Subphthalocyanines as fluorescence sensors for metal cations.

Subphthalocyanines (SubPcs) and their aza-analogues (SubTPyzPzs) are fluorophores with strong orange fluorescence emission; however, their sensing ability towards metal cations remains uncharted. To fill this gap, we have developed an efficient method for introducing aza-crown moieties at the axial position of SubPcs and SubTPyzPzs to investigate the structure-activity relationship for sensing alkali (Li+, Na+, K+) and alkaline earth metal (Ca2+, Mg2+, Ba2+) cations. SubPcs showed better photostability than SubTPyzPzs and even a commonly utilized dye, 6-carboxyfluorescein. Selectivity toward metal cations was driven by the size of the aza-crown, irrespective of the counter anion. The stoichiometry of binding was found to be 1 : 1 in all cases, and the interaction between SubPcs and cations was characterized by the corresponding apparent binding constants (Ka). Notably, an unusually strong interaction of all sensoric SubPcs with Ba2+ compared to other studied cations was demonstrated. The role of the surrounding environment, i.e. the addition of water or methanol, in sensing cations is shown in detail as well. Selectivity towards K+ over Na+ was demonstrated in aqueous media with SubPcs bearing the 1-aza-6-crown-18 moiety in Tween 80 micelles. In this case, a 5-fold increase of the fluorescence quantum yield was observed upon binding K+ ions. The high brightness, photostability, and sensing activity in aqueous media make SubPc macrocycles promising fluorophores for metal cation sensing.

Kinetic study of NADPH activation using ubiquinone-rhodol fluorescent probe and an IrIII-complex promoter at the cell interior.

Nicotine adenine dinucleotide derivatives NADH and NADPH are intimately involved in energy and electron transport within cells. The fluorescent ubiquinone-rhodol (Q-Rh) probe is used for NADPH activation monitoring. Q-Rh reacts with NADPH yielding its quenched hydroquinone-rhodol (H2Q-Rh) form with concurrent NADPH activation (i.e. NADP+ formation). NADPH activation can be enhanced by the addition of an IrIII-complex (i.e. [(η5-C5Me5)Ir(phen)(H2O)]2+) as a promoter. The rate of the Q-Rh fluorescence quenching process is proportional to the NADPH activation rate, which can be used to monitor NADPH. Experiments were performed in phosphate-buffered saline (PBS) solution and on HeLa cell cultures to analyze the kinetics of Q-Rh reduction and the influence of the IrIII-complex promoter on the activation of NADPH (in PBS) and of other intracellular reducing agents (in HeLa cells). There is a substantial increase in Q-Rh reduction rate inside HeLa cells especially after the addition of IrIII-complex promoter. This increase is partly due to a leakage process (caused by IrIII-complex-induced downstream processes which result in cell membrane disintegration) but also involves the nonspecific activation of other intracellular reducing agents, including NADH, FADH2, FMNH2 or GSH. In the presence only of Q-Rh, the activation rate of intracellular reducing agents is 2 to 8 times faster in HeLa cells than in PBS solution. When both Q-Rh and IrIII-complex are present, the rate of the IrIII-complex catalyzed reduction reaction is 7 to 23 times more rapid in HeLa cells. Concentration- and time-dependent fluorescence attenuation of Q-Rh with third-order reaction kinetics (reasonably approximated as pseudo-first-order in Q-Rh) has been observed and modelled. This reaction and its kinetics present an example of "bioparallel chemistry", where the activation of a molecule can trigger a unique chemical process. This approach stands in contrast to the conventional concept of "bioorthogonal chemistry", which refers to chemical reactions that occur without disrupting native biological processes.

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting.

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet-visible (UV-vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Selective Detection of Toxic C1 Chemicals Using a Hydroxylamine-Based Chemiresistive Sensor Array.

