Monomers Versus Prenucleation Clusters En Route to Polymorphism of Supramolecular Polymers.

Polymorphism in supramolecular polymers is strongly correlated with the polymerization pathways underlying their formation. To effectively control emerging polymorphs, a comprehensive understanding of nucleation pathways and mechanisms is essential. Herein, a coronene-dipeptide conjugate (Cr-o-FFOEt) is introduced and its self-assembly into two different stable 1D supramolecular polymorphs (Agg 1 and 2f) is observed in the same solvent composition (water/THF, 7:3 v/v) and same concentration at room temperature, following two competitive self-assembly pathways. The difference in the mode of solvent addition triggers the two self-assembly pathways. Furthermore, the isolated intermediate Agg 2i is found to transform into Agg 1 or Agg 2f under controlled experimental conditions. The supramolecular aggregates of Cr-o-FFOEt are thoroughly examined with the help of optical, chiroptical, and morphological techniques to understand the subtle difference in choosing the self-assembling pathways. The studies reveal that the nanotube formation of Agg 1 follows a classical nucleation-elongation supramolecular polymerization mechanism (involving monomers). In contrast, the helical fibers of Agg 2f are formed by the involvement of preorganized oligomers (nonclassical process). The observation highlights the underappreciated role of prenucleation clusters in pathway complexity and polymorphism of supramolecular 1D polymers.

Liquid-Liquid Phase Separation Mediated Formation of Chiral 2D Crystalline Nanosheets of a Co-Assembled System.

The role of macromolecule-macromolecule and macromolecule-H2O interactions and the resulting perturbation of the H-bonded network of H2O in the liquid-liquid phase separation (LLPS) process of biopolymers are well-known. However, the potential of the hydrated state of supramolecular structures (non-covalent analogs of macromolecules) of synthetic molecules is not widely recognized for playing a similar role in the LLPS process. Herein, LLPS occurred during the co-assembly of hydrated supramolecular vesicles (bolaamphiphile, BA1) with a net positive charge (zeta potential, ζ = +60 ± 2 mV) and a dianionic chiral molecule (disodium l-[+]-tartrate) is reported. As inferred from cryo-transmission electron microscopy (TEM), the LLPS-formed droplets serve as the nucleation precursors, dictating the structure and properties of the co-assembly. The co-assembled structure formed by LLPS effectively integrates the counter anion's asymmetry, resulting in the formation of ultrathin free-standing, chiral 2D crystalline sheets. The significance of the hydrated state of supramolecular structures in influencing LLPS is unraveled through studies extended to a less hydrated supramolecular structure of a comparable system (BA2). The role of LLPS in modulating the hydrophobic interaction in water paves the way for the creation of advanced functional materials in an aqueous environment.

Spontaneous Curvature Induction in an Artificial Bilayer Membrane.

Maintaining lipid asymmetry across membrane leaflets is critical for functions like vesicular traffic and organelle homeostasis. However, a lack of molecular-level understanding of the mechanisms underlying membrane fission and fusion processes in synthetic systems precludes their development as artificial analogs. Here, we report asymmetry induction of a bilayer membrane formed by an extended π-conjugated molecule with oxyalkylene side chains bearing terminal tertiary amine moieties (BA1) in water. Autogenous protonation of the tertiary amines in the periphery of the bilayer by water induces anisotropic curvature, resulting in membrane fission to form vesicles and can be monitored using time-dependent spectroscopy and microscopy. Interestingly, upon loss of the induced asymmetry by extensive protonation using an organic acid restored bilayer membrane. The mechanism leading to the compositional asymmetry in the leaflet and curvature induction in the membrane is validated by density functional theory (DFT) calculations. Studies extended to control molecules having changes in hydrophilic (BA2) and hydrophobic (BA3) segments provide insight into the delicate nature of molecular scale interactions in the dynamic transformation of supramolecular structures. The synergic effect of hydrophobic interaction and the hydrated state of BA1 aggregates provide dynamicity and unusual stability. Our study unveils mechanistic insight into the dynamic transformation of bilayer membranes into vesicles.

p-Phenylenediamine-derived carbon nanodots for probing solvent interactions.

