Mild selective photochemical oxidation of an organic sulfide using OxP-polyimide porous polymers as singlet oxygen generators.

A series of porous organic polymers based on a singlet oxygen generating oxoporphyinogen ('OxP') has been successfully prepared from a pseudotetrahedral OxP-tetraamine precursor (OxP(4-NH2Bn)4) by its reaction with tetracarboxylic acid dianhydrides under suitable conditions. Of the compounds studied, those containing naphthalene (OxP-N) and perylene (OxP-P) spacers, respectively, have large surface areas (~530 m2 g-1). On the other hand, the derivative with a simple benzene spacer (OxP-B) exhibits the best 1O2 generating capability. Although the starting OxP-tetraamine precursor is a poor 1O2 generator, its incorporation into OxP POPs leads to a significant enhancement of 1O2 productivity, which is largely due to the transformation of NH2 groups to electron-withdrawing diimides. Overall 1O2 production efficacy of OxP-POPs under irradiation by visible light is significantly improved over the common reference material PCN-222. All the materials OxP-B, OxP-N and OxP-P promote oxidation of thioanisole involving conversion of ambient triplet state oxygen to singlet oxygen under visible light irradiation and its reaction with the sulfide. Although the reaction rate of the oxidation promoted by OxP POPs is generally lower than for conventional materials (such as PCN-222) or previously studied OxP derivatives, undesired overoxidation of the substrate to methyl phenyl sulfone is suppressed. For organic sulfides, selectivity of oxidation is especially important for detoxification of mustard gas (bis(2-chloroethyl)sulfide) or similarly toxic compounds since controlled oxidation leads to the low toxicity bis(2-chloroethyl)sulfoxide while overoxidation leads to intoxification (since bis(2-chloroethyl)sulfone presents greater toxicity to humans than the sulfide substrate). Therefore, OxP POPs capable of promoting selective oxidation of sulfides to sulfoxides have excellent potential to be used as mild and selective detoxification agents.

Oxoporphyrinogen (OxP) is a unique chromophore compound in that it is intrinsically de-aggregated allowing large quantum yields of singlet oxygen generation. Due to its structure, OxP is also an ideal building block for porous systems. In this work, we describe the first incorporation of OxP in highly stable microporous polymers strongly enhanced singlet oxygen generation for selective oxidation of organic sulfides to sulfoxides (as a model reaction) under heterogeneous conditions. The novelty of this work lies in the high stability and easy recovery of the materials, the synergetic enhancement of singlet oxygen generation in the polymers over the starting OxP, and the excellent selectivity for the oxidation reaction.

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.

Photosensitizer Encryption with Aggregation Enhanced Singlet Oxygen Production.

Chromophores that generate singlet oxygen (1O2) in water are essential to developing noninvasive disease treatments using photodynamic therapy (PDT). A facile approach for formation of stable colloidal nanoparticles of 1O2 photosensitizers, which exhibit aggregation enhanced 1O2 generation in water toward applications as PDT agents, is reported. Chromophore encryption within a fuchsonarene macrocyclic scaffold insulates the photosensitizer from aggregation induced deactivation pathways, enabling a higher chromophore density than typical 1O2 generating nanoparticles. Aggregation enhanced 1O2 generation in water is observed, and variation in molecular structure allows for regulation of the physical properties of the nanoparticles which ultimately affects the 1O2 generation. In vitro activity and the ability of the particles to pass through the cell membrane into the cytoplasm is demonstrated using confocal fluorescence microscopy with HeLa cells. Photosensitizer encryption in rigid macrocycles, such as fuchsonarenes, offers new prospects for the production of biocompatible nanoarchitectures for applications involving 1O2 generation.

De novo creation of a naked eye-detectable fluorescent molecule based on quantum chemical computation and machine learning.

