Research Questions

I work in the area of supramolecular photophysics. Even though the research field has started way back in 1970s, supramolecular chemistry has started gaining wide attention during 1990s, where chemist realized that weak intermolecular forces can be ‘strong enough’ to hold complex chemical structures with desired functional properties. Our group has been focusing on the self-assembly of dendritic structures, small organic molecules and nano-particles in various phases (eg. solution, gel, liquid crystals and solid state), their light-matter interactions and applications.

The research questions that we ask fall in to three broad categories, as represented below.

• What are the molecular level interactions which develops the complex supramolecular structures?
• How these assemblies interact with light?
• How chemists can utilize these assemblies for useful materials applications?

In order to showcase the work done along the above mentioned lines, the findings from our research group are highlighted below.

1. Self-Assembly in Dendron Derivatives

Dendrimers are symmetrically branched molecules with potential applications in areas starting from materials chemistry to biomedical chemistry. We have selected dendritic structures for self-assembly, mainly because: (i) these molecules provide opportunity to utilize multiple non-covalent forces such as hydrogen bonding, p- p interactions, metal-ligand interactions and electro static interactions. Among several dendritic structures available, our research group focuses on the self-assembly of poly (aryl ether) type dendrimers/dendrons and poly (amido amine) {PAMAM} type dendrimers/dendrons.

a) Self-Assembly in Poly(aryl ether) Dendron Derivatives

Supramolecular chemistry of poly (aryl ether) Dendron derivatives has been studied in detail in solution and gel phase. Our research group has been publishing in this area since 2006 and the important findings can be summarized as:

• We have developed super gelators based on poly(aryl ether) dendrons
• Novel photophysical properties were identified in the self-assembled structures, compared to that from monomers.
• We have developed gels which are luminescent under UV/Visible light.
• We have developed colorimetric and fluorescent based sensors based on the self-assembly of poly(aryl ether) based structures
• We have achieved tunable morphology by proper design of monomer structures in poly(aryl ether) based compounds
• We have utilized the self-assembled systems as hosts for linearly arranging metal nanoparticles

The above said findings are published in international journals of high repute and a few representative papers are given below.

P. K. Lekha, E. Prasad “Aggregation Controlled Excimer Emission from Anthracene Containing PAMAM Dendrimers” Chem. Eur. J.2010,16, 3699-3706

P. K. Lekha, E. Prasad “Tunable Emission of Static Excimer in Pyrene Modified PAMAM Dendrimer Aggregate through Positive Solvatochromism”, Chem. -Eur. J.2011,17,8609

P. Rajamalli and Edamana Prasad “Low Molecular Weight Fluorescent Organogel for Fluoride Ion Detection” Org. Lett.2011 , 13, 3714.

P. Rajamalli, Partha Sarathi Sheet, Edamana Prasad, Glucose Cored Poly(Aryl Ether) Dendron based Low Molecular Weight Gels: pH Controlled Morphology and Hybrid Hydrogel Formation, Chem. Commun.2013 ,49, 6758-6760

P. Rajamalli, Supriya Atta, Sandeepan Maity, Edamana Prasad, Supramolecular Design for Two-Component Hydrogels with Intrinsic Emission in the Visible Region, Chem. Comm.2013,49, 1744-1746.

P. Rajamalli and Edamana Prasad, Tunable Morphology and Mesophase Formation by Naphthalene Containing Poly (aryl ether) Dendron Based Low Molecular Weight Fluorescent Gels, Langmuir,2013, 29, 1609-1617.