Formaldehyde (FA) is a deleterious C1 pollutant commonly found in the interiors of modern buildings. C1 chemicals are generally more toxic than the corresponding C2 chemicals, but the selective discrimination of C1 and C2 chemicals using simple sensory systems is usually challenging. Here, we report the selective detection of FA vapor using a chemiresistive sensor array composed of modified hydroxylamine salts (MHAs, ArCH2ONH2·HCl) and single-walled carbon nanotubes (SWCNT). By screening 32 types of MHAs, we have identified an ideal sensor array that exhibits a characteristic response pattern for FA. Thus, trace FA (0.02-0.05 ppm in air) can be clearly discriminated from the corresponding C2 chemical, acetaldehyde (AA). This system has been extended to discriminate methanol (C1) from ethanol (C2) in combination with the catalytic conversion of these alcohols to their corresponding aldehydes. Our system offers portable and reliable chemical sensors that discriminate the subtle differences between C1 and C2 chemicals, enabling advanced environmental monitoring and healthcare applications.

NMR lineshape analysis using analytical solutions of multi-state chemical exchange with applications to kinetics of host-guest systems.

Nuclear magnetic resonance (NMR) lineshape analysis is a powerful tool for the study of chemical kinetics. Here we provide techniques for analysis of the relationship between experimentally observed spin kinetics (transitions between different environments [Formula: see text]) and corresponding chemical kinetics (transitions between distinct chemical species; e.g., free host and complexed host molecule). The advantages of using analytical solutions for two-, three- or generally N-state exchange lineshapes (without J-coupling) over the widely used numerical calculation for NMR spectral fitting are presented. Several aspects of exchange kinetics including the generalization of coalescence conditions in two-state exchange, the possibility of multiple processes between two states, and differences between equilibrium and steady-state modes are discussed. 'Reduced equivalent schemes' are introduced for spin kinetics containing fast-exchanging states, effectively reducing the number of exchanging states. The theoretical results have been used to analyze a host-guest system containing an oxoporphyrinogen complexed with camphorsulfonic acid and several other literature examples, including isomerization, protein kinetics, or enzymatic reactions. The theoretical treatment and experimental examples present an expansion of the systematic approach to rigorous analyses of systems with rich chemical kinetics through NMR lineshape analysis.

Nonionic omnisoluble photosensitizer reference material for the estimation of singlet oxygen quantum yield.

Meso-Tetrakis-(3,4,5-tris{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}phenyl)porphyrin TEG12PH2 is reported as an 'omnisoluble' reference for singlet oxygen (1O2) generation quantum yield (ΦSO) estimation. TEG12PH2 is a highly soluble, nonionic compound possessing excellent 1O2 QY in a wide variety of common solvents, including water. TEG12PH2 was prepared on multigram scale by the 12-way O-alkylation of tetrakis(3,4,5-trihydroxyphenyl)porphyrin using 2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-toluenesulfonate as a reaction solvent. The corresponding Zn(II) complex TEG12PZn was also prepared and studied. The 1O2 QYs of TEG12PH2 in the different solvents studied were found to be 0.86 (acetone), 0.59 (acetonitrile), 0.66 (chloroform), 0.85 (methanol), 0.45 (toluene) and 0.51 (water). TEG12PH2 can be considered a reliable and easy to implement omnisoluble reference compound for the estimation of the 1O2 generating activities of new materials, especially new porphyrinic compounds.

Dynamics of Meso-Chiral Interconversion in a Butterfly-Shape Overcrowded Alkene Rotor Tunable by Solvent Properties.

Elucidation of dynamics of molecular rotational motion is an essential part and challenging area of research. We demonstrate reversible diastereomeric interconversion of a molecular rotor composed of overcrowded butterfly-shape alkene (FDF). Its inherent dual rotatory motion (two rotors, one stator) with interconversion between two diastereomers, chiral trans-FDF and meso cis-FDF forms, has been examined in detail upon varying temperatures and solvents. The free energy profile of 180° revolution of one rotor part has a bimodal shape with unevenly positioned maxima (transition states). FDF in aromatic solvents adopts preferentially meso cis-conformation, while in non-aromatic solvents a chiral trans-conformation is more abundant owing to the solvent interactions with peripheral hexyl chains (solvophobic effect). Moderate correlations between the trans-FDF/cis-FDF ratio and solvent parameters, such as refractive index, polarizability, and viscosity were found.

