About Supramolecular Assemblies of π-Conjugated Systems

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

In recent years there has been immense interest in studying gels derived from low molecular mass gelators (supramolecular, or simply molecular gels). The motivation for this is not only to understand the fundamental aggregate structures in the gels at different length scales, but also to explore their potential for futuristic technological applications. Gels have been made sensitive to external stimuli like light and chemical entities by incorporating a spectroscopically active or a receptor unit as part of the gelator molecule. This makes them suitable for applications such as sensing and actuating. The diversity of gel structural architectures has allowed them to be utilized as templates to prepare novel inorganic superstructures for possible applications in catalysis and separation. Gels derived from liquid crystals (anisotropy gels) that can act as dynamically functional materials have been prepared, for example, for (re-writable) information recording. Supramolecular gels can be important in controlled release applications, in oil recovery, for gelling cryogenic fuels etc. They can also serve as media for a range of applications. This tutorial review highlights some of the instructive work done by various groups to develop smart and functional gels, and covers a wide spectrum of scientific interest ranging from medicine to materials science.

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Supramolecular gels: Functions and uses

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

Functional π-Gelators and Their Applications

This Account is focused on the self-assembly of p-phenylenevinylenes, a linear π-system, which has been extensively studied over the years due to both fundamental and technological importance. A serendipitous observation of the gelation of an oligo(p-phenylenevinylene) (OPV) derivative in nonpolar hydrocarbon solvents that led to a new class of functional materials, namely, π-organogels, is described. Strategies to control the size, shape, and functions of the supramolecular architectures of OPV self-assemblies are highlighted. Formation of nano- to microsized helical architectures, control on chromophore packing, self-assembly induced modulation of optical properties, and application as light-harvesting assemblies are the important features of this novel class of photonically and electronically active soft materials.

π-Organogels of Self-Assembled p-Phenylenevinylenes: Soft Materials with Distinct Size, Shape, and Functions

The elegance and efficiency by which Nature harvests solar energy has been a source of inspiration for chemists to mimic such process with synthetic molecular and supramolecular systems. The insights gained over the years from these studies have contributed immensely to the development of advanced materials useful for organic based electronic and photonic devices. Energy transfer, being a key process in many of these devices, has been extensively studied in recent years. A major requirement for efficient energy transfer process is the proper arrangement of donors and acceptors in a few nanometers in length scale. A practical approach to this is the controlled self-assembly and gelation of chromophore based molecular systems. The present tutorial review describes the recent developments in the design of chromophore based organogels and their use as supramolecular scaffolds for excitation energy transfer studies.

Organogels as scaffolds for excitation energy transfer and light harvesting

Reaping the benefit: A supramolecular light-harvesting antenna has been developed by encapsulating small amounts of a π-conjugated oligomer (molecular wire) within a self-assembled gel-forming donor scaffold. The supramolecular tapes of oligo(p-phenylenevinylene)s facilitate fast exciton migration and funneling of the excitation energy to the encapsulated molecular wire, thereby resulting in intense red emission (see picture).

Molecular Wire Encapsulated into π Organogels: Efficient Supramolecular Light‐Harvesting Antennae with Color‐Tunable Emission

A white-light-emitting organogel is designed by the controlled self-assembly of a bischolesterol-functionalized OPV donor that allows slow energy migration and partial energy transfer in the gel state to an encapsulated acceptor. The white-light emission occurs because of the combination of blue-light emission from the OPV monomers, green-light emission from the OPV self-assembly, and red-light emission from the acceptor.

Vakayil K. Praveen

Papers

Functional π-Gelators and Their Applications

π-Organogels of Self-Assembled p-Phenylenevinylenes: Soft Materials with Distinct Size, Shape, and Functions

Organogels as scaffolds for excitation energy transfer and light harvesting

Molecular Wire Encapsulated into π Organogels: Efficient Supramolecular Light‐Harvesting Antennae with Color‐Tunable Emission

RGB Emission through Controlled Donor Self‐Assembly and Modulation of Excitation Energy Transfer: A Novel Strategy to White‐Light‐Emitting Organogels