Heavy Metal-Based Toxic Oxo-Pollutants Sequestration by Advanced Functional Porous Materials for Safe Drinking Water.

ConspectusWater scarcity as a consequence of either environmental or economic actions is the most compelling global concern of the 21st century, as ∼2 billion people (26% of the total population) struggle to access safe drinking water and ∼3.6 billion (46% of the total population) lack access to clean water sanitation. In this context, groundwater pollution by toxic heavy metals and/or their oxo-pollutants, such as CrO42-, Cr2O72-, AsO43-, SeO32-, SeO42-, TcO4-, UO22+, etc., have been becoming rapidly growing global concerns. The severe toxicity upon bioaccumulation of these oxo-anions has prompted the US Environment Protection Agency (EPA) to mark these persistent and hazardous substances as priority pollutants. Additionally, the heavy-metal-based pollutants are difficult to transform into eco-friendly substances, thus presenting serious challenges toward human health and environmental preservation. To this end, the emergence of advanced functional porous materials (AFPMs), including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), metal-organic polyhedrons (MOPs), porous organic polymers (POPs), etc., have presented extraordinary opportunities in material research and water treatment applications. The liberty in designing and structural tunability of AFPMs, facilitated by utilization of structure-encoded molecular building blocks, enables precise control over target-specificity and structure-property correlations. Bridging the gap between strategic material design and on-demand real-world application can facilitate the development of next-generation sorbents/ion-exchangers for efficient water treatment.In this Account, we summarize the recent advancements from our group toward the development of cutting-edge multifunctional ionic-porous sorbents, offering viable solutions toward providing clean and safe drinking water. Our vision allows us to comprehend this challenge through two strategic factors: efficient oxo-anion capture via ion-exchange and specific host-guest interactions via installation of modular functional groups. To provide an overview, we first highlight the different structural variants and coexistance of various toxic oxo-anions depending on the pH of the medium and their adverse effects. Next, we highlight the promising potential of water stable cationic MOFs toward selective remediation of toxic Cr(VI), Mn(VII), Tc(VI), Se(IV), Se(VI), U (VI), As(III), and As(V)-based toxic oxo-pollutants from water. In the subsequent sections, we summarize the target-specific design strategies and oxo-anion remediation performances of ionic porous organic polymers and hybrid functional porous materials. The key role of target-specific designability and/or structural fine-tuning of AFPMs toward preferential sorption of oxo-pollutants is systematically demonstrate. Particularly, the role of ion-exchange (anion-exchange) processes toward targeted oxo-pollutant capture by ionic AFPMs has been discussed in details. In several examples, the AFPMs were successful in reducing the toxic oxo-anion concentration levels lower than the permitted values for drinking water by the World Health Organizing Committee (WHO), showcasing their real-world applicability potency.Our contemporaneous endeavors in exploring ionic AFPMs for selective toxic oxo-anion sequestration may serve as a blueprint to researchers for future development of the next generation sorbent materials for energy-economically feasible water treatment methods.

Nanotrap Grafted Anionic MOF for Superior Uranium Extraction from Seawater.

On-demand uranium extraction from seawater (UES) can mitigate growing sustainable energy needs, while high salinity and low concentration hinder its recovery. A novel anionic metal-organic framework (iMOF-1A) is demonstrated adorned with rare Lewis basic pyrazinic sites as uranyl-specific nanotrap serving as robust ion exchange material for selective uranium extraction, rendering its intrinsic ionic characteristics to minimize leaching. Ionic adsorbents sequestrate 99.8% of the uranium in 120 mins (from 20,000 ppb to 24 ppb) and adsorb large amounts of 1336.8 mg g-1 and 625.6 mg g-1 from uranium-spiked deionized water and artificial seawater, respectively, with high distribution coefficient, Kd U ≥ 0.97 × 106  mL g-1 . The material offers a very high enrichment index of ≈5754 and it achieves the UES standard of 6.0 mg g-1 in 16 days, and harvests 9.42 mg g-1 in 30 days from natural seawater. Isothermal titration calorimetry (ITC) studies quantify thermodynamic parameters, previously uncharted in uranium sorption experiments. Infrared nearfield nanospectroscopy (nano-FTIR) and tip-force microscopy (TFM) enable chemical and mechanical elucidation of host-guest interaction at atomic level in sub-micron crystals revealing extant capture events throughout the crystal rather than surface solely. Comprehensive experimentally guided computational studies reveal ultrahigh-selectivity for uranium from seawater, marking mechanistic insight.