Pyrazinacenes exhibit on-surface oxidation-state-dependent conformational and self-assembly behaviours.

Acenes and azaacenes lie at the core of molecular materials' applications due to their important optical and electronic features. A critical aspect is provided by their heteroatom multiplicity, which can strongly affect their properties. Here we report pyrazinacenes containing the dihydro-decaazapentacene and dihydro-octaazatetracene chromophores and compare their properties/functions as a model case at an oxidizing metal substrate. We find a distinguished, oxidation-state-dependent conformational adaptation and self-assembly behaviour and discuss the analogies and differences of planar benzo-substituted decaazapentacene and octaazatetracene forms. Our broad experimental and theoretical study reveals that decaazapentacene is stable against oxidation but unstable against reduction, which is in contrast to pentacene, its C-H only analogue. Decaazapentacenes studied here combine a planar molecular backbone with conformationally flexible substituents. They provide a rich model case to understand the properties of a redox-switchable π-electronic system in solution and at interfaces. Pyrazinacenes represent an unusual class of redox-active chromophores.

Poly(N,N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure.

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N'-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N`-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van't Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

Enantiomeric Excess Dependent Splitting of NMR Signal through Dynamic Chiral Inversion and Coligand Exchange in a Coordination Complex.

Nuclear magnetic resonance (NMR) spectroscopy cannot be used to discriminate enantiomers, and NMR resonances of enantiomeric mixtures are generally not affected by enantiomeric excess (ee). Here, we report that a coordination complex (L·2Zn·3C), where L is a salen-like prochiral ligand and C is an exchangeable acetate coligand, exhibits symmetrical splitting of one of the 1H NMR resonances of L with the degree of splitting linearly proportional to ee of the chiral guest coligand C, 2-phenoxypropionic acid. Despite the well-defined chirality in the crystal structure of L·2Zn·3C, concurrent fast chiral inversion and coligand exchange in solution renders L·2Zn·3C the primary example of prochiral solvating agent (pro-CSA) based on a coordination complex. Notably, the NMR resonances remain split even in dilute solution due to the lack of chiral guest dissociation in the coligand exchange system. This work provides new insights into chiral transfer events in metal-ligand complexes.

Selective Phase Transfer Reagents (OxP-crowns) for Chromogenic Detection of Nitrates Especially Ammonium Nitrate.

Nitrogen and phosphorus-containing ions such as ammonium, nitrates and phosphates are anthropogenic pollutants while ammonium nitrate may be diverted for nefarious purposes in improvised explosive devices. Crown ether-oxoporphyrinogen conjugates (OxP-crowns) are used to selectively detect nitrates, especially their ion pairs with K+ and NH4 + , based on ion pair complexation of OxP-crowns under phase transfer conditions. The presence of phosphate and carbonate lead to deprotonation of OxP-crowns. OxP-1N18C6 is capable of extracting ion pairs with nitrate from aqueous phase leading to a selective chromogenic response. Deprotonation of the OxP moiety leads to [OxP- ]-1N18C6[K+ ] and is promoted by crown ether selective cation binding coupled with hydration of basic oxoanions, which are constrained to remain in the aqueous phase. This work illustrates the utility of molecular design to exploit partitioning and ion hydration effects establishing the selectivity of the chromogenic response.

Cascade Reaction-Based Chemiresistive Array for Ethylene Sensing.

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R0) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R0) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.

Nanomolecular singlet oxygen photosensitizers based on hemiquinonoid-resorcinarenes, the fuchsonarenes.