Carbon nanodots, the luminescent nanoparticles of carbon with size restriction below 10 nm, have attracted inordinate attention in materials science due to their widespread applications in optoelectronic and biological fields. Low toxicity and facile synthesis pathways render them favourites in the above-mentioned areas in the context of green chemistry. This work presents fine applications of p-phenylenediamine-derived carbon nanodots (PD-CNDs) achieved via a facile one-pot hydrothermal method. Adequate characterization using X-ray diffraction and spectroscopic and microscopic studies confirmed spherical particles with an average particle size of 2.8 nm, functionalised with amino, carboxyl, and hydroxyl groups. The carbon framework was functionalised with pyridinic and pyrrolic nitrogens. Upon 365 nm UV light illumination, an aqueous dispersion of PD-CNDs showed red-orange fluorescence. Detailed spectral analysis using UV-visible absorption and fluorescence spectroscopy identified edge states and surface groups as luminescent centres, with a significant contribution arising from the latter. The investigation conducted using a collection of solvents, categorized into polar and nonpolar, indicated the potential of the system for applications based on its solvatochromic nature. The feature enabled the determination of different polarity parameters of the solvents, as well as dielectric constants of solvents and solvent mixtures, with considerable accuracy. The system was potent for predicting the composition of a given pair of solvents. The service of the system is also extended for moisture sensing in organic solvents within an error percentage < 1. High quantum yield values (0.61) combined with solvent composition-dependent optical features ensure broader applications of the system to probe solvent interactions.

Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium.

A mechanistic understanding of the principles governing the hierarchical organization of supramolecular polymers offers a paradigm for tailoring synthetic molecular architectures at the nano to micrometric scales. Herein, the unconventional crystal growth mechanism of a supramolecular polymer of superbenzene(coronene)-diphenylalanine conjugate (Cr-FFOEt ) is demonstrated. 3D electron diffraction (3D ED), a technique underexplored in supramolecular chemistry, is effectively utilized to gain a molecular-level understanding of the gradual growth of the initially formed poorly crystalline hairy, fibril-like supramolecular polymers into the ribbon-like crystallites. The further evolution of these nanosized flat ribbons into microcrystals by oriented attachment and lateral fusion is probed by time-resolved microscopy and electron diffraction. The gradual morphological and structural changes reveal the nonclassical crystal growth pathway, where the balance of strong and weak intermolecular interactions led to a structure beyond the nanoscale. The role of distinct π-stacking and H-bonding interactions that drive the nonclassical crystallization process of Cr-FFOEt supramolecular polymers is analyzed in comparison to analogous molecules, Py-FFOEt and Cr-FF forming helical and twisted fibers, respectively. Furthermore, the Cr-FFOEt crystals formed through nonclassical crystallization are found to improve the functional properties.

Controlling the Morphological Features, Aspect Ratio and Emission Patterns of Supramolecular Copolymers by Restricted Dimensional Growth.

One of the bottlenecks associated with supramolecular polymerization of functional π-systems is the spontaneous assembly of monomers leading to one- or two-dimensional (1D or 2D) polymers without control over chain length and optical properties. In the case of supramolecular copolymerization of monomers that are structurally too diverse, preferential self-sorting occurs unless they are closely interacting donor-acceptor pairs. Herein, it is established that the spontaneous 1D polymerization of a phenyleneethynylene (PE) derivative and the 2D polymerization of a Bodipy derivative (BODIPY) can be controlled by copolymerizing them in different ratios, leading to unusual spindle-shaped structures with controlled aspect ratio, as evident by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) studies. For example, when the content of BODIPY is 50 % in the BODIPY-PE mixture, the 1D polymerization of PE is significantly restricted to form elongated spindle-like structures having an aspect ratio of 4-6. The addition of 75 % of BODIPY to PE resulted in circular spindles having an aspect ratio of 1-2.5, thereby completely restricting the 1D polymerization of PE monomers. Moreover, the resultant supramolecular copolymers exhibited morphology and aspect ratio dependent emission features as observed by the time-resolved emission studies.

