Controlling the Transition of Nanospheres to Superhelices in Aqueous Media by Using a "Smart" Pyrazine Building Block.

For the development of nature-inspired chiral nanostructures, controlling the chirality communication along the assembling pathways in aqueous media is a crucial task. Herein, we designed and synthesized pyrazine-based derivative Pyra-Met with an L-methionine group as a chiral handle. Due to the presence of flexible side chains in the Pyra-Met building block, its interactions with the solvent molecules in the assembled state are restricted, which leads to the generation of nanospheres in aqueous media. The Pyra-Met nanospheres selectively interact with Au3+ ions to generate dynamic helical fibers of the Pyra-Met:Au3+ ensemble with 'hidden' chirality. Finally, superhelices with the desired helical sense are achieved in the presence of L/D-methionine via a co-assembly process. This work presents a feasible strategy to achieve the desired handedness of supramolecular assemblies for better understanding of chirality translation in supramolecular chemistry and it highlights the importance of fine tailoring of building block to control the morphology of the assembled architecture.

'Light-Up' AIE-Active Materials: Self-Assembly, Molecular Recognition and Catalytic Applications.

Aggregation induced emission enhancement (AIEE) is one of the most widely explored phenomena to develop 'light up' (fluorescent) materials having potential applications in the field of supramolecular chemistry, analytical chemistry and material chemistry. By applying the principles of host-guest chemistry, we have developed a variety of aggregation induced emission (AIE/AEE) active materials having specific affinity for metal ions, electron deficient/electron rich analytes. The interactions between AIE active assemblies and metal ions are further tuned to prepare nanohybrids having potential applications as catalytic/photocatalytic systems in various organic transformations under eco-friendly conditions. This account summarizes various design strategies developed in our labortary for the preparation of AIE/AEE active building blocks having sensing and catalytic applications.

"Metal-Free" Nanoassemblies of AIEE-ICT-Active Pyrazine Derivative: Efficient Photoredox System for the Synthesis of Benzimidazoles.

Supramolecular nanoassemblies of an AIEE-ICT-active pyrazine derivative (TETPY) having strong absorption in the visible region and excellent transportability have been utilized as an efficient photoredox catalytic system for the synthesis of a variety of benzimidazoles having electron-withdrawing/electron-releasing/aliphatic groups under "metal-free" conditions. The reaction protocol involves the successful harvesting of visible light by TETPY assemblies to catalyze the coupling of o-phenylenediamine/substituted diamines and substituted aromatic/heterocyclic/aliphatic aldehydes under aerial conditions using mixed aqueous media as the reaction solvent. TETPY assemblies could activate aerial oxygen to generate superoxide for completing the vital proton abstraction step without the need for any external metal/base/oxidant. Moreover, all the products are purified by recrystallization from organic solvents. The TETPY assemblies also exhibited high efficiency in catalyzing the synthesis of 2-substituted benzothiazoles and quinazolines in excellent yields.

Phenazine-Based Donor Acceptor Systems as Organic Photocatalysts for "Metal-free" C-N/C-C Cross-Coupling.

With an aim to achieve a balance between ground-state and excited-state reduction potential of donor acceptor systems for efficient C-N/C-C cross-coupling, a series of donor acceptor systems DA1-DA4 have been synthesized by varying the donor strength and connecting positions. With an increase in donor strength, systematic elevation in the ground-state reduction potential and decrease in the HOMO-LUMO gap was observed. Interestingly, all the derivatives DA1-DA4 could catalyze the C-N bond formation reaction between activated aryl halides and amines at low catalytic loading under metal-free conditions without the need of any external base upon irradiation with white LED. A balance was realized in the case of derivative DA2, which exhibits high efficiency in C-N couplings. Different control experiments support the validity of the energy as well as electron transfer pathways in the visible light-mediated C-N bond formation. This study further reveals the potential of derivative DA1 in "metal-free" Sonogashira coupling involving activated aryl halides which is attributed to its high excited-state reduction potential.

Entropically Favoured Assembly of Pyrazine-Based Helical Fibers into Superstructures: Achiral/ Chiral Guest-Induced Chirality Transformation.

The development of synthetic helical structures undergoing stimuli-responsive chirality transformations is important for an understanding of the role of chirality in natural systems. However, controlling supramolecular chirality in entropically driven assemblies in aqueous media is challenging. To develop stimuli-responsive assemblies, we designed and synthesized pyrazine derivatives with l-alanine groups as chiral building blocks. These systems undergo self-assembly in aqueous media to generate helical fibers and the embedded alanine groups transfer their chirality to the assembled structures. Furthermore, these helical fibers undergo a Ni2+ -induced chirality transformation. The study demonstrates the role of intermolecular hydrogen bonding, π-π stacking, and the hydrophobic effect in the Ni2+ -mediated transition of helical fibers to supercoiled helical ensembles which mimic the formation of superstructures in biopolymers.