Singlet oxygen sensitization involving a class of hemiquinonoid-substituted resorcinarenes prepared from the corresponding 3,5-di-t-butyl-4-hydroxyphenyl-substituted resorcinarenes is reported. Based on variation in the molecular structures, quantum yields comparable with that of the well-known photosensitizing compound meso-tetraphenylporphyrin were obtained for the octabenzyloxy-substituted double hemiquinonoid resorcinarene reported herein. The following classes of compounds were studied: benzyloxy-substituted resorcinarenes, acetyloxy-substituted resorcinarenes and acetyloxy-substituted pyrogallarenes. Single crystal X-ray crystallographic analyses revealed structural variations in the compounds with conformation (i.e., rctt, rccc, rcct) having some influence on the identity of hemiquinonoid product available. Multiplicity of hemiquinonoid group affects singlet oxygen quantum yield with those doubly substituted being more active than those containing a single hemiquinone. Compounds reported here lacking hemiquinonoid groups are inactive as photosensitizers. The term 'fuchsonarene' (fuchson + arene of resorcinarene) is proposed for use to classify the compounds.

Amphiprotism-Coupled Near-Infrared Emission in Extended Pyrazinacenes Containing Seven Linearly Fused Pyrazine Units.

Peripherally substituted tetradecaazaheptacene (N14Hp) compounds, exhibiting amphiprotism-coupled emission, have been synthesized. X-ray crystallography reveals a planar acene-like chromophore, and electronic absorption and emission occur in the near-infrared biological transparency window (650-900 nm). The compounds exhibit long-wavelength emission with photoluminescence quantum yields ΦPL up to ∼0.61 at 686 nm, with the monodeprotonated state ΦPL ≈ 0.58 at 712 nm. This unprecedented highly nitrogenous chromophore illustrates the stability and utility of the pyrazinacenes for different applications based on their photophysical properties and chemical structures.

Knock-on synthesis of tritopic calix[4]pyrrole host for enhanced anion interactions.

Interactions of anionic guests with a tritopic peripherally functionalized conjugated calix[4]pyrrole host (1) prepared using a regioselective synthetic method is reported. The regioselectivity of synthesis relies on selective N-alkylation of the calix[4]pyrrole caused by peripheral substitution of one pyrrole group with subsequent N-alkylation at the opposing pyrrole group termed by us 'knock-on' regioselectivity. The resulting host molecule exhibits anion interactions with common chloride and nitrate anions enhanced by an order of magnitude over the parent conjugated calix[4]pyrrole. Combined analysis of 1H NMR and UV-vis spectroscopic titration data enabled an evaluation of binding strengths of anions with the host KA in a binding model where the salt dissociation process is also incorporated in the form of its dissociation constant Kd. Anions could be classified as two types based on their interactions with 1: Type A anions (chloride, nitrate, perchlorate, hydrogensulphate) associate as 1 : 1 complexes through hydrogen bonding while interactions involving Type B anions (acetate, fluoride, dihydrogenphosphate) are complicated by host deprotonation and/or countercation association. Hosts based on rim-functionalized calix[4]pyrroles such as 1 represent a promising new family of chromophores for estimation of biologically relevant anions or other species.

Multimodal switching of a redox-active macrocycle.

Molecules that can exist in multiple states with the possibility of toggling between those states based on different stimuli have potential for use in molecular switching or sensing applications. Multimodal chemical or photochemical oxidative switching of an antioxidant-substituted resorcinarene macrocycle is reported. Intramolecular charge-transfer states, involving hemiquinhydrones are probed and these interactions are used to construct an oxidation-state-coupled molecular switching manifold that reports its switch-state conformation via striking variation in its electronic absorption spectra. The coupling of two different oxidation states with two different charge-transfer states within one macrocyclic scaffold delivers up to five different optical outputs. This molecular switching manifold exploits intramolecular coupling of multiple redox active substituents within a single molecule.

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore.

Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) by structural modification. N-Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole-type binding sites of OxP and serial variation of the chromic properties of the products, double N-benzylated Bz2OxP and tetra N-benzylated Bz4OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz2OxP, and Bz4OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding-site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

NMR Spectroscopic Determination of Enantiomeric Excess Using Small Prochiral Molecules.

