Jatish Kumar

Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies

Circularly polarized luminescence, or CPL, is a luminescence phenomenon that provides the differential emission intensity of right and left circularly polarized light, thereby providing information on the excited state properties of the chiral molecular systems. In recent years, there has been a growing interest toward the development of organic chromophores capable of circularly polarized emission due to their potential applications in sensors, asymmetric synthesis as well as display and optical storage devices. The major drawback with organic molecules is the low dissymmetric factors exhibited by these systems. One of the recent strategies adopted for the improvement in luminescence dissymmetry of organic systems is through the controlled self-assembly of chromophores. In this Perspective, we highlight the recent experimental and theoretical developments in the field of chiral organic chromophoric systems and their self-assembly, that has produced promising results toward the enhancement of glum values in CPL.

Self-Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofibers†

Directing the supramolecular polymerization towards a preferred type of organization is extremely important in the design of functional soft materials. Proposed herein is a simple methodology to tune the length and optical chirality of supramolecular polymers formed from a chiral bichromophoric binaphthalene by the control of enantiomeric excess (ee). The enantiopure compound gave thin fibers longer than a few microns, while the racemic mixture favored the formation of nanoparticles. The thermodynamic study unveils that the heterochiral assembly gets preference over the homochiral assembly. The stronger heterochiral binding over homochiral one terminated the elongation of fibrous assembly, thus leading to a control over the length of fibers in the nonracemic mixtures. The supramolecular polymerization driven by π-π interactions highlights the effect of the geometry of a twisted π-core on this self-sorting assembly.

Circularly Polarized Luminescence in Supramolecular Assemblies of Chiral Bichromophoric Perylene Bisimides

Chiral bichromophoric perylene bisimides are demonstrated as active materials of circularly polarized emission. The bichromophoric system exhibited circularly polarized luminescence with dissymmetry factors typical of that of similar organic chiral chromophoric systems in the monomeric state. Variation in solvent composition led to the formation of stably soluble helical aggregates through intermolecular interactions. A large enhancement in the dissymmetry of circularly polarized luminescence was exhibited by the aggregated structures both in the solution and solid states. The sum of excitonic couplings between the individual chromophoric units in the self-assembled state results in relatively large dissymmetry in the circularly polarized luminescence, thereby giving rise to enhanced dissymmetry factors for the aggregated structures. The spacer between chiral center and chromophoric units played a crucial role in the effective enhancement of chiroptical properties in the self-assembled structures. These materials might provide opportunities for the design of a new class of functional bichromophoric organic nanoarchitectures that can find potential applications in the field of chiroptical memory and light-emitting devices based on supramolecular electronics.

Inversion of supramolecular chirality in bichromophoric perylene bisimides: influence of temperature and ultrasound.

The supramolecular helicity in the self-assembled nanostructures of two perylene bisimide bichromophoric systems could be controlled by varying the preparatory methods. The self-assembly of the compounds under different conditions was investigated in detail by using absorption, fluorescence, CD, FTIR, XRD, TEM, and SEM techniques. These studies reveal that the heating-cooling method results in aggregates with ordered molecular packing and enhanced optical chirality. Ultrasonication leads to molecular aggregates with less ordered packing wherein the supramolecular chirality was reversed relative to the sample prepared via a heating-cooling method. This heating-cooling method proved to be superior in terms of nanofiber synthesis, yielding fibers with extended length and a prominent helical twist. At higher concentration, both compounds exhibited a gelation property in benzonitrile. The tunable chiroptical properties in these supramolecular systems make them potential candidates for applications in the field of optical and electronic device fabrication based on organic nanostructures.

Surface plasmon coupling in end-to-end linked gold nanorod dimers and trimers

Colloidal gold nanorods were aligned end-to-end via dithiol coupling. The scattering properties of the resultant nanostructures were investigated at the single particle level by combining dark-field microscopy and high resolution scanning electron microscopy. The longitudinal surface plasmon resonance of end-to-end coupled Au nanorods exhibited a red-shift as the number of rods in the chain increased. The nanostructures exhibited polarization-dependent optical properties, due to selective excitation of collective bonding and anti-bonding modes. The surface plasmon peak energy was not strongly dependent on the angle of rod-sphere-rod trimers. The experimental scattering spectra were compared with the results obtained from theoretical calculations using the Finite Element Method (FEM) and found to be in good agreement.

Nanoscale chirality in metal and semiconductor nanoparticles

We discuss optical activity in metal nanoclusters and semiconductor quantum dots, broadly focusing on recent advances in nanoscale chirality in plasmonic nanoparticles and their assemblies.