Designing fluorescent molecules requires considering multiple interrelated molecular properties, as opposed to properties that straightforwardly correlated with molecular structure, such as light absorption of molecules. In this study, we have used a de novo molecule generator (DNMG) coupled with quantum chemical computation (QC) to develop fluorescent molecules, which are garnering significant attention in various disciplines. Using massive parallel computation (1024 cores, 5 days), the DNMG has produced 3643 candidate molecules. We have selected an unreported molecule and seven reported molecules and synthesized them. Photoluminescence spectrum measurements demonstrated that the DNMG can successfully design fluorescent molecules with 75% accuracy (n = 6/8) and create an unreported molecule that emits fluorescence detectable by the naked eye.

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.

Evaluation of the effects of natural isoquinoline alkaloids on low density lipoprotein receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9) in hepatocytes, as new potential hypocholesterolemic agents.

Nine different isoquinoline alkaloids, berberine, govaniadine, stylopine, adlumine, adlumidine, bicuculline, sanguinarine, protopine and californidine have been evaluated for their effects on a cellular model of hepatocyte for their effect on low density lipoprotein receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9) expression compared to simvastatin. Berberine, californidine and govaniadine induced LDLR with an effect similar to 2.5 µM simvastatin. Californidine and berberine at tested doses reduced the expression of PCSK9, with an opposite behaviour to simvastatin on this target. Govaniadine, on the other hand, showed a statin-like effect, although less potently, by increasing both LDLR and PCSK9 levels. Berberine californidine and govaniadine were then tested on the same cellular model to assess possible effect of reduction of total cholesterol, compared to simvastatin. All compounds were able to reduce total cholesterol level in the hepatocytes.

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.

Pushing property limits in materials discovery via boundless objective-free exploration.

Materials chemists develop chemical compounds to meet often conflicting demands of industrial applications. This process may not be properly modeled by black-box optimization because the target property is not well defined in some cases. Herein, we propose a new algorithm for automated materials discovery called BoundLess Objective-free eXploration (BLOX) that uses a novel criterion based on kernel-based Stein discrepancy in the property space. Unlike other objective-free exploration methods, a boundary for the materials properties is not needed; hence, BLOX is suitable for open-ended scientific endeavors. We demonstrate the effectiveness of BLOX by finding light-absorbing molecules from a drug database. Our goal is to minimize the number of density functional theory calculations required to discover out-of-trend compounds in the intensity-wavelength property space. Using absorption spectroscopy, we experimentally verified that eight compounds identified as outstanding exhibit the expected optical properties. Our results show that BLOX is useful for chemical repurposing, and we expect this search method to have numerous applications in various scientific disciplines.

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.

Electron and energy transfer in a porphyrin-oxoporphyrinogen-fullerene triad, ZnP-OxP-C60.

A multichromophoric triad, ZnP-OxP-C60 containing porphyrin (ZnTPP hereafter ZnP), oxoporphyrinogen (OxP) and fullerene (C60) has been synthesized to probe the intramolecular dynamics of its electron and energy transfer in relation to the presence of the closely linked electron deficient OxP-C60 'special pair', constructed as a mimic of the naturally occurring photosynthetic antenna-reaction center. The DFT optimized structure of the triad reveals the relative spatial remoteness of the ZnP entity with proximal OxP/C60 entities. Free-energetics of different energy and electron transfer events were estimated using spectral, computational and electrochemical studies, according to the Rehm-Weller approach. Femtosecond transient absorption spectral studies revealed energy transfer from 1ZnP* to OxP to yield ZnP-1OxP*-C60, and electron transfer to yield ZnP˙+-OxP-C60˙- and/or ZnP-OxP˙+-C60˙- charge seperated states. That is, the ZnP entity in the triad operates as both antenna and electron donor to generate relatively long-lived charge separated states thus mimicking the early photoevents of natural photosynthesis.

Rotaxanation as a sequestering template to preclude incidental metal insertion in complex oligochromophores.

The high yielding CuI-mediated click reaction is an effective procedure for the preparation of oligoporphyrinoid conjugates. However, the CuI catalyst leads to the adventitious and usually undesirable insertion of Cu ions into any non-metalated porphyrinoid centers during reaction. Here we report a "sacrificial rotaxane" strategy for the multifunctionalization of porphyrins with free base corroles without incidental copper insertion. This strategy can be considered a general method for implementing CuI-mediated click reactions with metal cation sequestration to avoid detrimental effects caused by the presence of copper cations.