Hemilabile Binding of Acetylene in an Amide-Rich Ultramicroporous MOF Enables Strong Acetylene Selectivity.

Thanks to a hemilabile amide-based binding site, a previously unreported amide-functionalized metal-organic framework (MOF) exhibits high acetylene affinity over ethylene, methane, and carbon dioxide, three-in-one.

Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin.

The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7-diethylaminothiocoumarin (TC1), a close analogue of the well-known probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1's potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule's photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations.

Transporting survival of an HIV clinical trial to the external target populations.

Due to the heterogeneity of the randomized controlled trial (RCT) and external target populations, the estimated treatment effect from the RCT is not directly applicable to the target population. For example, the patient characteristics of the ACTG 175 HIV trial are significantly different from that of the three external target populations of interest: US early-stage HIV patients, Thailand HIV patients, and southern Ethiopia HIV patients. This paper considers several methods to transport the treatment effect from the ACTG 175 HIV trial to the target populations beyond the trial population. Most transport methods focus on continuous and binary outcomes; on the contrary, we derive and discuss several transport methods for survival outcomes: an outcome regression method based on a Cox proportional hazard (PH) model, an inverse probability weighting method based on the models for treatment assignment, sampling score, and censoring, and a doubly robust method that combines both methods, called the augmented calibration weighting (ACW) method. However, as the PH assumption was found to be incorrect for the ACTG 175 trial, the methods that depend on the PH assumption may lead to the biased quantification of the treatment effect. To account for the violation of the PH assumption, we extend the ACW method with the linear spline-based hazard regression model that does not require the PH assumption. Applying the aforementioned methods for transportability, we explore the effect of PH assumption, or the violation thereof, on transporting the survival results from the ACTG 175 trial to various external populations.

Implication of the new VCS jurisdictional and nested REDD methodology on baselines of existing avoided deforestation projects.

A changing climate is poised to inflict massive-scale damage through extreme weather events. Preserving Earth's forests stands out as a critical resource in our battle to mitigate climate change. One pivotal approach for this endeavour is the Reduction of Emissions from Deforestation and Forest Degradation (REDD), a climate change mitigation solution currently being enacted through locally-based projects certified by the Verified Carbon Standard (VCS) Association. Nevertheless, these REDD projects have recently faced severe scrutiny for potentially overemphasizing their effectiveness. To address these concerns, the VCS has put forth a new jurisdictional and nested REDD methodology. This study, therefore, aims to assess the impact of the new REDD methodology on the baseline measurements of existing REDD projects within the VCS registry. For this assessment, we selected four REDD projects, each spanning across four continents and encompassing two major forest types. An in-depth analysis of these four projects reveals a noteworthy trend: under the new methodology, three of them are projected to experience a substantial reduction in the number of issued credits compared to the previous methodologies. Consequently, it appears that the new REDD methodology holds promise in generating higher-quality credits by reducing the potential for an inflated baseline.

Fast Model Selection and Hyperparameter Tuning for Generative Models.