Tweaking a BODIPY Spherical Self-Assembly to 2D Supramolecular Polymers Facilitates Excited-State Cascade Energy Transfer.

Excited state properties such as emission, exciton transport, electron transfer, etc., are strongly dependent on the shape, size and molecular arrangement of chromophore based supramolecular architectures. Herein, we demonstrate creation and control of distinct supramolecular energy landscapes for the reversible control of the excited-state emission processes through cascade energy transfer in chromophore assemblies, facilitated by an unprecedented solvent effect. In methylcyclohexane, a tailor-made Y-shaped BODIPY derivative self-assembles to form an unusual spherical architecture of 400-1200 nm size, which exhibits a single emission at 540 nm upon 475 nm excitation through a normal excitation deactivation process. However, in n-decane, the same BODIPY derivative forms two-dimensional supramolecular sheets, exhibiting multiple emission peaks at 540, 610, 650, 725 and 790 nm with 475 nm excitation due to cascade energy transfer. Further control on the morphology and excitation energy transfer is possible with variable solvent composition and ultrasound stimulation, resulting in enhanced near-infrared emission with an overall pseudo Stokes shift of 7105 cm-1 .

Helical supramolecular polymers with rationally designed binding sites for chiral guest recognition.

Since various helical supramolecular polymers became available, their application to molecular chirality recognition have been anticipated but not extensively studied. So far, only a few examples of chiral reactions have been reported, but none for chiral separation. Here, we report the application of a helical supramolecular polymer to the enantio-separation of chiral guest molecules. The monomer of this supramolecular polymer is the salt-pair of a dendritic carboxylic acid with an enantiopure amino alcohol. In an apolar solvent, this salt-pair stacks via hydrogen bonds to form a helical polymer. In conjunction with this carboxylic acid, various amino alcohols afford supramolecular polymers, whose helical handedness is determined by the stereochemistry of the amino alcohols. When two salts with the same chirality are mixed, they undergo copolymerization, while those with opposite chirality do not. Owing to this stereoselective copolymerizability, the helical supramolecular polymer could bias the enantiomeric composition of chiral amino alcohols.

Self-Assembled Extended π-Systems for Sensing and Security Applications.

Molecules and materials derived from self-assembled extended π-systems have strong and reversible optical properties, which can be modulated with external stimuli such as temperature, mechanical stress, ions, the polarity of the medium, and so on. In many cases, absorption and emission responses of self-assembled supramolecular π-systems are manifested several times higher when compared with the individual molecular building blocks. These properties of molecular assemblies encourage scientists to have a deeper understanding of their design to explore them for suitable optoelectronic applications. Therefore, it is important to bring in highly responsive optical features in π-systems, for which it is necessary to modify their structures by varying the conjugation length and by introducing donor-acceptor functional groups. Using noncovalent forces, π-systems can be put together to form assemblies of different shapes and sizes with varied optical band gaps through controlling intermolecular electronic interactions. In addition, using directional forces, it is possible to bring anisotropy to the self-assembled nanostructures, facilitating efficient exciton migration, resulting in the modulation of optical and electron-transport properties. In this Account, we mainly summarize our findings with optically tunable self-assemblies of extended π-systems such as p-phenylenevinylenes (PVs), p-phenyleneethynylenes (PEs), and diketopyrrolopyrroles (DPPs) as different stimuli-responsive platforms to develop sensors and security materials. We start with how PV self-assemblies and their coassemblies with appropriate electron-deficient systems can be used for the sensing of analytes in contact mode or in the vapor phase. For example, whereas the PV having electron-deficient terminal groups has high sensitivity toward trinitrotoluene (TNT) in contact mode, the supercoiled fibers formed by the coassembly of self-sorted stacks of C3-symmetrical PV and C3-symmetrical electron-deficient perylene bisimide are capable of sensing vapors of nitrobenzene and o-toluidine. The power of different functional groups in combination with PVs has been further illustrated by attaching CO2-sensitive tertiary amine moieties to a cyano-substituted PV, which allowed the bimodal detection of CO2 using fluorescence and Raman spectroscopy. Interestingly, the functionalization of PVs with terminal amide groups and chiral alkoxy side chains provided a mechanochromic system that allows self-erasable imaging. Whereas PVs exhibit quenching of fluorescence in most cases during self-assembly, PE derivatives exhibit aggregation-induced emission. This property of PEs has been exploited for the development of stimuli-responsive security materials, especially for currency and documents. For instance, the blue fluorescence of a PE attached to hydrophilic oxyethylene side chains coated on a filter paper upon contact with water changes to cyan emission due to the change in the molecular packing. Interestingly, the molecular packing of a Bodipy-attached PE-based gelator allowed a stress-induced change in the emission behavior, resulting in strong near-infrared (NIR) emission upon the application of mechanical stress or gelation. Finally, the use of DPP-based π-systems for the development of NIR transparent optical filters that block UV-vis light and their security- and forensic-related applications are described. These selected examples of the π-system self-assemblies provide an idea of the current status and future opportunities for scientists interested in this field of self-assembly and soft materials research.

