Research Projects

Molecular self-assembly can be a powerful strategy for fabricating dynamic and responsive two-dimensional (2D) nanomaterials. However, framing a rational design principle that can satisfy the strict thermodynamic and kinetic criteria enabling a uniform in-plane growth, presents a considerable challenge. Herein we propose to develop a design strategy for creating two-dimensional (2D) molecular assemblies based on naphthalene and perylene diimides (NDI and PDI, respectively) scaffolds. In addition to being bright chromophores and excellent n-type semiconductors, NDI and PDI are known to assemble into pi-stacked 1D nanostructures. Guiding these molecules to form 2D assembly requires a strategy that promotes strong noncovalent interactions along two orthogonal directions, and discourages any significant interaction along the third direction. The proposed approach builds on our serendipitous discovery of 2D assembly of cyano-NDI in water-dioxane mixtures, which is driven by a combination of pi-stacking, n-pi* and hydrophobic interactions. We propose to undertake a comprehensive investigation of factors that can modulate the strength and directionality of these interactions and affect 2D anisotropy of the assembled structures. This will involve design and synthesis of molecules with appropriate directional noncovalent interactions. Substitutions at the NDI core can alter the nature and orientation of the nonbonding electron-donor site, and change the -acidity of the NDI/PBI ring, which in turn can tune the strength and directionality of pi-stacking and n-pi* interactions. Substituents at the imide N can affect solvation, hydrophobic effects and/or introduce electrostatic interactions. The effect of asymmetric substitution at the NDI core and/or the imide positions will also be investigated. We further envisage that the proposed library of molecules will present the opportunity to explore various challenging and topical problems in molecular self-assembly. We propose to design molecules with dynamic self-assembly characteristics that can exhibit reversible polymorphism between different 2D arrangements, as well as 2D to 1D and 2D to 0D dimensional polymorphism. The nature and chemistry of the functional group(s) displayed at the exposed surface of a 2D assembly will be exploited to grow assembled heterostructures. Finally, we shall explore the possibility of using point as well as axial chirality of the molecular building block or enantiospecific surface interactions to introduce chirality in a 2D supramolecular array. Objectives 1. Understanding structure-property relationships in the context of pi-stacking and n-pi* and solvent-solute interactions in order to develop a design strategy for 2D molecular self-assembly. 2. Exploit competing noncovalent interactions to introduce polymorphism and stimuli-responsiveness in 2D self-assemblies. 3. Secondary nucleation at the 2D assembly surface to grow multi-component assemblies. 4. Incorporate point and axial chirality or utilize enantiospecific surface interactions to achieve global chirality in a 2D assembly.