Generative models have gained significant attention in recent years. They are increasingly used to estimate the underlying structure of high-dimensional data and artificially generate various kinds of data similar to those from the real world. The performance of generative models depends critically on a good set of hyperparameters. Yet, finding the right hyperparameter configuration can be an extremely time-consuming task. In this paper, we focus on speeding up the hyperparameter search through adaptive resource allocation, early stopping underperforming candidates quickly and allocating more computational resources to promising ones by comparing their intermediate performance. The hyperparameter search is formulated as a non-stochastic best-arm identification problem where resources like iterations or training time constrained by some predetermined budget are allocated to different hyperparameter configurations. A procedure which uses hypothesis testing coupled with Successive Halving is proposed to make the resource allocation and early stopping decisions and compares the intermediate performance of generative models by their exponentially weighted Maximum Means Discrepancy (MMD). The experimental results show that the proposed method selects hyperparameter configurations that lead to a significant improvement in the model performance compared to Successive Halving for a wide range of budgets across several real-world applications.

Achieving the Reverse Intersystem Crossing in Chalcone Based Donor-Acceptor System through Down-Conversion of Triplet Exciton.

In recent years, understanding the mechanism of thermally activated delayed fluorescence (TADF) has become the primary choice for designing high-efficiency, low-cost, metal-free organic light emitting diodes (OLEDs). Herein, we propose a strategically designed chalcone based donor-acceptor system, where intensification of delayed fluorescence with decrease in temperature (300 K to 100 K) is observed; the theoretical investigations of electronic states and orbital characters uncovered a new cold rISC pathway in donor-acceptor system, where rISC occurs through the down-conversation of higher triplet exciton (from T3 ) to lowest singlet state (S1 ), having negative energy splitting, thus no thermal energy is required. The comprehensive research described herein might open-up new avenues in donor-acceptor system over the conventional up-convention of triplet exciton and demonstrates that not necessarily all delayed fluorescence are thermally activated (TADF).

Ordered Macro/Microporous Ionic Organic Framework for Efficient Separation of Toxic Pollutants from Water.

In case of pollutant segregation, fast mass diffusion is a fundamental criterion in order to achieve improved performance. The rapid mass transport through porous materials can be achieved by availing large open pores followed by easy and complete accessibility of functional sites. Inducing macroporosity into such materials could serve as ideal solution providing access to large macropores that offer unhindered transport of analyte and full exposure to interactive sites. Moreover, the challenge to configure the ionic-functionality with macroporosity could emerge as an unparalleled avenue toward pollutants separation. Herein, we strategized a synthetic protocol for construction of a positively charged hierarchically-porous ordered interconnected macro-structure of organic framework where the size and number of macropores can easily be tuned. The ordered macropores with strong electrostatic interaction synergistically exhibited ultrafast removal efficiency towards various toxic pollutants.

Unfolding the Role of Building Units of MOFs with Mechanistic Insight Towards Selective Metal Ions Detection in Water.

The potential emergence of fluorescence-based techniques has propelled research towards developing probes that can sense trace metal ions specifically. Although luminescent metal-organic frameworks (MOFs) are well suited for this application, the role of building blocks towards detection is not fully understood. In this work, a systematic screening by varying number of Lewis basic (pyridyl-N atoms) sites is carried out in a series of isostructural, robust UiO-67 MOFs, and targeting a model metal ion-Fe3+ . All the three fluorescent MOFs are seen to present quenching response towards Fe3+ ions in water. However, UiO-67@N exhibits highly selective and sensitive response, whereas emission of both UiO-67 and UiO-67@NN is quenched by several metal ions. Detailed experimental and theoretical mechanistic investigation is carried out in addition to demonstration of UiO-67@N being able to sense trace amount of Fe3+ ions in synthetic biological water sample. Further, UiO-67@N based mixed-matrix membrane (MMM) has been prepared and employed to mimic the real time Fe3+ ions detection in water.

Palladium-Anchored N-Heterocyclic Carbenes in a Porous Organic Polymer: A Heterogeneous Composite Catalyst for Eco-Friendly C-C Coupling.