Bimodal detection of carbon dioxide using fluorescent molecular aggregates.

Cyano-substituted p-phenylenevinylene (R-1) aggregates exhibiting fluorescence and Raman spectroscopic responses towards CO2 are described. The aggregation-induced emission (AIE) as well as the aggregation-enhanced Raman scattering (AERS) of R-1 in aqueous conditions was reduced in the presence of a small amount of CO2, which enabled its easy and fast bimodal detection in different analytical samples.

Supramolecular Reassembly of Self-Exfoliated Ionic Covalent Organic Nanosheets for Label-Free Detection of Double-Stranded DNA.

Ionic covalent organic nanosheets (iCONs), a member of the two-dimensional (2D) nanomaterials family, offer a unique functional platform for a wide range of applications. Herein, we explore the potential of an ethidium bromide (EB)-based covalent organic framework (EB-TFP) that self-exfoliates in water resulting in 2D ionic covalent organic nanosheets (EB-TFP-iCONs) for the selective detection of double-stranded DNA (dsDNA). In an aqueous medium, the self-exfoliated EB-TFP-iCONs reassemble in the presence of dsDNA resulting in hybrid EB-TFP-iCONs-DNA crystalline nanosheets with enhanced fluorescence at 600 nm. Detailed steady-state and time-resolved emission studies revealed that the reassembly phenomenon was highly selective for dsDNA when compared to single-stranded DNA (ssDNA), which allowed us to use the EB-TFP-iCONs as a 2D fluorescent platform for the label-free detection of complementary DNA strands.

Transforming a C 3-Symmetrical Liquid Crystal to a π-Gelator by Alkoxy Chain Variation.

Rational understanding of the structural features involving different noncovalent interactions is necessary to design a liquid crystal (LC) or an organogelator. Herein, we report the effect of the number and positions of alkoxy chains on the self-assembly induced physical properties of a few π-conjugated molecules. For this purpose, we designed and synthesized three C 3-symmetrical molecules based on oligo(p-phenylenevinylene), C 3 OPV1-3. The self-assembly properties of these molecules are studied in the solid and solution states. All of the three molecules follow the isodesmic self-assembly pathway. Upon cooling from isotropic melt, C 3 OPV1 having nine alkoxy chains (-OC12H25) formed a columnar phase with two-dimensional rectangular lattice and retained the LC phase even at room temperature. Interestingly, when one of the -OC12H25 groups from each of the end benzene rings is knocked out, the resultant molecule, C 3 OPV2 lost the LC property, however, transformed as a gelator in toluene and n-decane. Surprisingly, when the -OC12H25 group from the middle position is removed, the resultant molecule C 3 OPV3 failed to form either the LC or the gel phases.

An unprecedented amplification of near-infrared emission in a Bodipy derived π-system by stress or gelation.