The use of chiral auxiliaries, which derivatize enantiomers to diastereomers, is an established technique for NMR spectroscopic analysis of chirality and enantiomeric excess ( ee). Here we report that some small prochiral molecules exhibit ee-dependent splitting of 1H NMR signals at room temperature based on acid/base interactions with chiral analytes, especially when either a chiral or prochiral acid contains a phenoxy group at the α-position of the carboxylic acid. As a representative case, the benzylamine (BA)/2-phenoxylpropionic acid (PPA) complex was comprehensively investigated by using various methods. Notably, X-ray crystallographic analysis shows that there are multipoint interactions in the BA/PPA complex, implying that "fixing" of molecular conformation is critical for efficient intermolecular transfer of magnetic anisotropy. Our results suggest that a wide range of prochiral molecules are available for NMR determination of ee when intermolecular interactions between prochiral molecules and chiral analytes are adequately designed.

Improving the Colloidal Stability of Temperature-Sensitive Poly(N-isopropylacrylamide) Solutions Using Low Molecular Weight Hydrophobic Additives.

Poly(N-isopropylacrylamide) (PNIPAM) is an important polymer with stimuli-responsive properties, making it suitable for various uses. Phase behavior of the temperature-sensitive PNIPAM polymer in the presence of four low-molecular weight additives tert-butylamine (t-BuAM), tert-butyl alcohol (t-BuOH), tert-butyl methyl ether (t-BuME), and tert-butyl methyl ketone (t-BuMK) was studied in water (D2O) using high-resolution nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering. Phase separation was thermodynamically modeled as a two-state process which resulted in a simple curve which can be used for fitting of NMR data and obtaining all important thermodynamic parameters using simple formulas presented in this paper. The model is based on a modified van't Hoff equation. Phase separation temperatures T p and thermodynamic parameters (enthalpy and entropy change) connected with the phase separation of PNIPAM were obtained using this method. It was determined that T p is dependent on additives in the following order: T p(t-BuAM) > T p(t-BuOH) > T p(t-BuME) > T p(t-BuMK). Also, either increasing the additive concentration or increasing pK a of the additive leads to depression of T p. Time-resolved 1H NMR spin-spin relaxation experiments (T 2) performed above the phase separation temperature of PNIPAM revealed high colloidal stability of the phase-separated polymer induced by the additives (relative to the neat PNIPAM/D2O system). Small quantities of selected suitable additives can be used to optimize the properties of PNIPAM preparations including their phase separation temperatures, colloidal stabilities, and morphologies, thus improving the prospects for the application.

Metallic versus Semiconducting SWCNT Chemiresistors: A Case for Separated SWCNTs Wrapped by a Metallosupramolecular Polymer.

As-synthesized single-walled carbon nanotubes (SWCNTs) are a mixture of metallic and semiconducting tubes, and separation is essential to improve the performances of SWCNT-based electric devices. Our chemical sensor monitors the conductivity of an SWCNT network, wherein each tube is wrapped by an insulating metallosupramolecular polymer (MSP). Vapors of strong electrophiles such as diethyl chlorophosphate (DECP), a nerve agent simulant, can trigger the disassembly of MSPs, resulting in conductive SWCNT pathways. Herein, we report that separated SWCNTs have a large impact on the sensitivity and selectivity of chemical sensors. Semiconducting SWCNT (S-SWCNT) sensors are the most sensitive to DECP (up to 10000% increase in conductivity). By contrast, the responses of metallic SWCNT (M-SWCNT) sensors were smaller but less susceptible to interfering signals. For saturated water vapor, increasing and decreasing conductivities were observed for S- and M-SWCNT sensors, respectively. Mixtures of M- and S-SWCNTs revealed reduced responses to saturated water vapor as a result of canceling effects. Our results reveal that S- and M-SWCNTs compensate sensitivity and selectivity, and the combined use of separated SWCNTs, either in arrays or in single sensors, offers advantages in sensing systems.