Aggregation-induced catalyst deactivation during the reaction in supported metal catalysts prevails as one of the pitfalls toward their practical implementation. Herein, a homogeneously dispersed palladium-coordinated N-heterocyclic carbene (NHC) was strategically integrated inside a microporous hyper-cross-linked polymer via post-synthesis structural modulation. Successful immobilization of spatially isolated Pd (II) units onto the polymer scaffold yielded highly robust heterogeneous catalysts 120-MI@Pd NHC and 120-EI@Pd NHC, respectively. 120-EI@NHC Pd (4.41 wt % Pd) illustrated a remarkable catalytic potency (yield up to >99%) toward the eco-friendly Suzuki-Miyaura coupling (SMC) reaction at room temperature. The superior catalytic efficiency of 120-EI@Pd NHC is further highlighted from its excellent functionality tolerance over 42 substrates bearing electronic diversity and a turnover frequency value reaching up to 4.97 × 103 h-1 at a very low catalyst dosage of 0.04 mol %. Pertaining to heterogenization, the polymer catalyst could be easily reused with intact catalytic efficiency for at least 10 cycles. The catalytic competence of 120-EI@NHC Pd in terms of scope, scalability, and sustainability advocates its proficiency, while processability was achieved by crafting 3D aerogel monoliths. The conceptual feasibility was further investigated by devising a cup-based nano-reactor with gram-scale product isolation over three catalytic cycles.

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins.

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC. A detailed study of the excited state dynamics that governs ISC is necessary to understand the mechanism of heavy atom effect (HAE). Incorporation of iodine at the 3 position of coumarin-1 reduces fluorescence quantum yield (ϕf) drastically as expected, whereas bromine substitution at the same position increased the ϕf. Such a contrasting effect of the two heavy atoms suggests that there are other features yet to be discovered to fully understand the HAE. Detailed steady state and femtosecond transient absorption studies along with theoretical calculations suggest that the C3-X (X = Br, I) bond vibration plays an important role in the ISC process. The study reveals that while in the case of the iodo-derivative there is no energy barrier in the singlet triplet crossing path, there is a barrier in the case of the bromo-derivative, which slows the ISC process. Such an unexpected phenomenon is not limited to halocoumarins as this rationalizes the photobehavior of 1-bromo-/iodo-substituted naphthalenes as well.

Trap Inlaid Cationic Hybrid Composite Material for Efficient Segregation of Toxic Chemicals from Water.

Metal-based oxoanions are potentially toxic pollutants that can cause serious water pollution. Therefore, the segregation of such species has recently received significant research attention. Even though several adsorbents have been employed for effective management of chemicals, their limited microporous nature along with non-monolithic applicability has thwarted their large-scale real-time application. Herein, we developed a unique anion exchangeable hybrid composite aerogel material (IPcomp-6), integrating a stable cationic metal-organic polyhedron with a hierarchically porous metal-organic gel. The composite scavenger demonstrated a highly selective and very fast segregation efficiency for various hazardous oxoanions such as, HAsO42- , SeO42- , ReO4- , CrO42- , MnO4- , in water, in the presence of 100-fold excess of other coexisting anions. The material was able to selectively eliminate trace HAsO42- even at low concentration to well below the AsV limit in drinking water defined by WHO.

Three-in-One C2 H2 -Selectivity-Guided Adsorptive Separation across an Isoreticular Family of Cationic Square-Lattice MOFs.

Energy-efficient selective physisorption driven C2 H2 separation from industrial C2-C1 impurities such as C2 H4 , CO2 and CH4 is of great importance in the purification of downstream commodity chemicals. We address this challenge employing a series of isoreticular cationic metal-organic frameworks, namely iMOF-nC (n=5, 6, 7). All three square lattice topology MOFs registered higher C2 H2 uptakes versus the competing C2-C1 gases (C2 H4 , CO2 and CH4 ). Dynamic column breakthrough experiments on the best-performing iMOF-6C revealed the first three-in-one C2 H2 adsorption selectivity guided separation of C2 H2 from 1:1 C2 H2 /CO2 , C2 H2 /C2 H4 and C2 H2 /CH4 mixtures. Density functional theory calculations critically examined the C2 H2 selective interactions in iMOF-6C. Thanks to the abundance of square lattice topology MOFs, this study introduces a crystal engineering blueprint for designing C2 H2 -selective layered metal-organic physisorbents, previously unreported in cationic frameworks.