We report an unprecedented strategy to generate and amplify near-infrared (NIR) emission in an organic chromophore by mechanical stress or gelation pathways. A greenish-yellow emitting film of π-extended Bodipy-1, obtained from n-decane, became orange-red upon mechanical shearing, with a 15-fold enhancement in NIR emission at 738 nm. Alternatively, a DMSO gel of Bodipy-1 exhibited a 7-fold enhancement in NIR emission at 748 nm with a change in emission color from yellow to orange-red upon drying. The reason for the amplified NIR emission in both cases is established from the difference in chromophore packing, by single crystal analysis of a model compound (Bodipy-2), which also exhibited a near identical emission spectrum with red to NIR emission (742 nm). Comparison of the emission features and WAXS and FT-IR data of the sheared n-decane film and the DMSO xerogel with the single crystal data supports a head-to-tail slipped arrangement driven by the N-H···F-B bonding in the sheared or xerogel states, which facilitates strong exciton coupling and the resultant NIR emission.

One-Pot MCR-Oxidation Approach toward Indole-Fused Heteroacenes.

A straightforward synthetic route toward indole-fused heteroacenes was developed. The strategy is composed of a one-pot process starting with a multicomponent reaction of cyclohexanone, primary amine and N-tosyl-3-nitroindole followed by an oxidation step. The one-pot approach was found to be general, affording both symmetric and nonsymmetric indolo[3,2-b]indoles in good yields. The strategy was also utilized for accessing 5-ring fused benzo[g]indolo[3,2-b]indole. We could extend the methodology for the synthesis of benzothieno[3,2-b]indoles starting from 3-nitrobenzothiophene. The importance of the developed method was exemplified by performing the reaction sequence on gram scale and also by the synthetic transformations of indolo[3,2-b]indoles. In addition, the change in photophysical properties with extension of conjugation of the synthesized heteroacenes was studied.

The Helix to Super-Helix Transition in the Self-Assembly of π-Systems: Superseding of Molecular Chirality at Hierarchical Level.

Higher-order super-helical structures derived from biological molecules are known to evolve through opposite coiling of the initial helical fibers, as seen in collagen protein. A similar phenomenon is observed in a π-system self-assembly of chiral oligo(phenyleneethylene) derivatives (S)-1 and (R)-1 that explains the unequal formation of both left- and right-handed helices from molecule having a specific chiral center. Concentration- and temperature-dependent circular dichroism (CD) and UV/Vis spectroscopic studies revealed that the initial formation of helical aggregates is in accordance with the molecular chirality. At the next level of hierarchical self-assembly, coiling of the fibers occurs with opposite handedness, thereby superseding the command of the molecular chirality. This was confirmed by solvent-dependent decoiling of super-helical structures and concentration-dependent morphological analysis.

The Rise of Near-Infrared Emitters: Organic Dyes, Porphyrinoids, and Transition Metal Complexes.

In recent years, the interest in near-infrared (NIR) emitting molecules and materials has increased significantly, thanks to the expansion of the potential technological applications of NIR luminescence in several areas such as bioimaging, sensors, telecommunications, and night-vision displays. This progress has been facilitated by the development of new synthetic routes for the targeted functionalization and expansion of established molecular frameworks and by the availability of simpler and cheaper NIR detectors. Herein, we present recent developments on three major classes of systems-i.e., organic dyes, porphyrinoids, and transition metal complexes-exhibiting the maximum of the emission band at λ > 700 nm. In particular, we focus on the design strategies that may increase the luminescence efficiency, while pushing the emission band more deeply in the NIR region. This overview suggests that further progress can be achieved in the near future, with enhanced availability of more robust, stronger, and cheaper NIR luminophores.

Supercoiled fibres of self-sorted donor-acceptor stacks: a turn-off/turn-on platform for sensing volatile aromatic compounds.