Hexaarylbenzene: Evolution of Properties and Applications of Multitalented Scaffold.

The easily rotatable peripheral aromatic rings around central benzene in hexaarylbenzene (HAB) derivatives create a very intriguing nonplanar, propeller-shaped geometry. Because of the very low susceptibility toward self-aggregation, HAB derivatives are much stronger candidates among various polyphenylenes/hetero-oligophenylenes when poor molecular cohesion and inefficient packing is required. However, the native properties of hexaphenylbenzene (HPB) can be varied by proper tailoring and substitution of the HAB core. The cohesion and packing in the structures of HAB aggregates induce effective structural variations so as to modify the fundamental features. Recently, HAB derivatives attracted a lot of research interest because of their significant role as liquid crystalline materials, organic light-emitting diodes, photochemical switches, redox materials, and molecular receptors. Herein, detailed attention is given to the pioneering work based on synthetic optimization of different HAB cores, elaborated study of their crystal engineering, various interesting applications of HAB derivatives, and future possibilities and capabilities of this still underexplored scaffold.

A Highly Selective Fluorescent Probe for Detection of Hydrogen Sulfide in Living Systems: In Vitro and in Vivo Applications.

A fluorescein-based fluorescent probe has been designed and synthesised that selectively detects H2 S in aqueous medium, among various analytes tested. This fluorescein-based fluorescent probe has also been successfully utilised for real-time imaging of exo- and endogenously produced H2 S in cancer cells and normal cells. Moreover, the probe can also detect H2 S in the rat brain hippocampus at variable depths and in living nematodes.

Fluorescent probes for hydrogen polysulfides (H2Sn, n > 1): from design rationale to applications.

Hydrogen polysulfides (H2Sn, n > 1) being active members of the reactive sulphur species family are gaining much research interest these days due to their involvement in signal transduction, anti-cancer activity and cytoprotection. The importance of H2Sn particularly in human health makes it desirable to explore selective and sensitive techniques for their detection. Out of the several techniques and tools available, fluorescence spectroscopy seems advantageous due to its high sensitivity, non-destructive nature and applicability to living systems. Remarkable progress has been made in this field and different approaches have been adopted for the selective detection of H2Snin vitro as well as in vivo. The present review discusses the importance of H2Sn in living systems and highlights recent advances in the design of fluorescent probes for the detection of H2Sn. It offers a critical overview of the designing strategies adopted for H2Sn detection with emphasis on various ways for achieving selectivity over other reactive sulphur species (RSS) and their applications in biological systems. Further, the fundamental challenges and future prospects in this field are also discussed.

A highly selective fluorescent probe for Fe3+ in living cells: a stress induced cell based model study.

A rhodamine-phenanthroline dyad based fluorescent probe 4 has been designed and synthesized which selectively monitors Fe3+ ions among the various metal ions tested. Furthermore, probe 4 has been explored for monitoring of dynamic changes in the Fe3+ ion level under aggressive Fenton reaction conditions using a cell based model study. More significantly, probe 4 has also been utilized for real time imaging of endogenous Fe3+ ions in living C6 cell lines, the results of which demonstrated that probe 4 acts as an efficient fluorescent tool for Fe3+ ion detection in biological systems.

Pentacenequinone-Stabilized Silver Nanoparticles: A Reusable Catalyst for the Diels-Alder [4 + 2] Cycloaddition Reactions.

Silver nanoparticles (AgNPs) stabilized by aggregates of derivative 4 have been used as catalyst for the construction of synthetically and biologically important [4 + 2] cycloadducts at room temperature.

Carbazole-functionalized polyphenylene-decorated solid state emissive D-A-D molecules: reduced donor-acceptor interaction and enhanced emission in the solid state.

A new series of D-A-D molecules (1-3) with carbazole as a donor and 2,5-diaryl-1,3,4-oxadiazole, dibenzothiophene-S,S-dioxide and benzo[1,2,5]thiadiazole as acceptors have been synthesized. Polyphenylene dendrons have also been incorporated to prevent the face to face π-π stacking in aggregates. Unlike traditional D-A systems, which show ACQ problems and large red shifts in the solid state, these derivatives (1-3) exhibit good solid state luminescence behavior without undergoing a red shift or fluorescence quenching. The present results indicate that just the combination of a donor-acceptor system with bulkier dendrons is not enough to generate efficient solid state emissions but the prevention of co-facial interaction between donors and acceptors in aggregates is more important. In the case of derivative 3, restriction of complete molecular planarization and steric hindrance near the acceptor can effectively prevent intermolecular donor-acceptor interactions, thus leading to enhanced emission in aggregates. While in the case of derivative 1, a sterically unhindered acceptor is easily approached by a donor due to close molecular packing in aggregates forming weakly luminescent species.