Unusual higher-order nonlinear optical properties in Au-coated triangular Ag-Au nanostructures.

Here, we report hitherto unobserved local field (LF)-assisted pump wavelength-dependent nonlinear optical (NLO) effects of three-photon (3PA)-induced four-photon absorption (4PA) at 532 nm and two-photon-induced 3PA at 730 nm in triangular-shaped core-shell Ag-Au nanoparticles (TrAg@Au) by femtosecond Z-scan. The shell thickness-dependent enhancement in the LF is observed by a COMSOL simulation. The intensity-dependent interplay between saturable and reverse-saturable absorptions along with switching of nonlinear (NL) phase is reported at 730 nm, showing the superiority of TrAg@Au in optical switching (OS). The optical limiting (OL) threshold (Fth) of 5.9(6.5)mJ/cm2 at 730 (532) nm boost their potential over benchmarked materials.

Imidazolium-Functionalized Chemically Robust Ionic Porous Organic Polymers (iPOPs) toward Toxic Oxo-Pollutants Capture from Water.

Fabricating new and efficient materials aimed at containment of water contamination, in particular removing toxic heavy metal based oxo-anions (e. g. CrO4 2- , TcO4 - ) holds paramount importance. In this work, we report two new highly stable imidazolium based ionic porous organic polymers (iPOPs) decorated with multiple interaction sites along with electrostatics driven adsorptive removal of such oxo-anions from water. Both the iPOPs (namely, iPOP-3 and iPOP-4) exhibited rapid sieving kinetics and very high saturation uptake capacity for CrO4 2- anions (170 and 141 mg g-1 for iPOP-3 and iPOP-4 respectively) and ReO4 - (515.5 and 350.3 mg g-1 for iPOP-3 and iPOP-4 respectively), where ReO4 - anions being the non-radioactive surrogative counterpart of radioactive TcO4 - ions. Noticeably, both iPOPs showed exceptional selectivity towards CrO4 2- and ReO4 - even in presence of several other concurrent anions such as Br- , Cl- , SO4 2- , NO3 - etc. The theoretical binding energy calculations via DFT method further confirmed the preferential interaction sites as well as binding energies of both iPOPs towards CrO4 2- and ReO4 - over all other competing anions which corroborates with the experimental high capacity and selectivity of iPOPs toward such oxo-anions.

Recognition and Sequestration of Toxic Inorganic Water Pollutants with Hydrolytically Stable Metal-Organic Frameworks.

Water pollution and crisis of freshwater is one of the most alarming concern globally, which threatens the development and survival of living beings. Recycling of contaminated water has been the prime demand of 21st century as the area of contamination in natural waterbodies increasing rapidly worldwide. Detoxification and purification of wastewater via adsorptive removal technology has been proven to be more efficient because of it's simplicity, lesser complexity and cost-effectiveness. As the most rapid-growing division of coordination chemistry, porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) with the liberty of crafting tailorable porous architecture and presence of numerous functional sites have become quintessential for recognition and sequestration of water pollutants. This personal account intends to highlight our recent contributions in the field of sensing and sequestration of toxic aquatic inorganic pollutants by functionalized water stable MOFs.

Synthesis and Characterization of Acid-Responsive Luminescent Fe(II) Metallopolymers of Rigid and Flexible Backbone N-Donor Multidentate Conjugated Ligands.

In this report we have disclosed the syntheses and properties of two new conjugated organic moieties bearing the same coordination sites but possessing different backbone rigidities and rotational flexibilities. Two new metallopolymers have been synthesized from the corresponding ligands under identical reaction conditions, and they have been thoroughly characterized through different techniques to understand the effect of backbone rigidity on the evolution of different properties in these metallopolymers. A FESEM micrograph of the rigid metallopolymer confirms the formation of a rigid nanorod type structure, while long agglomerated nanofiber strands are visible on the substrate in the case of the flexible analogue. All of the newly synthesized materials are fluorescence active. An Fe(II) metallopolymer of the flexible ligand showed huge changes in emission properties in the presence of different acids and showed a possibility of it being used as a thin film acid vapor sensor. All of the materials showed reversible electrochemical activity, and both of these polymers have shown electroluminescence when an appropriate potential is applied.