Bis(dipyrrinato)zinc(II) Complex Chiroptical Wires: Exfoliation into Single Strands and Intensification of Circularly Polarized Luminescence

One-dimensional (1D) coordination polymers (CPs) experiences limitations in exfoliation into individual strands, which hamper their utility as functional 1D nanomaterials. Here we synthesize chiral 1D-CPs that feature the bis(dipyrrinato)zinc(II) complex motif. They can be exfoliated into single strands upon sonication in organic media, retaining lengths of up to 3.19 μm (ca. 2600 monomer units). Their chiroptical structure allows the exfoliated wires to show circularly polarized luminescence at an intensity 5.9 times that of reference monomer complexes.

Enantioselective Light Harvesting on Self-Assembled Nanofibers

Self-assembling molecular systems often display amplified chirality compared to the monomeric state, which makes the molecular recognition more sensitive to chiral analytes. Herein, we report the almost absolute enantioselective recognition of a chiral perylenediimide (PDI) molecule by chiral supramolecular nanofibers of a bichromophoric naphthalenediimide (NDI) derivative. The chiral recognition was evaluated through the Förster resonance energy transfer (FRET) from the NDI-based host nanofibers to the guest PDI molecules. The excitation energy was successfully transferred to the guest molecule through efficient energy migration along the host nanofiber, thus demonstrating the light-harvesting capability of these hybrid systems. Furthermore, circularly polarized luminescence (CPL) was enantioselectively sensitized by the guest molecule as the wavelength band and sign of the CPL signal were switched in response to the chiral guest molecule.

Synthesis, characterisation and biocompatibility of surface-functionalised gold nanoparticles

Understanding and controlling the interactions between nanoparticles and living cells are of great importance in the diagnosis and therapeutic applications of nanosized materials. This article describes the synthesis, characterisation and interactions of gold nanoparticles (AuNPs) with different in vitro and in vivo experimental models. Preliminary cytotoxicity studies were carried out in tumour ascites (Daltons lymphoma ascites and Ehrlichs ascites carcinoma) and normal peritoneal cells for different concentrations and it was found that AuNPs did not cause any cell death or morphological changes even at 100 µM concentrations. The tissue distribution and toxicity of intravenously administered AuNPs were investigated in mice because of the fundamental importance of obtaining information about the localisation and biocompatibility of this material. Tissue distribution of AuNPs was studied in normal as well as skin tumour-induced Swiss albino mice using inductively coupled plasma-atomic emission spectrophotometer and transmission electron microscopy. Acute cytotoxicity and histopathology were carried out for toxicity evaluation in AuNP-injected normal mice and found that AuNPs did not cause any significant metabolic change. The health and behaviour of animals were normal throughout the study. The biocompatibility assessment of AuNPs both in vitro and in vivo confirmed that triethylene glycol-functionalised AuNPs are compatible to the biological system and can be used as a safe material for various biological applications.

Detection of amyloid fibrils in Parkinson’s disease using plasmonic chirality

Significance This contribution reports on the application of gold nanorods to the detection of amyloids in Parkinson’s and prion diseases. We found that gold nanorods show no interaction with monomeric proteins but adsorb onto helical protein fibrils. Chiral amyloid templates induce helical arrangement of nanorods, giving rise to intense optical activity at the plasmon resonance wavelengths. This report shows the use of protein fibrils as templates for chiral nanoparticle assembly and development of a biodetection technique. We show this effect on a model recombinant protein, α-synuclein (involved in Parkinson’s disease), using CD, cryogenic transmission EM tomography, and theoretical simulations supporting the experimental findings. We additionally show application to identify patients with Parkinson’s disease from human brain homogenates. Amyloid fibrils, which are closely associated with various neurodegenerative diseases, are the final products in many protein aggregation pathways. The identification of fibrils at low concentration is, therefore, pivotal in disease diagnosis and development of therapeutic strategies. We report a methodology for the specific identification of amyloid fibrils using chiroptical effects in plasmonic nanoparticles. The formation of amyloid fibrils based on α-synuclein was probed using gold nanorods, which showed no apparent interaction with monomeric proteins but effective adsorption onto fibril structures via noncovalent interactions. The amyloid structure drives a helical nanorod arrangement, resulting in intense optical activity at the surface plasmon resonance wavelengths. This sensing technique was successfully applied to human brain homogenates of patients affected by Parkinson’s disease, wherein protein fibrils related to the disease were identified through chiral signals from Au nanorods in the visible and near IR, whereas healthy brain samples did not exhibit any meaningful optical activity. The technique was additionally extended to the specific detection of infectious amyloids formed by prion proteins, thereby confirming the wide potential of the technique. The intense chiral response driven by strong dipolar coupling in helical Au nanorod arrangements allowed us to detect amyloid fibrils down to nanomolar concentrations.