Vakayil K. Praveen

White-light-emitting supramolecular gels.

Let there be light, let it be white: Recent developments in the use of chromophore-based gels as scaffolds for the assembly of white-light-emitting soft materials have been significant. The main advantage of this approach lies in the facile accommodation of selected luminescent components within the gel. Excitation-energy-transfer processes between these components ultimately generate the desired light output.

Bioinspired Superhydrophobic Coatings of Carbon Nanotubes and Linear π Systems Based on the “Bottom-up” Self-Assembly Approach†

Rough and clean: The physical interaction and self-assembly of oligo(p-phenylenevinylene)s (OPVs) on carbon nanotubes (CNTs) allow the dispersion of the latter in organic solvents. This well-dispersed nanocomposite can be coated on to glass, metal, and mica surfaces to give superhydrophobic self-cleaning surfaces with water contact angles of 165-170° and a sliding angle of less than 2° (see picture).

Self-Assembly of Oligo(para-phenylenevinylene)s through Arene—Perfluoroarene Interactions: π Gels with Longitudinally Controlled Fiber Growth and Supramolecular Exciplex-Mediated Enhanced Emission

The arene-perfluoroarene (ArH-ArF) interaction, which has been extensively studied in the field of solid-state chemistry, is exploited in the hierarchical self-assembly of oligo(para-phenylenevinylene)s (OPVs) with controlled longitudinal fiber growth that leads to gelation. The size of the self-assembled fibers of a pentafluorophenyl-functionalized OPV 5 could be controlled through C-FH--C hydrogen bonding and pi stacking. The ability of fluoroaromatic compounds to form excited-state complexes with aromatic amines has been utilized to form a supramolecular exciplex, exclusively in the gel state, that exhibits enhanced emission. Thus, the commonly encountered fluorescence quenching during the self-assembly of OPVs could be considerably prevented by exciplex formation with N,N-dimethylaniline (DMA), which only occurred for the fluorinated OPV and not for the non-fluorinated analogue 4. In the former case, a threefold enhancement in the emission intensity could be observed in the gel state, whereas no change in emission occurred in solution. Thus, the major limitations of spontaneous fiber growth and fluorescence self-quenching encountered in the self-assembly of OPVs could be controlled to a great extent by using the versatile ArH-ArF interaction.

Carbon Nanotube Triggered Self‐Assembly of Oligo(p‐phenylene vinylene)s to Stable Hybrid π‐Gels

Gel self-assembly: Addition of small amounts of carbon nanotubes (CNTs) to a solution of oligo(p-phenylene vinylene) OPV1 in toluene triggers their self-assembly to form a composite gel (see picture) with higher stability and better rheological properties than the OPV1 gel. Strong physical interactions between the species allow the encapsulation of CNTs into self-assembled tapes of OPV1, reinforcing the 3D gel network.

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.

Ultrasound Stimulated Nucleation and Growth of a Dye Assembly into Extended Gel Nanostructures

A squaraine dye functionalized with a bulky trialkoxy phenyl moiety through a flexible diamide linkage (GA-SQ) capable of undergoing self-assembly has been synthesized and fully characterized. Rapid cooling of a hot solution of GA-SQ to 0 °C results in self-assembled precipitates consisting of two types of nanostructures, rings and ill-defined short fibers. The application of ultrasound modifies the conditions for the supersaturation-mediated nucleation, generating only one kind of nuclei and prompting the formation of crystalline fibrous structures, inducing gelation of solvent molecules. The unique self-assembling behavior of GA-SQ under ultrasound stimulus has been investigated in detail by using absorption, emission, FT-IR, XRD, SEM, AFM and TEM techniques. These studies reveal a nucleation growth mechanism of the self-assembled material, an aspect rarely scrutinized in the area of sonication-induced gelation. Furthermore, in order to probe the effects of nanoscale substrates on the sonication-induced self-assembly, a minuscule amount of single-walled carbon nanotubes was added, which leads to acceleration of the self-assembly through a heterogeneous nucleation process that ultimately affords a supramolecular gel with nanotape-like morphology. This study demonstrates that self-assembly of functional dyes can be judiciously manipulated by an external stimulus and can be further controlled by the addition of carbon nanotubes.

Photoresponsive metal–organic materials: exploiting the azobenzene switch

Photoinduced cis–trans isomerisation of azobenzene has been an inspiration to chemists to design smart materials that respond to light. Lately, this chemistry has also made a mark in the emerging area of metal–organic materials. If one can regulate the properties of porous hybrid materials with light, their scope of applications can be expanded. The field is shown to be promising and gradually progressing from nano to mesospace. However, one needs more in-depth research to deliver materials that are truly smart and practically viable.

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.

Probing the initial stages of molecular organization of oligo(p-phenylenevinylene) assemblies with monolayer protected gold nanoparticles

Thiol-protected gold nanoparticles (GNPs) have been used to probe the initial stages of the molecular organization of oligo(p-phenylenevinylene) (OPV) gelators. The hybrid materials prepared by the self-assembly of OPVs and GNPs are characterized by optical microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. GNPs are located preferentially on the sides of the OPV structures, which implies the presence of alkyl chains at the edges, which makes the assemblies hydrophobic. TEM analyses at the early stages of self-assembly show tapes that have a width of 4 nm, which upon further self-assembly, form fibrils through hydrogen bonding. The experiment was performed with GNPs protected with dodecane and octadecane thiols. The existence of tapes, ribbons, fibrils, and fibers were confirmed by nanoparticle marking. Based on the experimental data, we have proposed a hierarchical model for the self-assembly of OPV molecules. The presence of nanoparticles does not alter the morphology or electronic properties of the OPV structures, as revealed by microscopic and spectroscopic studies.

Effect of the Bulkiness of the End Functional Amide Groups on the Optical, Gelation, and Morphological Properties of Oligo(p-phenylenevinylene) π-Gelators

Herein, we describe the role of end functional groups in the self-assembly of amide-functionalized oligo(p-phenylenevinylene) (OPV) gelators with different end-groups. The interplay between hydrogen-bonding and π-stacking interactions was controlled by the bulkiness of the end functional groups, thereby resulting in aggregates of different types, which led to the gelation of a wide range of solvents. The variable-temperature UV/Vis absorption and fluorescence spectroscopic features of gelators with small end-groups revealed the formation of 1D H-type aggregates in CHCl(3). However, under fast cooling in toluene, 1D H-type aggregates were formed, whereas slow cooling resulted in 2D H-type aggregates. OPV amide with bulky dendritic end-group formed hydrogen-bonded random aggregates in toluene and a morphology transition from vesicles into fibrous aggregates was observed in THF. Interestingly, the presence of bulky end-group enhanced fluorescence in the xerogel state and aggregation in polar solvents. The difference between the aggregation properties of OPV amides with small and bulky end-groups allowed the preparation of self-assembled structures with distinct morphological and optical features.