P. Rajamalli and Edamana Prasad, Non-amphiphilic pyrene cored poly(aryl ether) dendron based gels: tunable morphology, unusual solvent effects on the emission and fluoride ion detection by the self-assembled superstructures, Soft Matter,2012,8, 8896

Partha Malakkar and Edamana Prasad, ”Self-Assembly and Gelation of Poly(aryl Ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures”, Chem. Eur. J., 2015, 21, 5093

Neelakandan Vidhya Lakshmi, Thunga Madhu Babu and Edamana Prasad “Synthesis of Multi-Functional Materials through Self-Assembly of N-Alkyl Phenothiazine Linked Poly(Aryl Ether) Dendrons” Chem Commun.2016, 52, 617

b) Using gel medium for efficient resonance energy transfer:

In a recent work, we have utilized the gel medium made by poly aryl ether to align donor (poly cyclic hydrocarbons)-acceptor (lanthanide ions) molecules for efficient resonance energy transfer (RET). The self-assembled gelators were designed in such a way that the donor and acceptor molecules can co-ordinate to the gel fibers, resulting in an efficient resonance energy transfer, Conversely, in a solution medium, chances of non-radiative decays are very high due to the increased solvent dynamics and efficiency will drastically decreas. The metal-centered quantum yield values, through RET, as high as 11.9% were obtained from such assembly. Further, partial RET in the system has been utilized to generate white-light emission from the gel by incorporating an additional lanthanide ion, Tb(III), along with the organic donors and Eu(III).

Introducing new class of materials for organic electronics: Charge-transfer assisted gelation has been achieved in poly(aryl ether) dendron derivatives using a commonly used electron acceptor (trinitrofluorenone) through the selective incorporation of the acceptor unit in between the aromatic donor moieties in the dendron derivatives. As a result of this incorporation, the conductivity value increased by an order of magnitude (from 4.9 ×10−4 to 1.3×10−3 Sm−1), indicating that this class of compounds can be a new material for organic electronics.

c) Self-Assembly in PAMAM Dendrimer/Dendron Derivatives

Poly(amido amine) dendrimers consists of repeating branches of amido amine units, starting from an ethylene diamine core unit. The structure provides opportunities for intense hydrogen bonding network in the system and facilitates self-assembly. Initial study of self-assembly of PAMAM based dendrimers was focused to understand the mechanistic aspects of the self-assembly. We found that the fractal type assembly followed diffusion limited colloidal aggregation (DLCA), rather than activation limited colloidal aggregation (ALCA) pathway. The research findings in PAMAM self assembly are summarized below:

• We have utilized the intrinsic emission from PAMAM dendrimer to generate materials such as luminescent liquid crystals at room temperature
• We have generated tunable emissions from fluorophores substituted at the periphery of PAMAM dendrimers using the aggregation propensity of PAMAM dendrimers
• We have utilized PAMAM based gels for orienting donor-acceptor molecules with suitable orientation dipole, which facilitate resonance energy transfer

Some of the relevant publications:

M. J. Jasmine, M. Kavitha, Edamana Prasad, “Effect of Solvent Controlled Aggregation on the Intrinsic Emission Properties of PAMAM Dendrimers”, J. Luminescence2009,129,506-513

P. K. Sudhadevi Antharjanam, M. Jaseer, K. N. Ragi, Edamana Prasad, “Intrinsic Luminescence Properties of Ionic Liquid Crystals Based on PAMAM and PPI Dendrimers”, J. Photochem. Photobiol. A: Chemistry2009,203, 50-55

M.Kavitha, M. R. Parida, Edamana Prasad, C. Vijayan, P. C. Deshmukh, “Generation of Ag Nanoparticles by PAMAM Dendrimer and their Size Dependence on the Aggregation Behavior of Dendrimers”, Macromol. Chem. Phys.2009,210, 1310-1318

M. J. Jasmine, Edamana Prasad, “Fractal Growth of PAMAM Dendrimer Aggregates and Its Impact on the Intrinsic Emission Properties”, J. Phys. Chem. B,2010, 114, 7735

P. K. Lekha, E. Prasad, “Aggregation Controlled Excimer Emission from Anthracene Containing PAMAM Dendrimers”, Chem. Eur. J.2010,16, 3699-3706

P. K. Lekha and Edamana Prasad, “Tunable Emission of Static Excimer in Pyrene Modified PAMAM Dendrimer Aggregate through Positive Solvatochromism”, Chem. -Eur. J.2011,17,8609

d) Self-Assembly by Small Organic Molecules and Nano-Systems

Using small organic compounds/nano systems to generate stable molecular assembly for useful applications is a fascinating idea. We have worked towards this direction and achieved the following:

• Ideal white light emission has been achieved in solution and gel phase using commonly available anthracene derivatives and a dye (rhodamine)
• White light emission has been achieved using the aggregation propensity of carbon nano dots in solution and gel phase
• We have used chemicals available from nature to stabilize metal nano-systems and use such assemblies as sensors and photocatalysts

A few relevant works in this direction are:

S. Kaviya and Edamana Prasad “Sunlight Induced Synthesis of Reversible and Reusable Bio-capped Nanoparticles for Metal Ion Detection and SERS Studies” ACS Sustainable Chemistry and Engineering,2014 , 2, 699

S. Kaviya and Edamana Prasad, “Biogenic synthesis of ZnO–Ag nano custard apples for efficient photocatalytic degradation of methylene blue by sunlight irradiation” RSC Adv.,2015 , 5, 17179

Partha Malakar, Debadrita Modak and Edamana Prasad, “Pure White Light Emission from Organic Molecules Using Solvent Induced Selective Self-Assembly”, Chem. Commun. 2016, 52, 4309-4312

S. Kaviya and Edamana Prasad, “Eco-friendly Synthesis of ZnO Nano Pencils in Aqueous Medium: A Study of Photocatalytic Degradation of Methylene Blue under Direct Sunlight” RSC Advances 2016 , 6, 33821-33827

The recent work of generation of white light emission from small organic molecules (anthracene derivative and a dye) in water indicates that judicious molecular level design aspects can lead to achieve desired materials application.

Applications: The self-assembled materials can be utilized for a number of applications as sensors, controlled guest release systems, photocatalysts, gel based ink, hydrophobic surface, phase selective gels for oil spill removal and luminescent gels.

Composite Materials: We have recently developed materials with remarkable tunability of self-healing property for a low molecular weight hybrid gel, based on poly(aryl ether) dendron derivative (PAD). The hybrid gel has been synthesized using PAD and poly(amido amine) {PAMAM} dendrimer derivative (QPD), which are intercalated in the montmorillonite clay (MMT) layers. The self-healing of the hybrid gel (QPD-MMTPAD) was demonstrated through experiments where the distorted gel regained the initial value of storage modulus (G′) within a few minutes. Further, the propensity of self-healing of the gel has been tuned as a function of QPD concentration. The mechanically stable QPD-MMT-PAD hybrid gel has been utilized for the adsorption of ppm level concentration of polycyclic aromatic hydrocarbons (PAHs) such as β-naphthol, pyrene, and phenenathrene from water with excellent efficiency (80−98%).

Copies of the following three papers are attached to this document , at the end.

• Tufan Ghosh and Edamana Prasad, “”White Light Emission from Unmodified Graphene Quantum Dots” J. Phys. Chem. C. 2015, 119, 2733
• Partha Malakar,Debadrita Modak and Edamana Prasad, “Pure White Light Emission from Organic Molecules Using Solvent Induced Selective Self-Assembly”, Chem. Commun. 2016, 52, 4309-4312
• Sitakant Satpathy and Edamana Prasad, “Charge Transfer Modulated Self-Assembly in Poly(aryl ether) Dendron Derivatives with Improved Stability and Transport Characteristics”, ACS Applied Materials & Interfaces, 2016, 8, 26176–26189

Future Directions

Our research group plans the future activities in two broad categories:

• Using the knowledge generated using the past ten years, focus will be given to develop technologies in the areas of: (i) sensing toxic metal ions from water, (ii) degradation of dyes from waste water collected from industry, (iii) developing efficient solar cells, (iv) developing materials with hydrophobic surface, acid resistance property and shape-memory effect and (v) guest/drug release systems.
• Find out the answers for more fundamental questions in supramolecular photophysics: (i) how can we generate a long lived charge transfer state for energy harvesting? (ii) How do we develop assemblies where electron/energy transfer efficiency is very high?