Recent developments of thiacalixarene based molecular motifs.

Thiacalixarenes, a subclass of "third generation" calixarenes, exhibit many interesting features such as enlarged ring size, facile chemical modification, and metal complexation due to the presence of bridging sulfur atoms. The thiacalixarene scaffold is a unique host with vast possibilities for functionalization not only at the upper and lower rim but also at the bridging sulfide groups. Modified thiacalixarenes have been used for many applications such as the detection and separation of biologically important cations, anions, and bio-analytes, mimicking molecular logic gates and devices, and synthesis of self-assembled coordination cages, multinuclear complexes, magnetic materials and luminescent materials. This review article summarizes recent developments in the derivatization methods of thiacalixarenes and their utilization in various applications.

Resonance energy transfer-based fluorescent probes for Hg2+, Cu2+ and Fe2+/Fe3+ ions.

Resonance energy transfer (RET) between donor-acceptor architecture is an important physical mechanism which is used enormously for the development of fluorescent probes. The unique advantage of RET is its ability to transfer energy non-radiatively between molecules over biologically relevant distances. The distance dependency of RET makes this approach suitable for bioanalysis such as distances between biomolecules and molecular level interactions, both in vitro and in vivo. In addition, RET is a proficient approach for the development of fluorescent probes with ratiometric measurements. In the recent years, resonance energy transfer has been extensively applied for the design of fluorescent sensors for different types of analytes such as metal ions, anions, reactive oxygen species and molecules of biological interest. In this review, our aim is to highlight the applications of resonance energy transfer mechanisms, i.e. Förster or fluorescence resonance energy transfer (FRET) and through-bond energy transfer (TBET) for the development of fluorescent probes, mainly for Hg(2+), Cu(2+) and Fe(2+)/Fe(3+) ions.

Strategic Insertion of Heavy Atom to Tailor TADF OLED Material for the Development of Type I Photosensitizing Catalytic Red Emissive Assemblies.

The work presented in the manuscript describes a simple strategy for transforming thermally activated delayed fluorescent organic light-emitting diodes (TADF OLEDs) compound 10-(dibenzo[a,c]phenazin-11-yl)-10H-phenoxazine (DPZ-PXZ) into type I photosensitizer 10-(dibenzo[a,c]phenazin-11-yl)-10H-phenothiazine (DPZ-PHZ) by strategically introducing sulfur atom in the photosensitizing core. The synthesized compound DPZ-PHZ exhibits aggregation-induced enhancement (AIE) and through-space charge transfer (TSCT) characteristics and generates red emissive assemblies in mixed aqueous media. The original compound DPZ-PXZ exhibits well-separated HOMO and LUMO levels and is reported to have highly efficient reverse intersystem crossing (RISC). In comparison, the incorporation of sulfur atom in the phenothiazine donor regulates the electronic communication between donor and acceptor units and promotes the intersystem crossing (ISC) in DPZ-PHZ molecules. Interestingly, compound DPZ-PHZ exhibits rapid activation of aerial oxygen for instant generation of superoxide radical anion. Backed by excellent type I photosensitizing activity, DPZ-PHZ assemblies have high catalytic potential for the synthesis of benzimidazoles, benzothiazoles and quinazolines derivatives under mild reaction conditions. The work presented in the manuscript provides an insight into the combination of heavy atom approach and TSCT for achieving adequate electronic communication between donor and acceptor units, balanced RISC/ISC, and stabilized-charge separated state for the development of efficient type I photosensitizing assemblies.

Nanophotosensitizing through Space Charge Transfer Assemblies of Pentacenequinone Derivative for 'Metal-free' Photoamidation Reactions.

The present study demonstrates the influence of small portion (20 %) of organic co-solvent (DMSO/THF/ACN/MeOH) in mixed aqueous media (80 % water) in controlling the size, quantum yield and life time of the through space charge transfer assemblies (TSCT) of pentacenequinone derivative (TPy-PCQ-TPy). Among various solvent systems [H2 O : DMSO (8 : 2), H2 O : THF (8 : 2), H2 O : ACN (8 : 2) and H2 O : MeOH (8 : 2)] examined, highly emissive (Φf =12 %) and nano-sized assemblies having long life time (3.11 ns) were formed in H2 O : DMSO (8 : 2) solvent system. The solvent dependent differences in the size and excited state properties of TPy-PCQ-TPy assemblies are reflected in their photosensitizing behaviour in different solvent systems. Backed by excellent photosensitizing properties, TPy-PCQ-TPy assemblies smoothly catalyse the photoamidation reactions between unactivated/activated aldehydes and secondary amine under mild reaction conditions (visible light irradiation, aerial atmosphere, room temperature) in H2 O : DMSO (8 : 2) solvent mixture. The as prepared assemblies of TPy-PCQ-TPy also exhibit high potential to catalyse the oxidation of benzyl alcohols to aromatic aldehydes, thus, generating a possibility to use aromatic alcohols as the starting material in photoamidation reactions. The real time application of TSCT assemblies has also been demonstrated in gram scale transformation of aromatic aldehydes to aromatic amides and photooxidation of benzyl alcohol to aromatic aldehyde.