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.

Supramolecular ultrafast energy and electron transfer in a directly linked BODIPY-oxoporphyrinogen dyad upon fluoride ion binding.

A directly linked BODIPY-oxoporphyrinogen dyad has been newly synthesized and occurrence of sequential photoinduced energy and electron transfer upon fluoride anion binding to oxoporphyrinogen has been demonstrated by spectral, electrochemical and femtosecond transient absorption studies.

Coherent narrowband light source for ultrafast photoelectron spectroscopy in the 17-31 eV photon energy range.

Here, we report on a novel narrowband High Harmonic Generation (HHG) light source designed for ultrafast photoelectron spectroscopy (PES) on solids. Notably, at 16.9 eV photon energy, the harmonics bandwidth equals 19 meV. This result has been obtained by seeding the HHG process with 230 fs pulses at 515 nm. The ultimate energy resolution achieved on a polycrystalline Au sample at 40 K is ∼22 meV at 16.9 eV. These parameters set a new benchmark for narrowband HHG sources and have been obtained by varying the repetition rate up to 200 kHz and, consequently, mitigating the space charge, operating with ≈ 3 × 10 7 electrons/s and ≈ 5 × 10 8 photons/s. By comparing the harmonics bandwidth and the ultimate energy resolution with a pulse duration of ∼105 fs (as retrieved from time-resolved experiments on bismuth selenide), we demonstrate a new route for ultrafast space-charge-free PES experiments on solids close to transform-limit conditions.

State-Selective Dynamics of TiO2 Charge-Carrier Trapping and Recombination.

Time-resolved pump-probe photoemission spectroscopy has been used to study the dynamics of charge-carrier recombination and trapping on hydroxylated rutile TiO2(110). Two types of pump excitation were employed, one in the infrared (IR) (0.95 eV) and the other in the ultraviolet (UV) (3.5 eV) region. With IR excitation, electrons associated with defects are excited into the bottom of the conduction band from the polaronic states within the band gap, which are retrapped within 45 ± 10 fs. Under UV excitation, the electrons in these band-gap states (BGSs) and valence-band electrons are excited into the conduction band. In addition to the fast polaron trapping observed with IR excitation, we also observe a long lifetime (∼1 ps) component for both the depletion of hot electrons at the bottom of the conduction band and the refilling of the BGS. This points to a BGS-mediated recombination process with a picosecond lifetime.

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.

Derivatisation of parthenolide to address chemoresistant chronic lymphocytic leukaemia.

Parthenolide is a natural product that exhibits anti-leukaemic activity, however, its clinical use is limited by its poor bioavailability. It may be extracted from feverfew and protocols for growing, extracting and derivatising it are reported. A novel parthenolide derivative with good bioavailability and pharmacological properties was identified through a screening cascade based on in vitro anti-leukaemic activity and calculated "drug-likeness" properties, in vitro and in vivo pharmacokinetics studies and hERG liability testing. In vitro studies showed the most promising derivative to have comparable anti-leukaemic activity to DMAPT, a previously described parthenolide derivative. The newly identified compound was shown to have pro-oxidant activity and in silico molecular docking studies indicate a prodrug mode of action. A synthesis scheme is presented for the production of amine 7 used in the generation of 5f.

Ethylenation of aldehydes to 3-propanal, propanol and propanoic acid derivatives.

Methodology has been developed for the synthesis of 3-propanaldehydes through a five-step process in 11-67% yield from aldehydes. Aldehydes were reacted with Meldrum's acid through a Knoevenagel condensation to give materials that upon reduction with sodium borohydride and subsequent hydrolysis decarboxylation generated the corresponding 3-propanoic acid derivatives. The -propanoic acid derivatives were reduced to give 3-propanol derivatives, which were readily oxidised to target 3-propanal derivatives.

Non-Band-Gap Photoexcitation of Hydroxylated TiO2.

The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge-transfer processes involved, partly because the excited states involved are difficult to study. Here we present evidence of the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ∼3.1 eV. The onset is shifted to lower energy by the coadsorption of molecular water, which suggests a means of tuning the energy of the excited state.