To ensure the comfortable survival of living organisms, detection of different life threatening volatile organic compounds (VOCs) such as biological metabolites and carcinogenic molecules is of prime importance. Herein, we report the use of supercoiled supramolecular polymeric fibres of self-sorted donor-acceptor molecules as "turn-off/turn-on" fluorescent sensors for the detection of carcinogenic VOCs. For this purpose, a C3-symmetrical donor molecule based on oligo(p-phenylenevinylene), C3OPV, and a perylene bisimide based acceptor molecule, C3PBI, have been synthesized. When these two molecules were mixed together in toluene, in contrast to the usual charge transfer (CT) stacking, supramolecular fibres of self-sorted stacks were formed at the molecular level, primarily driven by their distinct self-assembly pathways. However, CT interaction at the macroscopic level allows these fibres to bundle together to form supercoiled ropes. An interfacial photoinduced electron transfer (PET) process from the donor to the acceptor fibres leads to an initial fluorescence quenching, which could be modulated by exposure to strong donor or acceptor type VOCs to regenerate the respective fluorescence of the individual molecular stacks. Thus, strong donors could regenerate the green fluorescence of C3OPV stacks and strong acceptors could reactivate the red fluorescence of C3PBI stacks. These supercoiled supramolecular ropes of self-sorted donor-acceptor stacks provide a simple tool for the detection of donor- or acceptor-type VOCs of biological relevance, using a "turn-off/turn-on" fluorescence mechanism as demonstrated with o-toluidine, which has been reported as a lung cancer marker.

Pyridyl-Amides as a Multimode Self-Assembly Driver for the Design of a Stimuli-Responsive π-Gelator.

An oligo(p-phenylenevinylene) (OPV) derivative connected to pyridyl end groups through an amide linkage (OPV-Py) resulted in a multistimuli-responsive π-gelator. When compared to the corresponding OPV π-gelator terminated by a phenyl-amide (OPV-Ph), the aggregation properties of OPV-Py were found to be significantly different, leading to multistimuli gelation and other morphological properties. The pyridyl moiety in OPV-Py initially interferes with the amide H-bonded assembly and gelation, however, protonation of the pyridyl moiety with trifluoroacetic acid (TFA) facilitated the formation of amide H-bonded assembly leading to gelation, which is reversible by the addition of N,N-diisopropyethylamine (DiPEA). Interestingly, addition of Ag(+) ions to a solution of OPV-Py facilitated the formation of a metallo-supramolecular assembly leading to gelation. Surprisingly, ultrasound-induced gelation was observed when OPV-Py was mixed with a dicarboxylic acid (A1). A detailed study using different spectroscopic and microscopic experimental techniques revealed the difference in the mode of assembly in the two molecules and the multistimuli-responsive nature of the OPV-Py gelation.

A slippery molecular assembly allows water as a self-erasable security marker.

Protection of currency and valuable documents from counterfeit continues to be a challenge. While there are many embedded security features available for document safety, they are not immune to forgery. Fluorescence is a sensitive property, which responds to external stimuli such as solvent polarity, temperature or mechanical stress, however practical use in security applications is hampered due to several reasons. Therefore, a simple and specific stimuli responsive security feature that is difficult to duplicate is of great demand. Herein we report the design of a fluorescent molecular assembly on which water behaves as a self-erasable security marker for checking the authenticity of documents at point of care. The underlying principle involves the disciplined self-assembly of a tailor-made fluorescent molecule, which initially form a weak blue fluorescence (λem = 425 nm, Φf = 0.13) and changes to cyan emission (λem = 488 nm,Φf = 0.18) in contact with water due to a reversible molecular slipping motion. This simple chemical tool, based on the principles of molecular self-assembly and fluorescence modulation, allows creation of security labels and optically masked barcodes for multiple documents authentication.

Detection of nitroaromatic explosives with fluorescent molecular assemblies and π-gels.

Molecular assemblies and gels made up of fluorescent π-systems through noncovalent interactions are fascinating materials with a wide range of properties and applications. Fluorescence is an extremely sensitive property, which gets perturbed upon molecular self-assembly and gelation. Further manipulation of fluorescence in such materials is possible with external stimuli, such as stress, temperature, or with different analytes. Explosives are a class of analytes that respond to certain fluorescent molecular systems; thus allowing their sensing in a required environment. In recent times, this research has become a topic of great demand, resulting in a large number of publications, due to their relevance in safety and security issues. In this account, we record some of the major developments in the field of explosive sensing with fluorescent molecular assemblies and gels.