane filtration of pNA and subsequent reaction with amplifying dyes. When interfaced with the 10 kD cellulose membrane, elastase sensor activity remained sensitive to 0.05 U/mL HNE. The nanocellulose surface properties, performance and design issues of the biosensor approach are discussed.     

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving C?H Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III O bonds.