A Water-Stable Ionic MOF for the Selective Capture of Toxic Oxoanions of SeVI and AsV and Crystallographic Insight into the Ion-Exchange Mechanism.

Selectively capturing toxic oxoanions of selenium and arsenic is highly desired for the remediation of hazardous waste. Ionic metal-organic frameworks (iMOFs) especially cationic MOFs (iMOF-C) as ion-exchange materials, featuring aqueous phase stability, present a robust pathway for sequestration of the oxoanions owing to their ability to prevent leaching because of their ionic nature. On account of scarcity of water-stable cationic MOFs, the capture of oxoanions of selenium and arsenic has been a major challenge and has not been investigated using iMOFs. Herein, we demonstrate large scale synthesis of cationic MOF, viz. iMOF-1C that exhibits selective capture of oxoanions of SeVI (SeO42- ) and AsV (HAsO42- ) in water with a maximum sorption capacity of 100 and 85 mg g-1 , respectively. This represents among the highest uptake capacities observed for selenate oxoanion in MOFs. Further, the ion-exchange mechanism was directly unveiled by single crystal analysis, which revealed variable modes of host-guest binding.

Switching of Trp-214 intrinsic rotamer population in human serum albumin: An insight into the aftermath of embracing therapeutic bioorganic luminophore azapodophyllotoxin into sudlow site I.

Human serum albumin is perceived to be the most abundant protein in human blood plasma and functions as a major carrier of different enzymes and drugs inside human body. The present article puts in an effort to demonstrate the attitude adopted by human serum albumin towards a potential therapeutic luminophore 4-(2-Hydroxyethyl)-10-phenyl-3,4,6,7,8,10-hexahydro-1H-cyclopenta[g]furo[3,4-b]quinoline-1-one (HPFQ). HPFQ is a prodigy from azapodophyllotoxin class of compounds, which have been synthesized from the perspective of improved bioactivity than its prologue podophyllotoxins. While, HPFQ has proved to be highly bioactive against most cancer cell lines with best GI50 values of <0.1 µM for a major number of cell lines; it also showed terrific fluorescent properties throughout the polarity scale, worthy of a promising imaging agent. The binding mechanism of HPFQ with HSA has been established by combining in vitro spectroscopic techniques, in silico molecular docking and induced fit docking (IFD). The competitive site-binding studies demonstrated that the otherwise anion-receptor sudlow site I of HSA nurtures neutral HPFQ with prudent affinity (Binding constant, Kb = 0.74 × 105 M-1). The time-resolve fluorescence studies reveal an appreciable reduction in HSA average radiative lifetime against an increase in HPFQ concentration and provided evidence for Forster's resonance energy transfer (FRET) being responsible for the dominant quenching mechanism, escorted by minor structural deformations in the backbone of protein structure. HPFQ institutes itself near Trp-214 within protein matrix, and subsequently the "hydrophobic amino acids" dominated cybotactic environment of Trp-214 experiences a reduction in the micropolarity. The allosteric modulation triggered by the stronger association of HPFQ with HSA leads towards minor deformation in secondary structure of protein. Sudlow site I of HSA proficiently embraces a favourable conformation like malleable dough to furnish space for arriving bioactive HPFQ molecule. HPFQ is also believed to administer the conformational regulation in HSA domain by affecting inter-conversion of HSA rotamers, which may prove to be an enlightening area to decode the preferable interaction between them. The juxtaposed spectroscopic research described herein is expected to embolden design of azapodophyllotoxin based anti-proliferative clinical agents for efficient in vivo bio-distribution employing HSA-centred drug delivery and administration systems.