Mercury-modulated supramolecular assembly of a hexaphenylbenzene derivative for selective detection of picric acid.

Spherical aggregates of hexaphenylbenzene derivative 5 undergo metal-induced modulation to form nanorods in the presence of Hg(2+) ions, which exhibit selective and sensitive response toward picric acid (PA) with a detection limit of 6.87 ppb.

Enzyme-/metal-free quinoxaline assemblies: direct light-up detection of cholesterol in human serum.

Supramolecular assemblies of quinoxaline derivative (QxPyA) have been developed for turn-on detection of cholesterol in human serum and in aqueous media with a detection limit of 4 nM.

Cobalt-Centered Supramolecular Nanoensemble for Regulated Aerobic Oxidation of Alcohols and "One-Pot" Synthesis of Quinazolin-4(3H)-ones.

The supramolecular assemblies of the donor-acceptor (D-A) system Im-Tpy, having phenanthro[9,10-d]imidazole as the donor and terpyridyl group as the acceptor unit, have been developed, which serve as supramolecular host to stabilize Co(II) in its nanoform. The as-prepared supramolecular nanoensemble Im-Tpy@Co in DMSO:water (7:3) shows high thermal stability and photostability. Even in the case of solvent mismatch, i.e., on dilution with cosolvent THF/DMSO, insignificant changes were observed in the size/morphology of the nanoensemble. The as-prepared Im-Tpy@Co nanoensemble in low catalytic loading (0.1 mol % of Co) catalyzes the oxidation of a wide variety of alcohols to aromatic aldehydes/ketones using visible light radiations as the source of energy without the need of any additive at room temperature. In comparison to already reported systems, the Im-Tpy@Co nanoensemble exhibits high turnover numbers (TONs) and turnover frequencies (TOFs). The practical application of the catalytic system has also been demonstrated in the gram-scale synthesis of 4-chlorobenzaldehyde. The Im-Tpy@Co nanoensemble exhibits recyclability up to four catalytic cycles with insignificant leaching and morphological changes. The present study also demonstrates the catalytic activity of the Im-Tpy@Co nanoensemble in "one-pot" synthesis of quinazolin-4(3H)-ones from 2-aminobenzamide and primary alcohols with better efficiency in comparison to other transition-metal-based catalytic systems.

Beyond Molecular Recognition: A Perylene Bisimide Derivative as a Functional Mimic of Chlorpyrifos.

Supramolecular assemblies of perylene bisimide derivative (PBI-SAH) have been developed which show 'turn-on' detection of chlorpyrifos in aqueous media, apple residue and blood serum. Differently from the already reported fluorescent probes for the detection of CPF, PBI-SAH assemblies also show affinity for acetylcholinesterase (AChE) which endow the PBI-SAH molecules with mixed inhibitory potential to restrict the AChE catalysed hydrolysis of acetylthiocholine (ATCh) in MG-63 cell lines (in vitro) and in mice (in vivo). The molecular docking studies support the inhibitory activity of PBI-SAH assemblies and their potential to act as safe insecticide with high benefit to harm ratio. The insecticidal potential of PBI-SAH derivative has been examined against Spodoptera litura (S. litura) and these studies demonstrate its excellent insecticidal activity (100 % mortality in nineteen days). To the best of our knowledge, this is the first report regarding development of PBI-SAH assemblies which not only detect chlorpyrifos but also mimic AChE inhibitory activity of CPF to show promising aptitude as safe insecticide.

Aggregation-Induced Emission (AIE), Life and Health.

Light has profoundly impacted modern medicine and healthcare, with numerous luminescent agents and imaging techniques currently being used to assess health and treat diseases. As an emerging concept in luminescence, aggregation-induced emission (AIE) has shown great potential in biological applications due to its advantages in terms of brightness, biocompatibility, photostability, and positive correlation with concentration. This review provides a comprehensive summary of AIE luminogens applied in imaging of biological structure and dynamic physiological processes, disease diagnosis and treatment, and detection and monitoring of specific analytes, followed by representative works. Discussions on critical issues and perspectives on future directions are also included. This review aims to stimulate the interest of researchers from different fields, including chemistry, biology, materials science, medicine, etc., thus promoting the development of AIE in the fields of life and health.